Proline accumulation: A parameter for evaluation of sensitivity of tomato varieties to drought stress?

The pattern of proline accumulation and the growth response were followed in several tomato ( Lycopersicon esculentum Mill.) varieties which were exposed to 7 days of drought stress followed by a 15-day period of rewatering. During dehydration, water potential and leaf elongation rates decreased more in var. 'Hosen' and 'S-5' than in 'LX-11', '1970', 'Pakmor', 'Faculty-16', 'Alcobaca' and '475'. Proline accumulation during stress was greatest in the first two varieties. In 'Hosen' and 'S-5' rewatering resulted in a decrease of proline to control levels, whereas in the other varieties accumulation of proline continued long after turgor had been regained. The extent of this continued accumulation was not correlated with the degree to which each variety was dehydrated. Upon rewatering of the plants the rate of leaf elongation was increased, but the final leaf size as well as whole shoot and root fresh weight of the recovered plants were not colated with the degree of "suffering" that each variety experienced during the drought period. Incubation of detached young tomato leaves in polyethylene glycol solution for 48 h resulted in a substantial accumulation of proline. The varietal differences observed under these conditions were reminiscent of the differential responses in proline accumulation obtained in the intact plants. It is concluded that proline accumulation at the time of dehydration signals drought stress in tomato plants but does not correlate with the overall varietal sensitivity to transient dehydration in recovered plants.     

The relations of stomatal closure and reopening to xylem ABA concentration and leaf water potential during soil drying and rewatering

Two tropical tree species, Acacia confusa and Leucaena leucocephala, were used to study the relationships among stomatal conductance, xylem ABA concentration and leaf water potential during a soil drying and rewatering cycle. Stomatal conductance of both A. confusa and L. leucocephala steadily decreased with the decreases in soil water content and pre-dawn leaf water potential. Upon rewatering, soil water content and pre-dawn leaf water potential rapidly returned to the control levels, whereas the reopening of stomata showed an obvious lag time. The length of this lag time was highly dependent not only upon the degree of water stress but also on plant species. The more severe the water stress, the longer the lag time. When A. confusa and L. leucocephala plants were exposed to the same degree of water stress (around –2.0 MPa in pre-dawn leaf water potential), the stomata of A. confusa reopened to the control level 6 days after rewatering. However, it took L. leucocephala about 14 days to reopen fully. A very similar response of leaf photosynthesis to soil water deficit was also observed for both species. Soil drying resulted in a significant increase in leaf and xylem ABA concentrations in both species. The more severe the water stress, the higher the leaf and xylem ABA concentrations. Both leaf ABA and xylem ABA returned to the control level following relief from water deficit and preceded the full recovery of stomata, suggesting that the lag phase of stomatal reopening was not controlled by leaf and/or xylem ABA. In contrast to drying the whole root system, drying half of the root system did not change the leaf water relations, but caused a significant increase in xylem ABA concentration, which could fully explain the decrease of stomatal conductance. After rewatering, the stomatal conductance of plants in which half of the roots were dried recovered more rapidly than those of whole-root dried plants, indicating that the leaf water deficit that occurred during the drying period was related to the post-stress stomatal inhibition. These results indicated that the decrease in stomatal conductance caused by water deficit was closely related to the increase in xylem ABA, but xylem ABA could not fully explain the reopening of stomata after relief of water stress, neither did the leaf ABA. Some unknown physiological and/or morphological processes in the guard cells may be related to the recovery process.     

Photosynthetic responses and acclimation of two castor bean cultivars to repeated drying–wetting cycles

To investigate the responses of castor bean to repeated drying–wetting cycles (RDWC), morpho-physiological parameters of two cultivars (Jiaxiang 2 and Hangbi 8) were determined by a pot experiment under well-watered control and RDWC. RDWC inhibited plant growth and leaf development, decreased water loss rate (WLR), and enhanced leaf mass per area (LMA) and chlorophyll content as indicated by spectral reflectance indices for both cultivars. Photosynthesis was inhibited by progressive drought stress but quickly recovered after rewatering for each cycle. Both cultivars exhibit a similar pattern of acclimation to RDWC: (1) higher LMA and lower WLR, (2) increased photosynthetic capacity under drought stress with increasing cycle numbers, (3) quick recovery and over-compensation for photosynthesis after rewatering, and (4) increased chlorophyll content. Jiaxiang 2 shows a high capacity for water preservation under drought stress and an over-compensation for photosynthesis after rewatering compared with Hangbi 8.     

干旱胁迫下牛心朴子幼苗相对含水量、质膜透性及保护酶活性变化

牛心朴子(Cynanchum komarovii)幼苗用不同浓度PEG-6000(聚乙二醇)模拟干旱处理,测定处理12d和复水24h中根、叶的RWC、质膜相对透性、MDA含量及几种保护酶(SOD、CAT、POD)活性变化情况,结果表明：高浓度胁迫后期,RWC明显下降,MDA含量增大,致使膜脂过氧化引起膜损伤;低、中浓度胁迫下,RWC下降程度低,细胞膜脂过氧化及膜透性影响小,且可能对膜脂过氧化起到一定的防御作用。复水后,低、中度胁迫下,各项指标能迅速恢复到CK水平,而高浓度胁迫下,除叶质膜透性、MDA含量、SOD活性不能恢复外,根、叶的其余指标均能达到CK水平。保护酶系统的作用,在一段时间内,可能是通过它们之间的相互协调而保持一个稳定的平衡态进行的。     

干旱胁迫及复水对海滨木槿光合作用和生理特性的影响

选择2年生海滨木槿扦插苗为材料,通过自然干旱20 d而后复水(21 d),研究了其在干旱和复水过程中的光合作用和生理特性.结果表明:经过20 d的干旱胁迫,在土壤含水量仅为5.9%的情况下海滨木槿全部成活,干旱胁迫显著降低了海滨木槿的净光合速率,实测值最高仅为1.1μmol·m^-2·s^-1,最大光化学效率为对照的84.3%.可溶性蛋白和抗氧化酶同时积累,稳定细胞渗透势并清除干旱造成的脂膜过氧化物质.复水7d后光合速率升高至对照的57.3%,超氧化物歧化酶和过氧化氢酶活性随着丙二醛含量下降而下降,复水21 d后,海滨木槿抗氧化酶、可溶性蛋白和叶片相对含水量均恢复至对照水平,干旱处理总生物量虽显著降低,但根冠比显著提高.海滨木槿具有极强的耐旱能力,是适用于沿海地区景观改良和道路绿化的重要树种.     

Drought resistance and recovery in mature Bituminaria bituminosa var. albomarginata

Few studies have investigated the response of perennial legumes to drought stress (DS) and their ability, following rewatering, to regrow and restore photosynthetic activity. We examined these responses for two genotypes of drought‐tolerant tedera (Bituminaria bituminosa var. albomarginata) and one genotype of lucerne (Medicago sativa). Plants were grown outdoors in 1‐m deep PVC pots with a reconstructed field soil profile, regularly watered for 8 months (winter to mid‐summer), and then moved to a glasshouse where either watering was maintained or drought was imposed for up to 47 days, before rewatering for 28 days. Drought stress greatly decreased shoot dry matter (DM) production in both species. Lucerne plants showed severe leaf desiccation after 21 days of withholding water. Relative leaf water content (RWC = 42%) and midday leaf water potential (LWP = ?6.5 MPa) decreased in tedera in response to DS, whereas leaf angle (85°) and lateral root DM both increased. Proline and pinitol accumulated in tedera leaves during DS, and their concentration declined after rewatering. Nine days after rewatering, previously drought‐stressed tedera had similar RWC and LWP to well‐watered control plants. In tedera and lucerne, 28 days after rewatering, photosynthesis and stomatal conductance were greater than in the well‐watered controls. The lateral root DM for one tedera genotype decreased during the recovery phase but for lucerne, the lateral root DM did not change during either the drought or the recovery phases. Overall, the root systems in tedera showed greater plasticity in response to DS and rewatering than in lucerne. In conclusion, tedera and lucerne showed different physiological and morphological strategies to survive and recover from DS. Proline and soluble sugars may act as a carbon source for regrowth in tedera during recovery. In comparison with lucerne, tedera's more rapid recovery after rewatering should contribute to a greater aboveground DM yield under alternating dry and wet periods. Tedera genotypes are highly heterogeneous and selecting genotypes with enhanced concentrations of pinitol and proline could be a valuable tool to improve plant performance during DS and recovery.     

Effects of water stress and rewatering on leaf water relations of lemon plants

Potted two-year-old lemon plants (Citrus limon (L.) Burm. fil.) cv. Fino, growing under field conditions were subjected to drought by withholding irrigation for 13 d. After that, plants were re-irrigated and the recovery was studied for 5 d. Control plants were daily irrigated maintaining the soil matric potential at about -30 kPa. Young leaves of control plants presented higher leaf conductance (g1) and lower midday leaf water potential (Ψmd) than mature ones. Young leaves also showed higher leaf water potential at the turgor loss point (Ψtlp) than mature leaves. In both leaf types g1 decreased with increased vapour pressure deficit of the atmosphere. From day 1 of the withholding water, predawn and midday leaf water potentials (Ψpd and Ψmd) decreased, reaching in both cases minimum values of -5.5 MPa, with no significant differences between mature and young leaves. Water stress induced stomatal closure, leaf rolling and partial defoliation. No osmotic adjustment was found in response to water stress in either leaf type, but both were able to enhance the cell wall elasticity (elastic adjustment). After rewatering, leaf water potential recovered quickly (within 2 d) but g1 did not. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlation of Activity and Amount of Ribulose 1,5-bisphosphate Carboxylase with Chloroplast Stroma Crystals in Water-Stressed Willow Leaves

Changes in the activity and amount of ribulose 1,5-bisphosphate(RuBP)carboxylase (E.C. 4.1.1.39 [EC] ) were studied in well-watered plantsof Salix ‘aquatica gigantea’ and in similar plantsduring three different water stress treatments and after rewatering.The chloroplast ultrastructure of these plants was examinedby electron microscopy. The amounts of crystallized proteinin the chloroplast stroma were assessed according to the areaof crystal structure seen in the thin sections. RuBP carboxylase activity decreased with decreasing leaf waterpotentials but recovered upon rewatering, except when leaveshad been exposed to severe water stress. The percentage of totalchloroplast area made up of crystal inclusions decreased withdecreasing leaf water potentials. After rewatering, the crystalseither disappeared or the amount decreased markedly. Both RuBPcarboxylase activity and the area of crystal inclusions increasedinitially with increased extractable RuBP carboxylase proteinbut decreased with further increases above 6700–7000 µgRuBP carboxylase protein mg^–1 chlorophyll. In well-wateredand water-stressed plants the activity of RuBP carboxylase,based on amount of chlorophyll, increased with an increasingamount of crystal inclusions in the chloroplast stroma. In rewateredplants no such correlation was observed, and the low percentageof crystal inclusions in the chloroplast area was independentof RuBP carboxylase activity. Key words: Chloroplast stroma crystals, ribulose 1,5-bisphosphate carboxylase, Salix, water stress     

Response of cassava leaf area expansion to water deficit: cell proliferation, cell expansion and delayed development

BACKGROUND AND AIMS: Cassava (Manihot esculenta) is an important food crop in the tropics that has a high growth rate in optimal conditions, but also performs well in drought-prone climates. The objectives of this work were to determine the effects of water deficit and rewatering on the rate of expansion of leaves at different developmental stages and to evaluate the extent to which decreases in cell proliferation, expansion, and delay in development are responsible for reduced growth. METHODS: Glasshouse-grown cassava plants were subjected to 8 d of water deficit followed by rewatering. Leaves at 15 developmental stages from nearly full size to meristematic were sampled, and epidermal cell size and number were measured on leaves at four developmental stages. KEY RESULTS: Leaf expansion and development were nearly halted during stress but resumed vigorously after rewatering. In advanced-stage leaves (Group 1) in which development was solely by cell expansion, expansion resumed after rewatering, but not sufficiently for cell size to equal that of controls at maturity. In Group 2 (cell proliferation), relative expansion rate and cell proliferation were delayed until rewatering, but then recovered partially, so that loss of leaf area was due to decreased cell numbers per leaf. In Group 3 (early meristematic development) final leaf area was not affected by stress, but development was delayed by 4-6 d. On a plant basis, the proportion of loss of leaf area over 26 d attributed to leaves at each developmental stage was 29, 50 and 21 % in Group 1, 2 and 3, respectively. CONCLUSIONS: Although cell growth processes were sensitive to mild water deficit, they recovered to a large extent, and much of the reduction in leaf area was caused by developmental delay and a reduction in cell division in the youngest, meristematic leaves.     

Effect of drought and rewatering on the cellular status and antioxidant response of Medicago truncatula plants

Effects of water stress on plants have been well-documented. However, the combined responses to drought and rewatering and their underlying mechanisms are relatively unknown. The present study attempts to describe spatiotemporal alterations in the physiology and cellular status of Medicago truncatula tissues that result from and subsequently follow a period of moderate water deficit. Physiological processes and cellular damage levels were monitored in roots and leaves by determining lipid peroxidation levels, as well as nitric oxide and hydrogen peroxide content, further supported by stomatal conductance and chlorophyll fluorescence measurements in leaves. During water stress, cells in both organs displayed increased damage levels and reactive oxygen and nitrogen species content, while leaves showed reduced stomatal conductance. Furthermore, both tissues demonstrated increased proline content. Upon rewatering, plants recovered displaying readings similar to pre-stress control conditions. Furthermore, molecular analysis of antioxidant gene expression by quantitative real-time RT-PCR revealed differential spatiotemporal regulation in a number of genes examined (including catalase, cytosolic ascorbate peroxidase, copper/zinc and iron superoxide dismutase and alternative oxidase). Overall, M. truncatula plants demonstrated increased sensitivity to drought-induced oxidative damage; however, this was reversed following rewatering indicating a great elasticity in the plant''s capacity to cope with free oxygen and nitrogen radicals.Key words: drought stress, antioxidants, cellular damage, medicago, proline, hydrogen peroxide, nitric oxide, reactive oxygen species, reactive nitrogen species     

Selenium improves recovery of wheat seedlings at rewatering after drought stress

Rewatering after drought is beneficial to plants subjected to moderate drought stress, and selenium (Se) could increase the tolerance of plants to stressful environment. The role of Se in rewatering of drought-treated wheat seedlings (Triticum aestivum L., cv Hengmai5229) was studied. The objective was to elucidate whether Se could improve recovery of wheat seedlings at rewatering after drought stress. Drought stress induced a significant reduction in growth parameters, total chlorophyll and soluble protein contents, and increased the rate of superoxide radical (O ₂ ^·? ) production, MDA content, and the activities of peroxidase, catalase (CAT), and superoxide dismutase in wheat seedlings. Rewatering after drought did not significantly affect biomass accumulation of seedlings over drought treatment, although it decreased the rate of O ₂ ^·? production and MDA content. However, the combined treatment of rewatering and Se evidently promoted biomass accumulation of seedlings over drought treatment and rewatering alone; and the rate of O ₂ ^·? production, MDA content, soluble protein content and CAT activity were recovered to the control values. This indicates that Se improved recovery of wheat seedlings at rewatering after drought stress.     

干旱和复水对文冠果生长及生理生态特性的影响

以文冠果2年生盆栽苗为材料,以21.8%～23.7%土壤含水量为对照,设置轻度干旱(14.3%～16.2%)、中度干旱(10.6%～12.5%)和重度干旱(6.8%～8.7%)3个干旱胁迫,研究干旱和复水对文冠果生长及生理生态特性的影响.结果表明: 随着干旱胁迫的加剧,文冠果单株干质量、株高、基径、叶数和叶面积减小,主根和一级侧根长度增加;文冠果叶片气体交换各参数均逐渐降低,在10:00-14:00,净光合速率(P_n)的大小主要受气孔因素影响,14:00以后影响P_n的主要因素是非气孔因素;中度干旱和重度干旱胁迫下,文冠果对CO₂利用效率显著提高.随着干旱胁迫的加剧,PSⅡ潜在活性(F_v/F_o)、PSⅡ最大光化学量子产量(F_v/F_m)、光化学猝灭系数(q_P)、表观电子传递效率(ETR)和实际光化学量子产量(Ф_PSⅡ)逐渐减小,非光化学淬灭系数(NPQ)逐渐增大,复水后变化相反.复水后第8天,F_v/F_o、F_v/F_m、q_P、ETR、Ф_PSⅡ显著高于各胁迫处理第30天.随着干旱胁迫的加剧,超氧化物歧化酶、过氧化物酶活性升高,过氧化氢酶活性降低,丙二醛含量增加,复水后变化相反.复水后各干旱胁迫处理根系活力明显升高.文冠果有极强的适应极度干旱环境的能力,可以作为能源或园林植物在华北园林绿化中广泛应用.     

Effects of mycorrhizal infection on drought tolerance and recovery in safflower and wheat

The influence of arbuscular mycorrhizal fungi on drought tolerance and recovery was studied in safflower (Carthamus tinctorius L.) and wheat (Triticum aestivum L.). Plants were grown with and without the mycorrhizal fungus, Glomus etunicatum Becker & Gerd., in nutrient-amended soil under environmentally-controlled conditions to yield mycorrhizal and nonmycorrhizal with similar leaf areas, root length densities, dry weights, and adequate tissue phosphorus. When drought stress was induced, mycorrhizal infection did not affect changes in leaf water, osmotic or pressure potentials, or osmotic potentials of leaf tissue rehydrated to full turgor in either safflower or wheat. Furthermore, in safflower, infection had little effect on drought tolerance as indicated by the level of leaf necrosis. Mycorrhizal wheat plants, however, had less necrotic leaf tissue than uninfected plants at moderate levels of drought stress (but not at severe levels) probably due to enhanced phosphorus nutrition. To determine the effects of infection on drought recovery, plants were rewatered at a range of soil water potentials from –1 to –4 MPa. We found that although safflower tended to recover more slowly from drought after rewatering than wheat, mycorrhizal infection did not directly affect drought recovery in either plant species. Daily water use after rewatering was reduced and was correlated to the extent that leaves were damaged by drought stress in both plant species, but was not directly influenced by the mycorrhizal status of the plants.     

Proline accumulation and nitrate reductase activity in contrasting sorghum lines during mid-season drought stress

Six lines of sorghum ( Sorghum bicolor L. Moench) with differing drought resistance (IS 22380, ICSV 213, IS 13441 and SPH 263, resistant and IS 12739 and IS 12744, susceptible) were grown under field conditions in the semi-arid tropics and analysed for proline and nitrate reductase activity (NRA; EC 1.6.6.1) during a mid-season drought. The resistant lines accumulated high levels of proline, while the susceptible lines showed no significant proline accumulation. Most of the proline was accumulated after growth of the plants had ceased. In a separate greenhouse experiment, most of the proline was found in the green rather than the fired portions of leaves. The levels returned to that of irrigated controls within 5 days of rewatering. Proline levels increased as leaf water potential and relative water content fell, and there was no apparent difference among the different sorghum lines with change in plant water status. Susceptible lines accumulated less proline than resistant lines as leaf death occurred at higher water potentials. Proline accumulation may, however, contribute to the immediate recovery of plants from drought. Leaf NRA reached high levels at about 35 days after sowing in both the stressed and irrigated plants, after which it declined. The decline in NRA was more pronounced in the stressed than in the irrigated plants and closely followed changes in the growth rate. Upon rewatering, NRA increased several-fold in all the lines and, in contrast to proline accumulation, genotypic differences in NRA were small, both during stress and upon rewatering. The high sensitivity of NRA to mild drought stress was reflected in the rapid decline of activity with small changes in leaf water potential and relative water content. The results are discussed in the light of a possible role for proline during recovery from drought, and the maintenance of NRA during stress and its recovery upon rewatering.     

Plasma membrane aquaporins play a significant role during recovery from water deficit

The role of plasma membrane aquaporins (PIPs) in water relations of Arabidopsis was studied by examining plants with reduced expression of PIP1 and PIP2 aquaporins, produced by crossing two different antisense lines. Compared with controls, the double antisense (dAS) plants had reduced amounts of PIP1 and PIP2 aquaporins, and the osmotic hydraulic conductivity of isolated root and leaf protoplasts was reduced 5- to 30-fold. The dAS plants had a 3-fold decrease in the root hydraulic conductivity expressed on a root dry mass basis, but a compensating 2.5-fold increase in the root to leaf dry mass ratio. The leaf hydraulic conductance expressed on a leaf area basis was similar for the dAS compared with the control plants. As a result, the hydraulic conductance of the whole plant was unchanged. Under sufficient and under water-deficient conditions, stomatal conductance, transpiration rate, plant hydraulic conductance, leaf water potential, osmotic pressure, and turgor pressure were similar for the dAS compared with the control plants. However, after 4 d of rewatering following 8 d of drying, the control plants recovered their hydraulic conductance and their transpiration rates faster than the dAS plants. Moreover, after rewatering, the leaf water potential was significantly higher for the control than for the dAS plants. From these results, we conclude that the PIPs play an important role in the recovery of Arabidopsis from the water-deficient condition.     

干旱复水对柑橘幼苗叶片光合、叶绿素荧光和根系构型的影响

干旱严重影响柑橘的生长和发育.为探索柑橘对干旱胁迫的响应机制,本试验以抗旱性不同的三湖红橘和三湖化红为材料,通过盆栽控水进行干旱胁迫和复水处理,研究处理后植株叶片光合、叶绿素荧光和根系构型的变化.结果表明: 干旱显著降低了两种柑橘幼苗的净光合作用速率、气孔导度、蒸腾速率和胞间CO₂浓度,而三湖红橘的下降幅度更小;复水后,光合参数均有所恢复,但仍低于对照.三湖红橘水分利用率在干旱15 d后开始显著高于对照,而三湖化红除干旱15 d外的其他处理时间均低于对照.干旱提高了两种幼苗的PSII最大光合效率,但抑制了三湖化红的PSII实际光合效率.干旱到一定程度后,两种幼苗的PSII电子传递速率和光化学淬灭均下降,干旱和复水后非光化学淬灭在三湖红橘中下降,但在三湖化红中上升.根系构型分析表明,干旱导致两种幼苗的根表面积和根体积下降,同时抑制了三湖化红的总根长,但能够提高三湖红橘的总根长和总根尖数.进一步分析不同直径的侧根长度发现,三湖红橘的一类侧根长度在干旱胁迫10 d后开始增加,而三湖化红的一类侧根长度在干旱前期没有变化,干旱20 d时显著下降;除三湖红橘的三类侧根外,两种幼苗其余直径等级侧根的生长均受干旱抑制.除总根尖数外,复水后根系生长各参数均没有恢复.干旱对三湖红橘光合性能的影响小于三湖化红,并且前者能够维持更高的水分和光能利用率.干旱后三湖红橘根尖数和细根长度增加,可能有助于提高其对水分的吸收能力.     

Elevated CO2 reduces the drought effect on nitrogen metabolism in barley plants during drought and subsequent recovery

The objective of this study was to determine the response of nitrogen metabolism to drought and recovery upon rewatering in barley (Hordeum vulgare L.) plants under ambient (350 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂ conditions. Barley plants of the cv. Iranis were subjected to drought stress for 9, 13, or 16 days. The effects of drought under each CO₂ condition were analysed at the end of each drought period, and recovery was analysed 3 days after rewatering 13-day droughted plants. Soil and plant water status, protein content, maximum (NR_max) and actual (NR_act) nitrate reductase, glutamine synthetase (GS), and aminant (NADH-GDH) and deaminant (NAD-GDH) glutamate dehydrogenase activities were analysed. Elevated CO₂ concentration led to reduced water consumption, delayed onset of drought stress, and improved plant water status. Moreover, in irrigated plants, elevated CO₂ produced marked changes in plant nitrogen metabolism. Nitrate reduction and ammonia assimilation were higher at elevated than at ambient CO₂, which in turn yielded higher protein content. Droughted plants showed changes in water status and in foliar nitrogen metabolism. Leaf water potential (Ψ_w) and nitrogen assimilation rates decreased after the onset of water deprivation. NR_act and NR_max activity declined rapidly in response to drought. Similarly, drought decreased GS whereas NAD-GDH rose. Moreover, protein content fell dramatically in parallel with decreased leaf Ψ_w. In contrast, elevated CO₂ reduced the water stress effect on both nitrate reduction and ammonia assimilation coincident with a less-steep decrease in Ψ_w. On the other hand, Ψ_w practically reached control levels after 3 days of rewatering. In parallel with the recovery of plant water status, nitrogen metabolism was also restored. Thus, both NR_act and NR_max activities were restored to about 75-90% of control levels when water supply was restored; the GS activity reached 80-90% of control values; and GDH activities and protein content were similar to those of control plants. The recovery was always faster and slightly higher in plants grown under elevated CO₂ conditions compared to those grown in ambient CO₂, but midday Ψ_w dropped to similar values under both CO₂ conditions. The results suggest that elevated CO₂ improves nitrogen metabolism in droughted plants by maintaining better water status and enhanced photosynthesis performance, allowing superior nitrate reduction and ammonia assimilation. Ultimately, elevated CO₂ mitigates many of the effects of drought on nitrogen metabolism and allows more rapid recovery following water stress.     

Changes in content of free,conjugated and bound polyamines and osmotic adjustment in adaptation of vetiver grass to water deficit

Osmotic adjustment and alteration of polyamines (PAs) have been suggested to play roles in plant adaptation to water deficit/drought stress. In this study, the changes in cell intactness, photosynthesis, compatible solutes and PAs [including putrescine (Put), spermidine (Spd) and spermine (Spm) each in free, conjugated and bound forms] were investigated in leaves of vetiver grass exposed to different intensity of water deficit stress and subsequent rewatering. The results showed that, when vetiver grass was exposed to the moderate (20% and 40% PEG-6000 solutions) and severe (60% PEG solution) water deficit for 6 days, the plant injury degree (expressed as the parameters of plant growth, cell membrane integrity, water relations and photosynthesis) increased and contents of free and conjugated Put decreased with the rise of PEG concentration. Under the moderate water deficit, the plants could survive by the reduced osmotic potential (ψ_s), increased free and conjugated Spd and Spm in leaves. After subsequent rewatering, the osmotic balance was re-established, most of the above investigated physiological parameters were fully or partly recovered to the control levels. However, it was not the case for the severely-stressed and rewatering plants. It indicates that, vetiver grass can cope well with the moderate water deficit/drought stress by using the strategies of osmotic adjustment and maintenance of total contents of free, conjugated and bound PAs in leaves.     

水分胁迫对小麦幼苗及悬浮培养细胞中诱导蛋白表达的影响

丰抗8号小麦幼苗及成熟胚诱导的悬浮培养细胞在水分胁迫（－1．0MPa PEG6000）下,可溶性蛋白含量与蛋白组分变化有差异,幼苗可溶性蛋白含量高于对照,并随生长的延长呈降低趋势;悬浮培养细胞可溶性蛋白含量低于对照,且略有上升;复水后均可恢复对照水平,SDS－PAGE电泳及薄层扫描分析结果表明,幼苗受水分胁迫诱导,出现44．2kD蛋白亚基,该蛋白亚基含量可随胁迫时间延长上升,复水后消失,在正常条件下悬浮培养细胞中含有44．2kD蛋白亚基表达,轻度胁迫处理时,该蛋白亚基含量上升,对悬浮培养细胞进行水分胁迫,该蛋白则表现下降趋势,复水后又可上升。