Grapevine Structure and Function

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5
Grapevine Structure and Function
Edward W. Hellman
This chapter presents an overview of grapevine structure and function to provide a
basic understanding of how grapevines grow. Such understanding is the foundation
of good vineyard management, and the practical application of this knowledge is
emphasized throughout this book. The reader should consult the references cited in this chapter
for more complete coverage of these topics. Much of the common viticultural terminology is
introduced in this chapter.
Grapevine Structure
Cells and Tissues
The basic unit of plant structure and function is the
cell. All cells have the same general organization,
consisting of a cell wall, protoplasm (liquid-filled
region containing living organelles), and the vacuole
(region containing the cell sap). This basic cell
structure is modified to create different cell types that
are capable of specialized functions. Organized groups
of specialized cells that perform specific functions are
called tissues. For example, the outside protective
“skin” of grape leaves, the epidermis, consists of one
to several layers of specialized cells. A thorough
discussion of the cell and tissue anatomy of grapevines
has been prepared by Pratt (1974), or a general
discussion of plant anatomy can be found in any
introductory botany textbook.
Meristems
Certain plant cells, termed meristematic cells, perform
the specialized function of growth by the creation of
new cells through cell division. Groups of these cells
are organized into meristems (or growing points),
positioned at various locations on the vine. The apical
meristem is a tiny growing point, hidden from view
within the unfolding leaves at the tip of an expanding
shoot. In addition to the apical meristem, the shoot
produces many additional growing points at the base
of each leaf, called buds; these are described in more
detail below. Each root tip also contains a growing
point. Two specialized meristems, the vascular
cambium and the cork cambium, are responsible for
the radial growth (diameter increase) of woody parts
of the vine.
New xylem and phloem tissues (described below
under Vascular System) are produced every year from
a specialized meristem called the vascular cambium
(or simply cambium). The location and arrangement
of cambium, xylem, and phloem vary between plant
parts (e.g., shoots and roots) and with the develop-
mental stage of the part. The cambium consists of a
single layer of meristematic cells, which produce
xylem cells to the inside and phloem cells outside
(Figure 1). Thus, the annual increase in girth of woody
tissues such as the trunk is a result of the addition of
new xylem and phloem cells from the cambium.
Xylem cells are larger and produced in much greater
abundance than phloem cells, which form tissue only
a few cell layers thick. This causes the cambium always
to be positioned close to the outer surface of a woody
stem. Older xylem can remain functional for up to
seven years, but is mostly inactivated after two or three
years. Some phloem cells continue to function for
three to four years.
The exterior of woody parts of the vine is protected
by periderm, which comprises cork cells and is covered
by an outer bark consisting of dead tissues. Once a
year, some of the cells within the outer, nonfunctional
phloem become meristematic, creating the cork
cambium. The cork cambium produces a layer of new
cells that soon become impregnated with an imper-
meable substance, cutting off the water supply to the
cork cells and older phloem that are external to the
layer. These cells die and add to the layers of bark.
Older bark cracks from the expansion growth of new
bark beneath it, creating the peeling bark that is
characteristic of older wood on grapevines.
Figure 1. Cross section of 3-year-old grapevine arm.
Redrawn from Esau (1948) by Scott Snyder.
Periderm
(Bark)
Phloem
Vascular
Cambium
Xylem
Annual
Ring
Ray

6 Oregon Viticulture
Vascular System
The interior of all the plant parts described below
contains groups of specialized cells organized into a
vascular system that conducts water and dissolved
solids throughout the vine. There are two principal
parts to the vascular system: xylem is the conducting
system that transports water and dissolved nutrients
absorbed by the roots to the rest of the vine, and
phloem is the food-conducting system that transports
the products of photosynthesis from leaves to other
parts of the vine. The xylem and phloem tissues each
consist of several different types of cells, some of which
create a continuous conduit throughout the plant, and
others provide support functions to the conducting
cells, such as the storage of food products in xylem
cells. A group of specialized cells are arranged in
narrow bands of tissue called rays, which extend out
Trunk
Cordon
Arm
Fruiting
Spur
Shoot
Dormant Season Growing Season
Root
System
Crown
Figure 2. Grapevine structures and
features: self-rooted vine. Drawing
by Scott Snyder.
perpendicular from the center of a stem, through the
xylem and phloem. Ray cells facilitate the radial
transfer of water and dissolved substances between
and among xylem and phloem cells and are a site for
storage of food reserves. The vascular system con-
stitutes the wood of older stems, and the thick cell
walls of the xylem provide the principal structural
support for all plant parts.
Parts of the Vine
The shape of a cultivated grapevine is created by
pruning and training the vine into a specific arrange-
ment of parts according to one of many training
systems. Over the centuries, innumerable training
systems have been developed and modified in efforts
to facilitate vine management and provide a favorable
growing environment for the production of grapes.

Grapevine Structure and Function 7
Figures 2 and 3 illustrate a mature grapevine as it might
appear at two representative time periods, trained to
two different systems. The parts of the vine are labeled
with commonly used viticultural terms that often
reflect how we manage the vine rather than describing
distinct morphological structures as defined by
botanists.
The Root System
In addition to anchoring the vine, roots absorb water
and nutrients, store carbohydrates, other foods, and
nutrients for the vine’s future use, and produce
hormones that control plant functions. The root
system of a mature grapevine consists of a woody
framework of older roots (Richards, 1983) from which
permanent roots arise and grow either horizontally or
vertically. These roots are typically multi-branching,
producing lateral roots that can further branch into
smaller lateral roots. Lateral roots produce many
short, fine roots, which has the effect of increasing the
area of soil exploited. Certain soil fungi, mycorrhizae,
live in a natural, mutually beneficial association with
grape roots. Mycorrhizae influence grapevine nut-
rition and growth and have been shown to increase
the uptake of phosphorus.
The majority of the grapevine root system is usually
reported to be within the top 3 feet of the soil, although
individual roots can grow much deeper under
favorable soil conditions. Distribution of roots is
influenced by soil characteristics, the presence of
hardpans or other impermeable layers, the rootstock
variety (see below), and cultural practices such as the
type of irrigation system.
Grapevines can be grown “naturally” on their own
root system (own-rooted or self-rooted vines) or they
may be grafted onto a rootstock. A grafted vine (Figure
3) consists of two general parts, the scion variety (e.g.,
Pinot noir), which produces the fruit, and the rootstock
variety (often denoted by numbers, e.g., 101-14),
which provides the root system and lower part of the
trunk. The position on the trunk where the two
varieties were joined by grafting and subsequently
grew together is called the graft union. Successful
healing of the graft union requires that the vascular
cambiums of the stock and scion be in contact with
each other, since these are the only tissues having the
meristematic activity necessary for the production of
new cells to complete the graft union. Healing of the
graft union often results in the production of abundant
callus (a wound healing tissue composed of large thin-
walled cells that develop in response to injury) tissue,
often making the area somewhat larger than adjacent
parts of the trunk. Because rootstock and scion
varieties may grow at different rates, trunk diameter
can vary above and below the graft union.
Rootstock varieties were developed primarily to
provide a root system for Vitis vinifera L. (“European”
winegrape) varieties that is resistant or tolerant to
phylloxera, a North American insect to which V.
vinifera roots have no natural resistance. Most
phylloxera-resistant rootstocks are either native North
American species or hybrids of two or more of these
species, including V. riparia, V. berlandieri, and V.
rupestris. The rooting pattern and depth, as well as
other root system characteristics, vary among the
species and hybrid rootstocks, so the rootstock can
Bud Arms
Cane Head
Shoo
t
Growing Season
Dormant Season
Watersprout
Sucker
(from Rootstock)
Scion
Graft
Union
Rootstock
Figure 3. Grapevine structures and
features: grafted vine. Drawing by Scott
Snyder.

8 Oregon Viticulture
influence aspects of vine growth, including vigor,
drought tolerance, nutrient uptake efficiency, and pest
resistance. Rootstock variety selection is, therefore, an
important factor in vineyard development.
The Trunk
The trunk, formerly an individual shoot, is perm-
anent and supports the aboveground vegetative and
reproductive structures of the vine. The height of the
trunk varies among training systems, and the top of
the trunk is referred to as the head. The height of the
head is determined by pruning during the initial stages
of training a young grapevine. The trunk of a mature
vine has arms, short branches from which canes or
spurs (defined below) originate; arms are located in
different positions depending on the system. Some
training systems utilize cordons (Figure 2), semi-
permanent branches of the trunk, usually trained
horizontally along a trellis wire, with arms spaced at
regular intervals along their length. Other systems
utilize canes (Figure 3), one-year-old wood arising
from arms usually located near the head of the vine.
The crown refers to the region of the trunk near the
ground, from slightly below to slightly above ground
level.
Shoots and Canes
The shoot is the primary unit of vine growth and the
principal focus of many viticultural practices. Shoots
are the stemlike green growth arising from a bud.
Primary shoots arise from primary buds (described
below) and are normally the fruit-producing shoots
on the vine. The components of the shoot are
illustrated in Figure 4, and the stages of grapevine
growth and flower and fruit development are shown
in Figure 5. The main axis of the shoot consists of
structural support tissues and conducting tissues to
transport water, nutrients, and the products of
photosynthesis. Arranged along the shoot in regular
patterns are leaves, tendrils, flower or fruit clusters,
and buds. General areas of the shoot are described as
basal (closest to its point of origin), mid-shoot, and
apex (tip). The term canopy is used to denote the
collective arrangement of the vine’s shoots, leaves and
fruit; some viticulturists also consider the trunk,
cordons, and canes to be parts of the canopy.
Shoot Tip. The shoot has many points of growth,
but the extension growth of the shoot occurs from the
shoot tip (growing tip). New leaves and tendrils unfold
from the tip as the shoot grows. Growth rate of the
shoot varies during the season. Grapevine shoots do
not stop expanding by forming a terminal bud as some
plants do; they may continue to grow if there is
sufficient heat, soil moisture, and nutrients.
Leaves. Leaves are produced at the apical meristem.
The shoot produces two or more closely spaced bracts
(small scalelike leaves) at its base before it produces
the first true foliage leaf. Leaves are attached at the
slightly enlarged area on the shoot referred to as a
node. The area between nodes is called the internode.
The distance between nodes is an indicator of the rate
of shoot growth, so internode length varies along the
cane corresponding to varying growth rates during the
season.
Leaves consist of the blade, the broad, flat part of
the leaf designed to absorb sunlight and CO2 in the
food manufacturing process of photosynthesis (see
below), and the petiole, the stemlike structure that
connects the leaf to the shoot. The lower surface of
leaf blades contains thousands of microscopic pores
called stomata (s., stomate), through which diffusion
of CO2, O2, and water vapor occurs. Stomata are open
in the light and closed in the dark. The petiole
conducts water and food material to and from the leaf
blade and maintains the orientation of the leaf blade
to perform its functions.
Flowers and Fruit. A fruitful shoot usually produces
from one to three flower clusters (inflorescences)
depending on variety, but typically two under Oregon
conditions. Flower clusters develop opposite the
leaves, typically at the third to sixth nodes from the
base of the shoot, depending on the variety. If three
Shoot
Tip
Tendril
Lateral
Shoot
Flower
Cluster
Petiole
Bud
(compound)
Node
Internode
Leaf Blade
Figure 4. Principal features of a grapevine shoot prior to
bloom. Drawing by Scott Snyder.

Grapevine Structure and Function 9
Figure 5. Stages of grapevine growth. Adapted with
permission from Meier (2001).
Principal growth stage 0: Sprouting/Bud development
00 Dormancy: buds pointed to round, light
or dark brown according to variety; bud
scales more or less closed according to
variety
01 Beginning of bud swelling: buds begin to
expand inside the bud scales
03 End of bud swelling: buds swollen, but not
green
05 "Wool Stage": brown wool clearly visible
07 Beginning of bud burst: green shoot tips
just visible
08 Bud burst: green shoot tips clearly visible
Principal growth stage 1: Leaf development
11 First leaf unfolded and spread away from
shoot
12 2nd leaves unfolded
13 3rd leaves unfolded
14 Stages continuous until...
19 9 or more leaves unfolded
Principal growth stage 5: Inflorescence emerge
53 Inflorescences clearly visible
55 Inflorescences swelling, flowers closely
pressed together
57 Inflorescences fully developed; flowers
separating
Principal growth stage 6: Flowering
60 First caps detached from the receptacle
61 Beginning of flowering: 10% of caps fallen
62 20% of caps fallen
63 Early flowering: 30% of caps fallen
64 40% of caps fallen
65 Full flowering: 50% of caps fallen
66 60% of caps fallen
67 70% of caps fallen
68 80% of caps fallen
69 End of flowering
Principal growth stage 7: Development of fruits
71 Fruit set: young fruits begin to swell,
remains of flowers lost
73 Berries swelling, clusters begin to hang
75 Berries pea-sized, clusters hang
77 Berries beginning to touch
79 Majority of berries touching
Principal growth stage 8: Ripening of berries
81 Beginning of ripening: berries begin to
develop variety-specific color
83 Berries developing color
85 Softening of berries
89 Berries ripe for harvest
Principal growth stage 9: Senescence
91 After harvest; end of wood maturation
92 Beginning of leaf discoloration
93 Beginning of leaf-fall
95 50% of leaves fallen
97 End of leaf-fall
99 Harvested product

10 Oregon Viticulture
The successful union is termed fertilization, and the
subsequent growth of berries is called fruit set. The
berry develops from the tissues of the pistil, primarily
the ovary. The ovule together with its enclosed embryo
develops into the seed.
Because there are four ovules per flower, there is a
maximum potential of four seeds per berry. Un-
favorable environmental conditions during bloom,
such as cool, rainy weather, can reduce both fruit set
(number of berries) and berry size. Berry size is related
to the number of seeds within the berry but can also
be influenced by growing conditions and practices,
particularly water management. Some immature
berries may be retained by a cluster without com-
pleting their normal growth and development, a
phenomenon known as millerandage or “hens and
chicks.” See Pratt (1971) for a more complete botanical
description of grapevine reproductive anatomy and
process.
Tendrils. The shoot also produces tendrils—slender
structures that coil around smaller objects (e.g., trellis
wires, small stakes, and other shoots) to provide
support for growing shoots. Tendrils grow opposite a
leaf in the absence of a flower cluster, except the first
two or three leaves and thereafter skipping every third
leaf. Flower clusters and tendrils have a common
developmental origin (Mullins et al., 1992), so
occasionally a few flowers develop on the end of a
tendril.
Buds. A bud is a growing point that develops in the
leaf axil, the area just above the point of connection
between the petiole and shoot. The single bud that
develops in this area is described in botanical terms
as an axillary bud. It is important to understand that
on grapevines a bud develops in every leaf axil,
including the inconspicuous basal bracts (scalelike
leaves). In viticultural terminology, we describe two
buds associated with a leaf—the lateral bud, and the
dormant bud (or latent bud). The lateral bud is the true
axillary bud of the foliage leaf, and the dormant bud
forms in the bract axil of the lateral bud. Because of
Calyptra
(cap) Anther Stigma
Stamen
Pistil
Ovary
Figure 6. Grape flower at two stages of bloom. Left,
early bloom with cap separating from flower base.
Right, flower in full bloom. Drawing by Scott
Snyder.
flower clusters develop, two develop on adjacent
nodes, the next node has none, and the following node
has the third flower cluster. The number of flower
clusters on a shoot is dependent upon the grape
variety and the conditions of the previous season
under which the dormant bud (that produced the
primary shoot) developed. A cluster may contain
several to many hundreds of individual flowers,
depending on variety.
The grape flower does not have conspicuous petals
(Figure 6); instead, the petals are fused into a green
structure termed the calyptra but commonly referred
to as the cap. The cap encloses the reproductive organs
and other tissues within the flower. A flower consists
of a single pistil (female organ) and five stamens, each
tipped with an anther (male organ). The pistil is
roughly conical in shape, with the base dispro-
portionately larger than the top and the tip (the
stigma) slightly flared. The broad base of the pistil is
the ovary, which consists of two internal com-
partments, each having two ovules containing an
embryo sac with a single egg. The anthers produce
many yellow pollen grains, which contain the sperm.
The time during which flowers are open (the
calyptra has fallen) is called bloom (also flowering or
anthesis) and can last from one to three weeks
depending on weather. Viticulturists variously refer to
full bloom as the stage at which either roughly one-
half or two-thirds of the caps have loosened or fallen
from the flowers. Bloom typically occurs between 50
and 80 days after budburst in Oregon.
The stages of bloom (60-69) are illustrated in Figure
5. When the flower opens, the cap separates from the
base of the flower, becomes dislodged, and usually
falls off, exposing the pistil and anthers. The anthers
may release their pollen either before or after capfall.
Pollen grains randomly land upon the stigma of the
pistil. This event is termed pollination. Multiple pollen
grains can germinate, each growing a pollen tube
down the pistil to the ovary and entering an ovule,
where a sperm unites with an egg to form an embryo.

Grapevine Structure and Function 11
their developmental association, the two buds are
situated side-by-side in the main leaf axil.
Although the dormant bud (sometimes called an
eye) looks like a simple structure, it is actually a
compound bud consisting of three growing points,
sometimes referred to as the primary, secondary, and
tertiary buds. The distinction between secondary and
tertiary buds is sometimes difficult to make and often
of little importance, so it is common to refer to both
of the smaller buds as secondary buds. The collection
of buds is packaged together within a group of external
protective bud scales (Figure 7). Continuing the bud
development pattern, the primary growing point is the
axillary bud of the lateral bud; the secondary and
tertiary growing points are the axillary buds of the first
two bracts of the primary growing point.
The dormant bud is of major concern at pruning,
since it contains cluster primordia (the fruit-
producing potential for the next season). It is referred
to as dormant to reflect the fact that it does not
normally grow out in the same season in which it
develops.
The dormant bud undergoes considerable develop-
ment during the growing season. The three growing
points each produce a rudimentary shoot that
ultimately will contain primordia (organs in their
earliest stages of development) of the same basic
components of the current season’s fully grown shoot:
leaves, tendrils, and in some cases flower clusters. The
primary bud develops first, so it is the largest and most
fully developed. If it is produced under favorable
environmental and growing conditions, it will contain
flower cluster primordia before the end of the growing
season. The flower cluster primordia thus represent
the fruiting potential of the bud in the following
season. Reflecting the sequence of development, the
secondary and tertiary buds are progressively smaller
and less developed. They are generally less fruitful
(have fewer and smaller clusters) than the primary
bud. Bud fruitfulness (potential to produce fruit) is a
Secondary
Bud
Primary
Bud
Tertiary
Bud
Leaf Petiole
Figure 7. Cross
section of
dormant grape
bud in leaf axil,
showing primary,
secondary, and
tertiary buds.
Lateral bud not
shown. Drawing
by Scott Snyder.
function of the variety, environmental conditions, and
growing practices. Dormant buds that develop under
unfavorable conditions produce fewer flower cluster
primordia.
In most cases, only the primary bud grows, pro-
ducing the primary shoot. The secondary bud can be
thought of as a “backup system” for the vine; normally,
it grows only when the primary bud or young shoot
has been damaged, often from freeze or frost.
However, under some conditions such as severe
pruning, destruction of part of the vine, or boron
deficiency, it is possible for two or all three of the buds
to produce shoots (Winkler et al., 1974). Tertiary buds
provide additional backup if both the primary and
secondary buds are damaged, but they usually have
no flower clusters. If only the primary shoot grows, the
secondary and tertiary buds remain alive, but dor-
mant, at the base of the shoot.
The lateral bud grows in the current season, but
growth may either cease soon after formation of the
basal bract or continue, producing a lateral shoot
(summer lateral) of highly variable length. Regardless
of the extent of lateral bud development, a compound
bud develops in the basal bract, forming the dormant
bud. Long lateral shoots sometimes produce flower
clusters and fruit, which is known as second crop. But
because they develop later in the season than fruit on
the primary shoot, second crop fruit does not mature
fully in Oregon. If a lateral bud does not grow in the
current season, it will die.
Suckers and Watersprouts. Shoots may also arise
from bud locations on older wood such as cordons and
trunks. Suckers are shoots that grow from the crown
area of the trunk. Watersprout is a term sometimes
used to refer to a shoot arising from the upper regions
of the trunk or from cordons. Buds growing from older
wood are not newly initiated buds; rather, they
developed on green shoots as axillary buds that never
grew out. These buds are known as latent buds,
because they can remain dormant indefinitely until
an extreme event such as injury to the vine or severe
pruning stimulates renewed development and shoot
growth.
Suckers often arise from latent buds at under-
ground node positions on the trunk. In routine vine
management, suckers are removed early in the season
before axillary buds can mature in basal bracts of the
sucker shoots. Similarly, aboveground suckers are
typically stripped off the trunk manually so that a
pruning stub does not remain to harbor additional
latent buds that could produce more suckers in the
following year.
Latent buds come into use when trunk, cordon, or
spur renewal is necessary. Generally, numerous latent
buds exist at the “renewal positions” (a pruning term)

12 Oregon Viticulture
on the trunk or cordons. Dormant secondary and
tertiary buds exist in the stubs that remain after canes
or spurs have been removed by pruning.
Canes. The shoot begins a transitional phase about
midseason, when it begins to mature, or ripen. Shoot
maturation begins as periderm develops, starting at
the shoot base, appearing initially as a yellow, smooth
“skin.” Periderm continues to extend development
toward the shoot tip through summer and fall. As
periderm develops, it changes from yellow to brown
and becomes a dry, hard, smooth layer of bark. During
shoot maturation, the cell walls of ray tissues thicken
and there is an accumulation of starch (storage carbo-
hydrates) in all living cells of the wood and bark. Once
the leaves fall off at the beginning of the dormant
season, the mature shoot is considered a cane.
The cane is the principal structure of concern in
the dormant season, when the practice of pruning is
employed to manage vine size and shape and to
control the quantity of potential crop in the coming
season. Because a cane is simply a mature shoot, the
same terms are used to describe its parts. Pruning
severity is often described in terms of the number of
buds retained per vine, or bud count. This refers to the
dormant buds, containing three growing points,
described above. The “crown” of buds observed at the
base of a cane includes the secondary and tertiary
growing points of the compound bud that gave rise to
the primary shoot, as well as the axillary buds of the
shoot’s basal bracts (Pratt, 1974). These basal buds are
generally not fruitful and do not grow out, so they are
not included in bud counts and may be referred to as
noncount buds.
Canes can be pruned to varying lengths, and when
they consist of only one to four buds they are referred
to as spurs, or often as fruiting spurs since fruitful
shoots arise from spur buds. Grapevine spurs should
not be confused with true spurs produced by apple,
cherry, and other fruit trees, which are the natural
fruit-bearing structures of these trees. On grapevines,
spurs are created by short-pruning of canes. Figure 2
illustrates a vine that is cordon-trained, spur-pruned.
Training systems that use cane-pruning (Figure 3)
sometimes also use spurs for the purpose of growing
shoots to be trained for fruiting canes in the following
season. These spurs are known as renewal spurs,
indicating their role in replacing the old fruiting cane.
Major Physiological Processes
Photosynthesis
Grapevines, like other green plants, have the capacity
to manufacture their own food by capturing the energy
within sunlight and converting it to chemical energy
(food). This multi-stage process is called photo-
synthesis. In simple terms, sunlight energy is used to
split water molecules (H2O), releasing molecular
oxygen (O2) as a byproduct. The hydrogen (H) atoms
donate electrons to a series of chemical reactions that
ultimately provide the energy to convert carbon
dioxide (CO2) into carbohydrates (CH2O). Details of
this complex process are beyond the scope of this
chapter and have been summarized elsewhere
(Mullins et al., 1992).
Photosynthesis occurs in chloroplasts, highly
specialized organelles containing molecules called
chlorophyll, which are abundant in leaf cells. The
structure of a leaf is well adapted to carry out its
function as the primary site of photosynthesis. Leaves
provide a large sunlight receptor surface, an
abundance of specialized cells containing many
chloroplasts, numerous stomata to enable uptake of
atmospheric carbon dioxide, and a vascular system to
transport water and nutrients into the leaf and export
food out.
The products of photosynthesis are generally
referred to as photosynthates (or assimilates), which
include sugar (mostly sucrose) and other carbo-
hydrates. Sucrose is easily transported throughout the
plant and can be used directly as an energy source or
converted into other carbohydrates, proteins, fats, and
other compounds. The synthesis of other compounds
often requires the combination of carbon (C) based
products with mineral nutrients such as nitrogen,
phosphorus, sulfur, iron, and others that are taken up
by the roots. Starch, a carbohydrate, is the principal
form of food energy that the vine stores in reserve for
later use. The carbohydrates cellulose and hemi-
cellulose are the principal structural materials used
to build plant cells. Organic acids (malic, tartaric,
citric) are another early product of photosynthesis and
are used directly or converted into amino acids by the
addition of nitrogen. Amino acids can be stored or
combined to form proteins.
Photosynthates are the food energy used to fuel
plant growth and maintain plant function. The
allocation of photosynthates to different parts of the
vine is described in terms of “sources” and “sinks.”
Leaves are the source of photosynthates, and any plant
part—such as shoots, fruit, or roots—or metabolic
process that utilizes photosynthates is considered a
sink. The amounts of food materials moved to different
points of need (sinks) varies through the season,
depending upon photosynthate production and
demand from the various plant parts (Williams, 1996).
Thus, the majority of foods and food materials are first
sent to actively growing areas such as shoot tips,
developing fruit, and root tips. Later, when growth has
slowed and a full canopy is producing more photo-
synthates than are demanded by growing points,

Grapevine Structure and Function 13
increasing quantities of food are directed to the roots,
trunk, and other woody tissues for storage as reserves.
However, during the ripening phase of fruit dev-
elopment, the fruit cluster is the main sink for
photosynthates, and only surpluses go to reserves.
After harvest, all woody tissues, especially roots, are
the principal sinks. Food reserves in the roots and
woody parts of the vine provide the energy for initial
shoot growth in the spring, before new leaves are
capable of producing more food than they consume.
Sunlight. The process of photosynthesis is
obviously dependent upon sunlight, and it is generally
assumed that between one-third and two-thirds full
sunlight is needed to maximize the rate of photo-
synthesis. The optimization of sunlight captured by
the vine is an important component of canopy
management that not only affects the rate of photo-
synthesis but also directly influences fruit quality.
Sunlight exposure on a vine is highly dependent upon
the training system and the shoot density and can be
influenced by the orientation of the rows and row
spacing. The term canopy management encompasses
many vineyard practices designed to optimize the
sunlight exposure of the grapevine.
Other Environmental Influences. The rate of
photosynthesis in grapevines is also influenced by leaf
temperature; the apparently broad optimum range of
25–35ºC (77–95ºF) may be attributable to differences
in grape variety, growing conditions, or seasonal
variation ( Williams et al., 1994). Leaf temperature can
be highly dependent upon vine water status but
otherwise cannot be influenced to the same extent as
sunlight exposure in the canopy, so it is of less concern
to vineyard management.
Water status of grapevines can have a strong impact
on photosynthetic rate through its control over the
closing of leaf stomata, the sites of gas exchange
critical for photosynthesis. A water deficit exists when
the plant loses more water (via transpiration,
described below) than it takes up from the soil. One
consequence of water deficits is the closure of
stomata, which reduces water loss but also reduces
the uptake of CO2 necessary for photosynthesis. The
extent of stomatal closure, and therefore the impact
on photosynthetic rate, is related to the severity of
water deficit. Vines are considered to be under water
stress when the deficit is extreme enough to reduce
plant functions significantly. The major impact of
water deficits on vine photosynthesis is the reduction
of leaf area (Williams, 1996).
Inadequate supply of certain nutrients (nitrogen
and phosphorus) may also limit photosynthesis
directly, or indirectly by reduced availability of
elements (iron and magnesium) for the synthesis of
chlorophyll.
Respiration
The process by which the stored energy within food is
released for the plant’s use is called respiration. In
simple terms, the end result can be considered to be
the reverse chemical reaction of the photosynthetic
process, although the multiple reactions and sites of
activity are completely different. Respiration involves
the reaction of oxygen with the carbon and hydrogen
of organic compounds, such as carbohydrates, to form
water and CO2 and release energy. Many forms of
carbohydrates, including sugars and starch, can be
oxidized (broken down) by respiration, as can fats,
amino acids, organic acids, and other substances. The
decrease of malic acid, and to a lesser extent tartaric
acid, in ripening fruit is largely attributed to
respiration.
The respiration process has been reviewed by
Mullins et al. (1992), and this summary is primarily
based upon their review. Respiration can be con-
sidered to perform two functions: supplying energy
for growth, and supplying energy for organ main-
tenance. It is probable that a large portion of the daily
photosynthate produced by a grapevine is consumed
in maintenance respiration. The food energy demands
of maintenance respiration are considerable even
during times of little vine growth, and it is significant
that respiration, unlike photosynthesis, occurs
continuously. The energy derived from maintenance
respiration is used to meet the demands of many
physiological processes, including carbohydrate
translocation, protein turnover, nitrogen assimilation,
and nutrient uptake in the roots. The synthesis of
substances integral to vine maintenance and growth,
including proteins, enzymes, colors, aromas, flavors,
acids, and tannins, is fueled by respiration.
The rate of maintenance respiration is dependant
upon grapevine size, whereas growth respiration rates
vary with the level of growth activity. Temperature is
the most influential environmental factor affecting the
rate of respiration. Increasing temperatures cause a
progressive increase in respiration rate up to a point
where tissue damage occurs. At 50ºF (10ºC), respir-
ation of a mature grape leaf is close to zero, but
respiration rate approximately doubles with every
18ºF (10ºC) increase in temperature.
Translocation
The long-distance movement of water, mineral
nutrients, food, and other materials through the
vascular system is called translocation. Water and
dissolved mineral nutrients absorbed by the roots are
moved upward in the xylem to all parts of the
grapevine. The phloem is the conduit primarily for
food materials and their derivatives to be moved
throughout the plant.

14 Oregon Viticulture
Movement of photosynthates in the phloem
throughout the growing season has been described by
Kliewer (1981) and Williams (1996) and is summarized
here. Beginning at budburst and continuing for about
two to three weeks, carbohydrates and nitrogenous
compounds are moved upward from their storage
locations in roots and woody parts of the vine to
support the new shoot growth. When the shoot and
leaves develop to the point that some leaves (those
greater than 50% of their final size) produce more
photosynthates then they consume, food materials
begin to move in both directions in the phloem.
Mature leaves from the apical portion of the shoot
supply the growing shoot tip, and the remaining leaves
export photosynthates out of the shoot to the parent
vine: canes, arms, trunk, and roots. This pattern
continues until about bloom, when growth from the
shoot tip generally begins to slow down. From fruit
set until the beginning of fruit ripening, photo-
synthates move primarily to three sinks: shoot tip, fruit
cluster, and the parent vine. The fruit cluster is the
primary sink from the start of ripening until harvest;
the parent vine and growing tips of primary and lateral
shoots are weaker sinks. After harvest most of the
photosynthates moves out of the shoot into the
storage reserve parts of the vine: roots and woody
tissues. Generally there is a period of root growth after
harvest, so the growing root tips would further favor
carbohydrate movement to the roots.
In grapevines, sucrose is the main carbohydrate
translocated, so starch and other carbohydrates must
first be broken down to release sucrose for transport.
Plant hormones, which have a role in controlling plant
functions, are also moved through the xylem and
phloem. Some cross-movement (radial translocation)
of water and materials between the xylem and phloem
occurs through vascular rays, which also function as
storage sites for food reserves.
Mineral nutrients absorbed by the roots (see
discussion below) are moved into the xylem of the
root, and from there they are translocated upward to
the shoot and distributed in the plant to the areas of
use. Nutrient reserves are stored in the roots and
woody parts of the vine and are remobilized and
translocated in the phloem when uptake from soil is
inadequate to meet the current need. Remobilization
from storage reserves is an important source of
nutrients, especially nitrogen, during the early stages
of shoot growth in the spring, before roots have begun
active growth.
Transpiration
Transpiration is the loss of water, in the form of vapor,
through open stomata. Stomatal pores open into the
empty spaces between mesophyll (interior cells) cells
of the leaf. This creates an uninterrupted path between
the outside environment and the inner environment
of the leaf. The outside environment almost always
has a lower relative humidity than the protected
interior of the leaf, which is assumed to be 100%. Thus
a vapor pressure gradient exists, causing water vapor
to move out of the leaf from the area of high vapor
pressure (high water content) to the area of lower
vapor pressure. When the thin-walled mesophyll cells
lose water from transpiration, their absorptive power
is increased due to concentration of the dissolved
solids in the cell sap and partial drying of solid and
semisolid materials of the cell. The partially dried cells
then have a greater potential to absorb water, which
they obtain from the xylem. Thus, the absorptive force,
called transpirational pull, is applied to the cont-
inuous column of water (transpiration stream) in the
xylem that extends from the leaves to the roots.
The rate of transpiration is dependent upon the
extent to which stomata are open, which is primarily
related to light levels and secondarily influenced by
external environmental conditions: humidity, temp-
erature, and wind. Stomata can, however, be partially
or completed closed in response to varying degrees of
water deficit, overriding the influence of light and
other environmental conditions. Transpiration also
has an evaporative cooling effect on the leaf because
water molecules absorb heat energy during the con-
version of water from the liquid phase to the gas phase
within the leaf.
Absorption of Water and Nutrients
Water. The suction force of transpirational water loss
is transmitted throughout the unbroken column of
water in the xylem all the way to the roots, providing
the major mechanism by which water is taken up from
the soil and moved throughout the vine. Water is
pulled into the root from the soil. Young roots absorb
the majority of water, primarily through root hairs and
other epidermal (outer layer) cells. But older suberized
(“woody”) roots uptake water at a lower, but constant,
rate. Water then moves through the cells of the inner
tissues of the root and into the xylem ducts, where it
continues its movement upward, reaching all parts of
the vine, and is eventually lost via the stomata.
The effect of transpiration on the rate and quantity
of water uptake is obvious, but new root growth is also
necessary because roots eventually deplete the
available water in their immediate area and soil water
movement is slow at best. Therefore, conditions that
influence root growth affect the rate of water uptake.
Nutrients. Mineral nutrients must be dissolved in
water for uptake by roots. Nutrient uptake often occurs
against a concentration gradient; that is, the con-
centration of a mineral nutrient in the soil solution is

Grapevine Structure and Function 15
usually much lower than its concentration in root cells.
Thus an active process, consuming energy, is required
to move nutrients against the concentration gradient.
Active transport is a selective method of nutrient
uptake, and some nutrients can be taken up in much
greater quantity than others. Nitrates and potassium
are absorbed several times as rapidly as calcium,
magnesium, or sulfate. There are also interactions
between nutrient ions that influence their absorption.
For example, potassium uptake is affected by the
presence of calcium and magnesium. In rapidly
transpiring vines, nutrient uptake also occurs by mass
flow (a passive process) with water from the soil
solution (Mullins et al., 1992).
Major Developmental Processes
Shoot Growth
Shoot growth begins with budburst (or budbreak),
when previously dormant buds begin to grow after
they have received adequate heat in the spring. This
usually occurs when average daily temperature
reaches about 50ºF. Representative stages in the
growth and fruiting of a grapevine are illustrated in
Figure 5. At budburst, the primary growing point
usually contains 10–12 leaf primordia and one or two
cluster primordia, located opposite leaf primordia at
node positions three to six. Development of these
structures continues as the shoot grows out from the
bud. Early shoot growth is relatively slow, but soon it
enters a phase of rapid growth called the grand period
of growth, which typically continues until just after
fruit set. Even when the shoot is only a few inches long,
developing flower clusters can be seen opposite the
young leaves.
As each new leaf unfolds, the lateral bud and
dormant bud begin to develop in its axil. Some lateral
buds in the leaf axils grow into lateral shoots, but many
produce only one or a few small leaves, then stop
growing. Other laterals grow out to varying lengths.
Under some circumstances, such as excessive vine
vigor, or in response to summer pruning (tipping or
hedging) of primary shoots, the lateral shoot grows out
with substantial vigor.
After fruit set, shoot growth generally continues to
slow, to a halt or nearly so, by about the time the fruit
begins to ripen. Under circumstances of high vigor,
however, shoot growth may continue at a steady rate
throughout the season. This situation can arise from
one or more of the following causes: abundant water,
excessive nitrogen fertilization, severe pruning, or
extreme undercropping. Smart and Robinson (1991)
describe the “ideal” shoot to be 2–3 feet long with 10–
15 full-sized leaves.
Flower Cluster Initiation
As the shoot grows, considerable development takes
place within the dormant buds in the leaf axils. Of
greatest interest is the development of flower cluster
primordia, since they represent the fruiting potential
of the vine for the following season. The period at
which flower cluster primordia begin to form on the
rudimentary shoot is called flower cluster initiation.
The process occurs first in the midsection of the
primary shoot at node positions four through eight,
beginning soon after bloom of the current season’s
flower clusters (initiated in the previous season) and
continuing for up to six weeks. The buds at basal nodes
one to three undergo cluster initiation a little later, and
initiation continues progressively in buds toward the
growing tip. Usually, by the end of the season, fruitful
buds exist along the cane to the extent to where it is
fully ripened.
Grape flower initiation is described and illustrated
in Mullins et al. (1992) and summarized below. Flower
development in V. vinifera is described as a three-step
process, occurring within the developing dormant
buds. The first step is the formation of uncommitted
primordia by the growing points of developing
dormant buds (which are not dormant at this early
developmental stage) in leaf axils of the current
season’s shoots. The primordia are described as
uncommitted at this point because they can develop
into either flower clusters or tendrils, depending on
environmental and growing conditions experienced
by the specific bud and the shoot in general. In the
second stage the primordia become committed to
becoming a flower cluster or a tendril. Mullins et al.
report flower cluster initiation coinciding with the
beginning of periderm development on the shoot, but
others have found it to begin before bloom with some
varieties (L. E. Williams, personal communication) or
at about the time of bloom (Winkler et al., 1974).
Cluster primordia develop during the current season,
and the final step, formation of flowers from the cluster
primordia, begins after budburst in the following
spring. The later stages of flower development are
completed as bloom time is approached.
Sunlight and temperature are the most influential
environmental factors on grapevine flower cluster
initiation, although opinions vary on which is the
dominant factor. According to Williams et al. (1994),
the development of uncommitted primordia into
either flower clusters or tendrils is dependent upon
the amount of sunlight striking the bud during
development. The number and size of cluster pri-
mordia increase with increasing sunlight levels.
Mullins et al. (1992) conclude that it is probable that a
combination of exposure to high temperature and
high light intensity is necessary for maximum

16 Oregon Viticulture
fruitfulness of dormant buds. They also report that
sunlight and temperature requirements for initiation
of flower cluster primordia are known to vary among
varieties. From a vineyard management perspective,
it appears that, for a grape variety with demonstrated
adaptation to a region’s temperatures, sunlight
exposure of the developing buds is the most critical
concern. Thus training systems and canopy manage-
ment practices that facilitate good sunlight exposure
promote better fruitfulness than those that create
conditions of shade.
Dormancy, Acclimation, and Cold Hardiness
In autumn, the vine enters dormancy—the stage with
no leaves or growth activity, which extends until
budburst the following spring. Despite the apparent
inactivity of this stage, it can be a critical time for
grapevines when they may be exposed to potentially
damaging low temperatures. The ability of a dormant
grapevine to tolerate cold temperatures is referred to
as its cold hardiness. Grapevine cold hardiness is a
highly dynamic condition, influenced by environ-
mental and growing conditions, and varying among
grapevine varieties and tissues and over time. There-
fore, cold hardiness cannot be viewed or described in
absolute terms such as “Variety X is cold hardy to -
8ºF.”
There are three stages of the dormant season:
acclimation, the period of transition from the non-
hardy to the fully hardy condition; midwinter, the
period of most severe cold and greatest cold hardiness;
and deacclimation, the period of transition from fully
hardy to the non-hardy condition and active growth.
Acclimation is a gradual process, beginning after
shoot growth ceases and continuing through autumn
and early winter. The combination of declining day
length and decreasing temperatures in autumn are
important factors influencing acclimation and cold
hardiness. The process of acclimation in grapevines
is not well understood, but it involves many simul-
taneous activities that collectively increase cold
hardiness. Water content of some tissues decreases,
while increases occur in cells’ solute (dissolved solids)
concentration, membrane permeability, and the
thermal stability of several enzymes.
Howell (2000) has reviewed the mechanisms by
which grapevines survive cold temperatures. The
primordial tissues of dormant buds survive by
avoiding the formation of ice crystals in the tissue by
supercooling—a process by which a liquid remains
fluid below its normal freezing temperature. Other
tissues survive by increasing their capacity to tolerate
both ice in the tissue and increased concentration of
solutes in the cell. Increased solute concentration in
the cell lowers its freezing point.
Because of the different mechanisms involved,
tissues vary in tolerance to freezing temperatures.
Woody tissues of the trunk, cordon, and canes
generally have greater cold hardiness than dormant
buds and roots. In comparisons of grapevine woody
tissues, the vascular cambium is thought to be the last
tissue to be damaged by cold temperatures, followed
in sequence by younger xylem, older xylem, and
phloem (Wample et al., 2000). Within dormant buds,
primary buds are typically less cold hardy than
secondary buds, and tertiary buds are the most hardy.
Species and varieties of grapes exhibit a broad range
of potential cold hardiness based on their inherent
genetic characteristics. This fundamental genetic
potential for cold hardiness is influenced by both
environmental conditions and the circumstances
under which the vine grew in the previous season.
Poor management practices or growing conditions
can inhibit the acclimation process, resulting in
reduced cold hardiness. Acclimation is promoted by
exposure of shoots and leaves to sunlight and is
associated with periderm development and low
relative water content. Cold hardiness can vary
considerably between and within vines. Reduced
hardiness has been associated with large, dense
(shaded) canopies, canes with either long internodes
or large internode diameter, and canes with large
persistent lateral canes. Additionally, heavy fruit loads
or defoliation (early leaf fall due to stress, disease, or
pest activity) inhibit acclimation, probably through
reduced availability of photosynthates. Contrary to
popular belief, neither nitrogen fertilization nor
irrigation practices reduce grapevine cold hardiness,
unless nonstandard practices are used that encourage
continued late-season growth, which inhibits ac-
climation (Wample et al., 2000).
Cold hardiness of buds is fairly stable through the
winter months, but sharp increases in temperature
can cause buds to deacclimate and lose hardiness, and
the extent of deacclimation can vary by variety or
species. Bud hardiness has been correlated with air
temperature of the preceding five-day period. Cold
hardiness decreases as the grapevine rapidly de-
acclimates in response to warm temperatures in the
spring. Deacclimation is much less gradual than cold
acclimation in the fall, and the rate of deacclimation
accelerates through the dormant season.
Fruit Growth
Berry development commences after successful
pollination and fertilization of ovules within a flower.
Flowers with unfertilized ovules soon shrivel and die,
while those remaining begin growth into berries. Many
of these tiny berries, abscise (drop off) within the first
two to three weeks. Following this drop period (called

Grapevine Structure and Function 17
shatter), the retained berries generally continue to
develop to maturity. Commonly, only 20–30% of
flowers on a cluster develop into mature berries, but
this is adequate to produce a full cluster of fruit.
The berry develops from the tissues of the pistil,
primarily the ovary. Although pollination and fert-
ilization initiate fruit growth, seed development seems
to provide the greatest growth stimulus, as evidenced
by the relationship of fruit size to the number of seeds
within the berry. The maximum number of seeds is
four, but lack of ovule fertilization or ovule abortion
reduces the number of developing seeds, generally
resulting in smaller berry size.
Berry growth occurs in three general stages—rapid
initial growth, followed by a shorter period of slow
growth, and finishing with another period of rapid
growth. A graph of grape berry growth thus appears
as a double sigmoid pattern. Berry growth during the
first stage is due to a rapid increase in cell numbers
during the first three to four weeks, followed by two to
three weeks of rapid cell enlargement. During this
stage the berries are firm, dark green in color, and
rapidly accumulating acid. Seeds have attained their
full size by the end of the first growth stage.
The middle stage, called the lag phase, is a time of
slow growth. The embryo is rapidly developing within
each seed, and the seed coat becomes hardened.
Berries reach their highest level of acid content and
begin to accumulate sugar slowly. Toward the end of
lag phase, berries undergo a reduction in chlorophyll
content, causing their color to change to a lighter
green.
The final stage of berry growth coincides with the
beginning of fruit maturation (ripening). The begin-
ning of ripening, referred to by the French term
veraison, is discernable by the start of color
development and softening of the berry. The color
change is most easily visible on dark-colored varieties,
but “white” varieties continue to become lighter green,
and some varieties turn a yellowish or whitish-green
color by harvest. Softening of the berry and rapid sugar
accumulation occur abruptly and simultaneously.
Berry growth, occurring by cell enlargement, becomes
rapid again in this final stage of ripening. It is thought
that most of the water entering the berry after veraison
comes from phloem sap, since xylem at the junction
of the berry and its pedicel (stem) appears to become
blocked at this time (Coombe, 1992).
During ripening, acid content declines and sugar
content increases. It is widely believed that flavors
develop in the later stages of ripening. Berries begin
to accumulate sugar rapidly at the start of the ripening
period, and the rate tends to remain steady until
accumulation slows as the end of the maturation
period is approached. Sugar is translocated as sucrose
to the fruit, where it is quickly converted into glucose
and fructose. Both sugars and acids primarily accum-
ulate in cells constituting the pulp (flesh) of the berry,
although a small amount of sugar accumulates in the
skin.
The skin (epidermis) and the thin tissue layer
immediately below it contain most of the color, aroma
and flavor constituents, and tannins contained in the
berry. Thus, all things being equal, small berries have
greater color, tannins, and flavor constituents than
large berries because the skin constitutes a larger
percentage of the total mass of small berries. Seeds
also contain tannins that can contribute to the overall
astringency of wine.
The chemical composition of grape berries is
complex, consisting of hundreds of compounds, many
in tiny quantities, which may contribute to fruit
quality attributes. The single largest component is
water, followed by the sugars fructose and glucose,
then the acids tartaric and malic. Other important
classes of chemical compounds within grape berries
include amino acids, proteins, phenolics, antho-
cyanins, and flavonols. The reader is referred to a
review of the biochemistry of grape ripening by
Kanellis and Roubelakis-Angelakis (1993) for a
thorough discussion of this topic.
Berries are considered to be fully ripe when they
achieve the desired degree of development for their
intended purpose, and they are generally harvested
at this time. Ripeness factors of the fruit that are
typically considered when scheduling harvest are
sugar content, acid content, pH, color, and flavor. The
combination of these factors determines the fruit
quality of the harvest. Ripening processes in the fruit
cease upon harvest, but while fruit is on the vine
ripening is a continuous process. So there is usually a
short time, influenced by weather, during which the
fruit remains within the desired ripeness parameters.
Berries can become overripe if harvest is delayed until
the fruit has developed beyond the desired range of
ripeness. Consider also that ripeness parameters can
vary considerably depending on the intended use. For
example, Pinot noir grapes for sparkling wine pro-
duction are harvested much earlier, at lower sugar and
higher acid content, than Pinot noir for non-sparkling
red wine. Thus, the terms “fruit ripeness” and “fruit
quality” do not have absolute values but are defined
subjectively.
Fruit ripening can be delayed, and the attainment
of desired ripeness parameters inhibited, by an
excessive crop load (amount of fruit per vine). A vine
that is allowed to produce more fruit than it can
develop to the desired level of ripeness is considered
to be overcropped. Severe overcropping can negatively
impact vine health as well as fruit quality by precluding

18 Oregon Viticulture
the vine from allocating adequate photosynthates to
weaker sinks: shoots, roots, and storage reserves.
Viticulturists generally seek to attain vine balance, the
condition of having a canopy of adequate, but not
excessive, leaf area to support the intended crop load
to the desired level of fruit ripeness.
Climatic factors, particularly temperature, have
long been recognized to have a major influence on the
fruit quality of grapes and subsequent wine quality.
The principal effect is on the rates of change in the
constituents of the fruit during development and the
composition at maturity. Hot climates favor higher
sugar content and lower acidity; cool climates tend to
slow sugar accumulation and retain more acidity.
Grape varieties tend to ripen their fruit with a desirable
combination of quality components most consistently
in specific climates. Thus, some varieties, such as Pinot
noir and Gewürztraminer, are considered to be “cool
climate varieties,” whereas others such as Carignane
and Souzão are considered to be “warm” or “hot
climate varieties.” A few varieties, most notably Char-
donnay, are capable of producing high-quality wines
in different climates by adjusting the wine style for the
varying expression of fruit characteristics in each
climate. The relationship of climate, and in particular
temperature, to fruit ripening and wine quality has
been incorporated into methods of matching grape
varieties to climate; Winkler’s heat summation (degree
days) system for California (Winkler et al., 1974) is one
such system, and there are other more elaborate
methods (see, e.g., Jackson and Cherry, 1988;
Gladstones, 1992). Phenology is the study of the
relationship between climatic factors and the pro-
gression of plant growth stages and developmental
events that recur seasonally.
Thus, the first step in the production of high-quality
winegrapes is the selection of a site with appropriate
climatic characteristics for fruit ripening of the
varieties to be grown. Vineyard practices, including
training systems and canopy management, are
utilized to optimize the sunlight and temperature
characteristics of the canopy for fruit ripening. In cool
climates, canopy management practices that provide
good exposure of leaves and fruit to sunlight have
generally improved grape and wine composition.
Vines in which the canopy interiors are well exposed
to sunlight usually produce fruit with higher rates of
sugar accumulation, greater concentrations of
anthocyanins and total phenols, lower pH, and
decreased levels of malic acid and potassium com-
pared to vines with little interior canopy exposure
(Williams et al., 1994). Improved fruit quality under
such circumstances may be due to higher temp-
eratures in addition to better sunlight exposure, but it
is extremely difficult to separate these factors.
The Annual Cycle of Growth
The annual growth cycle of the grapevine involves
many processes and events that have been briefly
introduced above. Figure 8 illustrates the sequence of
major processes and events in a timeline. It should be
recognized that the timing and duration of develop-
mental events are subject to variations due to the
grape variety, local climate, and seasonal weather, but
the sequence of events remains constant. It is
significant that many of these events overlap others
for a period of time, requiring the vine to allocate its
resources among competing activities. For example,
during the time that the vine is developing and
ripening the current season’s fruit, flower cluster
initiation and development is underway in dormant
buds and carbohydrates are being moved into storage
reserves. Therefore, it is of critical importance for the
long-term growth and productivity of grapevines that
adequate photosynthates be produced to supply the
complete needs of the vine. This goal can be achieved
by supplying adequate water and nutrients to the vine,
maintaining a healthy canopy, providing good sunlight
exposure, and developing an appropriate balance
between crop load and canopy size.
Acknowledgment
The author gratefully acknowledges Nick Dokoozlian,
Larry Williams, and Robert Wample for their critical
review of this chapter.
Figure 8. Annual
cycle of grapevine
growth.Figure by Ed
Hellman.
April M J J A S O N D J F M
Bloom Veraison Fruit Maturation and Harvest
Budburst
Cane Maturation
Flower Cluster Initiation
and Development

Grapevine Structure and Function 19
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