Water Movement and Storage in a Desert Succulent: Anatomy and Rehydration Kinetics for Leaves of Agave deserti

Smith, J. A. C. and Nobel, P. S. 1986. Water movement and storagein a desert succulent: anatomy and rehydration kinetics forleaves of Agave deserti.—J. exp. Bot. 37: 1044–1053. Anatomic and kinetic aspects of water storage were investigatedfor the succulent leaves of the desert CAM plant, Agave deserti.An approximately linear relationship was found between the numberof vascular bundles and leaf surface area, both for leaves ofdifferent sizes and also along the length of a single leaf.The bundles, which were distributed throughout the leaf cross-section,were separated from each other by about eight water-storagecells. Even though the cell walls of the water-storage groundtissue made up only 2?5% of the cell volume, they provided about10% of the total cross-sectional area available for water transportradial to the xylem because cell-cell contact in such a directionaveraged 25% of the cell surface area. The rehydration kineticsof partially dehydrated leaf segments were resolved into threephases: (1) a relatively rapid movement into the vascular tissue(half-time of 2 min); (2) water movement into storage in theground tissue (half-time of 59 min); and (3) water movementinto the intercellular air spaces (half-time of about 10 h).Using the observed kinetics for water movement into the storagetissue and standard diffusion theory, the bulk-averaged diffusivityof water in the relatively homogeneous ground tissue (D₁) was2?0 ? 10^–10 m² s^–1 Using this (D₁) and pathway analysis,most of the water moving from the xylem into storage in themassive leaves of A. deserti apparently occurred from cell tocell across the cell membranes rather than through the cellwalls. Key words: Agave deserti, capacitance, diffusivity, leaf anatomy, succulence, water storage     

Electrical Impedance Analysis in Plant Tissues8

Electrical impedance spectra (100 Hz–800 kHz) were measuredin leaves of Peperomia obtusifolia L. (a succulent) and Brassicaoleracea L. (cabbage). By measuring impedances at three or moreinter-electrode distances in a single leaf, electrode impedanceand specific tissue impedance were separated. Analysis of impedance data from B. oleracea leaves in relationto an equivalent circuit model showed that leaf developmentwas accompanied by increases in extracellular resistance, cytoplasmicresistance and vacuole interior resistance, together with decreasesin plasma membrane capacitance and tonoplast capacitance. AfterB. oleracea leaves were subjected to a –6 °C freeze-thawstress, extracellular resistance, cytoplasmic resistance andvacuole interior resistance decreased, but plasma membrane capacitanceand tonoplast capacitance did not change. These results indicatethat useful measurements of leaf parameters can be obtainedby this technique. Examination of the electrode impedance spectrum showed thatelectrode insertion produced a damaged collar, 0·4–0·5mm wide, around the electrode. This was confirmed by visualobservation of the damage in P. obtusifolia leaf. Key words: Peperomia obtusifolia L., Brassica oleracea L. (cabbage), electrical impedance, equivalent circuit, electrode polarization     

VvPIP2;4N aquaporin involvement in controlling leaf hydraulic capacitance and resistance in grapevine

Hydraulic capacitance (C) in a plant tissue buffers the xylem tension, storing and releasing water and has been highlighted in recent years as an important factor that affects water relations such as drought tolerance and embolism formation. Aquaporins (AQPs) are well known to control leaf hydraulic resistance (Rh) but their role in the control of C is unknown. Here, we assess Rh and C on detached grapevines wild‐type (WT) (cv. Brachetto) leaves and over‐expressing the aquaporin gene VvPIP2;4N (OE). For this purpose, we developed a new method inspired from the pressure‐volume curve technique and the rehydration‐kinetic‐method, which allowed us to monitor the dynamics of dehydration and rehydration in the same leaf. The recovery after dehydration was measured in dark, light non‐transpirative conditions, light‐transpirative conditions and light‐transpirative condition adding abscisic acid. Pressurizing to dehydrate leaves in the OE line, the recorded Rh and C were respectively lower and higher than those in the WT. The same results were obtained in the dark recovery by rehydration treatment. In the presence of light, either when leaves transpired or not (by depressing vapor pressure deficit), the described effects disappeared. The change in Rh and C did not affect the kinetics of desiccation of detached leaves in dark in air, in OE plants compared to WT ones. Our study highlighted that both Rh and C were influenced by the constitutive over‐expression of VvPIP2;4N. The effect of AQPs on C is reported here for the first time and may involve a modulation of cell reflection coefficient.     

Coordination and trade‐offs between leaf and stem hydraulic traits and stomatal regulation along a spectrum of isohydry to anisohydry

The degree of plant iso/anisohydry, a widely used framework for classifying species‐specific hydraulic strategies, integrates multiple components of the whole‐plant hydraulic pathway. However, little is known about how it associates with coordination of functional and structural traits within and across different organs. We examined stem and leaf hydraulic capacitance and conductivity/conductance, stem xylem anatomical features, stomatal regulation of daily minimum leaf and stem water potential (Ψ), and the kinetics of stomatal responses to vapour pressure deficit (VPD) in six diverse woody species differing markedly in their degree of iso/anisohydry. At the stem level, more anisohydric species had higher wood density and lower native capacitance and conductivity. Like stems, leaves of more anisohydric species had lower hydraulic conductance; however, unlike stems, their leaves had higher native capacitance at their daily minimum values of leaf Ψ. Moreover, rates of VPD‐induced stomatal closure were related to intrinsic rather than native leaf capacitance and were not associated with species' degree of iso/anisohydry. Our results suggest a trade‐off between hydraulic storage and efficiency in the leaf, but a coordination between hydraulic storage and efficiency in the stem along a spectrum of plant iso/anisohydry.     

Dynamics of leaf hydraulic conductance with water status: quantification and analysis of species differences under steady state

Leaf hydraulic conductance (K(leaf)) is a major determinant of photosynthetic rate in well-watered and drought-stressed plants. Previous work assessed the decline of K(leaf) with decreasing leaf water potential (Ψ(leaf)), most typically using rehydration kinetics methods, and found that species varied in the shape of their vulnerability curve, and that hydraulic vulnerability correlated with other leaf functional traits and with drought sensitivity. These findings were tested and extended, using a new steady-state evaporative flux method under high irradiance, and the function for the vulnerability curve of each species was determined individually using maximum likelihood for 10 species varying strongly in drought tolerance. Additionally, the ability of excised leaves to recover in K(leaf) with rehydration was assessed, and a new theoretical framework was developed to estimate how rehydration of measured leaves may affect estimation of hydraulic parameters. As hypothesized, species differed in their vulnerability function. Drought-tolerant species showed shallow linear declines and more negative Ψ(leaf) at 80% loss of K(leaf) (P(80)), whereas drought-sensitive species showed steeper, non-linear declines, and less negative P(80). Across species, the maximum K(leaf) was independent of hydraulic vulnerability. Recovery of K(leaf) after 1 h rehydration of leaves dehydrated below their turgor loss point occurred only for four of 10 species. Across species without recovery, a more negative P(80) correlated with the ability to maintain K(leaf) through both dehydration and rehydration. These findings indicate that resistance to K(leaf) decline is important not only in maintaining open stomata during the onset of drought, but also in enabling sustained function during drought recovery.     

Insight into the physiological role of water absorption via the leaf surface from a rehydration kinetics perspective

Soil water transported via the petiole is a primary rehydration pathway for leaves of water‐stressed plants. Leaves may also rehydrate by absorbing water via their epidermal surfaces. The mechanisms and physiological relevance of this water pathway, however, remain unclear, as the associated hydraulic properties are unknown. To gain insight into the foliar water absorption process, we compared rehydration kinetics via the petiole and surface of Prunus dulcis and Quercus lobata leaves. Petiole rehydration could be described by a double exponential function suggesting that 2 partly isolated water pools exist in leaves of both species. Surface rehydration could be described by a logistic function, suggesting that leaves behave as a single water pool. Whereas full leaf rehydration via the petiole required approximately 20 min, it took over 150 and 300 min via the surface of P. dulcis and Q. lobata , respectively. Such differences were attributed to the high resistance imposed by the leaf surface and especially the cuticle. The minimum resistance to surface rehydration was estimated to be 6.6 × 10² (P. dulcis ) and 2.6 × 10³ MPa·m²·s·g^?1 (Q. lobata ), which is remarkably higher than estimated for petiole rehydration. These results are discussed in a physiological context.     

Water balance in Hemizonia luzulifolia: the role of extracellular polysaccharides

Abstract. The role of extracellular water in ameliorating drought stress was examined in Hemizonia luzulifola , an annual composite that comprises two subspecies differing significantly in the amount of extracellular polysaccharide within basal leaves. Rosette leaves of the high polysaccharide (HP) ssp were more than 30% pectin on a dry weight basis in contrast to only 4% in the low polysaccharide (LP) ssp. Concomitant with this difference in polysaccharide content was a significant difference in the dehydration response of leaves. Near full hydration, relative capacitances of HP leaves (0.7 MPa^-1) were an order of magnitude greater than the LP leaves (0.08 MPa^-1). Relative capacitance of the polysaccharide alone was 1.5 MPa^-1. The weight of water per unit leaf area was not significantly different in the two taxa. Nevertheless, the pectin-like polysaccharides significantly altered the distribution of metabolically available water from primarily cell-stored water in the LP ssp to apoplasmic and symplasmic capacitors in leaves of the HP ssp. The consequence of this apoplasmic water store with colloidal properties is that the Ψ-dependence of leaf capacitance ceases to be linked directly to cell water relations. Transfer resistances for water movement between capacitors and the xylem near full hydration (0 to -0.5 MPa) were significantly larger in the HP leaf. This difference in transfer resistance was interpreted to be the result of a large resistance to water movement between the polysaccharide and the xylem. Because of these large transfer resistances, the apoplasmic capacitor probably buffers cells at lower water potentials under transient water fluxes than expected from laboratory measurements made during slow desiccation. Field measurements support this conclusions; HP leaves were better buffered than LP leaves at midday water potentials.     

Temperature and Water Relations for Sun and Shade Leaves of a Desert Broadleaf, Hyptis emoryi

The temperature and water relations of sun versus shade leavesof Hyptis emoryi Torr. were evaluated from field measurementsmade in late summer. Throughout most of the day sun leaves hadhigher temperatures and higher resistances to water vapour diffusion,but lower transpiration rates and lower stem water potentials,than did shade leaves. Leaf absorptivity to solar irradiationwas less for 1.5-cm-long sun leaves (0.44) than for 4.0-cm shadeleaves (0.56). For both leaf types the stomatal resistance increasedas the water vapour concentration drop from the leaf to theair increased. Energy balance equations were used together with the measuredtemperature dependence of photosynthesis to predict the effectof variations in leaf absorptivity, length, and resistance onnet photosynthesis. The influence of leaf dimorphism on wholeplants was determined by calculating daily photosynthesis andtranspiration for plants with various percentages of sun andshade leaves. A hypothetical plant with all sun leaves in thesun had about twice the photosynthesis and half the transpirationratio as did plants with sun leaves in the shade or shade leavesin the sun or shade. Plants with both sun and shade leaves hadthe highest predicted photosynthesis per unit ground area. Thepossible adaptive significance of the seasonal variation insun and shade leaf percentages observed for individual H. emoryibushes is discussed in terms of water economy and photosynthesi     

Hydraulic conductivity of red oak (Quercus rubra L.) leaf tissue does not respond to light

The permeability of leaf tissue to water has been reported to increase under illumination, a response reputed to involve aquaporins. We studied this ‘light response’ in red oak (Quercus rubra L.), the species in which the phenomenon was first detected during measurements of leaf hydraulic conductance with the high‐pressure flow meter (HPFM). In our HPFM measurements, we found that pre‐conditioning leaves in darkness was not sufficient to bring them to their minimum conductance, which was attained only after an hour of submersion and pressurization. However, pre‐conditioning leaves under anoxic conditions resulted in an immediate reduction in conductance. Leaves light‐ and dark‐acclimated while on the tree showed no differences in the time course of HPFM measurement under illumination. We also studied the effect of light level and anoxia on rehydration kinetics, finding that anoxia slowed rehydration, but light had no effect either in the lab (rehydration under low light, high humidity) or on the tree (acclimation under high light, 10 min of dark prior to rehydration). We conclude that the declines in conductance observed in the HPFM must involve a resistance downstream of the extracellular air space, and that in red oak the hydraulic conductivity of leaf tissue is insensitive to light.     

Turning over a new ‘leaf’: multiple functional significances of leaves versus phyllodes in Hawaiian Acacia koa

Hawaiian endemic tree Acacia koa is a model for heteroblasty with bipinnately compound leaves and phyllodes. Previous studies suggested three hypotheses for their functional differentiation: an advantage of leaves for early growth or shade tolerance, and an advantage of phyllodes for drought tolerance. We tested the ability of these hypotheses to explain differences between leaf types for potted plants in 104 physiological and morphological traits, including gas exchange, structure and composition, hydraulic conductance, and responses to varying light, intercellular CO₂, vapour pressure deficit (VPD) and drought. Leaf types were similar in numerous traits including stomatal pore area per leaf area, leaf area‐based gas exchange rates and cuticular conductance. Each hypothesis was directly supported by key differences in function. Leaves had higher mass‐based gas exchange rates, while the water storage tissue in phyllodes contributed to greater capacitance per area; phyllodes also showed stronger stomatal closure at high VPD, and higher maximum hydraulic conductance per area, with stronger decline during desiccation and recovery with rehydration. While no single hypothesis completely explained the differences between leaf types, together the three hypotheses explained 91% of differences. These findings indicate that the heteroblasty confers multiple benefits, realized across different developmental stages and environmental contexts.     

Altered Synthesis and Composition of Cell Wall of Grape (Vitis vinifera L.) Leaves during Expansion and Growth-Inhibiting Water Deficits

The rate and composition of cell wall polysaccharide synthesisduring development and growth-inhibiting water deficits wereinvestigated in leaves of grape (Vitis vinifera L.). The rateof leaf expansion was monitored as plant water status was manipulatedby modulating the supply of irrigation water to potted plantsover several days. The corresponding wall synthesis was determinedby incubating leaf tissue with [¹⁴C]glucose and quantifyingincorporation into wall components. Samples were obtained fromrapidly expanding and mature leaves before, during, and following(recovery from) moderate water deficits. Uptake was approximately2-fold greater for mature leaf tissue than for rapidly expandingtissue at both high and low water status. In contrast, incorporationinto cell wall polysaccharides was 18 to 41% (under low andhigh water status) of uptake in expanding leaves but less than4% in mature tissue. Incorporation of precursor into wall polysaccharideswas insensitive to plant water status in mature leaves, butwas inhibited to less than 50% of well-watered controls in expandingleaves at low water potential. Incorporation of label into cellulose,uronic acid, and neutral sugar fractions was differentiallyaffected by water deficits, with cellulose synthesis apparentlyexhibiting the greatest sensitivity to low water status. Afterrewatering, growth, as well as uptake and incorporation of labelrecovered, although the latter did not attain prestress rates.The results indicate a high sensitivity of wall polysaccharide(particularly cellulose) synthesis to growth-inhibiting waterdeficits. ¹ Supported by United States Department of Agriculture, CompetitiveResearch grant GAM 8502539. (Received November 15, 1989; Accepted January 17, 1990)     

Comparative hydraulic and anatomic properties in palm trees (Washingtonia robusta) of varying heights: implications for hydraulic limitation to increased height growth

As trees grow taller, the energetic cost of moving water to the leaves becomes higher and could begin to limit carbon gain and subsequent growth. The hydraulic limitation hypothesis states that as trees grow taller, the path length and therefore frictional resistance of water flow increases, leading to stomatal closure, reduced photosynthesis and decreased height growth in tall trees. Although this hypothesis is supported by the physical laws governing water movement in trees, its validation has been complicated by the complex structure of most tree species. Therefore, this study tested the hydraulic limitation hypothesis in Washingtonia robusta (H. Wendl.), a palm that, while growing to tall heights, is still structurally simple enough to act as a model organism for testing. There were no discernable relationships between tree height and stomatal conductance, stomatal densities, guard cell lengths, leaf dry mass per unit area (LMA) or sap flux, suggesting that these key aspects of hydraulic limitation are not reduced in taller palms. Taller palms did, however, have higher maximum daily photosynthetic assimilation rates, lower minimum leaf water potentials that occurred earlier in the day and fewer, smaller leaves than did shorter palms. Leaf epidermal cells were also smaller in taller palms compared with shorter ones. These findings are consistent with hydraulic compensation in that tall palms may be overcoming the increased path length resistance through smaller, more efficient leaves and lower leaf water potentials than shorter palms.     

An abscisic acid-related reduced transpiration promotes gradual embolism repair when grapevines are rehydrated after drought

* Proposed mechanisms of embolism recovery are controversial for plants that are transpiring while undergoing cycles of dehydration and rehydration. * Here, water stress was imposed on grapevines (Vitis vinifera), and the course of embolism recovery, leaf water potential (Psi(leaf)), transpiration (E) and abscisic acid (ABA) concentration followed during the rehydration process. * As expected, Psi(leaf) and E decreased upon water stress, whereas xylem embolism and leaf ABA concentration increased. Upon rehydration, Psi(leaf) recovered in 5 h, whereas E fully recovered only after an additional 48 h. The ABA content of recovering leaves was higher than in droughted controls, both on the day of rewatering and the day after, suggesting that ABA accumulated in roots during drought was delivered to the rehydrated leaves. In recovering plants, xylem embolism in petioles, shoots, and roots decreased during the 24 h following rehydration. * A model is proposed to describe plant recovery after rehydration based on three main points: embolism repair occurs progressively in shoots and further in roots and in petioles, following an almost full recovery of Psi(leaf); hydraulic conductance recovers during diurnal transpiring hours, when formation and repair of embolisms occurs in all plant organs; an ABA residual signal in rehydrated leaves hinders stomatal opening even when water relations have recovered, suggesting that an ABA-induced transpiration control promotes gradual embolism repair in rehydrated grapevines.     

Desiccation-tolerant Sporobolus stapfianus: lipid composition and cellular ultrastructure during dehydration and rehydration

The desiccation-tolerant plant Sporobolus stapfianus was subjectedto slow dehydration and to rehydration either as a silica gel-drieddetached leaf or as an airdried plant. In detached leaves dehydrationresulted in a lower relative water content in comparison withleaves dried on the plant. Water loss caused a reduction inchlorophyll, carotenoid and lipid contents and an increase inconjugated dienes. In detached leaves, ultrastructure was alsoaffected by dehydration, showing damaged cells with alteredchloroplasts which retained large quantities of starch and lipid-likeinclusions in the stroma. Upon rehydration a continuous degradationof the chemical composition and cell organization was observedwith a further increase in peroxidation. Leaves dehydrated onthe plant showed degradation of chlorophyll and lipids, whereascarotenoids increased and conjugated dienes decreased. Desiccationcaused a vacuolar fragmentation and a decline in starch, whereaschloroplasts underwent slight alterations. Following rewateringa full recovery of chlorophyll and lipids occurred, while carotenoidsand dienes remained constant. Starch increased in the chloroplastsand there was complete recovery of the ordered cell arrangementand chloroplast organization. Of the chloroplast polar lipids,in both sets of leaves desiccation caused a reduction only inmonogalactosyldiacylglycerol, while phospholipids showed anopposite pattern, increasing in air-dried leaves and decreasingin detached leaves. Rewatering of leaves desiccated on the plantled to a complete recovery of the lipid composition, whereasdetached leaves suffered a complete lipid degradation with theloss of polyunsaturated fatty acids. Key words: Desiccation tolerance, lipids, resurrection plants, Sporobolus stapfianus, ultrastructure     

散孔材与环孔材树种枝干、叶水力学特性的比较研究

为揭示散孔材与环孔材树种树木水分生理特性的差异,选取了常见的3种散孔材落叶树种(毛白杨、法国梧桐和樱花)和3种环孔材落叶树种(刺槐、合欢和白蜡),研究了其枝干与叶水力学性质的差异及其协调性。结果表明:3种环孔材树种枝干横截面积基础上的最大比导水率(Ks-max)大于3种散孔材树种,但其木质部对空穴化的脆弱性(P50branch)高于散孔材树种,6种树木枝干的水分传输能力和抵抗空穴化能力之间存在一种相互制约的权衡关系。3种散孔材与3种环孔材树种的叶最大水力导度(Kl-max)和水力脆弱性(P50leaf)并无显著差异;对于3种散孔材树种,叶的水力脆弱性要高于枝干,但对3种环孔材树种而言,枝干的水力脆弱性要高于叶。6种树木枝干和叶的水力学性质(Kmax、P50)之间并无相关关系。这些结果表明:散孔材与环孔材树种的枝干水力学特性有明显差异,但叶水力学特性无差异;枝干与叶水力学性质之间是相互独立的。     

The Partitioning of Hydraulic Conductances within Mature Orange Trees

Sap flow (F) and leaf water potential (LWP) were followed diurnallyin mature Valencia and Shamouti orange trees in an orchard.The hydraulic conductance of these trees was computed from thediurnal relationship between the LWP and F. The driving forcefor water movement was estimated from a weighted average ofsunlit and shaded LWP, assuming that leaves in the shade transpireto some extent. LWP of covered, non-transpiring leaves was alsomeasured hourly. It was assumed to represent the xylem waterpotential within the axial conduit of the trunk. Relating coveredLWP to F on an hourly basis enables the computation of the hydraulicconductance of the root system, including axial conductances.The hydraulic conductance of the transpiring crown was computed.Its magnitude was comparable to the root system hydraulic conductance. Key words: Orange trees, hydraulic conductance, sap flow, leaf water potential     

Kinetics of recovery of leaf hydraulic conductance and vein functionality from cavitation-induced embolism in sunflower

The kinetics of leaf vein recovery from cavitation-induced embolism was studied in plants of sunflower cv. Margot, together with the impact of vein embolism on the overall leaf hydraulic conductance (Kleaf). During the air-dehydration of leaves to leaf water potentials (Psi L) of -1.25 MPa, Kleaf was found to decrease by about 46% with respect to values recorded in well-hydrated leaves. When leaves, previously dehydrated to Psi L= -1.1 MPa (corresponding to the turgor loss point), were put in contact with water, Kleaf recovered completely in 10 min and so did leaf water potential. Functional vein density was estimated in both dehydrating and rehydrating leaves in terms of total length of red-stained veins infiltrated with a Phloxine B solution per unit leaf surface area. Veins were found to embolize (unstained) with kinetics showing a linear relationship with Kleaf so that about a 70% loss of functional veins corresponded with a Kleaf loss of 46%. Cavitated veins recovered from embolism within 10 min from the beginning of leaf rehydration. These data indicate that: (a) leaves of sunflower underwent substantial vein embolism during dehydration; (b) vein embolism and leaf hydraulic efficiency apparently recovered from dehydration completely and rapidly upon rehydration; (c) vein refilling occurred while conduits were still at more negative xylem pressures than those required for spontaneous bubble dissolution on the basis of Henry's law. The possible consistent contribution of vital mechanisms for vein refilling is discussed.     

Water uptake profile response of corn to soil moisture depletion

The effects of soil moisture distribution on water uptake of drip‐irrigated corn were investigated by simultaneously monitoring the diurnal evolution of sap flow rate in stems, of leaf water potential, and of soil moisture, during intervals between successive irrigations. The results invalidate the steady‐state resistive flow model for the continuum. High hydraulic capacitance of wet soil and low hydraulic conductivity of dry soil surrounding the roots damped significantly diurnal fluctuations of water flow from bulk soil to root surface. By contrast, sap flow responded directly to the large diurnal variation of leaf water potential. In wet soil, the relation between the diurnal courses of uptake rates and leaf water potential was linear. Water potential at the root surface remained nearly constant and uniformly distributed. The slope of the lines allowed calculating the resistance of the hydraulic path in the plant. Resistances increased in inverse relation with root length density. Soil desiccation induced a diurnal variation of water potential at the root surface, the minimum occurring in the late afternoon. The increase of root surface water potential with depth was directly linked to the soil desiccation profile. The development of a water potential gradient at the root surface implies the presence of a significant axial resistance in the root hydraulic path that explains why the desiccation of the soil upper layer induces an absolute increase of water uptake rates from the deeper wet layers.     

Stomatal Control and Leaf Thermal and Hydraulic Capacitances under Rapid Environmental Fluctuations

Leaves within a canopy may experience rapid and extreme fluctuations in ambient conditions. A shaded leaf, for example, may become exposed to an order of magnitude increase in solar radiation within a few seconds, due to sunflecks or canopy motions. Considering typical time scales for stomatal adjustments, (2 to 60 minutes), the gap between these two time scales raised the question whether leaves rely on their hydraulic and thermal capacitances for passive protection from hydraulic failure or over-heating until stomata have adjusted. We employed a physically based model to systematically study effects of short-term fluctuations in irradiance on leaf temperatures and transpiration rates. Considering typical amplitudes and time scales of such fluctuations, the importance of leaf heat and water capacities for avoiding damaging leaf temperatures and hydraulic failure were investigated. The results suggest that common leaf heat capacities are not sufficient to protect a non-transpiring leaf from over-heating during sunflecks of several minutes duration whereas transpirative cooling provides effective protection. A comparison of the simulated time scales for heat damage in the absence of evaporative cooling with observed stomatal response times suggested that stomata must be already open before arrival of a sunfleck to avoid over-heating to critical leaf temperatures. This is consistent with measured stomatal conductances in shaded leaves and has implications for water use efficiency of deep canopy leaves and vulnerability to heat damage during drought. Our results also suggest that typical leaf water contents could sustain several minutes of evaporative cooling during a sunfleck without increasing the xylem water supply and thus risking embolism. We thus submit that shaded leaves rely on hydraulic capacitance and evaporative cooling to avoid over-heating and hydraulic failure during exposure to typical sunflecks, whereas thermal capacitance provides limited protection for very short sunflecks (tens of seconds).     

Biophysical limitation of cell elongation in cereal leaves

Grass leaves grow from the base. Unlike those of dicotyledonous plants, cells of grass leaves expand enclosed by sheaths of older leaves, where there is little or no transpiration, and go through developmental stages in a strictly linear arrangement. The environmental or developmental factor that limits leaf cell expansion must do so through biophysical means at the cellular level: wall-yielding, water uptake and solute supply are all candidates. This Botanical Briefing looks at the possibility that tissue hydraulic conductance limits cell expansion and leaf growth. A model is presented that relates pathways of water movement in the elongation zone of grass leaves to driving forces for water movement and to anatomical features. The bundle sheath is considered as a crucial control point. The relative importance of these pathways for the regulation of leaf growth and for the partitioning of water between expansion and transpiration is discussed.