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© 2012 Science From Israel / LPPLtd., Jerusalem
Israel Journal of Plant Sciences Vol. 60 2012 pp. 85–95
DOI: 10.1560/IJPS.60.1-2.85
*Author to whom correspondence should be addressed.
E-mail: jgamon@gmail.com
Facultative and constitutive pigment effects on the Photochemical Reectance Index
(PRI) in sun and shade conifer needles
John A. GAmona,* And Joseph A. Berryb
aDepartments of Earth and Atmospheric Sciences & Biological Sciences, University of Alberta, Edmonton,
Alberta T6G 2E3, Canada
bDepartment of Global Ecology, Carnegie Institution for Science, Stanford, California 94305, USA
(Received 30 January 2012; accepted in revised form 12 February 2012)
Honoring Anatoly Gitelson on the occasion of his 70th birthday
ABSTRACT
Leaf pigment content and spectral reectance were examined in four conifer species
from the Pacic Northwest and Canadian boreal forest. Our goal was to evaluate
the causes of within- and between-stand variation in the Photochemical Reectance
Index (PRI), an indicator of xanthophyll cycle activity and carotenoid pigment con-
tent that often scales with photosynthetic light-use efciency. Both the dark-state
PRI values and the change in PRI upon dark–light transition (ΔPRI) were measured
in situ in leaves from different canopy positions (top vs. bottom) having contrasting
light histories (sun vs. shade). PRI varied with species, canopy position, and with the
pool sizes of several photoprotective carotenoid pigments (relative to chlorophyll).
Upper-canopy leaves had a greater Δ PRI than their shaded counterparts lower in
the canopy, reecting a higher investment of the photoprotective xanthophyll cycle
pigments for sun-exposed top-canopy leaves. These results indicate that the rela-
tive concentration of different pigment groups and associated PRI responses varied
with canopy position and light history over more than one time scale, and included
rapidly changing (facultative) and slowly changing (constitutive) components. Most
of the PRI variability among the forest trees sampled was due to constitutive pig-
ment pool size variation associated with species and canopy position. We conclude
that both facultative and constitutive pigment components should be considered
when applying PRI to photosynthetic studies of forest stands with remote sensing.
Leaf-level measurements of PRI and ΔPRI provide non-destructive probes of both
facultative and constitutive pigment changes within plant canopies that could help
interpret variation in PRI signal viewed from remote sensing platforms.
Keywords: Photochemical Reectance Index (PRI), leaf pigments, irradiance, coni-
fers, carotenoids, xanthophyll cycle
INTRODUCTION
Plant leaves balance their requirements for light harvest-
ing and photoprotection many ways, including chloro-
phlast movement (Brugnoli and Björkman, 1992; Park
et al., 1996; Zygielbaum et al., 2012), leaf movement
(Gamon and Pearcy, 1989; Jiang et al., 2006), pigment
pool size changes (Thayer and Bjorkman, 1990; García-
Plazaola et al., 1997; Demmig-Adams, 1998; Stylinski
et al., 2002), and diurnally changing energy distribution
through xanthophyll pigment conversion and non-
photochemical dissipation of absorbed photons (Dem-
mig-Adams and Adams, 1992). Slow or irreversible
adjustments are often termed constitutive properties,
whereas rapid, reversible adjustments can be considered
Israel Journal of Plant Sciences 60 2012
86
facultative. Examples of facultative responses include
the changing epoxidation state of the xanthophyll
cycle pigments in response to diurnally changing irradi-
ance (Demmig-Adams and Adams, 1992). Examples
of constitutive adjustments include enhanced levels
(pool sizes) of carotenoid pigments with increased sun
exposure (Demmig-Adams, 1998; Niinemets et al.,
2003). Similarly, pigment pool sizes change gradually
during leaf development (Gamon and Surfus, 1999),
seasonal change (García-Plazaola et al., 1997; Stylin-
ski et al., 2002), or leaf age (Gitelson and Merzlyak,
1994; Merzlyak and Gitelson, 1995). In this study, we
dene “facultative effects” as those that change on a
daily or shorter time scale, and “constitutive effects” as
those that change over much longer time scales (e.g.,
due to changing pigment pool sizes in response to sun
exposure, leaf age, or chronic stress). These facultative
and constitutive pigment effects both inuence leaf
reectance properties and can be monitored with the
appropriate “reectance index.”
Many leaf pigment indices have been developed to
measure chlorophyll, carotenoid, or anthocyanin levels
using spectral reectance (Gamon and Surfus, 1999;
Merzlyak et al., 1999, 2003; Sims and Gamon, 2002;
Gitelson et al., 2006, 2009; Steele et al., 2009; Ustin
et al., 2009). When hyperspectral detectors comprised
of many narrow wavebands are used, spectral reec-
tance can resolve narrow-band features diagnostic of
individual pigments, pigment groups, or relative levels
of major pigment groups (Ustin et al., 2009). With the
appropriate foreoptics, these instruments can be used for
assessing leaf pigments in the eld. A particular advan-
tage of this approach is that it provides a rapid, non-de-
structive in situ sampling method that can preclude the
need for extensive and costly laboratory analyses.
The Photochemical Reectance Index (PRI) was
originally developed as an optical measure of leaf
xanthophyll cycle pigment activity that detects the in-
terconversion of xanthophyll cycle pigments in leaves
and plant stands over diurnal time spans (Gamon et al.,
1992, 1993; Peñuelas et al., 1995). Because xanthophyll
pigment conversion is closely linked to non-photo-
chemical quenching and PSII photochemical efciency
(Demmig-Adams and Adams, 1992), PRI can be related
to leaf- and stand-level photosynthetic activity and of-
fers promise as a “scaleable” remote sensing measure of
photosynthetic rate or light-use efciency (LUE) over
diurnal time scales (Gamon et al., 1992 and 2001).
In recent years, many tests of PRI as a remote in-
dicator of stand- or ecosystem-level photosynthetic
activity have been conducted using aircraft or satellite
measurements, and several have found signicant cor-
relations between PRI and whole-ecosystem photosyn-
thetic activity or LUE measured using eddy covariance
(Nichol et al., 2000; Rahman et al., 2001, 2004; Drolet
et al., 2005; Garbulsky et al., 2008; Hilker et al., 2009;
Goerner et al., 2011). These encouraging results have
led to much recent speculation that satellite-based mea-
sures employing PRI (or a similar index) could be used
to improve remote sensing of “photosynthesis from
space” (Grace et al., 2007; Coops et al., 2010; Hall et
al., 2011).
Remote sensing studies of PRI as a measure of stand
or ecosystem photosynthetic activity generally have not
considered the potential for constitutive changes in leaf
pigment content inuencing the observed PRI. Since
most eld and remote sensing studies do not actually
measure pigment levels, they often assume that varia-
tion in PRI primarily reects short-term xanthophyll
cycle pigment activity, and ignore the potential contri-
bution of constitutive pool size differences. However,
many long-term eld studies have now shown that PRI
can also be strongly affected by changing pigment pool
sizes, particularly over seasonal time spans or across
species (Stylinski et al., 2002; Filella et al., 2004,
2009; Sims et al., 2006), In some ecosystems, these
constitutive effects can be the primary source of PRI
variation, and are linked to prevailing environmental
conditions (Sims et al., 2006), which can involve irradi-
ance (Niinemets et al., 2003), temperature (Sims et al.,
2006), nutrients (Gamon et al., 1997), and water status
(Sims et al., 2006). Currently, the effect of constitutive
pigment changes on the PRI–LUE relationship has not
been well-studied, and we are not aware of any stud-
ies demonstrating a consistent relationship between
the constitutive PRI level and other measures of LUE.
Consequently, caution must be applied when attempting
to use PRI as a LUE indicator, particularly when these
individual factors have not been controlled or dened,
as is the case for most landscape-level remote sensing
studies.
For the facultative component of PRI, irradiance
is particularly critical, in part because the asymptotic
shape of the photosynthetic light-response curve results
in a steadily increasing level of excess radiation and
progressive lowering of LUE as a leaf is exposed to
progressively higher irradiance (Björkman and Dem-
mig-Adams, 1994). Photoprotective pigment levels are
enhanced in response to this excessive irradiance, and
diurnal operation of the xanthophyll cycle causes rapid
PRI changes that scale with LUE (Gamon et al., 1992).
However, this effect on LUE typically relaxes when
leaves are returned to low light (e.g., overnight).
Over longer time scales (days to months), levels of
carotenoids (including carotenes and xanthophylls) can
all change in response to sun exposure during leaf devel-
Gamon and Berry / PRI in sun and shade conifer needles
87
opment (Thayer and Bjorkman, 1990; Demmig-Adams,
1998). Even within a pigment class (e.g., chlorophylls),
the relative levels of pigment types (chl a and b) vary
with sun and shade (Thayer and Björkman, 1990), as
plants adjust the sizes of their photosynthetic light
harvesting complexes relative to their reaction centers
(Bjorkman, 1981). Clearly, pigment levels tend to fol-
low irradiance gradients within the canopy (Niinemets
et al., 2003), which could cause changes in the PRI.
However, to our knowledge, the relative responses of
facultative and constitutive pigment levels to these
within-canopy gradients, and the implications for LUE,
have not been well studied.
In this study, we hypothesized that both facultative
and constitutive changes would occur within a single
canopy due to strong gradients in irradiance, and that
top-canopy leaves would have a higher capacity for
facultative changes characterized by rapid xanthophyll
cycle pigment conversion. Similarly, we expected that
carotenoid–chlorophyll pool sizes would vary with
canopy position and light history, causing reductions
in PRI towards the top of the canopy. We also expected
that the relative levels of individual carotenoid pigments
(α- and β-carotene, lutein, neoxanthin, violaxanthin,
antheraxanthin, and zeaxanthin) would vary due to the
photoprotective and antenna functions ascribed to these
pigments (Thayer and Björkman, 1990). Our goal was
to explore naturally varying responses with leaf posi-
tion and light history in several conifer forests, using
leaf reectance as a rapid probe of both facultative and
constitutive pigment effects on PRI.
MATERIALS AND METHODS
Four conifer species were included in this study: Doug-
las-r (Pseudotsuga menziesii (Mirb.) Franco), western
hemlock (Tsuga heterophylla (Raf.) Sarg.), ponderosa
pine (Pinus ponderosa Douglas ex C. Lawson), and
jack pine (Pinus banksiana, Lamb.). The eld sites in-
cluded a mixed Douglas-r/hemlock stand near Carson,
Washington, USA (Wind River Canopy Crane site),
a ponderosa pine stand at Black Butte, Oregon, USA
(access tower provided by B. Yoder and M. Ryan), and
a jack pine forest near Thompson, Manitoba, Canada
(BOREAS OJP site).
At the jack pine site, top-canopy branches in full sun
and bottom-canopy branches in the shade were accessed
from a scaffolding tower in July 1996 as part of the
BOREAS study (Sellers et al., 1995, 1997). At the Wind
River site, top-canopy branches of three hemlock trees
and three Douglas-r trees in full sun were accessed
by the crane gondola and selected for measurement in
September 1996. In addition, a single shade branch of
a young hemlock tree growing in deep shade was also
selected. The study did not include a “shade” Douglas-
r because no shade branches could be reached from the
forest oor. At the Black Butte ponderosa pine site in
September 1996, four top-canopy “sun” branches were
accessed from a scaffolding tower, and four bottom-
canopy “shade” branches were reached from the ground
(approx. 2 m high).
Branch tips were covered with black cloth bags either
the previous evening or early in the morning (before
sunrise) to maintain needles in their dark state prior
to reectance sampling. Needle reectance was then
measured using a “leaf reectometer” by attaching the
ber optic probe (UNI410, PP Systems, Haverhill MA,
USA) onto the needle with a leaf clip (UNI501, PP Sys-
tems, Haverhill, MA, USA). This ber probe provided
both a white measuring and actinic light (PPFD equal
to full sun), and a path for reected light to reach the
detector (see Gamon and Surfus, 1999, for further de-
tails and instrument description). The leaf clip provided
a xed optical geometry during sampling, which was
essential for accurate and repeatable needle reectance
measurements. To calculate reectance, each leaf scan
was divided by a scan of a white reference (Spectralon,
LabSphere, North Sutton, NH, USA) taken immediately
prior to leaf measurements. The Photochemical Reec-
tance Index (PRI) was calculated as follows:
PRI = (R531 – R570)/(R531 + R570)
where R indicates reectance, and the subscript in-
dicates the wavelength (in nm) (Gamon et al., 1993;
Peñuelas et al., 1995)
By using dark-adapted leaves as a baseline for PRI
measurements, we were able to experimentally separate
constitutive from facultative pigment effects on PRI.
The assumption was that when measuring dark-adapted
leaves, the prior night’s reversion to the dark state (re-
epoxidation to violaxanthin in the dark) minimized the
inuence of diurnal xanthophyll cycle activity on the
measured PRI. Consequently, under these conditions,
PRI variation was primarily due to the constitutive
changes in pigment pool sizes; in subsequent discussion
we refer to this dark-state PRI measurement as “cPRI.”
To examine reectance kinetics indicative of faculta-
tive xanthophyll cycle conversion, dark-adapted leaves
were sampled continuously during several minutes of
high-light exposure, causing rapid transition from the
“dark” to “light” state. The change in reectance dur-
ing this “dark-to-light transition” was expressed as the
change in the PRI (ΔPRI), by subtracting the PRI value
at 10 min of light exposure from the PRI value from the
initial “dark state” (rst scan upon illumination with the
instrument). Similarly, difference spectra (Δ reectance
Israel Journal of Plant Sciences 60 2012
88
spectra) were calculated as the difference between light-
and dark-state reectance, and were used to visualize the
facultative xanthophyll cycle pigment effects on spec-
tral reectance. Previous studies have shown that this
“dark-to-light” method scales closely with the amount
of de-epoxidized xanthophyll cycle pigments formed, as
well as the total pool size of xanthophyll cycle pigments
(violaxanthin + antheraxanthin + zeaxanthin) present in
the leaf (Gamon and Surfus, 1999).
We hypothesized that dark-state PRI values (cPRI,
reecting constitutive effects) would be lower in top-
canopy leaves relative to bottom-canopy leaves, and
that ΔPRI (reecting facultative effects of the xan-
thophyll cycle) would be higher in top-canopy leaves
than bottom-canopy leaves. We also partitioned total
variation in PRI caused by the xanthophyll cycle (dark-
to-light effects on ΔPRI) and due to pool size changes
(canopy position and species effects on cPRI).
A small amount of leaf material was collected
from each leaf species for later pigment analysis via
HPLC according to Thayer and Björkman, 1990.
These samples were from needles of canopy position
adjacent to that of needles sampled for reectance, but
not necessarily from the same needles or branches, the
assumption being that pigment levels for all leaves of a
given age and light environment adjusted similarly to
ambient light levels. Pigment samples were limited to
several needles from 3–4 “representative” branch tips
per species per light environment (sun or shade) and
were generally collected late in the afternoon without
control of prior light conditions. Because the eld sites
were far from laboratory facilities, direct freezing of
leaf samples in the eld was not possible. For these
reasons, direct assessment of xanthophyll cycle pigment
epoxidation state (EPS) was not possible, and EPS data
were not included in the results presented here. The jack
pine samples were maintained on dry ice for three days,
then transferred to a freezer (–80 °C) prior to pigment
extraction. All other samples were stored in an ice chest
followed by refrigeration for several days before pig-
ment extraction could be completed (about 1 week after
initial collection). Because of the difculties in obtain-
ing comparable samples from conifer needles of varying
shapes and thicknesses, pigment pool sizes were rst ex-
pressed on a projected leaf area basis (μmoles m–2), then
expressed relative to chlorophyll content also expressed
on a leaf area basis (μmoles m–2), resulting in unitless
pigment pool size ratios.
RESULTS
Needle reectance spectra were typical of green
leaves (Gausman, 1985), showing the characteristic
green “hump” near 550 nm due to greater chlorophyll
absorption in the blue and red regions, and increased
reectance in the near-infrared due to increased light
scattering (Fig. 1). Spectra from top-canopy sun needles
(Fig. 1A) and bottom-canopy shade needles (Fig. 1B)
varied slightly in the visible region, with shade leaves
appearing slightly darker (lower green reectance peak)
than sun leaves. This difference was more clearly visible
when directly comparing dark-state spectra of sun and
shade needles side-by-side (Fig. 2A), providing direct
visual evidence for constitutive pigment differences in
sun and shade leaves. Examination of the wavelengths
used for PRI calculation (Fig. 2A) suggested that the
contrasting reectance spectra for sun and shade leaves
could easily inuence PRI, as further discussed below.
Upon conversion from the dark to light state, all
leaves showed subtle declines in apparent reectance
(compare solid and dashed lines, Fig. 1). These declines
were more visible when the changes were expressed as
difference spectra (Fig. 2B), representing the facultative
response of the xanthophyll cycle to sudden irradiance.
This sudden dark–light transition is analogous to the
reectance change that leaves undergo in response
to diurnal changes in irradiance (Gamon et al., 1993;
Gamon and Surfus, 1999). These difference spectra
showed a dip centered at 531 nm that was characteristic
of the xanthophyll cycle pigment conversion (Gamon
Fig. 1. Reectance spectra of dark-adapted jack pine needles
in the initial “dark” state and “light” state after ten minutes of
illumination with bright light (approx. 2000 μmol photons m–2
s–1). Panel A: top-canopy sun leaf. Panel B: bottom-canopy
shade leaf.
Gamon and Berry / PRI in sun and shade conifer needles
89
and Surfus, 1999) and a double dip at 685 and 738 nm,
characteristic of chlorophyll uorescence quenching
(Gamon et al., 1990; Gamon and Surfus, 1999). For a
given species, the dip at 531 nm due to the xanthophyll
cycle was deeper for top-canopy sun needles than for
bottom-canopy shade needles (Fig. 2), suggesting larger
xanthophyll pigment pool sizes and greater potential for
diurnal pigment conversion in the leaves from upper
canopy regions.
Upon sudden exposure to high light, all dark-adapted
leaves (regardless of species or canopy position) showed
a rapid (facultative) decline of PRI that continued over
several minutes (representative example shown in
Fig. 3). Typically, at the range of temperatures encoun-
tered during kinetic measurement, this decline reached
an asymptote by 10 min, allowing us to dene a “delta
PRI” (ΔPRI) as the difference between the initial PRI
value measured in the dark state and the nal PRI value
measured at 10 min of light exposure. In Fig. 3, the
offset between the two dark state PRI values illustrates
constitutive effects of pigment pool sizes, and the PRI
kinetics summarized by ΔPRI illustrate the facultative
effects of xanthophyll cycle activity. Collectively, the
results shown in Figs. 1–3 demonstrate that both consti-
tutive and facultative effects on PRI differed for sun and
shade leaves collected from a single tree canopy (see
further discussion below).
Both cPRI (Fig. 4A) and ΔPRI (Fig. 4B) varied
consistently between top-canopy sun needles and bot-
tom-canopy shade needles for all three species where
both canopy positions were sampled (Douglas-r shade
leaves were not available so were not included in this
comparison). The cPRI values were lower for top-
canopy sun leaves than bottom-canopy shade leaves,
suggesting pigment pool size differences. At the same
time, ΔPRI of dark-adapted leaves given sudden light
exposure was higher for sun leaves than shade leaves,
indicating greater capacity for facultative photoconver-
sion of xanthophyll cycle pigments in sun leaves rela-
tive to shade leaves.
Pool sizes for several carotenoid groups (expressed
relative to chlorophyll) varied consistently between sun
and shade leaves for all species sampled (Fig. 5). For ex-
ample, pool sizes of lutein, xanthophyll cycle pigments
(violaxanthin + antheraxanthin + zeaxanthin combined),
and total carotenoid (all carotenes and xanthophylls
combined) were higher for sun leaves than shade leaves
(note that these were expressed as lower pigment ratios
since chlorophyll was the numerator). Similarly, com-
bined xanthophyll levels (violaxanthin, antheraxanthin,
Fig. 2. Top panel: initial dark-state reectance for sun and
shade jack pine needles, showing wavelengths used for PRI
calculation. Bottom panel: difference spectra (“Δ reec-
tance”—light-state reectance minus dark-state reectance)
for single jack pine needles from the sun-exposed, top of the
canopy (“sun leaf,” dotted line) and the shaded bottom of
the canopy (“shade leaf,” solid line). Changing reectance at
531 nm due to xanthophyll cycle activity and the double dip
near 700 nm due to chlorophyll uorescence quenching are
illustrated. Data from Fig. 1.
Fig. 3. Representative PRI kinetics for top-canopy (“sun”)
and bottom-canopy (“shade”) jack-pine needles. In each case,
dark-adapted needles were exposed to full-sun illumination,
reaching a steady state PRI value after approximately 10 min
(derived from data in Figs. 1 and 2). Also illustrated are the
initial, dark-state PRI (cPRI) values and the ΔPRI values for
each needle.
Israel Journal of Plant Sciences 60 2012
90
zeaxanthin, and lutein together) were consistently
higher for sun leaves than shade leaves. This observa-
tion provided independent support for the observations
based on spectral reectance (e.g., dark-state reectance
plots in Fig. 2A, and PRI comparisons presented in Figs.
3 and 4) that constitutive levels of photoprotective carot-
enoid pigments were higher in sun-exposed, top-canopy
leaves relative to shaded leaves deeper in the canopy.
Comparisons of cPRI to pigment pool sizes revealed
several statistically signicant correlations when all
species and treatments (sun and shade) were combined
(Table 1). The cPRI values were strongly related to
the ratios of chlorophyll to several carotenoid pigment
groups, including total carotenoids, lutein, and total
xanthophylls (violaxanthin, antheraxanthin, zeaxanthin,
and lutein combined) (Table 1). Surprisingly, cPRI was
not closely correlated with the pool size of xanthophyll
cycle pigments (violaxanthin + antheraxanthin + zea-
xanthin), unless lutein (another xanthophyll pigment not
directly involved in the xanthophyll cycle) was added to
this pool. These comparisons between cPRI and carot-
enoid pigment levels are illustrated for lutein (Fig. 6A)
and total carotenoids (all carotenes and xanthophylls
combined, Fig. 6B), the two pigment groupings show-
ing the strongest correlations with cPRI (Table 1). These
results suggest that constitutive carotenoid pigment pool
sizes, not xanthophyll cycle pigments, are the primary
source of PRI variability across species and canopy
positions.
To help put these results in context, we compared
absolute values of PRI variation due to facultative causes
(ΔPRI from “dark–light” PRI transitions), and due to
constitutive causes (cPRI variation with canopy posi-
tion and species) (Table 2). Using the facultative PRI
variation (ΔPRI) in top-canopy sun leaves as a reference
(100%), it was clear that shaded leaves had a smaller
(60%) variation in PRI associated with dark-light xan-
thophyll cycle conversion (ΔPRI). On the other hand,
cPRI differences associated with canopy position were
slightly greater (107%) than the range in PRI due to fac-
ultative effects (ΔPRI in top-canopy leaves). When only
top-canopy leaves were considered, cPRI differences
associated with species were almost three times larger
(272%) than facultative effects for the same top-canopy
leaves. When only shade leaves were considered, cPRI
variation due to species was 2.5 times (247%) the facul-
tative effects (ΔPRI) in top-canopy leaves, and 4.5 times
the facultative effects (ΔPRI) of bottom-canopy leaves.
From this analysis, we conclude that total variation in
PRI due to pigment pool sizes associated with canopy
position and species differences were larger than the PRI
variation due to the activity of the xanthophyll cycle.
DISCUSSION
The reectance and Δ reectance spectra for conifer
needles reported here are similar to those reported
previously for broad leaves measured with the same
instrument (Gamon and Surfus, 1999). All species and
sun conditions showed the characteristic feature at
531 nm due to xanthophyll pigment conversion and the
double dip near 700 nm due to chlorophyll uorescence
quenching (Gamon et al., 1990; Gamon and Surfus,
1999). The PRI kinetics upon transition from the dark
state to the light state (full-sun illumination for 10 min)
are similar to patterns reported previously for other
species (Gamon and Surfus, 1999). These similarities
with other reectance spectra and with reectance ki-
Fig. 4. Panel A: Dark-state PRI (cPRI) values for sun and
shade leaves for four conifer species. Panel B: Delta (Δ) PRI
values for sun and shade leaves for the same species (Psme
= Pseudotsuga menziesii, Tshe = Tsuga heterophylla, Pipo =
Pinus ponderosa, and Piba = Pinus banksiana). P. mensiezii
shade leaves were not sampled. Bars and error bars indicate
mean values and standard deviation, respectively.
Gamon and Berry / PRI in sun and shade conifer needles
91
Fig. 5. Pigment ratios for top-canopy (“sun”) and bottom-canopy (“shade”) leaves. Psme = Pseudotsuga menziesii, Tshe = Tsuga
heterophylla, Pipo = Pinus ponderosa, Piba = Pinus banksiana. Chl = total chlorophyll, V = violaxanthin, A = antheraxanthin,
Z = zeaxanthin, L = lutein, Carotenoid = all carotenes and xanthophylls. Bars and error bars indicate mean values and standard
deviation, respectively.
Table 1
R2 values (and p values in parentheses) for linear regressions
between dark-state PRI (cPRI) and the pigment ratios indicated
in the left column. Chl = total chlorophyll, L = lutein,V+A+Z
= total pool size of xanthophyll cycle pigments (violaxanthin,
antherazanthin, and zeaxanthin), V+A+Z+L = total pool size
of xanthophyll cycle pigments plus lutein, Car = all carotenes
and xanthophylls together
Pigment ratios R2 (p)
Chl/(V+A+Z) 0.113 (0.462)
Chl/(V+A+Z+L) 0.7423 (0.013)
Chl/L 0.953 (< 0.001)
Chl/Car 0.879 (0.002)
and xanthophylls combined) in sun leaves versus shade
leaves indicate greater investment in photoprotection
for sun-exposed, top-canopy leaves relative to shade
leaves. This nding is consistent with previous reports
of constitutive pool size differences in sun and shade
leaves (Thayer and Bjorkman, 1990; Demmig-Adams,
1998; Niinemets et al., 2003). The lower cPRI values in
top-canopy sun leaves (relative to shade leaves), along
with the strong correlations with several chl:carotenoid
ratios (Table 1, Fig. 6), demonstrate that leaf-level PRI
within conifer canopies is strongly inuenced by con-
stitutive changes in photoprotective pigment pool sizes.
The weaker correlations between cPRI and xanthophyll
cycle pigment pools (violaxanthin + antheraxanthin +
zeaxanthin) suggest that variation in PRI was not pri-
marily driven by short-term xanthophyll cycle pigment
activity in this case. These results support our hypoth-
esis that natural variation in PRI within plant stands
can be largely explained by constitutive pigment pool
size changes rather than facultative xanthophyll cycle
effects alone.
netics from broadleaved species demonstrate that this
sampling method can be effectively applied to narrow
conifer needles in situ, providing a portable tool for rap-
idly probing pigment light responses for intact conifer
needles in the eld.
The higher levels of carotenoid pool sizes (carotenes
Israel Journal of Plant Sciences 60 2012
92
relation with cPRI is obtained; this correlation was not
signicant with the xanthophyll cycle pigments alone
(Table 1). Similarly, the highest correlation between
cPRI and carotenoid pigment pool sizes was found with
the chl:lutein ratio (Table 1). Recently, a second “xan-
thophyll cycle” involving lutein deepoxidase has been
discovered (Bungard et al., 1999). Like the xanthophyll
cycle involving violaxanthin deepoxidation, the lutein
deepoxidase cycle appears to be involved in energy dis-
sipation (García-Plazaola et al., 2007). As of this writ-
ing, the lutein deepoxidation cycle has been observed in
many species, including some trees (García-Plazaola et
al., 2002; Matsubara et al., 2007), but to our knowledge
its presence or function in conifers has not yet been
conclusively demonstrated. Similarly, the inuence of
lutein deepoxidation on reectance spectra and PRI has
not yet been explored. The high correlation reported
here between cPRI and lutein suggests that lutein may
exert a particular inuence on PRI, perhaps through a
reversible lutein epoxidation. Exploration of this hy-
pothesis was beyond the scope of this initial eld study,
and further experimental work is needed to explore the
possible inuence of lutein deepoxidation on PRI vari-
ability in nature.
In our study, facultative effects associated with xan-
thophyll cycle activity were a smaller source of PRI
variation than constitutive effects; cPRI variation asso-
ciated with canopy position was slightly larger than the
amount of PRI variation (ΔPRI) caused by xanthophyll
cycle activity. When different species were considered,
the cPRI variation was 3–4 times larger than that due to
facultative xanthophyll cycle effects (ΔPRI). In some
ecosystems (e.g., Sims et al., 2006), seasonal variation
in PRI values is much larger than diurnal PRI values.
Together, these observations suggest that PRI variation
in nature is largely driven by constitutive effects, with
facultative effects a smaller component of the total PRI
variation, particularly when all seasons, species, and
stand levels are included in the analysis. The methods
presented here, employing a combination of dark-state
sampling and dark–light conversion, provide one way
to experimentally isolate the two effects in situ without
extensive destructive sampling.
Within each species, the larger ΔPRI values in sun
vs. shade leaves is consistent with the greater invest-
ment in xanthophyll cycle pigment pools (violaxanthin
+ antheraxanthin, + zeaxanthin) in sun-exposed, top-
canopy leaves relative to shade leaves (Figs. 4B and
5A). Consequently, the capacity for short-term changes
in PRI (i.e., facultative effects associated with rapid
xanthophyll pigment conversion) appears to vary with
canopy position in a manner similar to constitutive vari-
ation in pigment pool sizes, a nding that is consistent
Table 2
Variation in PRI attributable to xanthophyll cycle activity
(ΔPRI) or to constitutive pigment changes (cPRI) according
to sun-shade, and species differences. Percent variation is
expressed relative to xanthophyll cycle activity in sun leaves
(100%)
Source of Average PRI %
variation variation variation
Xanthophyll cycle activity in sun 0.0363 100
(ΔPRI)
Xanthophyll cycle activity in shade 0.0219 60
(ΔPRI)
Sun-shade differences (cPRI) 0.0390 107
Species differences, sun leaves (cPRI) 0.0989 272
Species difference, shade leaves (cPRI) 0.0898 247
Fig. 6. Dark-state PRI (cPRI) vs pigment ratios for chl/lu-
tein and chl/carotenoids (all carotenes and xanthophylls
combined). Each point is an average of samples from 3 or 4
branches, and error bars indicate SEM. Data replotted from
Figs. 4 and 5.
Our pigment data (Table 1, Fig. 6) suggest that cPRI
values may be strongly affected by variation in lutein
levels within the canopy. For example, when lutein is
added to the xanthophyll cycle pigments (violaxanthin +
antheraxanthin + zeaxanthin + lutein), a signicant cor-
Gamon and Berry / PRI in sun and shade conifer needles
93
with other eld studies showing correlation between
xanthophyll cycle activity and carotenoid pool sizes
(Stylinki et al., 2002; Filella et al., 2009). Because these
facultative and constitutive effects on PRI are often
strongly interrelated, it is often hard to isolate the short-
er-term xanthophyll effects on PRI from the longer-term
pool size effects on PRI without careful analysis of leaf
pigments or reectance properties, an analysis that is
usually lacking in remote sensing studies of PRI.
Our results have important implications for recent
attempts to apply PRI from aircraft and satellite plat-
forms as a measure of photosynthetic activity. Most
publications on this topic invoke the xanthophyll cycle
as a primary driver of variation in PRI, and most do not
consider constitutive pigment pool size variation with
season, species, or canopy position. Few of these stud-
ies measure actual leaf reectance or pigment levels to
identify the cause of PRI variation, leaving the mecha-
nistic interpretation of remote PRI measurements unre-
solved. Our results suggest that part of the PRI variation
with canopy position (and possibly with sensor view
angle) could be due to constitutive pool size effects on
PRI. Consequently, more work is clearly needed to bet-
ter understand the extent to which constitutive pool size
variation is determining PRI variation when measured
from aircraft or satellite. Without directly considering
various pigment pools, the causes and time scales of
variation in pigment content, and the environmental
context (light history, temperature, moisture, nutrient
conditions, etc.), remote sensing studies employing PRI
from aircraft or satellite platforms could be misconstru-
ing the cause of variation in PRI. Constitutive pigment
effects on PRI could explain some of the scatter in re-
mote sensing studies of the PRI–LUE relationships, and
may partly explain why different vegetation stands do
not always appear to have the same PRI–LUE relation-
ships (Grace et al., 2007; Goerner et al., 2011).
Because few of these recent studies have considered
constitutive pigment effects on photosynthetic activity,
it is unclear to what extent these pool size variations
scale with variation in LUE or photosynthetic activity.
Because these constitutive pool size effects can be larger
than the facultative xanthophyll cycle effects (Table 2),
they can “magnify” the PRI signal, making it appear
more effective as an index of photosynthetic rate or
light-use efciency, particularly if the xanthophyll cycle
and long-term pigment pool size changes are coordi-
nately regulated. We hypothesize that such combined
(facultative and constitutive) pigment effects on PRI
may partly explain why so many remote sensing studies
are able to correlate PRI variation with LUE variation
(Nichol et al., 2000; Rahman et al., 2001, 2004; Drolet
et al., 2005; Garbulsky et al., 2008; Hilker et al., 2009;
Goerner et al., 2011). In this case, variation in pigment
pool sizes makes PRI differences more detectable with
remote sensing. However, due to the different time con-
stants of facultative and constitutive pigment responses,
we might also expect these two types of responses to
inuence light-use efciency over different time scales,
and this can cause disjunct PRI–LUE patterns when
observed over many months or years (Filella et al.,
2004; Sims et al., 2006). The varying contribution of
facultative and constitutive effects could also explain
part of the scatter in many published PRI–LUE rela-
tionships, particularly if the facultative and constitu-
tive components are out of phase. Similarly, we do not
know to what extent the lutein deepoxidase cycle may
contribute to the observed variation in PRI within forest
canopies and further confound (or improve) PRI as a
photosynthetic index. Until these issues are resolved, it
will be difcult to attain the goal of reliably “measuring
photosynthesis from space” (Grace et al., 2007; Coops
et al., 2010; Hall et al., 2011) with PRI. Recent studies
that correct for sunlit fraction (Hall et al., 2008) are an
exciting step in this direction, but may need to consider
additional pigment responses besides the facultative
xanthophyll cycle alone.
Our ndings provide clear evidence for both facul-
tative and constitutive pigment effects on PRI within
a single vegetation canopy. The constitutive pigment
effects explained most of the PRI variation between spe-
cies and within a single canopy. Facultative and consti-
tutive effects operate over different time scales and are
only part of a wide array of mechanisms (physiological
to structural) that plants employ to adjust their radiation
balance under changing environmental and physiologi-
cal conditions. A better appreciation of the full range of
plant responses, from the fast physiological adjustments
to the slower structural and ontogenetic changes, is now
needed to improve our understanding of photosynthetic
physiology from remote sensing. A clearer understand-
ing of both constitutive and facultative changes in leaf
pigment pools will be needed to develop effective
photosynthesis models using PRI. The leaf reectance
methods demonstrated here, along with additional opti-
cal methods for assessing relative pigment levels (e.g.,
Gitelson et al., 2006), could be instrumental in realizing
this goal.
ACKNOWLEDGMENTS
Completion of this study was inspired by the work of
Anatoly Gitelson, who has long been an innovator in as-
sessing leaf pigment content from spectral reectance.
John Scott Surfus provided technical assistance for all
parts of this study. Measurements at the Wind River
Israel Journal of Plant Sciences 60 2012
94
Crane site were conducted with the help of the skilled
staff of the Wind River Crane facility. Canopy access
and sampling advice at the Black Butte site was pro-
vided by Barbara Bond and Mike Ryan. Measurements
at the BOREAS Old Jack Pine site were conducted with
eld assistance by Art Fredeen. Financial support for
this work was provided by grants from the US-EPA,
NASA, and NSF to J. Gamon and a NASA BOREAS
grant to J. Berry.
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