Quercitol plays a key role in stress tolerance of Eucalyptus leptophylla (F. Muell) in naturally occurring saline conditions

Understanding trees adaptation to arid, saline conditions is a major challenge for catchment revegetation in Australia. The accumulation of low molecular weight solutes is an established response of trees to the effects of salt and/or drought stress. Recent studies have shown that quercitol – a cyclitol – contributes significantly towards the adjustment of osmotic potential in some species of Eucalyptus. The present study investigated the role of quercitol in leaf tissues of Eucalyptus leptophylla (F. Muell) under fluctuating environmental stresses. Analysis of leaf tissues from trees growing at distances between 0 and 125 m from hyper-saline lakes suggests that quercitol contributes significantly to the adjustment of osmotic potential induced by drought in E. leptophylla. The presence of substantial concentrations of quercitol in xylem sap suggests that quercitol plays additional roles in signalling amelioration of osmotic stress in myrtaceous species. Quercitol concentrations fluctuate in both xylem and leaf tissues on a seasonal basis, suggesting a form of environmental regulation of solutes. The capacity of soil profiles to store rainwater, rather than proximity to hyper-saline groundwater largely determined osmotic stress in studied trees. Understanding such avoidance/tolerance mechanisms will be crucial to advance tree selection and breeding for stress tolerance.     

Adaptive responses of Populus kangdingensis to drought stress

We measured dry matter accumulation and allocation, photosynthesis, lipid peroxidation, osmotic adjustment, antioxidative defences and ABA content of Populus kangdingensis C. Wang et Tung under three different watering regimes (100%, 50% and 25% of the field capacity) to characterize the morphological, physiological and biochemical basis of drought resistance in woody plants. The results showed that drought stress caused pronounced inhibition of the growth and photosynthesis rate, and that the stomatal limitation to photosynthesis was dominant. The decrease in stomatal conductance effectively controlled water loss and increased water use efficiency. Drought also affected many physiological and biochemical processes, including increases in free proline, malondialdehyde and ABA content, and superoxide dismutase activity. On the other hand, the ABA content of leaves was significantly higher than that of stem and roots under all watering regimes; the high level of ABA in the leaf may result from the large import of ABA to leaves from other organs. These results demonstrate that there are a large set of parallel changes in the morphological, physiological and biochemical responses when plants are exposed to drought stress; these changes may enhance the capability of plants to survive and grow during drought periods.     

小麦叶片光合作用与其渗透调节能力的关系

在缓慢干旱条件下,小麦叶片渗透调节能力在一定范围内随胁迫程度的加剧而增加,而在快速干旱下,渗透调节能力丧失。小麦叶片通过渗透调节使光合速率和气孔导度对水分胁迫的敏感性降低,叶片维持较高的电子传递能力、RuBP羧化酶活性和叶绿体光合能量转换系统活性,并推迟了小麦叶片光合速率受气孔因素限制向叶肉细胞光合活性限制转变的时间。     

Effects of water stress on internal water relations of apple leaves

The capacity of apple ( Malus pumila Mill. cv. James Grieve and Golden Delicious) pot- and orchard-grown trees to adjust osmotically in response to drought was investigated. Stressed leaves exhibited alterations in the moisture release curves when compared to well hydrated control leaves. Results suggest that osmotic adjustment occurred in both field- and pot-grown trees. Water potential for zero turgor was lowered by 0.5 MPa in leaves of potted trees and by 1.1 MPa in leaves of field-grown trees as a result of stress treatments. A decrease in the osmotic potential was responsible for that adjustment allowing the leaf to maintain turgor at lower water potentials and relative water contents. The extent of adjustment was similar for both potted and orchard trees despite the difference in the rate of stress imposition and its intensity. Changes in the concentration of sugars apparently contributed to this adjustment.     

Diurnal and Seasonal Changes in Leaf Water Potential Components and Elastic Properties in Response to Water Stress in Apple Trees

Apple trees are very drought tolerant, having the capability to grow and carry on photosynthesis even at low water potentials. Much of the tolerance is due to the ability of apple leaves to maintain turgor potentials at levels conducive to growth and stomatal opening. Diurnally, leaf turgor is maintained through decreases in osmotic potentials (due to active solute accumulation), osmotic adjustment, or to concentration of solutes via tissue water loss. These two processes combined may decrease osmotic potentials by as much as 1.65 MPn during the day. Seasonally, osmotic potentials remain fairly constant, but leaf elasticity increases, allowing growth to continue and stomata to remain open us water and turgor potentials become progressively lower. Release of stored water from plant tissues to the transpiration stream is another means of preventing water potentials from reaching critical values for stomatal closure. A combination of a number of these physiological adaptations may account for much of the drought tolerance in apple trees.     

The role of organic and inorganic solutes in the osmotic adjustment of drought-stressed Jatropha curcas plants

This study aimed to assess the accumulation of organic and inorganic solutes and their relative contribution to osmotic adjustment in roots and leaves of Jatropha curcas subjected to different water deficit intensity. Plants were grown in vermiculite 50% (control), 40%, 30%, 20% and 10% expressed in gravimetric water content. The water potential, osmotic potential and turgor potential of leaves decreased progressively in parallel to CO₂ photosynthetic assimilation, transpiration and stomatal conductance, as the water deficit increased. However, the relative water content, succulence and water content in the leaves did not show differences between the control and stressed plants, indicating osmotic adjustment associated with an efficient mechanisms to prevent water loss by transpiration through stomatal closure. The K⁺ ions had greater quantitative participation in the osmotic adjustment in both leaves and roots followed by Na⁺ and Cl⁻, while the NO₃⁻ ion only showed minor involvement. Of the organic solutes studied, the total soluble sugars showed the highest relative contribution to the osmotic adjustment in both organs and its concentration positively increased with more severe water deficit. The free amino acids and glycinebetaine also effectively contributed to the osmotic potential reduction of both the root and leaves. The role of proline was quantitatively insignificant in terms of osmotic adjustment, in both the control and stressed roots and leaves. Our data reveal that roots and leaves of J. curcas young plants display osmotic adjustment in response to drought stress linked with mechanisms to prevent water loss by transpiration by means of the participation of inorganic and organic solutes and stomatal closure. Of all the solutes studied, soluble sugars uniquely display a prominent drought-induced synthesis and/or accumulation in both roots and leaves.     

Evaluation of physiological traits for improving drought tolerance in faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.)

Among grain legumes, faba bean is becoming increasingly popular in European agriculture due to recent economic and environmental interests. Faba bean can be a highly productive crop, but it is sensitive to drought stress and yields can vary considerably from season to season. Understanding the physiological basis of drought tolerance would indicate traits that can be used as indirect selection criteria for the development of cultivars adapted to drought conditions. To assess genotypic variation in physiological traits associated with drought tolerance in faba bean and to determine relationships among these attributes, two pot experiments were established in a growth chamber using genetic materials that had previously been screened for drought response in the field. Nine inbred lines of diverse genetic backgrounds were tested under adequate water supply and limited water conditions. The genotypes showed substantial variation in shoot dry matter, water use, stomatal conductance, leaf temperature, transpiration efficiency, carbon isotope discrimination (Δ¹³C), relative water content (RWC) and osmotic potential, determined at pre-flowering vegetative stage. Moisture deficits decreased water usage and consequently shoot dry matter production. RWC, osmotic potential, stomatal conductance and Δ¹³C were lower, whereas leaf temperature and transpiration efficiency were higher in stressed plants, probably due to restricted transpirational cooling induced by stomatal closure. Furthermore, differences in stomatal conductance, leaf temperature, Δ¹³C and transpiration efficiency characterized genotypes that were physiologically more adapted to water deficit conditions. Correlation analysis also showed relatively strong relationships among these variables under well watered conditions. The drought tolerant genotypes, ILB-938/2 and Melodie showed lower stomatal conductance associated with warmer leaves, whereas higher stomatal conductance and cooler leaves were observed in sensitive lines (332/2/91/015/1 and Aurora/1). The lower value of Δ¹³C coupled with higher transpiration efficiency in ILB-938/2, relative to sensitive lines (Aurora/1 and Condor/3), is indeed a desirable characteristic for water-limited environments. Finally, the results showed that stomatal conductance, leaf temperature and Δ¹³C are promising physiological indicators for drought tolerance in faba bean. These variables could be measured in pot-grown plants at adequate water supply and may serve as indirect selection criteria to pre-screen genotypes.     

Poor stomatal control of water balance and the abscission of green needles from a declining stand of spruce trees [Picea abies (L.) Karst.] from the northern Alps

On a site on the west slope of the Wank in the northern Alps changes in water potential, osmotic potential and transpiration rate were measured in spruce trees during the dry summer months of 1991. The pattern of decrease in water potential and osmotic potential on days of varying evaporative demand from trees of widely different decline conditions was used to describe the relative ability of the trees to withstand drought stress. Stress diagrams served as a tool for interpreting the state of health of each tree. The criterion is independent of the water situation of the tree and the other external conditions of the respective experiment. These diagrams clearly show that the foliage of spruces with high needle losses reaches the limit of endurance relatively early. For equal evaporative demand much lower turgor levels were observed in spruces with high needle loss compared to undamaged trees. Associated with the occurrence of low turgor values was the shedding of green needles. The abscission zone was shown in sections. The accumulation of highly fluorescent substances in the bundle sheath cells of the same material was described. Incomplete to non-existent stomatal control over water loss was attributed to modifications in the cell walls of the stomata which appear to alter the ability of the guard cells to sense changes in either atmospheric or cellular hydration. Our studies point to the following situation: air pollution directly affects stomatal control, the loss of stomatal control changes the drought avoidance abilities of the foliage and, as a consequence, low turgor levels occur and premature needle abscission is induced. As site water balance decreases, either due to a dry year or to poor moisture holding abilities of the soil, these conditions become apparent.     

岷江干旱河谷区典型灌木对干旱胁迫的生理生化响应

以岷江干旱河谷区6种典型山地灌木[沙棘(Hippophae rhamnoides)、羊蹄甲(Bauhinia faberi)、白刺花(Sorphora davidii)、锦鸡儿(Caragana arborescens)、三颗针(Berberis sargentiana)、黄栌(Cotinus szechuanensis)]为试材,采用盆栽试验研究了土壤自然干旱胁迫对苗木叶片抗氧化保护酶活性、膜伤害程度以及渗透调节物质的影响,探讨植物应对土壤干旱逐渐加剧的生理生态适应机理。结果表明,植物不同抗氧化保护酶对干旱胁迫及随之而来的氧化胁迫的响应存在一定差异。各灌木的超氧化物歧化酶随着干旱胁迫的增强呈先上升后下降,过氧化物酶在整个胁迫期间表现为逐渐升高,过氧化氢酶的升高则主要发生在胁迫的中后期,表明不同胁迫时期对植物体起主导作用的保护酶不同,三者表现为相互协调的作用方式。随着干旱胁迫的增强,各灌木的丙二醛呈缓慢增加趋势,细胞的膜脂过氧化作用逐渐加强,植物开始遭受一定程度的毒害。沙棘和羊蹄甲叶片的质膜相对透性随胁迫的增强呈现先升后降最终回到初始水平的特点,表明植物可以通过干旱锻炼获得一定的抗旱能力。白刺花和黄栌叶片的质膜相对透性在胁迫的0—4 d保持不变,从8 d开始大幅上升并一直维持在较高水平,说明胁迫初期植物细胞膜的结构和功能还很完整,生理活动仍能正常进行,但从中期开始细胞膜遭受严重破坏。随着干旱胁迫的增强,各灌木叶片的脯氨酸总体均呈增加趋势,表明干旱胁迫下植物通过积累脯氨酸以提高细胞的渗透调节能力。主成分分析表明,6种灌木的抗旱能力由强到弱依次为:羊蹄甲、沙棘、锦鸡儿、黄栌、白刺花、三颗针。综合分析表明,干旱河谷区几种典型灌木可通过提高抗氧化酶活性并积累渗透调节物质对干旱胁迫进行积极的反馈,以减弱逆境胁迫下活性氧的危害,提高细胞的渗透调节能力,减轻细胞遭受的损伤。     

Modification of Vegetative Phenology in a Tropical Semi‐deciduous Forest by Abnormal Drought and Rain1

The control of vegetative phenology in tropical trees is not well understood. In dry forest trees, leaf abscission may be enhanced by advanced leaf age, increasing water stress, or declining photoperiod. Normally, it is impossible to dissect the effects of each of these variables because most leaves are shed during the early dry season when day length is near its minimum and leaves are relatively old. The 1997 El‐Niño Southern Oscillation caused a ten‐week long, severe abnormal drought from June to August in the semi‐deciduous forests of Guanacaste, Costa Rica. We monitored the effect of this drought on phenology and water status of trees with young leaves and compared modifications of phenology in trees of different functional types with the pattern observed during the regular dry season. Although deciduous trees at dry sites were severely water stressed (Ψstem < ‐7MPa) and their mesic leaves remained wilted for more than two months, these and all other trees retained all leaves during the abnormal drought. Many trees exchanged leaves three to four months earlier than normal during the wet period after the abnormal drought and shed leaves again during the regular dry season. Irrigation and an exceptional 70 mm rainfall during the mid‐dry season 1998/1999 caused bud break and flushing in all leafless trees except dormant stem succulents. The complex interactions between leaf age and water stress, the principal determinants of leaf abscission, were found to vary widely among trees of different functional types.     

Morphophysiological differences in leaves of <Emphasis Type="Italic">Lavoisiera campos</Emphasis>-<Emphasis Type="Italic">portoana</Emphasis> (Melastomataceae) enhance higher drought tolerance in water shortage events

Lavoisiera campos-portoana Barreto (Melastomataceae) has two kinds of leaves, pubescent and glabrous, and branches may have one or both types of leaves at the same moment. The plant is endemic to high altitude rocky fields in Brazil where rainfall is very seasonal. We predicted that these two leaf types are adaptations to different regimes of water availability. In experimental conditions of drought stress, we measured relative water content (RWC), photosynthetic pigments, chlorophyll a fluorescence and osmotic potential, and we counted stomates and measured stomatal conductance on both sides of leaves and compared these between the two leaf types. Stomatal conductance and electron transport rate at a given photosynthetic photon flux were greater in pubescent leaves than in glabrous leaves, and both declined during drought stress. Excessive photon flux density in glabrous leaves was greater during stress and after rehydration. Photosynthetic pigment content and RWC did not change between leaves, and values reduced during the stress period. Both types of leaves showed osmotic adjustment capacity, which occurred earlier in glabrous ones. These morphophysiological differences highlight the adaptation strategies of this plant to withstand drought, since the glabrous portion of the plant presents a preventive behavior, while the pubescent portion only shows the same responses in more advanced stages of drought stress.     

硅和干旱胁迫对水稻叶片光合特性和矿质养分吸收的影响

硅被认为是植物生长的有益元素,它能增强植物对非生物逆境和生物逆境胁迫的抗性。以抗旱性不同的一对水稻近等基因系w-14-和w-20为实验材料,采用盆栽实验,研究了干旱胁迫下硅处理对水稻生长性状、光合生理特性和矿质养分吸收的影响。结果表明,在正常水分条件下硅处理对水稻的生长及生理特性没有明显影响。干旱胁迫显著降低水稻植株的生长,叶绿素含量、叶绿素荧光参数Fv/Fm及Fv/F0值显著降低,光合作用受到明显抑制。加硅能提高干旱胁迫条件下水稻植株的生物量、水分利用效率、叶片叶绿素含量、净光合速率和蒸腾速率,而气孔导度和细胞间隙CO2浓度则下降。无论干旱与否,施硅后水稻的叶片硅含量均显著上升。两个水稻品系叶片的无机离子含量在干旱胁迫条件下均呈显著增加的趋势,而硅处理后材料w-14的叶片K+、Na+、Ca2+、Mg2+、Fe3+含量分别降低16.38%,24.50%,19.70%,21.52%,18.58%,w-20则分别降低11.64%,12.11%,16.06%,11.11%和19.15%,并使之回复到与对照更接近的水平。研究结果表明了硅提高水稻植株的抗旱性与光合作用的改善和矿质养分的调节有关。     

狭叶红景天幼苗对水分及遮阴的生长及生理生化响应

研究植物对水分和遮阴胁迫的响应及其生理机制对制定合理的栽培管理措施十分必要。以红景天属植物为研究对象,设置土壤含水量分别为田间持水量的80%(过湿水分)、70%(正常水分)、60%(轻度干旱)、40%(中度干旱)、20%(重度干旱)5个水分梯度;设置2个遮阴处理,以全光照(遮阴率为0)为对照、黑色遮阴网遮阴(遮阴率为85%),研究狭叶红景天生长及生理生化指标的变化特征。结果表明:在不同水分处理下,与对照相比,叶绿素含量、茎干重和茎重比(SMR)显著增加(P0.05),株高、总生物量、叶面积、叶干重、叶重比(LMR)、比叶面积(SLA)、叶面积比(LAR)和叶面积根干重比(LARMR)增加,根冠比和根重比(RMR)减少;随着干旱程度加剧,丙二醛(MDA)、脯氨酸(Pro)和可溶性糖(Ss)含量增加,超氧化物歧化酶(SOD)活性总体呈先增加后减小的趋势。在遮阴处理下,株高、SMR、SLA、LAR和LARMR显著增加(P0.05),叶绿素SPAD值和叶面积增加,总生物量、根干重、根冠比和LMR显著减少(P0.05),茎干重和叶干重减少,MDA含量显著增加,Pro含量略有下降,Ss含量减少。在水分胁迫下,狭叶红景天中度干旱时通过增加酶活性抵御伤害,重度干旱超过其阈值,SOD活性下降,植物体受到伤害,Ss可能是主要的渗透调节物质。在遮阴处理下,狭叶红景天通过增加SLA避免遮阴伤害。狭叶红景天在受到环境胁迫时会通过形态改变、调节MDA含量、抗氧化酶活性和渗透调节物质来保证自身正常的生长发育。     

Causes of reduced leaf‐level photosynthesis during strong El Niño drought in a Central Amazon forest

Sustained drought and concomitant high temperature may reduce photosynthesis and cause tree mortality. Possible causes of reduced photosynthesis include stomatal closure and biochemical inhibition, but their relative roles are unknown in Amazon trees during strong drought events. We assessed the effects of the recent (2015) strong El Niño drought on leaf‐level photosynthesis of Central Amazon trees via these two mechanisms. Through four seasons of 2015, we measured leaf gas exchange, chlorophyll a fluorescence parameters, chlorophyll concentration, and nutrient content in leaves of 57 upper canopy and understory trees of a lowland terra firme forest on well‐drained infertile oxisol. Photosynthesis decreased 28% in the upper canopy and 17% in understory trees during the extreme dry season of 2015, relative to other 2015 seasons and was also lower than the climatically normal dry season of the following non‐El Niño year. Photosynthesis reduction under extreme drought and high temperature in the 2015 dry season was related only to stomatal closure in both upper canopy and understory trees, and not to chlorophyll a fluorescence parameters, chlorophyll, or leaf nutrient concentration. The distinction is important because stomatal closure is a transient regulatory response that can reverse when water becomes available, whereas the other responses reflect more permanent changes or damage to the photosynthetic apparatus. Photosynthesis decrease due to stomatal closure during the 2015 extreme dry season was followed 2 months later by an increase in photosynthesis as rains returned, indicating a margin of resilience to one‐off extreme climatic events in Amazonian forests.     

Stress responses in Salix gracilistyla cuttings subjected to repetitive alternate flooding and drought

To determine the tolerance of Salix gracilistyla to repetitive alternate flooding and drought, we measured leaf stomatal conductance, pre-dawn water potential, osmotic adjustment, and biomass production under greenhouse conditions. We used a control and nine crossed treatments (F1-D1–F3-D3) in which we combined 1-, 2-, or 3-week floodings (F) and droughts (D). Leaf stomatal conductance was lowest in 3 weeks of flooding or drought when the preceding event (flood or drought) was also of a 3-week duration. Leaf pre-dawn water potential was reduced in 3 weeks of drought when preceded by 2 or 3 weeks of flooding. Cuttings had slight osmotic adjustments in repetitions of long floodings and droughts. During longer durations of drought in crossed experiments, plants had low root and shoot mass, few hypertrophic lenticels, and reduced leaf mass; when flooding duration increased in crossed experiments, root mass was reduced, there were more hypertrophic lenticels, and the leaf area was reduced. Cuttings achieved stress tolerance by inhibition of transpiration, osmotic adjustment, reduction of transpiration area, and development of hypertrophic lenticels. Stress tolerance was weak when repetitive 2- or 3-week floodings were combined with 3-week droughts. The duration of flooding and drought periods under which S. gracilistyla achieves stress tolerance may be critical in determining distributions along riverbanks.     

Expression analysis of dehydrin multigene family across tolerant and susceptible barley (Hordeum vulgare L.) genotypes in response to terminal drought stress

Dehydrins are one of the characteristic families of plant proteins that usually accumulate in response to drought. In the present study, gene expressions of dehydrin multigene family (13 genes) were examined in flag leaves of tolerant (Yousef) and susceptible (Moroco) barley varieties under terminal drought to characterize the involvement of dehydrins in the adaptive processes. The stomatal conductance, RWC, and Chl a, b contents had more reduction in Moroco than the Yousef which has more elevated osmotic adjustment. Drought stress increased significantly MDA and electrolyte leakage levels, but greater in Moroco, indicating a poor protection of cell and cytoplasmic membrane in this variety. Yousef variety had no reduction in grain yield under drought condition. Five genes (Dhn1, Dhn3, Dhn5, Dhn7 and Dhn9) were exclusively induced in Yousef under drought stress. In the stress condition, relative gene expression of Dhn3, Dhn9 had the direct correlations (P < 0.05) with Chl a, b contents, osmotic adjustment, stomatal conductance, plant biomass and grain yield, and the negative correlations (P < 0.05) with MDA and electrolyte leakage levels. The results supported the impending functional roles of dehydrin K_n and particularly Y_nSK_n types in dehydration tolerance of barley during the reproductive stage.     

Salinity and drought tolerance of mannitol-accumulating transgenic tobacco

Tobacco plants (Nicotiana tabacum L.) were transformed with a mannitol-1-phosphate dehydrogenase gene resulting in mannitol accumulation. Experiments were conducted to determine whether mannitol provides salt and/or drought stress protection through osmotic adjustment. Non-stressed transgenic plants were 20–25% smaller than non-stressed, non-transformed (wild-type) plants in both salinity and drought experiments. However, salt stress reduced dry weight in wild-type plants by 44%, but did not reduce the dry weight of transgenic plants. Transgenic plants adjusted osmotically by 0.57 MPa, whereas wild-type plants did not adjust osmotically in response to salt stress. Calculations of solute contribution to osmotic adjustment showed that mannitol contributed only 0-003-0-004 MPa to the 0.2 MPa difference in full turgor osmotic potential (π_o) between salt-stressed transgenic and wild-type plants. Assuming a cytoplasmic location for mannitol and that the cytoplasm constituted 5% of the total water volume, mannitol accounted for only 30–40% of the change in π_o of the cytoplasm. Inositol, a naturally occurring polyol in tobacco, accumulated in response to salt stress in both transgenic and wild-type plants, and was 3-fold more abundant than mannitol in transgenic plants. Drought stress reduced the leaf relative water content, leaf expansion, and dry weight of transgenic and wild-type plants. However, π_o was not significantly reduced by drought stress in transgenic or wild-type plants, despite an increase in non-structural carbohydrates and mannitol in droughted plants. We conclude that (1) mannitol was a relatively minor osmolyte in transgenic tobacco, but may have indirectly enhanced osmotic adjustment and salt tolerance; (2) inositol cannot substitute for mannitol in this role; (3) slower growth of the transgenic plants, and not the presence of mannitol per se, may have been the cause of greater salt tolerance, and (4) mannitol accumulation was enhanced by drought stress but did not affect π_o or drought tolerance.     

Carbon isotope discrimination in irrigated and droughted potato (Solanum tuberosum L.)

Carbon isotope discrimination (Δ) was measured in irrigated and droughted potato. Under irrigation, Δ in leaflets at given nodes increased (P < 0.001) between 21 and 63 d after emergence (DAE), which was attributed to increasing stomatal conductance (g_s) during leaf expansion. The effect of leaf position on Δ was non-significant in mature leaves. Under drought, Δ decreased (P < 0.001) in successive leaves up the stem, reflecting changes in g_s and water stress. At each node Δ remained constant or decreased, suggesting that effects of water stress were greater than changes with leaf expansion. There were significant differences in Δ between cultivars in both treatments, and in the progressive decrease in Δ up the stem under drought. Differences in Δ between cultivars were consistent with differences in stomatal control of leaf water status following water stress. Values for Δ in tubers were consistently lower than in stem and leaf, and decreased more rapidly. Differences in Δ between cultivars did not reflect dry matter production in either treatment, and differences in water use were non-significant between cultivars under drought. So, plants can achieve similar dry matter production through different growth strategies when irrigated or droughted, and Δ does not provide a simple, indirect method of selecting for dry matter production under water stress.     

Osmotic adjustment in Fraxinus excelsior L . : malate and mannitol accumulation in leaves under drought conditions

 In leaves of Fraxinus excelsior L., malate and mannitol were characterized by ¹³C NMR spectroscopy and enzymatic specific assays as the major constituents of a soluble carbon fraction involved in an osmotic adjustment. During a summer drought where predawn leaf water potential of adult trees growing in a mesoxerophilic stand fell to – 4 MPa in August, malate and mannitol leaf contents increased by a factor of 1.8 and 2.2 respectively, compared to control trees growing on a flood plain. This drought stress led to concentrations as high as 280 mM and 600 mM for mannitol and malate, respectively. The effects of gradually developing water deficit were also studied in a semi-controlled environment in 3-year-old seedlings. When predawn leaf water potential reached -6 MPa, leaves displayed a low turgor pressure but stomatal conductance was still measurable. Malate and mannitol were also the main osmoticum involved. After rewatering, gas exchange capacities were largely restored. Altogether, these results show that the strong water-stress tolerance of Fraxinus excelsior is in part related to an accumulation of malate and mannitol. Received: 3 January 1996 / Accepted: 19 March 1996     

Seasonal patterns in water use and leaf turnover of different plant functional types in a species-rich woodland,south-western Australia

Woodlands in south-western Australia are evergreen and transpire throughout the year despite the long, hot and dry summers of the Mediterranean climate. Results from a case study in a species-rich Banksia woodland are used to discuss the ecological and physiological properties that appear to be essential features of this and similar communities. Tree, shrub and perennial herbaceous species with long-lived leaves dominate the community, whereas winter-green herbaceous species with short-lived leaves constitute a minor group. The total leaf area index is therefore reasonably constant in all seasons. Leaf area index is low and canopies are open, causing good coupling between the vegetation and the atmosphere, and making stomatal control an effective regulator of transpiration. Mean maximum (winter) stomatal conductances were high at approximately 300 mmol m^–2 s^–1. Deep-rootedness allows the dominant species to access soil moisture throughout the unsaturated zone, and down to the capillary fringe of the saturated zone. Shrubs and herbs with shallow roots experience greater drought stress during summer. Rates of community evapotranspiration are limited by leaf area index in the wet season, and further reduced by stomatal closure in the dry season. Deep-rooted plants appear to decrease their stomatal conductance before the development of severe drought stress. Such conservative behaviour, possibly related to plant hydraulic constraints, is a contributing factor to the limited seasonality in community water use.