Cytokinins and Water Stress

It is almost impossible to find a single process in plant life that is not affected by both stress and hormones directly or indirectly. This minireview is focused on the interactions between water stress and cytokinins (CKs). The attention was paid mainly to changes in endogenous CK content and composition under water stress, involvement of CK in plant responses to water stress mainly in stomatal regulation of gas exchange, water relations of transgenic plants with elevated CK content, and possibilities to ameliorate the negative effects of water stress by application of exogenous CKs.     

The Effect of Water Stress on the Vegetative Growth of White Clover (Trifolium repens L): Comparison of Long-term Water Deficit and a Short-term Developing Water Stress

White clover plants were subjected to either a short-term developingwater stress or long-term stable levels of water deficit on‘water stress columns’. The short-term stress reducedplant water status to –2?0 MPa over 15 d. The water stresscolumns imposed only mild levels of water stress (a reductionof 0?35 MPa in leaf water potential for the more severe treatment)but these were maintained for several weeks. The absolute growthof plants on the control columns was maintained throughout theexperimental period. Vegetative growth was measured. Stolon, petiole, and laminagrowth were all reduced to some extent when plants were grownsymbiotically. The two regimes gave comparable results. Whennitrate was supplied there was no effect of water stress. Aconsiderably reduced absolute growth rate did not result ina similar decrease in final organ size. Stolon growth was mostreduced by water stress. Leaf death during water stress wasas important as changes in growth in determining final dry matteryield. Consequently, the yield of petiole and lamina from plantsgrown without supplied nitrate on the water stress columns waslower than that of stolon at the end of the treatment period. The merits of the water stress column system for imposing long-termwater deficit are discussed. Key words: Trifolium repens, white clover, water stress, vegetative growth     

Photosynthetic Enzymes of the C4 Grass Setaria sphacelata Under Water Stress: A Comparison Between Rapidly and Slowly Imposed Water Deficit

Two stress imposing systems were used: a rapid stress developed by allowing excised leaves to loose water by transpiration, and a slow stress developed by withholding watering of potted plants. Carboxylating enzymes reacted differently on both types of stress. Rapid stress increased ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activation, but both activities (initial and total) showed little variation with stress. Under slow stress the activation did not change, although both activities decreased much under stress. Phosphoenolpyruvate carboxylase (PEPC) showed a deep decrease of activity under rapid stress, nevertheless, a certain recovery was found under extreme stress. On the other hand, under slow stress the activity of PEPC showed a linear increase with decreasing relative water content. The ratio between physiological and maximal activity increased slightly under both types of stress. The activity of malic enzyme did not change under rapid stress, and decreased linearly under slow stress.     

不同氮素形态及水分胁迫对水稻苗期水分吸收、光合作用及生长的影响

采用室内营养液培养, 聚乙二醇(PEG6000)模拟水分胁迫处理、HgCl2抑制水通道蛋白活性的方法, 在3种供氮形态下(NH4+-N/ NO 3--N为100/0、50/50和0/100), 研究了水稻苗期水分吸收、光合及生长的状况。结果表明, 在非水分胁迫下, 水稻单位干重吸水量以单一供NO3--N处理最高, 加HgCl2抑制水通道蛋白活性后, 单一供NO3--N、NH4+-N和NH4+-N/ NO3--N为50/50处理的水稻水分吸收分别下降了9.6%、20.7%和16.0%; 但在水分胁迫下, 单一供NO3--N的处理水分吸收量显著降低, 低于其它2个处理, 加HgCl2抑制水通道蛋白活性后, 水分吸收量分别降低了1.0%、18.8%和23.5%。在2种水分条件(水分胁迫与非水分胁迫)下, 净光合速率、气孔导度、蒸腾速率和细胞间隙CO2浓度等指标均以单一供NH4+-N处理最大,NH4+-N/ NO3--N为50/50处理次之, 单一供NO3--N处理最小。HgCl2处理结果表明, 不同形态氮素营养能够影响水稻幼苗根系水通道蛋白活性。在2种水分条件下, NH4+-N/ NO3--N为50/50处理的生物量(干重)均最大。本研究为水稻苗期合理施肥以壮苗提供了理论依据。     

Leaf Age as a Determinant in Stomatal Control of Water Loss from Cotton during Water Stress

The stomatal resistance of individual leaves of young cotton plants (Gossypium hirsutum L. var. Stoneville 213) was measured during a period of soil moisture stress under conditions of constant evaporative demand. When plants were subjected to increasing soil water stress, increases in stomatal resistance occurred first on the lower leaves and the stomata on the upper surfaces were the most sensitive to decreasing leaf-water potential. Stomatal closure proceeded from the oldest leaves to the youngest as the stress became more severe. This apparent effect of leaf age was not due to radiation differences during the stress period. Radiation adjustments on individual leaves during their development altered the stomatal closure potential for all leaves, but did not change the within-plant pattern. Our data indicate that no single value of leaf water potential will adequately represent a threshold for stomatal closure in cotton. Rather, the stomatal resistance of each leaf is uniquely related to its own water potential as modified by age and radiation regime during development. The effect of age on stress-induced stomatal closure was not associated with a loss of potassium from older leaves. Increases in both the free and bound forms of abscisic acid were observed in water-stressed plants, but the largest accumulations occurred in the youngest leaves. Thus, the pattern of abscisic acid accumulation in response to water stress did not parallel the pattern of stomatal closure induced by water stress.     

不同氮素形态及水分胁迫对水稻苗期水分吸收、光合作用及生长的影响

采用室内营养液培养,聚乙二醇（PEG6000）模拟水分胁迫处理、HgCl2抑制水通道蛋白活性的方法,在3种供氮形态下（NH4^＋-N／NO36-N为100／0、50／50和0／100）,研究了水稻苗期水分吸收、光合及生长的状况。结果表明,在非水分胁迫下,水稻单位干重吸水量以单一供NO3^--N处理最高,加HgCl2抑制水通道蛋白活性后,单一供NO3^--N、NH4^＋-N和NH4^＋-N／NO3^--N为50,50处理的水稻水分吸收分别下降了9．6％、20．7％和16．0％;但在水分胁迫下,单一供N03^--N的处理水分吸收量显著降低,低于其它2个处理,加HgCl2抑制水通道蛋白活性后,水分吸收量分别降低了1．0％、18．8％和23．5％。在2种水分条件（水分胁迫与非水分胁迫）下,净光合速率、气孔导度、蒸腾速率和细胞间隙CO2浓度等指标均以单一供NH4^＋-N处理最大,NH4^＋-N／NO3^--N为50,50处理次之,单一供NO3^--N处理最小。HgCl2处理结果表明,不同形态氮素营养能够影响水稻幼苗根系水通道蛋白活性。在2种水分条件下,NH4^＋-N／N03^--N为50,50处理的生物量（干重）均最大。本研究为水稻苗期合理施肥以壮苗提供了理论依据。     

Response of Indoor-Grown Cassava to Water Deficits and Recovery of Leaf Water Potential and Stomatal Activity after Water Stress

Wilting of the leaves occurred in acropetal succession at leafwater potentials between –0.9 and –1.1 MPa. Onlysevere water stress caused the discoloration and abscissionof the basal leaves. Leaf resistance was independent of leafwater potential above –0.5 MPa but increased as the potentialdropped below this value. When the stressed plants were rewatered,leaf water potentials recovered rapidly within the first h.Subsequently, the rate of recovery declined gradually. The maximumvalue of leaf water potential after rewatering was dependenton the severity of the water stress.     

Proline Metablism During Water Stress in Mulberry

The influence of water stress on proline metabolism was studiedin 3-month-old mulberry plants at four levels of water stress.Leaf water potential was drastically decreased in all treatments.Though leaf area and relative water content were decreased,drastic decrease was observed only in very severe stress treatments.Proline accumulation was observed both in roots and leaves instress treatments; but accumulation was greater in roots thanin leaves. The enzymes, proline dehydrogenase and proline oxidase,were inhibited under stress conditions. Proline oxidase wasmore inhibited in roots than in leaves. The significance ofthe relative activities of these two enzymes is discussed. Key words: Water stress, proline dehydrogenase, proline oxidase     

Response of Tea Plants to Water Stress

Two-year-old potted plants of six Camellia sinensis cultivars (TV-18, TV-26, UPASI-3, UPASI-26, T-78 and HV-39) were subjected to water stress for 4, 8 and 12 d. Relative water content (RWC) of leaves of all cultivars declined with water stress, but in the two drought tolerant cultivars (UPASI-3 and UPASI-26), higher RWC were maintained in comparison to the others. Phenol content and activities of phenylalanineammonialyase, polyphenoloxidase and peroxidase initially increased, but decreased during extended drought. Chlorophyll contents decreased, whereas proline contents increased during water stress. SDS-PAGE analysis of proteins revealed increased accumulation of proteins of intermediate molecular masses (42 – 44 kDa) and low molecular masses (14 – 26 kDa). After 12 d of water stress, most of these proteins disappeared in T-78 and HV-39, but in the other cultivars they were still detectable.     

Growing More Potatoes with Less Water

Demand for potato is steadily rising in developing countries, where actual per-hectare production levels reach mere fractions of the yields achieved in Europe or North America. Improving abiotic stress tolerance, e.g., against drought, could increase these low potato yields and thus help to satisfy the growing demand for this crop. Hypotheses about genes and traits that could mitigate yield decreases caused by drought have been driven by information obtained from model plants and have recently been complemented with data of high throughput gene expression profiling and metabolite studies on potato genotypes under water stress. Principal tolerance traits that could diminish the vulnerability of potato yields to drought stress include improved detoxification of reactive oxygen species produced during stress, optimized stomatal control under drought to reduce water loss but at the same time allow for continuous CO₂ access for photosynthesis, and mechanisms to protect proteins and membranes from damage by water stress. Candidate genes underlying these traits as well as genotypes that express them are available and, after appropriate validation, could be used for breeding.     

Proline Accumulation and Relative Water Content in Soya Bean (Glycine max) Varieties under Water Stress

The effects of water stress on proline accumulation and relativewater content of four soya bean varieties were studied. Twolocal strains, Kalitur and T-49, showed a smaller increase inproline content and greater reduction in relative water contentthan did JS-2 or Bragg under the same conditions of stress. Glycine max, proline accumulation, relative water content, water stress     

Effects of Water Stress on Energy Transduction in Chloroplasts

Water stress inhibited the photosynthetic O2 evolution rate of wheat leaves. It was shown that water stress decreased the electron transport rate, the activities of photophosphorylation and, coupling factor, and, the synthesis of ATP in chloroplasts. PS Ⅱ electron transport was more senstitive to water stress than PS Ⅰ. The reduction in photophosphorylation activity might be the results of reduction in electron transport rate and coupling factor activity, as well as the uncoupling effect of water stress on chloroplasts. The uncoupling effect could be due to the inhibition of light induced proton translocation in chloroplasts.     

Abscisic Acid Production and Water Relations in Wilty Tomato Mutants Subjected to Water Deficiency

Neill, S. J. and Horgan, R. 1985. Abscisic acid production andwater relations in wilty tomato mutants subjected to water deficiency.—J.exp. BoL 36: 1222-1231. Abscisic acid (ABA) concentrations were determined in shootsof Lycopersicon esculentum Mill. cv. Ailsa Craig wild type andthe three wilty mutants notabilis (not), flacca (flc) and sitiens(sit). ABA content of unstressed wild type leaves was 1.5 nmolg^–1 fr. wt.; concentrations in not, flc and sit were 49,26 and 15% of this respectively. Gradual water stress was imposed on potted plants and a morerapid stress imposed on detached leaves. Leaves of the wildtype and not responded to both stresses by increasing theirABA content but leaves of flc and sit did not produce any moreABA under stress. Transpiration rates of flc plants were three times greater thanthose of the wild type and stomatal resistances correspondinglylower. Stomata of both flc and the wild type responded to darknessand externally supplied ABA by closing. However, only wild typestomata responded to water stress by dosing; those of flc leavesremained open until the leaves were severely desiccated. Thus,there was some relationship between the lack of stomatal responseto water stress and the failure to synthesize ABA. Key words: ABA, biosynthesis, stomata, water shortage, wilty mutants     

Effects of Water Stress and Hardening on the Internal Water Relations and Osmotic Constituents of Cotton Leaves

Experiments were designed to test the hypothesis that the internal water relations of leaves are altered when cotton plants (Gossypium hirsutum L.‘Acala SJ-2′) are conditioned by several cycles of water stress. Preliminary experiments suggested that plants so conditioned are less sensitive to water deficits and that the change might be partly explained by an accumulation of solutes or by structural alterations attendant on development under conditions of water stress. Leaves of preconditioned plants maintained turgor to lower values of water potential than did leaves of well-watered plants. Accompanying this change was a lower osmotic potential at any given leaf water content in preconditioned plants. Tissue analysis of several osmotically active solutes indicated that soluble sugars and malate accumulate to about the same levels (dry-weight basis) in both conditioned and unconditioned plants exposed to stress. These accumulations could not account for the turgor change. Analysis of the data on relative water content indicated that the leaves of conditioned plants had less water per unit dry weight than did leaves of controls. This change accounts for a substantial fraction of the difference between the osmotic potential of conditioned and control plants. The results of a simple model suggest that structural changes may play a significant role in explaining differences in the responses of conditioned and control plants to water stress.     

Influence of Rate of Development of Leaf Water Deficits upon Photosynthesis, Leaf Conductance, Water Use Efficiency, and Osmotic Potential in Sorghum

This study reports the effect of rate of development of leaf water deficits in soil-grown sorghum (Sorghum bicolor) on the relationship of net photosynthesis, leaf conductance, and water use efficiency to leaf water potential, and on the degree of solute accumulation (osmotic adjustment). Recovery of these processes on rewatering, and responses during a second stress cycle were also studied. The most rapid rate of stress (1.2 MPa day^?1) resulted in no solute accumulation and the lowest rate of net photosynthesis and leaf conductance for any given leaf water potential during stress. Stress at 0.7 and 0.15 MPa day^?1 led to equal solute accumulations of approximately 0.6 MPa, but net photosynthesis, leaf conductance, and water use efficiency at a given leaf water potential were lower with the faster rate of stress (0.7 MPa day^?1). Additionally, leaf conductance at a given leaf turgor potential was lowest at the 1.2 MPa day^?1 stress rate, slightly higher at the intermediate rate of stress, and clearly highest at the slowest rate of stress. Recovery of both net photosynthesis and leaf conductance upon rewatering was rapid, taking less than 3 days, but full recovery of osmotic potential took between 6 and 11 days. One slow stress cycle had no influence on relationships during a second cycle. The concept of a threshold leaf water potential for stomatal closure is discussed and the conclusion reached that stomatal closure occurs slowly over a wide range of leaf water potential (> 1.0 MPa), the range being greater for slower rates of stress.     

Influence of Water Stress on Water Relations, Photosynthetic Parameters and Nitrogen Metabolism of Moth Bean Genotypes

Effects of water stress at pre-flowering stage were studied in three genotypes (RMO-40, Maru moth and CZM-32 E) of moth bean [Vigna aconitifolia (Jacq.) Marechal]. Increasing water stress progressively decreased plant water potential, leaf area, net photosynthetic rate, starch and soluble protein contents and nitrate reductase activity while contents of reducing sugars, total soluble sugar, free amino acids and free proline progressively increased. Significant genotypic differences were observed and genotype CZM-32-E displayed a better drought tolerance than other genotypes.     

Transient Water Stress in Carnation Flowers: Effect of Amino-oxyacetic Acid

A short and temporary water stress imposed on cut carnationflowers (Dianthus caryophyllus L., cv. White Sim) flowers advancedsenescence symptoms, including ethylene production and wilting.Pretreatment with amino-oxyacetic acid (AOA) resulted in anincrease of the resistance of the flowers to water stress: waterloss during stress was reduced, recovery was more rapid andwilting was delayed. Water stress accelerated the decrease inlevel of membrane phospholipids, but pretreatment with AOA counteractedthis effect. Since the content of membrane sterols was not affectedby the treatments, the mole ratio of sterol to phospholipidincreased in water-stressed flower petals but not in stressedflowers pretreated with AOA. Membrane permeability and fluiditywere also adversely affected by water stress and AOA: waterstress alone resulted in an increase in permeability and a decreasein fluidity, but in AOA-pretreated stressed flower petals theseparameters were similar to those of nonstressed control flowerpetals. On the basis of these results two main conclusions can be drawn:(a) Water stress induces alterations in the physical and compositionalproperties of carnation petal membranes, (b) Pretreatment ofthe flowers with AOA influences petal membrane traits, mostprobably via modifications in phospholipid turnover, in a waywhich counteracts the effects of water stress. Key words: Amino-oxyacetic acid, Water stress, Carnation flowers     

水分胁迫积累的ABA诱导抗氧化防护系统的信号级联

水分胁迫是限制植物生长发育的主要胁迫因子之一。植物通过感受刺激,产生和传递信号、启动多种防御机制对水分胁迫做出响应和适应。脱落酸(ABA)作为一种重要的植物体内胁迫激素,参与了许多这样的反应。研究表明,ABA增强植物水分胁迫的忍耐力与ABA诱导的抗氧化剂防护系统有关;且细胞溶质Ca2 ([Ca2 ]i)、活性氧(ROS)等许多第二信使参与了ABA诱导的信号转导过程。本文就这些信号分子在水分胁迫积累的内源ABA诱导的抗氧化剂防护系统中的作用作一综述。     

干旱和复水对崖柏光合特性及水分利用效率的影响

以4年生崖柏(Thuja sutchuenensis Franch.)盆栽实生苗为材料,试验干旱和复水对崖柏光合特性及水分利用效率的影响。结果显示,在自然干旱处理过程中,叶片相对含水量(LRWC)在土壤相对含水量(SRWC)降至30%之后开始出现下降;净光合速率(P_n)、蒸腾速率(T_r)、胞间CO₂浓度(C_i)、光饱和点(LSP)和最大光合速率(P_max)随着土壤可利用水分的减少而逐渐降低;而表观量子效率(Q)、表观量子需要量(1/Q)及光补偿点(LCP)未发生明显变化;水分利用效率(WUE)则随着SRWC的下降而逐渐提高。在停止浇水50 d后(SRWC下降约95%),叶片萎蔫,净光合速率接近零,复水后3 d,净光合速率(P_n)恢复至对照的88.59%,WUE降至对照的88.63%。通过干旱-复水一系列生理指标的变化分析认为,崖柏为干旱避免型植物。