Physiological and biochemical traits of drought tolerance in Argania spinosa

The objective of this study was to understand and characterize the physiological and biochemical tolerance mechanisms of Argania spinosa under drought stress for selection tolerant ecotypes. Significant differences were observed among ecotypes in indices of leaf water status studied: stomatal conductance (g_s), predawn leaf water potential (Ψ_pd) and leaf relative water content. There was a significant decrease in these physiological traits with increasing degree of drought stress in all ecotypes. Drought stress significantly increased endogenous H₂O₂ and lipid peroxidation. Moderate and severe drought stress increased significantly the catalase, superoxide dismutase, peroxidase, polyphenoloxidase and lipoxygenase activities, depending on time. Their constitutive activities were higher in inland ecotypes than in coastal ecotypes. According to canonical discriminant analysis, the inland ecotypes were essentially distinguished from the coastal ecotypes by the following physiological and biochemical traits: Ψ_pd, g_s, polyphenol oxidase, superoxide dismutase and malonyldialdehyde. Inland ecotypes seem to be more tolerant to drought stress than coastal ecotypes.     

ABA application to maize hybrids contrasting for drought tolerance: changes in water parameters and in antioxidant enzyme activity

The objective of this study was to evaluate the effects of abscisic acid (ABA) related to the increase of water-stress tolerance in two drought contrasting maize hybrids: DKB 390 (tolerant) and BRS 1030 (sensitive). The characterization of water status (pre-dawn leaf water potential, Ψ_pd; midday leaf water potential, Ψ_md and stem water potential, Ψ_st) and antioxidant enzyme activity was conducted on greenhouse grown plants. The ABA, hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) contents were also analyzed. Water deficit was imposed for 10 days at the flowering stage and a dosage of 100 μM ABA was applied to plant canopy. Measurements were taken during 10 days after the water recovery. With 5 days of stress, the tolerant hybrid showed lower MDA content, decrease in the water status, and higher activity of the enzymes superoxide dismutase, catalase, ascorbate peroxidase, as well as guaiacol, glutathione reductase, dehydroascorbate reductase, polyphenol oxidase, and l-phenylalanine ammonia-lyase, as compared to the sensitive hybrid. With 10 days of stress, DKB 390 had a decrease in the activity of enzymes whereas BRS 1030 showed a higher activity. In addition, the latter showed greater amounts of H₂O₂ and MDA. ABA application led to a higher tolerance only in DKB 390, due to the increase of water status and the enzymatic activity, mainly the catalase.     

Root to shoot communication and abscisic acid in calreticulin (CR) gene expression and salt-stress tolerance in grafted diploid potato clones

Potato is an important world crop but its cultivation is relatively limited by its sensitivity to salt-stress. Auto- and hetero-grafting was used to examine the effect of rootstock and abscisic acid (ABA) on expression of the Ca²⁺-storage protein calreticulin (CR) and salt-stress tolerance in potato. Sibling-selected diploid clones of potato (S. tuberosum) were utilized that are distinguished by differential root Na⁺ absorption; including type: late-maturing, LM and excluding type, early-maturing, EM under salt treatment; salt-stress sensitivity (S/T, sensitive or tolerant); and abscisic acid production (AD/AN, ABA-deficient or-normal sibling lines). CR expression, osmotic potential (OP) and leaf Ca²⁺ were measured at the end of a 5 days NaCl stress treatment applied at tuber initiation. Increased CR expression was induced by NaCl stress and associated with salt tolerance in early-maturing tolerant (EMT) and late-maturing tolerant (LMT) clones with higher levels of CR in LMT compared to the EMT clone. Early-maturing sensitive (EMS) clone salt tolerance increased when grafted onto LMT but not onto EMT rootstocks. EMS scions maintained less negative leaf OP when grafted onto LMT rootstocks than grafting onto the EMT rootstock. Exogenous ABA application induced a less negative upper leaf OP in the salt-stress sensitive AD clone but not in the AN clone. AD clones were characterized by low CR levels, which did not increase after stress. However, grafting the AD clone onto LMT increased CR expression in the AD portion of the graft combination. Salt-stress induced CR expression and is positively associated with the presence of ABA and the salt-stress tolerant phenotypes. Both, elevation in CR expression and salt tolerance in the tolerant rootstocks, were translocated to sensitive scions although highest permeation depended on the LM type. Calreticulin expression appears to be involved in ABA-induced salt tolerance and both salt-stress tolerance and CR expression appear to be regulated by the roots.     

Leaf gas exchange and multiple enzymatic and non-enzymatic antioxidant strategies related to drought tolerance in two oil palm hybrids

Key message

The drought tolerance in young oil palm plants is related to greater efficiency in preventing oxidative damage by activating enzymatic and non-enzymatic antioxidant strategies simultaneously.

Abstract

Drought is a major environmental constraint limiting growth and yield of oil palm trees. In this study, two oil palm hybrids (BRS Manicoré and BRS C 2501) were grown in large containers and subjected to a water deficit during 57 days. Leaf gas exchange analysis was combined with an in-depth assessment of the antioxidant system over the drought imposition. Under drought, leaf water potential at predawn (Ψ _pd) decreased similarly in both hybrids. In parallel, there were decreases in the net CO₂ assimilation rate (A), chlorophyll concentrations and Rubisco total activity. Overall, these decreases were more pronounced in BRS C 2501 than in BRS Manicoré. BRS C 2501 plants triggered more markedly its enzymatic antioxidant system earlier (Ψ _pd = ?2.1 MPa) than did BRS Manicoré, but these responses were accompanied by higher concentrations of H₂O₂ and malondialdehyde in BRS C 2510 than in BRS Manicoré. With the progress of drought stress (Ψ _pd = ?2.9 MPa and below), BRS Manicoré was better able to cope with oxidative stress through a more robust antioxidant system. In addition, significant decreases in drought-induced NAD⁺-malate dehydrogenase activities were only observed in stressed BRS C 2501 plants. Regardless of watering regimes, the total carotenoid, ascorbate and glutathione concentrations were higher in BRS Manicoré than in BRS C 2501. In conclusion, BRS Manicoré is better able to tolerate drought than BRS C 2501 by triggering multiple antioxidant strategies involved both in reactive oxygen species scavenging and dissipation of excess energy and/or reducing equivalents particularly under severe drought stress.

     

Chemical and hydraulic signals regulate stomatal behavior and photosynthetic activity in maize during progressive drought

The objectives of this study were to investigate stomatal regulation in maize seedlings during progressive soil drying and to determine the impact of stomatal movement on photosynthetic activity. In well-watered and drought-stressed plants, leaf water potential (Ψ _leaf), relative water content (RWC), stomatal conductance (g _s), photosynthesis, chlorophyll fluorescence, leaf instantaneous water use efficiency (iWUE_leaf), and abscisic acid (ABA) and zeatin-riboside (ZR) accumulation were measured. Results showed that g _s decreased significantly with progressive drought and stomatal limitations were responsible for inhibiting photosynthesis in the initial stages of short-term drought. However, after 5 days of withholding water, non-stomatal limitations, such as damage to the PSII reaction center, became the main limiting factor. Stomatal behavior was correlated with changes in both hydraulic and chemical signals; however, changes in ABA and ZR occurred prior to any change in leaf water status. ABA in leaf and root tissue increased progressively during soil drying, and further analysis found that leaf ABA was negatively correlated with g _s (R ² = 0.907, p < 0.05). In contrast, leaf and root ZR decreased gradually. ZR in leaf tissue was positively correlated with g _s (R ² = 0.859, p < 0.05). These results indicate that ABA could induce stomatal closure, and ZR works antagonistically against ABA in stomatal behavior. In addition, the ABA/ZR ratio also had a strong correlation with g _s, suggesting that the combined chemical signal (the interaction between ABA and cytokinin) plays a role in coordinating stomatal behavior. In addition, Ψ _leaf and RWC decreased significantly after only 3 days of drought stress, also affecting stomatal behavior.     

Rootstock-induced physiological and biochemical mechanisms of drought tolerance in sweet orange

The poorly understood physiological and biochemical drought responses induced in sweet orange by citrus rootstocks of contrasting drought tolerance were investigated during a drought/rewatering cycle under controlled conditions. Long-term exposure of the grafted trees to a gradually increasing water deficit and subsequent recovery revealed distinct strategies of drought acclimation that were induced by the different rootstocks. Trees grafted onto the drought-tolerant rootstock ‘Cravo’ rangpur lime were less water conservative, exhibiting an increased cell-wall elasticity that contributes to turgor maintenance and its related processes of growth and photosynthesis over a wider range of soil–water potentials. On the other hand, the drought-tolerant ‘Sunki Tropical’ mandarin and drought-sensitive ‘Flying Dragon’ trifoliate orange rootstocks induced a water conservation strategy by increasing tissue rigidity under drought. ‘Sunki Tropical’ was also able to induce osmotic adjustment, conferring thereby a more efficient water conservation strategy than ‘Flying Dragon’ by allowing for turgor maintenance at lower soil–water potentials while attenuating cell dehydration and shrinkage. In contrast to ‘Cravo’ and ‘Sunki Tropical’, trees grafted onto ‘Flying Dragon’ exhibited a significant photoinhibition of the photosystem II reaction centers, as well as an increased H₂O₂ production and lipid peroxidation under drought treatment. A significantly higher activity of the antioxidant enzyme GPX was also observed in drought stressed trees grafted onto ‘Flying Dragon’. Collectively, these results support the involvement of elastic and osmotic adjustments, as well as the control of oxidative stress, as functional leaf traits associated with the rootstock-induced drought tolerance in sweet orange.     

Mesophyll conductance decreases in the wild type but not in an ABA‐deficient mutant (aba1) of Nicotiana plumbaginifolia under drought conditions

Under drought conditions, leaf photosynthesis is limited by the supply of CO₂. Drought induces production of abscisic acid (ABA), and ABA decreases stomatal conductance (g_s). Previous papers reported that the drought stress also causes the decrease in mesophyll conductance (g_m). However, the relationships between ABA content and g_m are unclear. We investigated the responses of g_m to the leaf ABA content [(ABA)_L] using an ABA‐deficient mutant, aba1, and the wild type (WT) of Nicotiana plumbaginifolia. We also measured leaf water potential (Ψ_L) because leaf hydraulics may be related to g_m. Under drought conditions, g_m decreased with the increase in (ABA)_L in WT, whereas both (ABA)_L and g_m were unchanged by the drought treatment in aba1. Exogenously applied ABA decreased g_m in both WT and aba1 in a dose‐dependent manner. Ψ_L in WT was decreased by the drought treatment to ?0.7 MPa, whereas Ψ_L in aba1 was around ?0.8 MPa even under the well‐watered conditions and unchanged by the drought treatment. From these results, we conclude that the increase in (ABA)_L is crucial for the decrease in g_m under drought conditions. We discuss possible relationships between the decrease in g_m and changes in the leaf hydraulics.     

Drought tolerance in two oil palm hybrids as related to adjustments in carbon metabolism and vegetative growth

Drought tolerance was examined in two oil palm hybrids (BRS Manicoré and BRS C 2501) grown in large pots and subjected to long-term drought (57 days) and rehydration. Regardless of hybrids, predawn water potential (Ψ _pd), net photosynthesis rates, and stomatal conductance decreased similarly upon drought imposition, but the absolute values of these variables were lower in BRS C 2501 than in BRS Manicoré. Overall, drought-induced decreases in activities of key enzymes associated with carbon metabolism, including Rubisco, ADP-glucose pyrophosphorylase, and sucrose-phosphate synthase, were stronger in BRS C 2501 than in BRS Manicoré. Our data suggest that synthesis of starch (and possibly sucrose) was fundamentally limited by a lower substrate availability rather than by enzymatic constraints, particularly until day 34 after suspending watering. Drought stress provoked similar decreases in biomass accumulation in either hybrid; however, BRS Manicoré plants displayed higher total leaf area and root length coupled with greater relative decreases in dry matter of above-ground parts than of roots as compared to BRS C 2501, thereby leading to an increased root-to-above ground ratio in the former. Upon resuming irrigation, the slow recovery of Ψ _pd preceded the recovery of stomatal conductance and photosynthesis irrespective of hybrid. BRS Manicoré was better able to recover enzyme activities and carbohydrate status than BRS C 2501. In conclusion, both hybrids could tolerate (or survive) the severe drought conditions, but, overall, BRS Manicoré plants were better able to adjust its physiological, morphological, and biochemical traits to cope with drought than did BRS C 2501.     

The ecosystem wilting point defines drought response and recovery of a Quercus-Carya forest

Soil and atmospheric droughts increasingly threaten plant survival and productivity around the world. Yet, conceptual gaps constrain our ability to predict ecosystem-scale drought impacts under climate change. Here, we introduce the ecosystem wilting point (Ψ_EWP), a property that integrates the drought response of an ecosystem's plant community across the soil–plant–atmosphere continuum. Specifically, Ψ_EWP defines a threshold below which the capacity of the root system to extract soil water and the ability of the leaves to maintain stomatal function are strongly diminished. We combined ecosystem flux and leaf water potential measurements to derive the Ψ_EWP of a Quercus-Carya forest from an “ecosystem pressure–volume (PV) curve,” which is analogous to the tissue-level technique. When community predawn leaf water potential (Ψ_pd) was above Ψ_EWP (=−2.0 MPa), the forest was highly responsive to environmental dynamics. When Ψ_pd fell below Ψ_EWP, the forest became insensitive to environmental variation and was a net source of carbon dioxide for nearly 2 months. Thus, Ψ_EWP is a threshold defining marked shifts in ecosystem functional state. Though there was rainfall-induced recovery of ecosystem gas exchange following soaking rains, a legacy of structural and physiological damage inhibited canopy photosynthetic capacity. Although over 16 growing seasons, only 10% of Ψ_pd observations fell below Ψ_EWP, the forest is commonly only 2–4 weeks of intense drought away from reaching Ψ_EWP, and thus highly reliant on frequent rainfall to replenish the soil water supply. We propose, based on a bottom-up analysis of root density profiles and soil moisture characteristic curves, that soil water acquisition capacity is the major determinant of Ψ_EWP, and species in an ecosystem require compatible leaf-level traits such as turgor loss point so that leaf wilting is coordinated with the inability to extract further water from the soil.     

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.     

Growth, gas exchange, water-use efficiency, and carbon isotope composition of ‘Gale Gala’ apple trees grafted onto 9 wild Chinese rootstocks in response to drought stress

To determine the effects of rootstock choice on the scion response to drought stress, we compared the vegetative growth, biomass accumulation, gas exchange, and water-use efficiency (WUE) of ??Gale Gala?? apple (Malus domestica Borkh.) trees grafted onto nine wild Chinese Malus rootstocks. Compared with the well-watered control, drought treatment limited growth, as manifested by smaller increments in plant height (PH), trunk diameter (TD), total fresh biomass (TB), total dry biomass (TDB), total leaf area (LA), and relative growth rate (RGR). The extent of this effect differed among rootstocks. Stress conditions led to increases in the root/shoot ratio (RSR), leaf thickness (LT), water-holding capacity (WHC), carbon isotope composition (??¹³C), and WUE. Decreases were noted in stomatal density (SD), leaf relative water content (RWC), chlorophyll content (Chl), net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s), again varying by rootstock. Those that are generally considered more drought-tolerant, e.g., M. sieversii, M. prunifolia, and M. toringoides, had smaller declines in PH, TD, TB, TDB, LA, RGR, SD, RWC, Chl, P _N, E, and g _s and proportionally greater increases in RSR, LT, WHC, ??¹³C, and WUE compared with the droughtsensitive M. hupehensis and M. sieboldii. These results suggest that moisture stress has a significant dwarfing effect in the latter two species. Based on WUE calculations, trees on drought-tolerant rootstocks showed higher tolerance when stressed, whereas those on drought-sensitive rootstocks were less tolerant, as indicated by their lower WUE values.     

Linking the patterns in soil moisture to leaf water potential, stomatal conductance, growth, and mortality of dominant shrubs in the Florida scrub ecosystem

Patterns in soil moisture availability affect plant survival, growth and fecundity. Here we link patterns in soil moisture to physiological and demographic consequences in Florida scrub plants. We use data on different temporal scales to (1) determine critical soil moisture content that leads to loss of turgor in leaves during predawn measurements of leaf water status (Ψ _crit), (2) describe the temporal patterns in the distribution of Ψ _crit, (3) analyze the strength of relationship between rainfall and soil moisture content based on 8 years of data, (4) predict soil moisture content for 75 years of rainfall data, and (5) evaluate morphological, physiological and demographic consequences of spring 2006 drought on dominant shrubs in Florida scrub ecosystem in the light of water-uptake depth as determined by stable isotope analysis (δ¹⁸O). Based on 1998–2006 data, the soil moisture content at 50 cm depth explained significant variation in predawn leaf water potential of two dominant shrubs, Quercus chapmanii and Ceratiola ericoides (r ²?=?0.69). During 8 years of data collection, leaves attained Ψ _crit only during the peak drought of 2000 when the soil moisture fell below 1% by volume at 50 and 90 cm depth. Precipitation explained a significant variation in soil moisture content (r ²?=?0.62). The patterns in predicted soil moisture for 75 year period, suggested that the frequency of drought occurrence has not increased in time. In spring 2006, the soil reached critical soil moisture levels, with consequences for plant growth and physiological responses. Overall, 24% of plants showed no drought-induced damage, 51% showed damage up to 50%, 21% had intense leaf shedding and 2% of all plants died. Over the drought and recovery period (May–October 2006), relative height growth was significantly lower in plants with greater die-back. All species showed a significant depression in stomatal conductance, while all but deep-rooted palms Sabal etonia and Serenoa repens showed significantly lower predawn (Ψ _pd) and mid-day (Ψ _md) leaf water potential in dry compared to wet season. Plants experiencing less severe die-back exhibited greater stomatal conductance, suggesting a strong relationship between physiology and morphology. Based on results we suggest that the restoration efforts in Florida scrub should consider the soil moisture requirements of key species.     

OsASR5 enhances drought tolerance through a stomatal closure pathway associated with ABA and H2O2 signalling in rice

Drought is one of the major abiotic stresses that directly implicate plant growth and crop productivity. Although many genes in response to drought stress have been identified, genetic improvement to drought resistance especially in food crops is showing relatively slow progress worldwide. Here, we reported the isolation of abscisic acid, stress and ripening (ASR) genes from upland rice variety, IRAT109 (Oryza sativa L. ssp. japonica), and demonstrated that overexpression of OsASR5 enhanced osmotic tolerance in Escherichia coli and drought tolerance in Arabidopsis and rice by regulating leaf water status under drought stress conditions. Moreover, overexpression of OsASR5 in rice increased endogenous ABA level and showed hypersensitive to exogenous ABA treatment at both germination and postgermination stages. The production of H₂O₂, a second messenger for the induction of stomatal closure in response to ABA, was activated in overexpression plants under drought stress conditions, consequently, increased stomatal closure and decreased stomatal conductance. In contrast, the loss‐of‐function mutant, osasr5, showed sensitivity to drought stress with lower relative water content under drought stress conditions. Further studies demonstrated that OsASR5 functioned as chaperone‐like protein and interacted with stress‐related HSP40 and 2OG‐Fe (II) oxygenase domain containing proteins in yeast and plants. Taken together, we suggest that OsASR5 plays multiple roles in response to drought stress by regulating ABA biosynthesis, promoting stomatal closure, as well as acting as chaperone‐like protein that possibly prevents drought stress‐related proteins from inactivation.     

Long‐distance ABA transport can mediate distal tissue responses by affecting local ABA concentrations

Environmental stresses that perturb plant water relations influence abscisic acid (ABA) concentrations, but it is unclear whether long‐distance ABA transport contributes to changes in local ABA levels. To determine the physiological relevance of ABA transport, we made reciprocal‐ and self‐grafts of ABA‐deficient flacca mutant and wild‐type (WT) tomato plants, in which low phosphorus (P) conditions decreased ABA concentrations while salinity increased ABA concentrations. Whereas foliar ABA concentrations in the WT scions were rootstock independent under conditions, salinity resulted in long‐distance transport of ABA: flacca scions had approximately twice as much ABA when grafted on WT rootstocks compared to flacca rootstocks. Root ABA concentrations were scion dependent: both WT and flacca rootstocks had less ABA with the flacca mutant scion than with the WT scion under conditions. In WT scions, whereas rootstock genotype had limited effects on stomatal conductance under conditions, a flacca rootstock decreased leaf area of stressed plants, presumably due to attenuated root‐to‐shoot ABA transport. In flacca scions, a WT rootstock decreased stomatal conductance but increased leaf area of stressed plants, likely due to enhanced root‐to‐shoot ABA transport. Thus, long‐distance ABA transport can affect responses in distal tissues by changing local ABA concentrations.     

Morpho-physiological and molecular characterization of drought tolerance traits in Gossypium hirsutum genotypes under drought stress

Drought stress is one of the major abiotic stresses affecting lint yield and fibre quality in cotton. With increase in population, degrading natural resources and frequent drought occurrences, development of high yielding, drought tolerant cotton cultivars is critical for sustainable cotton production across countries. Six Gossypium hirsutum genotypes identified for drought tolerance, wider adaptability and better fibre quality traits were characterized for various morpho-physiological and biochemical characters and their molecular basis was investigated under drought stress. Under drought conditions, genotypes revealed statistically significant differences for all the morpho-physiological and biochemical traits. The interaction (genotype × treatment) effects were highly significant for root length, excised leaf water loss and cell membrane thermostability indicating differential interaction of genotypes under control and stress conditions. Correlation studies revealed that under drought stress, relative water content had significant positive correlation with root length and root-to-shoot ratio while it had significant negative correlation with excised leaf water loss, epicuticular wax, proline, potassium and total soluble sugar content. Analysis of expression of fourteen drought stress related genes under water stress indicated that both ABA dependent and ABA independent mechanisms of drought tolerance might be operating differentially in the studied genotypes. IC325280 and LRA5166 exhibited ABA mediated expression of stress responsive genes and traits. Molecular basis of drought tolerance in IC357406, Suraj, IC259637 and CNH 28I genotypes could be attributed to ABA independent pathway. Based on physiological phenotyping, the genotypes IC325280 and IC357406 were identified to possess better root traits and LRA5166 was found to have enhanced cellular level tolerance. Variety Suraj exhibited good osmotic adjustment and better root traits to withstand water stress. The identified drought component trait(s) in specific genotypes would pave way for their pyramiding through marker assisted cotton breeding.Electronic supplementary materialThe online version of this article (10.1007/s12298-020-00890-3) contains supplementary material, which is available to authorized users.     

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.     

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.     

Ectomycorrhiza affect architecture and nitrogen partitioning of beech (Fagus sylvatica L.) seedlings under shade and drought

The aim of this study was to investigate the influence of ectomycorrhizal fungi (EMF) on the architecture of and nitrogen (N) partitioning in young beech (Fagus sylvatica) plants in response to different light regimes and water deprivation. We hypothesized that EMF modify biomass partitioning and architecture of young beech plants by increased N uptake in comparison with non-mycorrhizal (NM) plants and that therefore, the drought responses of EM and NM plants diverge. We anticipated that full light-exposed plants were more drought tolerant due to improved water status and nutrition, whereas shade-acclimated EM plants were more drought susceptible because of decreased mycorrhizal colonization. To test these hypotheses seedlings were grown in native or sterilized forest soil. To avoid effects of soil pretreatment NM and EM plants were transplanted into sand-peat culture systems and exposed to shade, drought or the combination of both factors. Shade resulted in reduced root biomass production decreasing the root-to-shoot ratio. Mild drought stress (pre-dawn water potential [Ψ_pd] = −1.3 MPa) did not affect biomass partitioning. EMF colonization did not increase plant biomass, but had strong effects on root architecture: the numbers of root tips as well as the absolute and specific root lengths were increased because of formation of thin roots, especially in the diameter classes from 0.2 to 0.8 mm. In contrast to our expectation N uptake of well irrigated EM plants was not increased despite their larger potential for soil exploitation. Overall, EM plants exhibited higher amounts of carbon fixed per unit of N taken up than NM plants and shifted N partitioning towards the roots. Beneficial effects of EMFs were apparent under mild drought but the responses differed depending on the light availability: shaded EM plants showed a delay in the decrease of Ψ_pd; light exposed EM plants showed increased N uptake compared with NM beeches. These results indicate that EMFs are involved in mediating divergent responses of beech to drought depending on the light availability.