Differences in osmotic adjustment,foliar abscisic acid dynamics,and stomatal regulation between an isohydric and anisohydric woody angiosperm during drought

Species are often classified along a continuum from isohydric to anisohydric, with isohydric species exhibiting tighter regulation of leaf water potential through stomatal closure in response to drought. We investigated plasticity in stomatal regulation in an isohydric (Eucalyptus camaldulensis) and an anisohydric (Acacia aptaneura) angiosperm species subject to repeated drying cycles. We also assessed foliar abscisic acid (ABA) content dynamics, aboveground/belowground biomass allocation and nonstructural carbohydrates. The anisohydric species exhibited large plasticity in the turgor loss point (Ψ_TLP), with plants subject to repeated drying exhibiting lower Ψ_TLP and correspondingly larger stomatal conductance at low water potential, compared to plants not previously exposed to drought. The anisohydric species exhibited a switch from ABA to water potential‐driven stomatal closure during drought, a response previously only reported for anisohydric gymnosperms. The isohydric species showed little osmotic adjustment, with no evidence of switching to water potential‐driven stomatal closure, but did exhibit increased root:shoot ratios. There were no differences in carbohydrate depletion between species. We conclude that a large range in Ψ_TLP and biphasic ABA dynamics are indicative of anisohydric species, and these traits are associated with exposure to low minimum foliar water potential, dense sapwood and large resistance to xylem embolism.     

Vein cavitation and stomatal behaviour of sunflower (Helianthus annuus) leaves under water limitation

The impact of leaf vein cavitation and embolism on stomatal response and leaf hydraulic conductance was studied in potted plants of sunflower subjected to water limitation. Plant dehydration was achieved either by cutting well‐watered plants near their base and leaving them dehydrating in air or by depriving intact plants of irrigation. The vein cavitation threshold (Ψ_CAV) was estimated in terms of ultrasound acoustic emissions (UAE) from the leaf blade versus leaf water potential (Ψ_L). This was found to be the same (Ψ_CAV ≈ ?0.6 MPa) for leaves of both cut and intact plants where stomata began to close in coincidence with starting vein cavitation. Vein embolism was detected by infiltrating leaves at different Ψ_L with 0.7 mM fluorescein and measuring the percentage fluorescent area as percentage of total leaf surface area. A distinct loss of vein functionality (up to 50%) was found to occur in leaves at progressively decreasing Ψ_L, starting when leaves reached Ψ_CAV. A linear positive relationship with high statistical significance was found to exist between g_L and percentage leaf fluorescent area, thus indicating that stomata were sensitive to vein embolism. The hydraulic conductance (K_L) of the leaf was affected by leaf dehydration less than expected (K_L decreased by about 20% between near full turgor and Ψ_L = ?1.3 MPa). When the extravascular leaf compartment was excluded either by killing cells by immersing leaves in 70% ethanol or by cutting the main leaf venous system through to allow flow to bypass it, K_L turned out to increase 5.5 times, thus suggesting that the high dominance of the hydraulic resistance of the extravascular leaf compartment over the total leaf resistance might buffer or mask possibly large local changes in K_L inducing stomatal closure.     

Stem water storage capacity and efficiency of water transport: their functional significance in a Hawaiian dry forest

We investigated the contribution of internal water storage and efficiency of water transport to the maintenance of water balance in six evergreen tree species in a Hawaiian dry forest. Wood‐saturated water content, a surrogate for relative water storage capacity, ranged from 70 to 105%, and was inversely related to its morphological correlate, wood density, which ranged between 0·51 and 0·65 g cm^?3. Leaf‐specific conductivity (k_L) measured in stem segments from terminal branches ranged from 3 to 18 mmol m^?1 s^?1 MPa^?1, and whole‐plant hydraulic efficiency calculated as stomatal conductance (g) divided by the difference between predawn and midday leaf water potential (Ψ_L), ranged from 70 to 150 mmol m^?2 s^?1 MPa^?1. Hydraulic efficiency was positively correlated with k_L (r² = 0·86). Minimum annual Ψ_L ranged from ? 1·5 to ? 4·1 MPa among the six species. Seasonal and diurnal variation in Ψ_L were associated with differences among species in wood‐saturated water content, wood density and k_L. The species with higher wood‐saturated water content were more efficient in terms of long‐distance water transport, exhibited smaller diurnal variation in Ψ_L and higher maximum photosynthetic rates. Smaller diurnal variation in Ψ_L in species with higher wood‐saturated water content, k_L and hydraulic efficiency was not associated with stomatal restriction of transpiration when soil water deficit was moderate, but avoidance of low minimum seasonal Ψ_L in these species was associated with a substantial seasonal decline in g. Low seasonal minimum Ψ_L in species with low k_L, hydraulic efficiency, and wood‐saturated water content was associated with higher leaf solute content and corresponding lower leaf turgor loss point. Despite the species‐specific differences in leaf water relations characteristics, all six evergreen tree species shared a common functional relationship defined primarily by k_L and stem water storage capacity.     

Osmotic and elastic adjustments in cold desert shrubs differing in rooting depth: coping with drought and subzero temperatures

Physiological adjustments to enhance tolerance or avoidance of summer drought and winter freezing were studied in shallow- to deep-rooted Patagonian cold desert shrubs. We measured leaf water potential (Ψ_L), osmotic potential, tissue elasticity, stem hydraulic characteristics, and stomatal conductance (g _S) across species throughout the year, and assessed tissue damage by subzero temperatures during winter. Species behavior was highly dependent on rooting depth. Substantial osmotic adjustment (up to 1.2?MPa) was observed in deep-rooted species exhibiting relatively small seasonal variations in Ψ_L and with access to a more stable water source, but having a large difference between predawn and midday Ψ_L. On the other hand, shallow-rooted species exposed to large seasonal changes in Ψ_L showed limited osmotic adjustment and incomplete stomatal closure, resulting in turgor loss during periods of drought. The bulk leaf tissue elastic modulus (ε) was lower in species with relatively shallow roots. Daily variation in g _S was larger in shallow-rooted species (more than 50?% of its maximum) and was negatively associated with the difference between Ψ_L at the turgor loss point and minimum Ψ_L (safety margin for turgor maintenance). All species increased ε by about 10?MPa during winter. Species with rigid tissue walls exhibited low leaf tissue damage at ?20?°C. Our results suggest that osmotic adjustment was the main water relationship adaptation to cope with drought during summer and spring, particularly in deep-rooted plants, and that adjustments in cell wall rigidity during the winter helped to enhance freezing tolerance.     

Common trade‐offs between xylem resistance to cavitation and other physiological traits do not hold among unrelated Populus deltoides ×Populus nigra hybrids

We examined the relationships between xylem resistance to cavitation and 16 structural and functional traits across eight unrelated Populus deltoides×Populus nigra genotypes grown under two contrasting water regimes. The xylem water potential inducing 50% loss of hydraulic conductance (Ψ₅₀) varied from ?1.60 to ?2.40 MPa. Drought‐acclimated trees displayed a safer xylem, although the extent of the response was largely genotype dependant, with Ψ₅₀ being decreased by as far as 0.60 MPa. At the tissue level, there was no clear relationship between xylem safety and either xylem water transport efficiency or xylem biomechanics; the only structural trait to be strongly associated with Ψ₅₀ was the double vessel wall thickness, genotypes exhibiting a thicker double wall being more resistant. At the leaf level, increased cavitation resistance was associated with decreased stomatal conductance, while no relationship could be identified with traits associated with carbon uptake or bulk leaf carbon isotope discrimination, a surrogate of intrinsic water‐use efficiency. At the whole‐plant level, increased safety was associated with higher shoot growth potential under well‐irrigated regime only. We conclude that common trade‐offs between xylem resistance to cavitation and other physiological traits that are observed across species may not necessarily hold true at narrower scales.     

Does the turgor loss point characterize drought response in dryland plants?

The water potential at turgor loss point (Ψ_tlp) has been suggested as a key functional trait for determining plant drought tolerance, because of its close relationship with stomatal closure. Ψ_tlp may indicate drought tolerance as plants, which maintain gas exchange at lower midday water potentials as soil water availability declines also have lower Ψ_tlp. We evaluated 17 species from seasonally dry habitats, representing a range of life‐forms, under well‐watered and drought conditions, to determine how Ψ_tlp relates to stomatal sensitivity (pre‐dawn water potential at stomatal closure: Ψg_s0) and drought strategy (degree of isohydry or anisohydry; ΔΨ_MD between well‐watered conditions and stomatal closure). Although Ψg_s0 was related to Ψ_tlp, Ψg_s0 was better related to drought strategy (ΔΨ_MD). Drought avoiders (isohydric) closed stomata at water potentials higher than their Ψ_tlp; whereas, drought tolerant (anisohydric) species maintained stomatal conductance at lower water potentials than their Ψ_tlp and were more dehydration tolerant. There was no significant relationship between Ψ_tlp and ΔΨ_MD. While Ψ_tlp has been related to biome water availability, we found that Ψ_tlp did not relate strongly to stomatal closure or drought strategy, for either drought avoiders or tolerators. We therefore suggest caution in using Ψ_tlp to predict vulnerability to drought.     

Relationships between stomatal behavior,xylem vulnerability to cavitation and leaf water relations in two cultivars of Vitis vinifera

Current understanding of physiological mechanisms governing stomatal behavior under water stress conditions is still incomplete and controversial. It has been proposed that coordination of stomatal kinetics with xylem vulnerability to cavitation [vulnerability curve (VC)] leads to different levels of isohydry/anisohydry in different plant species/cultivars. In this study, this hypothesis is tested in Vitis vinifera cultivars displaying contrasting stomatal behavior under drought stress. The cv Montepulciano (MP, near‐isohydric) and Sangiovese (SG, anisohydric) were compared in terms of stomatal response to leaf and stem water potential, as possibly correlated to different petiole hydraulic conductivity (k_petiole) and VC, as well as to leaf water relations parameters. MP leaves showed almost complete stomatal closure at higher leaf and stem water potentials than SG leaves. Moreover, MP petioles had higher maximum k_petiole and were more vulnerable to cavitation than SG. Water potential at the turgor loss point was higher in MP than in SG. In SG, the percentage reduction of stomatal conductance (PLg_s) under water stress was almost linearly correlated with corresponding percentage loss of k_petiole (PLC), while in MP PLg_s was less influenced by PLC. Our results suggest that V. vinifera near‐isohydric and anisohydric genotypes differ in terms of xylem vulnerability to cavitation as well as in terms of k_petiole, and that the coordination of these traits leads to their different stomatal responses under water stress conditions.     

Physiological acclimation to drought stress in Solidago canadensis

Solidago canadensis is an invasive species from North America that is spreading across Europe, Australia and temperate Asia. We hypothesized that the species' wide ecological amplitude is also based on its potential in hydraulic acclimation, and analyzed hydraulic and anatomical properties along a transect with decreasing soil humidity. Stem hydraulic conductivity, vulnerability to drought‐induced embolism, stomatal closure during dehydration and xylem‐anatomical parameters were quantified at three sites. At the humid site, specific hydraulic conductivity of stems (1.0 ± 0.2 kg m^–1 MPa^–1 s^–1) was about twofold higher, and leaf‐specific conductivity about 1.5 times higher (3.1 ± 0.5 kg m^–1 MPa^–1 s^–1) than at the dry site. Water potential (Ψ) at 50% loss of conductivity was ?3.7 ± 0.1 MPa at the dry site and ?3.1 ± 0.2 MPa at the humid site (September). Vulnerability to drought‐induced embolism decreased along the transect and over the vegetation period. At drier sites, stomata started closing at lower Ψ while complete stomatal closure was reached at less negative Ψ (12% of maximum stomatal conductance: –2.5 ± 0.0 and ?3.0 ± 0.2 MPa at the dry and humid site). The safety margin between stomatal closure and 50% loss of conductivity was 1.2 and 0.2 MPa at the dry and humid sites. The observed variability indicated an efficient acclimation in hydraulic conductivity and safety: plants at dry sites exhibited lower specific hydraulic conductivity, higher embolism resistance and broader safety margins, signifying a trade‐off between the hydraulic safety and efficiency. The observed intraspecific plasticity in hydraulic and anatomical traits may help to explain the invasive potential of this species.     

Rapid hydraulic recovery in Eucalyptus pauciflora after drought: linkages between stem hydraulics and leaf gas exchange

In woody plants, photosynthetic capacity is closely linked to rates at which the plant hydraulic system can supply water to the leaf surface. Drought‐induced embolism can cause sharp declines in xylem hydraulic conductivity that coincide with stomatal closure and reduced photosynthesis. Recovery of photosynthetic capacity after drought is dependent on restored xylem function, although few data exist to elucidate this coordination. We examined the dynamics of leaf gas exchange and xylem function in Eucalyptus pauciflora seedlings exposed to a cycle of severe water stress and recovery after re‐watering. Stomatal closure and leaf turgor loss occurred at water potentials that delayed the extensive spread of embolism through the stem xylem. Stem hydraulic conductance recovered to control levels within 6 h after re‐watering despite a severe drought treatment, suggesting an active mechanism embolism repair. However, stomatal conductance did not recover after 10 d of re‐watering, effecting tighter control of transpiration post drought. The dynamics of recovery suggest that a combination of hydraulic and non‐hydraulic factors influenced stomatal behaviour post drought.     

Does turgor limit growth in tall trees?

The gravitational component of water potential contributes a standing 0.01 MPa m^?1 to the xylem tension gradient in plants. In tall trees, this contribution can significantly reduce the water potential near the tree tops. The turgor of cells in buds and leaves is expected to decrease in direct proportion with leaf water potential along a height gradient unless osmotic adjustment occurs. The pressure–volume technique was used to characterize height‐dependent variation in leaf tissue water relations and shoot growth characteristics in young and old Douglas‐fir trees to determine the extent to which growth limitation with increasing height may be linked to the influence of the gravitational water potential gradient on leaf turgor. Values of leaf water potential (Ψ_l), bulk osmotic potential at full and zero turgor, and other key tissue water relations characteristics were estimated on foliage obtained at 13.5 m near the tops of young (approximately 25‐year‐old) trees and at 34.7, 44.2 and 55.6 m in the crowns of old‐growth (approximately 450‐year‐old) trees during portions of three consecutive growing seasons. The sampling periods coincided with bud swelling, expansion and maturation of new foliage. Vertical gradients of Ψ_l and pressure–volume analyses indicated that turgor decreased with increasing height, particularly during the late spring when vegetative buds began to swell. Vertical trends in branch elongation, leaf dimensions and leaf mass per area were consistent with increasing turgor limitation on shoot growth with increasing height. During the late spring (May), no osmotic adjustment to compensate for the gravitational gradient of Ψ_l was observed. By July, osmotic adjustment had occurred, but it was not sufficient to fully compensate for the vertical gradient of Ψ_l. In tall trees, the gravitational component of Ψ_l is superimposed on phenologically driven changes in leaf water relations characteristics, imposing potential constraints on turgor that may be indistinguishable from those associated with soil water deficits.     

Leaf morphology of 40 evergreen and deciduous broadleaved subtropical tree species and relationships to functional ecophysiological traits

We explored potential of morphological and anatomical leaf traits for predicting ecophysiological key functions in subtropical trees. We asked whether the ecophysiological parameters stomatal conductance and xylem cavitation vulnerability could be predicted from microscopy leaf traits. We investigated 21 deciduous and 19 evergreen subtropical tree species, using individuals of the same age and from the same environment in the Biodiversity‐Ecosystem Functioning experiment at Jiangxi (BEF‐China). Information‐theoretic linear model selection was used to identify the best combination of morphological and anatomical predictors for ecophysiological functions. Leaf anatomy and morphology strongly depended on leaf habit. Evergreen species tended to have thicker leaves, thicker spongy and palisade mesophyll, more palisade mesophyll layers and a thicker subepidermis. Over 50% of all evergreen species had leaves with multi‐layered palisade parenchyma, while only one deciduous species (Koelreuteria bipinnata) had this. Interactions with leaf habit were also included in best multi‐predictor models for stomatal conductance (g_s) and xylem cavitation vulnerability. In addition, maximum g_s was positively related to log ratio of palisade to spongy mesophyll thickness. Vapour pressure deficit (vpd) for maximum g_s increased with the log ratio of palisade to spongy mesophyll thickness in species having leaves with papillae. In contrast, maximum specific hydraulic conductivity and xylem pressure at which 50% loss of maximum specific xylem hydraulic conductivity occurred (Ψ₅₀) were best predicted by leaf habit and density of spongy parenchyma. Evergreen species had lower Ψ₅₀ values and lower maximum xylem hydraulic conductivities. As hydraulic leaf and wood characteristics were reflected in structural leaf traits, there is high potential for identifying further linkages between morphological and anatomical leaf traits and ecophysiological responses.     

Water loss regulation to soil drought associated with xylem vulnerability to cavitation in temperate ring-porous and diffuse-porous tree seedlings

Key message

Sustainable stomatal opening despite xylem cavitation occurs in ring-porous species and stomatal closure prior to cavitation in diffuse-porous species during soil drought.

Abstract

To elucidate the relationship between water loss regulation and vulnerability to cavitation associated with xylem structure, stomatal conductance (g _s), defoliation, vulnerability curves, and vessel features were measured on seedlings of ring-porous Zelkova serrata and Melia azedarach, and diffuse-porous Betula platyphylla var. japonica, Cerasus jamasakura and Carpinus tschonoskii. Under prolonged drought conditions, the percentage loss of hydraulic conductivity (PLC) increased and g _s decreased gradually with decreasing predawn (Ψ_pd) or xylem water potential (Ψ_xylem) in Z. serrata. During the gentle increase of PLC in M. azedarach, g _s increased in the early stages of dehydration while leaves were partly shed. A sharp reduction in g _s was observed before the onset of an increase in the PLC for drying plants of the three diffuse-porous species, suggesting cavitation avoidance by stomatal regulation. In the ring-porous species, xylem-specific hydraulic conductivity (K _s) was higher, whereas the vessel multiple fractions, the ratio of the number of grouped vessels to total vessels, was lower than that in the diffuse-porous species, suggesting that many were distributed as solitary vessels. This may explain the gradual increase in the PLC with decreasing Ψ_xylem because isolated vessels provide less opportunity for air seeding. Different water loss regulation to soil drought was identified among the species, with potential mechanisms being sustainable gas exchange at the expense of xylem dysfunction or partial leaf shedding, and the avoidance of xylem cavitation by strict stomatal regulation. These were linked to vulnerability to cavitation that appears to be governed by xylem structural properties.     

Homeostasis in leaf water potentials on leeward and windward sides of desert shrub crowns: water loss control vs. high hydraulic efficiency

Phenotypic plasticity in morphophysiological leaf traits in response to wind was studied in two dominant shrub species of the Patagonian steppe, used as model systems for understanding effects of high wind speed on leaf water relations and hydraulic properties of small woody plants. Morpho-anatomical traits, hydraulic conductance and conductivity and water relations in leaves of wind-exposed and protected crown sides were examined during the summer with nearly continuous high winds. Although exposed sides of the crowns were subjected to higher wind speeds and air saturation deficits than the protected sides, leaves throughout the crown had similar minimum leaf water potential (Ψ_L). The two species were able to maintain homeostasis in minimum Ψ_L using different physiological mechanisms. Berberis microphylla avoided a decrease in the minimum Ψ_L in the exposed side of the crown by reducing water loss by stomatal control, loss of cell turgor and low epidermal conductance. Colliguaja integerrima increased leaf water transport efficiency to maintain transpiration rates without increasing the driving force for water loss in the wind-exposed crown side. Leaf physiological changes within the crown help to prevent the decrease of minimum Ψ_L and thus contribute to the maintenance of homeostasis, assuring the hydraulic integrity of the plant under unfavorable conditions. The responses of leaf traits that contribute to mechanical resistance (leaf mass per area and thickness) differed from those of large physiological traits by exhibiting low phenotypic plasticity. The results of this study help us to understand the unique properties of shrubs which have different hydraulic architecture compared to trees.     

Different vulnerabilities of Quercus ilex L. to freeze- and summer drought-induced xylem embolism: an ecological interpretation

Quercus ilex L. growing in the southern Mediterranean Basin region is exposed to xylem embolism induced by both winter freezing and summer drought. The distribution of the species in Sicily could be explained in terms of the different vulnerability to embolism of its xylem conduits. Naturally occurring climatic conditions were simulated by: (1) maintaining plants for 3h at ambient temperatures of 0, -1.5, -2.5, -5.0 and -11°C; and (2) allowing plants to dry out to ratios of their minimum diurnal leaf water potentials (Ψ₁) to that at the turgor loss point (Ψ_tlp) of 0.6, 0.9, 1.05, 1.20 and 1.33. The loss of hydraulic conductivity of one-year-old twigs reached 40% at -1.5°C and at Ψ₁/Ψ_tlP= 1.05. Recovery from these strains was almost complete 24 h after the release of thermal stress or after one irrigation, respectively. More severe stresses reduced recovery consistently. The percentages of xylem conduits embolized following application of the two stresses, were positively related to xylem conduit diameter. The capability of the xylem conduits to recover from stress was positively related to the conduit diameter in plants subjected to summer drought, but not in the plants subjected to winter freezing stress. The ecological significance of the different vulnerabilities to embolism of xylem conduits under naturally occurring climatic conditions is discussed.     

Vulnerability to cavitation of leaf minor veins: any impact on leaf gas exchange?

Vulnerability to cavitation of leaf minor veins and stems of Laurus nobilis L. was quantified together with that of leaflets, rachides and stems of Ceratonia siliqua L. during air‐dehydration of 3‐year‐old branches. Embolism was estimated by counting ultrasound acoustic emissions (UAE) and relating them to leaf water potential (Ψ_L). The threshold Ψ_L for cavitation was less negative in L. nobilis than in C. siliqua according to the known higher drought resistance of the latter species. Leaf minor vein cavitation was also quantified by infiltrating leaves with fluorescein at different dehydration levels and observing them under microscope. Distinct decreases in the functional integrity of minor veins were observed during leaf dehydration, with high correlation between the two variables. The relationship between leaf conductance to water vapour (g_L) and Ψ_L showed that stomata of L. nobilis closed in response to stem and not to leaf cavitation. However, in C. siliqua, g_L decreased in coincidence to the leaf cavitation threshold, which was, nevertheless, very close to that of the stem. The hypothesis that stem cavitation acts as a signal for stomatal closure was confirmed, while the same role for leaf cavitation remains an open problem.     

Vulnerability to embolism differs in roots and shoots and among three Mediterranean conifers: consequences for stomatal regulation of water loss?

We investigated the potential links between stomatal control of transpiration and the risk of embolism in root and shoot xylem of seedlings of three Mediterranean conifers (Cupressus sempervirens, Pinus halepensis and P. nigra) grown in a greenhouse under semi-controlled conditions. We measured the intrinsic vulnerability to embolism in roots and current year shoots by the air injection method. Root and shoot segments were subjected to increasing pressures, and the induced loss of hydraulic conductivity recorded. The three species displayed very different vulnerabilities in shoots, with P. nigra being much more vulnerable than P. halepensis and C. sempervirens. Roots were distinctly more vulnerable than shoots in C. sempervirens and P. halepensis (50% loss of conductivity induced at 3.0 MPa and 1.7 MPa higher xylem water potential in roots vs shoots). In P. nigra, no significant difference of vulnerability between shoots and roots was found. Seedlings were subjected to soil drought, and stomatal conductance, twig hydraulic conductivity and needle water potential were measured. The water potential resulting in almost complete stomatal closure (90%) was very close to the threshold water potential inducing loss of conductivity (10%) in twigs in P nigra, resulting in a very narrow safety margin between stomatal closure and embolism induction. The safety margin was larger in P. halepensis and greatest in C. sempervirens. Unexpectedly, this water potential threshold produced a 30–50% loss of conductivity in 3–5 mm diameter roots, depending on the species. The implications of this finding are discussed.     

Hydraulic architecture of sugarcane in relation to patterns of water use during plant development*

Hydraulic conductance was measured on leaf and stem segments excised from sugarcane plants at different stages of development. Maximum transpiration rates and leaf water potential (Ψ_L) associated with maximum transpiration were also measured in intact plants as a function of plant size. Leaf specific hydraulic conductivity (L_sc) and transpiration on a unit leaf area basis (E) were maximal in plants with approximately 0.2 m² leaf area and decreased with increasing plant size. These changes in Fand L_sc were nearly parallel, which prevented φ_L in larger plants from decreasing to levels associated with substantial loss in xylem conductivity caused by embolism formation. Coordination of changes in E and leaf hydraulic properties was not mediated by declining leaf water status, since φ_L increased with plant size. Hydraulic constrictions were present at nodes and in the node-leaf sheath-leaf blade pathway. This pattern of constrictions is in accord with the idea of plant segmentation into regions differing in water transport efficiency and would tend to confine embolisms to the relatively expendable leaves at terminal positions in the pathway, thereby preserving water transport through the stem.     

Aridity-dependent sequence of water potentials for stomatal closure and hydraulic dysfunctions in woody plants

The sequence of physiological events during drought strongly impacts plants' overall performance. Here, we synthesized the global data of stomatal and hydraulic traits in leaves and stems of 202 woody species to evaluate variations in the water potentials for key physiological events and their sequence along the climatic gradient. We found that the seasonal minimum water potential, turgor loss point, stomatal closure point, and leaf and stem xylem vulnerability to embolism were intercorrelated and decreased with aridity, indicating that water stress drives trait co-selection. In xeric regions, the seasonal minimum water potential occurred at lower water potential than turgor loss point, and the subsequent stomatal closure delayed embolism formation. In mesic regions, however, the seasonal minimum water potential did not pose a threat to the physiological functions, and stomatal closure occurred even at slightly more negative water potential than embolism. Our study demonstrates that the sequence of water potentials for physiological dysfunctions of woody plants varies with aridity, that is, xeric species adopt a more conservative sequence to prevent severe tissue damage through tighter stomatal regulation (isohydric strategy) and higher embolism resistance, while mesic species adopt a riskier sequence via looser stomatal regulation (anisohydric strategy) to maximize carbon uptake at the cost of hydraulic safety. Integrating both aridity-dependent sequence of water potentials for physiological dysfunctions and gap between these key traits into the hydraulic framework of process-based vegetation models would improve the prediction of woody plants' responses to drought under global climate change.     

Influence of leaf water status on stomatal response to humidity,hydraulic conductance,and soil drought in Betula occidentalis

Whole-canopy measurements of water flux were used to calculate stomatal conductance (g _s ) and transpiration (E) for seedlings of western water birch (Betula occidentalis Hook.) under various soil-plant hydraulic conductances (k), evaporative driving forces (ΔN; difference in leaf-to-air molar fraction of water vapor), and soil water potentials (Ψ_s). As expected, g _s dropped in response to decreased k or Ψ_S, or increased ΔN(> 0.025). Field data showed a decrease in mid-day g _s with decreasing k from soil-to-petiole, with sapling and adult plants having lower values of both parameters than juveniles. Stomatal closure prevented E and Ψ from inducing xylem cavitation except during extreme soil drought when cavitation occurred in the main stem and probably roots as well. Although all decreases in g _s were associated with approximately constant bulk leaf water potential (ψ_l), this does not logically exclude a feedback response between Ψ_L and g _s . To test the influence of leaf versus root water status on g _s , we manipulated water status of the leaf independently of the root by using a pressure chamber enclosing the seedling root system; pressurizing the chamber alters cell turgor and volume only in the shoot cells outside the chamber. Stomatal closure in response to increased ΔN, decreased k, and decreased Ψ_S was fully or partially reversed within 5 min of pressurizing the soil. Bulk Ψ_L remained constant before and after soil pressurizing because of the increase in E associated with stomatal opening. When ΔN was low (i.e., < 0.025), pressurizing the soil either had no effect on g _s , or caused it to decline; and bulk Ψ_L increased. Increased Ψ_l may have caused stomatal closure via increased backpressure on the stomatal apparatus from elevated epidermal turgor. The stomatal response to soil pressurizing indicated a central role of leaf cells in sensing water stress caused by high ΔN, low k, and low Ψ_S. Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.     

Neither xylem collapse,cavitation, or changing leaf conductance drive stomatal closure in wheat

Identifying the drivers of stomatal closure and leaf damage during stress in grasses is a critical prerequisite for understanding crop resilience. Here, we investigated whether changes in stomatal conductance (g_s) during dehydration were associated with changes in leaf hydraulic conductance (K_leaf), xylem cavitation, xylem collapse, and leaf cell turgor in wheat (Triticum aestivum). During soil dehydration, the decline of g_s was concomitant with declining K_leaf under mild water stress. This early decline of leaf hydraulic conductance was not driven by cavitation, as the first cavitation events in leaf and stem were detected well after K_leaf had declined. Xylem vessel deformation could only account for <5% of the observed decline in leaf hydraulic conductance during dehydration. Thus, we concluded that changes in the hydraulic conductance of tissues outside the xylem were responsible for the majority of K_leaf decline during leaf dehydration in wheat. However, the contribution of leaf resistance to whole plant resistance was less than other tissues (<35% of whole plant resistance), and this proportion remained constant as plants dehydrated, indicating that K_leaf decline during water stress was not a major driver of stomatal closure.