Concurrent measurements of stem density, leaf and stem water potential, stomatal conductance and cavitation on a sapling of Thuja occidentalis L
Abstract
Abstract Concurrent estimates of stem density, leaf and stem water potential, stomatal conductance and ultrasonic acoustic emissions (cavitations) in an excised sapling of Thuja occidentalis L. were made. As the sapling dehydrated in air, the decline in leaf water potential to about - 2.0 MPa was followed by apparent rehydration of the foliage while the stem showed no sign of rehydration. The rate of acoustic emissions peaked prior to the onset of rehydration which coincided with virtual stomatal closure. There was a significant decline in stem density until maximum foliage rehydration level was reached. From this point, leaf water potential, stem water potential and stem density continued a relatively slow decline while acoustic emission rate and stomatal conductance remained low.
Removal of the bark and majority of foliage from the sapling resulted in increased cavitation and more rapid deelines in leaf and stem water potential and stem density.
... In the 1980s, piezoelectric elements were applied to the measurement of AE. Measurement equipment and filtering were improved to prevent detection of noise, and AE was detected in the ultrasonic frequency range (Dixon et al., 1984;Sandford and Grace, 1985;Raschi et al., 1989). AE measurement has been applied to herbaceous plants as well as woody plants, but widespread application of the method has been limited (Tyree et al., 1986). ...
Acoustic emission (AE) can be used to evaluate drought stress (water stress) in plants. Although bubble motion in plant vessels is suspected of being the source of AE generated by plants (burst-type AE), this has not yet been demonstrated experimentally. This study examined whether burst-type AE is caused by bubble motion by comparing the detected frequency with the theoretical frequency estimated based on a bubble motion equation. Four experimental treatments with different soil dry densities and water supply regimes were established, and the effect of the treatment conditions on AE characteristics was clarified. AE induced by water stress was extracted by focusing on the maximum amplitude and the centroid frequency calculated by wavelet transform analysis. The results showed that the measured frequency and the estimated frequency were almost equal, which demonstrates that the model agreed with the actual measurements and that the burst-type AE can be attributed to bubble motion. The AE induced by water stress had a smaller maximum amplitude and a higher centroid frequency than the AE detected during the water supply regime. Therefore, focusing on the centroid frequency may facilitate the extraction of AE induced by water stress.
... High wood density is a marker of root and stem growth, which augments a plant's ability to intercept light [29]. The rest of the created biomass invests in increasing the hardness of branches (specific stem density) and leaf tissues (leaf dry matter content) to resist low temperature and drought environments [30]. ...
Leaf habits (e.g., evergreen or deciduous) can reflect strategies of species adapting to varied environmental conditions. However, how species with different leaf habits coexist within a plant community is still poorly understood. Trait gradient analysis is a new approach to partition plant functional trait variations into alpha (within-community) and beta (among-community) components to quantify the effects of environmental filtering and biotic competition on community assembly. Here, on the basis of establishing forty-eight forest dynamic plots in a subtropical evergreen and deciduous broadleaved mixed forest in central China and measuring of seven functional traits, we compared the trait variation patterns and influencing factors of evergreen and deciduous species by using the trait gradient analysis method. The results showed that there were significant differences between functional traits for evergreen and deciduous species. Alpha trait components consistently varied more widely than beta components. The correlation between species trait mean and the alpha of each trait was highly significant, but there was no significant correlation between beta and alpha trait values. There were relatively weak or nonexistent significant correlations among species mean trait values and alpha trait values of different functional traits. However, the beta trait value showed high and significant correlations in both evergreen and deciduous species. Our results indicated that evergreen and deciduous species adopt similar adaptation strategies (beta component) in the context of environmental change in the community. However, the species initially came to coexist via the ecological positioning of traits (alpha component), which helped reduce competition so individuals could obtain more resources.
... The analysis of tree rings in vines allows calculating wood anatomical and isotopic traits (especially δ 13 C and δ 18 O) about water conductivity and water use efficiency that are parameters linked with several processes in plant metabolism including: gas exchanges and photosynthesis (the carbon source), biomass accumulation through the activity of vascular cambium (one carbon sink), plant hydraulics and resource allocation (Farquhar et al., 1989;Scheidegger et al., 2000;Cernusak et al., 2003;Saurer et al., 2004;Sperry et al., 2006;Barbour, 2007;Roden and Farquhar, 2012;Beeckman, 2016;De Micco et al., 2019). The vine water status is traditionally evaluated through the measurement of water potentials in leaves and stem, although attempts to evaluate it through carbon isotope composition have been recently increasing (Scholander et al., 1965;Dixon et al., 1984;Gaudillère et al., 2002;Brillante et al., 2018). The analysis of carbon isotopes in must has been shown to be linked with water availability and is considered a continuous integrator of the vine water status throughout the ripening period, although still controversial results are reported (Brillante et al., 2018). ...
In this century, one of the main objectives of agriculture is sustainability addressed to achieve food security, based on the improvement of use efficiency of farm resources, the increasing of crop yield and quality, under climate change conditions. The optimization of farm resources, as well as the control of soil degradation processes (e.g., soil erosion), can be realized through crop monitoring in the field, aiming to manage the local spatial variability (time and space) with a high resolution. In the case of high profitability crops, as the case of vineyards for high-quality wines, the capability to manage and follow spatial behavior of plants during the season represents an opportunity to improve farmer incomes and preserve the environmental health. However, any field monitoring represents an additional cost for the farmer, which slows down the objective of a diffuse sustainable agriculture. Satellite multispectral images have been widely used for production management in large areas. However, their observation is limited by the pre-defined and fixed scale with relatively coarse spatial resolution, resulting in limitations in their application. In this paper, encouraged by recent achievements in convolutional neural network (CNN), a multiscale full-connected CNN is constructed for the pan-sharpening of Sentinel-2A images by UAV images. The reconstructed data are validated by independent multispectral UAV images and in-situ spectral measurements. The reconstructed Sentinel-2A images provide a temporal evaluation of plant responses using selected vegetation indices. The proposed methodology has been tested on plant measurements taken either in-vivo and through the retrospective reconstruction of the eco-physiological vine behavior, by the evaluation of water conductivity and water use efficiency indexes from anatomical and isotopic traits recorded in vine trunk wood. In this study, the use of such a methodology able to combine the pro and cons of space-borne and UAVs data to evaluate plant responses, with high spatial and temporal resolution, has been applied in a vineyard of southern Italy by analyzing the period from 2015 to 2018. The obtained results have shown a good correspondence between the vegetation indexes obtained from reconstructed Sentinel-2A data and plant hydraulic traits obtained from tree-ring based retrospective reconstruction of vine eco-physiological behavior.
... Acoustic emission events during drought stress have been attributed mainly to cavitation events in xylem conduits (Milburn, 1973;Dixon et al., 1984). It was surprising to observe increased AE rates in the petioles during the beginning of drought before the onset of stomatal closure, because leaf water potential (Ψ leaf ) did not reach values that would result in xylem embolism formation until the end of the experiment (Figs 4, 6). ...
Signal coordination in response to changes in water availability remains unclear, as does the role of embolism events in signaling drought stress.
Sunflowers were exposed to two drought treatments of varying intensity while simultaneously monitoring changes in stomatal conductance, acoustic emissions (AE), turgor pressure, surface‐level electrical potential, organ‐level water potential and leaf abscisic acid (ABA) concentration. Leaf, stem and root xylem vulnerability to embolism were measured with the single vessel injection technique.
In both drought treatments, it was found that AE events and turgor changes preceded the onset of stomatal closure, whereas electrical surface potentials shifted concurrently with stomatal closure. Leaf‐level ABA concentration did not change until after stomata were closed. Roots and petioles were equally vulnerable to drought stress based on the single vessel injection technique. However, anatomical analysis of the xylem indicated that the increased AE events were not a result of xylem embolism formation. Additionally, roots and stems never reached a xylem pressure threshold that would initiate runaway embolism throughout the entire experiment.
It is concluded that stomatal closure was not embolism‐driven, but, rather, that onset of stomatal closure was most closely correlated with the hydraulic signal from changes in leaf turgor.
... Several studies involving acoustic sensing of woody tissues in other species showed that cavitation often occurred when the water potential falls below -1.5 MPa (Dixon et al., 1984;Peña & Grace, 1986;Tyree & Sperry, 1989). Then the recovery delay observed in this study may also be related to the complexity of the process for the refilling of xylem cavitation. ...
Little information is available on the diurnal behaviour of water potential and leaf conductance on pistachio trees despite their relevance to fine tune irrigation strategies. Mature pistachio trees were subject to simultaneous measurements of stem water potential (Ψx) and leaf conductance (gl) during the day, at three important periods of the irrigation season. Trees were grown on three different rootstocks and water regimes. An initial baseline relating Ψx to air vapor pressure deficit (VPD) is presented for irrigation scheduling in pistachio. Ψx was closely correlated with VPD but with a different fit according to the degree of water stress. No evidence of the variation of Ψx in relation to the phenology of the tree was observed. Furthermore, midday Ψx showed more accuracy to indicate a situation of water stress than predawn water potential. Under well irrigated conditions, gl was positively correlated with VPD during stage II of growth reaching its peak when VPD reached its maximum value (around 4 kPa). This behaviour changed during stage III of fruit growth suggesting a reliance of stomatal behaviour to the phenological stage independently to the tree water status. The levels of water stress reached were translated in a slow recovery of tree water status and leaf conductance (more than 40 days). Regarding rootstocks, P. integerrima showed little adaptation to water shortage compared to the two other rootstocks under the studied conditions.
The measurement of sap movement in xylem sapwood tissue using heat pulse velocity sap flow instruments has been commonly used to estimate plant transpiration. In this study, sap flow sensors (SFM1) based on the heat ratio method (HRM) were used to assess the sap flow performance of three different tree species located in the eastern suburbs of Melbourne, Australia over a 12-month period. A soil moisture budget profile featuring potential evapotranspiration and precipitation was developed to indicate soil moisture balance while the soil-plant-atmosphere continuum was established at the study site using data obtained from different monitoring instruments. The comparison of sap flow volume for the three species clearly showed that the water demand of Corymbia maculata was the highest when compared to Melaleuca styphelioides and Lophostemon confertus and the daily sap flow volume on the north side of the tree on average was 63% greater than that of the south side. By analysing the optimal temperature and vapour pressure deficit (VPD) for transpiration for all sampled trees, it was concluded that the Melaleuca styphelioides could better cope with hotter and drier weather conditions.
The determination of leaf water potential is useful in the establishment of irrigation guidelines for agricultural crop management practices and requires the use of various methods, among which thermocouple psychrometers (TCP). TCP have been widely used for this purpose. However, the psychrometric technique is complex and difficult to understand and the instrumentation required is difficult to handle. For this reason, a profound knowledge of the different aspects involved in the technique-which have not been clearly explained in the literature-is required. This paper reviews a number of research areas of TCP and focuses on three very specific fields: a) main applications of phsychrometry in field studies; b) determination of the measurement accuracy of psychrometric equipment, accuracy per se and accuracy tested against alternative methods; c) main errors and handling difficulties of TCP in the field. Research in these areas provides an updated overview of TCP as a method for determining water relations in plant material that will contribute criteria to select the most suitable technique according to the type of plant material and the purpose of the research and will highlight the types of instruments, accuracies and errors that have detrimental effects on measurements.
Leaf specific conductivities (LSC's) were measured on stem segments excised from various points within the canopy of eastern white cedar trees, Thuja occidentalis L. LSC is defined as the water flow rate (kilograms per second) through a stem caused by a unit of pressure potential gradient (megapascals per metre) per unit leaf surface area supplied by the stem (square metres). LSC's were measured on stems of various diameters and were found to vary over a factor of 30 in magnitude from 1 × 10−5 kg s−1 m−1 MPa−1 for stems 1 mm in diameter to 3 × 10−4 kg s−1 m−1 MPa−1 for stems 100 mm in diameter. LSC was found to be related to stem diameter (D (millimetres)) by the following empirical formula: LSC = 9.58 × 10−6 × D0.727. LSC's measured on stem segments including a node had significantly lower LSC's than internodal stem segments of the same length. Various water relations parameters were measured on cedar trees on a diurnal basis including evaporative flux, leaf resistance to evaporation, shoot water potential, air temperature, and air humidity. Water potential isotherms were also measured on excised green shoots. From the above data, we estimate that about 15% of the total water evaporated from green shoots comes from stored water while the shoot water potentials are growing progressively negative. The typical peak evaporative flux in midday was 1.5 × 10−5 kg s−1 m−2; using this value and our measured LSC's we estimate that the pressure potential gradients in the stems must be 50 kPa m−1 in stems 100 mm in diameter and 1500 kPa m−1 in stems 1 mm in diameter. Pressure potential gradients were measured in stems 30 to 50 mm in diameter by the pressure bomb technique and were found to be 69 kPa m−1 during a typical afternoon and this confirms the accuracy of the above estimates.
Ultrasonic acoustic emissions (AE) in the frequency range of 0.1 to 1 megahertz appear to originate in the sapwood of Thuja occidentalis L. The AE are vibrations of an impulsive nature. The vibrations can be transduced to a voltage waveform and amplified. The vibrations of each AE event begin at a large amplitude which decays over 20 to 100 microseconds. Strong circumstantial evidence indicates that the ultrasonic AE result from cavitation events because: (a) they occur only when the xylem pressure potential Psi(xp) is more negative than a threshold level of about -1 megapascal; (b) the rate of AE events increases as Psi(xp) decreases and when the net rate of water loss increases; (c) the AE can be stopped by raising Psi(xp) above -1 megapascal. Ultrasonic AE have been measured in whole terminal shoots allowed to dry in the laboratory, in isolated pieces of sapwood as they dried in the laboratory, and in whole terminal shoots in a pressure bomb when Psi(xp) was decreased by lowering the gas pressure in the pressure bomb.
This chapter consists of two parts, the first part (by M.H.Z.) describes the structural basis of water conduction from roots to leaves. Xylem anatomy has been known for a long time. However, those aspects which concern water movement through the plant as a whole have remained rather elusive and therefore neglected. The problem was partly that we are dealing here with microscopic structures spread throughout the whole plant body. We need the microscope to look at the structure, but it is too myopic to give us the information we seek. Whole-plant-xylem anatomy is an essential factor in plant competition and survival. For example, a tree can afford to lose a branch by cavitation of the xylem water, but it could not survive extensive cavitation in the trunk. How does it control this? The answer to this question seems to be “clever” construction.
Water, solute and pressure potentials have been investigated in shoots in the canopy of a Sitka spruce stand using the pressure chamber. Water potentials of - 15 bar or lower frequently occurred in the upper parts of the canopy on warm, sunny days of low vapour pressure deficit although there was never a shortage of water in the soil. Root water potentials ranged between -0.5 and -4.5 bar. Gradients of water potential of up to 2 bar m-1 occurred in the trunk and larger gradients occurred in the primary and secondary branches. On overcast or wet days the water potentials in the canopy were higher and the gradients smaller. It was concluded that the resistance to flow in the trunk was largely responsible for the large drop in potential between roots and leaves. The resistance per unit length was estimated as 107 bar s m-4 in the trunk and was considerably higher than in the angiosperms or pines for which comparable data are available. The solute potential of shoots in the canopy varied between -13 and -24 bar depending on the time of year. There was some tendency for solute potentials to be lower higher up in the canopy. From typical Hofler diagrams constructed from the water and solute potentials, the change in pressure potential (turgor pressure) with water content was determined, and a pressure dependent bulk modulus of elasticity derived. A water potential of -15 bar was equivalent to a fall in pressure potential to about 40% of that at full turgor. Empirical equations relating solute, pressure and water potential to water content were determined.
The influence of sapwood water content on the conductivity of sapwood to water was measured on stem sections of Pinus contorta. A reduction in relative water content from 100 to 90% caused permeability to fall to about 10% of the saturated value.Pressure–volume curves of branchwood and stem sapwood of Pinus contorta and Picea sitchensis have been analysed to definè the tissue capacitance and the time constant and resistance for water movement between stored water and the functional xylem as functions of tissue water potential. Three phases in water loss were discernible. In the initial phase at high water potentials (> –0.5 MPa), the capacitance was large, the time constant long and the resistance to flow large in comparison with intermediate water potentials (−0.5 to −1.5 MPa). At still lower water potentials (−1.5 to −3.0 MPa), the time constant and resistance declined still further but the capacitance had a tendency to increase again, especially in the stemwood of Sitka spruce. Typical values in the second phase were for the time constant 5 s, for the resistance 4 × 10−13 N s m−5 and for the capacitance (change in relative water content per unit change in potential) 1×10−11 m3 Pa−1. These parameters define the availability of stored water and are being used in a dynamic model of water transport in trees.
A technique is described for the nondestructive measurement of water content variations in the stems of trees.
The density of intact tree stems in the forest was estimated using attenuation of gamma radiation. Water content was calculated after subsequent derivation of the volume fraction of solid matrix.
A radial sequence of such measurements was obtained by measuring a sequence of chords, and assuming homogeneity within concentric tori. Analysis of the source of errors showed that the system was a close approximation of an ideal, rigid-geometry, mono-energetic source/detector system, with basic precision dependent on the quantity of gamma radiation measured. Correlation coefficients between the gamma-attenuation technique and subsequent gravimetric estimates of water content for two field experiments reported here were 0.882 and 0.938.
The technique was shown to be capable of describing the radial and diurnal variation in water content at two heights in the stem of a 20-year-old tree of Pinus contorta.
Attempts to correlate values of stomatal conductance and leaf water potential with particular environmental variables in the field are generally of only limited success because they are simultaneously affected by a number of environmental variables. For example, correlations between leaf water potential and either flux of radiant energy or vapour pressure deficit show a diurnal hysteresis which leads to a scatter diagram if many values are plotted. However, a simple model may be adequate to relate leaf water potential to the flow of water through the plant. The stomatal conductance of illuminated leaves is a function of current levels of temperature, vapour pressure deficit, leaf water potential (really turgor pressure) and ambient CO 2 concentration. Consequently, when plotted against any one of these variables a scatter diagram results. Physiological knowledge of stomatal functioning is not adequate to provide a mechanistic model linking stomatal conductance to all these variables. None the less, the parameters describing the relationships with the variables can be conveniently estimated from field data by a technique of non-linear least squares, for predictive purposes and to describe variations in response from season to season and plant to plant.