Diurnal variations of photosynthesis and dew absorption by leaves in two evergreen shrubs growing in Mediterranean field conditions

The effects of summer drought, dew deposition on leaves and autumn rainfall on plant water relations and diurnal variations of photosynthesis were measured in two evergreen shrubs, rosemary ( Rosmarinus officinalis ) and lavender ( Lavandula stoechas ), grown in Mediterranean field conditions. Withholding water for 40 d caused a similar decrease in predawn shoot water potential (ψ_pd) from c. −0.4 to c. −1.3 MPa in both species, but a 50% decrease in the relative leaf water content in L. stoechas compared with 22% in R. officinalis . A similar decrease in CO₂ assimilation rates by c. 75% was observed in water-stressed plants of both species, although L. stoechas showed smaller photosynthesis: stomatal conductance ratio than R. officinalis (35 vs 45 μmol CO₂:mol H₂O). The relative quantum efficiency of photosystem II photochemistry also decreased by c. 45% at midday in water- stressed plants of both species. Nevertheless, neither L. stoechas nor R. officinalis suffered drought-induced damage to photosystem II, as indicated by the maintenance of the ratio F _v: F _m throughout the experiment, associated with an increase in the carotenoid content per unit of chlorophyll by c. 62% and c. 30%, respectively, in water-stressed plants. Only L. stoechas absorbed dew by leaves. In this species the occurrence of 6 d of dew over a 15-d period improved relative leaf water content by c. 72% and shoot water potential by c. 0.5 MPa throughout the day in water-stressed plants, although the photosynthetic capacity was not recovered until the occurrence of autumn rainfall. The ability of leaves to absorb dew allowed L. stoechas to restore plant water status, which is especially relevant in plants exposed to prolonged drought.     

Variations in water status, gas exchange, and growth in Rosmarinus officinalis plants infected with Glomus deserticola under drought conditions

The influence of the arbuscular mycorrhizal fungus Glomus deserticola on the water relations, gas exchange parameters, and vegetative growth of Rosmarinus officinalis plants under water stress was studied. Plants were grown with and without the mycorrhizal fungus under glasshouse conditions and subjected to water stress by withholding irrigation water for 14 days. Along the experimental period, a significant effect of the fungus on the plant growth was observed, and under water stress, mycorrhizal plants showed an increase in aerial and root biomass compared to non-mycorrhizal plants. The decrease in the soil water potential generated a decrease in leaf water potential (psi(l)) and stem water potential (psi(x)) of mycorrhizal and non-mycorrhizal plants, with this decrease being lower in mycorrhizal water-stressed plants. Mycorrhization also had positive effects on the root hydraulic conductivity (Lp) of water stressed plants. Furthermore, mycorrhizal-stressed plants showed a more important decrease in osmotic potential at full turgor (psi(os)) than did non-mycorrhizal-stressed plants, indicating the capacity of osmotic adjustment. Mycorrhizal infection also improved photosynthetic activity (Pn) and stomatal conductance (g(s)) in plants under water stress compared to the non-mycorrhizal-stressed plants. A similar behaviour was observed in the photochemical efficiency of PSII (Fv/Fm) with this parameter being lower in non-mycorrhizal plants than in mycorrhizal plants under water stress conditions. In the same way, under water restriction, mycorrhizal plants showed higher values of chlorophyll content than did non-mycorrhizal plants. Thus, the results obtained indicated that the mycorrhizal symbiosis had a beneficial effect on the water status and growth of Rosmarinus officinalis plants under water-stress conditions.     

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.     

Effects of dew on eco-physiological traits and leaf structures of Leymus chinensis and Agropyron cristatum grown under drought stress北大核心CSCD

Aims: To investigate the effects of dew on plants, we conducted the experiment to determine the physiological characteristics and leaf structures of Leymus chinensis and Agropyron cristatum in response to increasing dew under drought stress. Methods: Four treatments (no dew, three times dew and five times dew per week under drought stress, and well-watering) were designed to examine leaf relative water content, water potential, net photosynthetic rate, water use efficiency, biomass, and leaf structures of L. chinensis and A. cristatum. Important findings: There was a significant increase in the relative water content and water potential by simulated dew increase for two plants species under drought stress (p < 0.05). For A. cristatum, simulated dew increase significantly enhanced the net photosynthetic rate, stomatal conductance, and transpiration rate of plants under drought stress (p < 0.05). On the other hand, there was no significant difference in the stomatal conductance and transpiration rate for L. chinensis among treatments. Simulated dew increase improved the aboveground biomass and root biomass of two species. The ratio of yellow leaves to the total leaves was decreased by simulated dew increase for two species. Dew increase also protected leaf structures against the drought stress, suggesting that the dew increase can slow down the death process of leaves resulted from drought stress. Therefore, the study demonstrated that dew increased the available water for the leaves of L. chinensis and A. cristatum grown in the drought stress and thus had positive effects on the photosynthesis, water physiology and plant development.     

Effects of dew on eco-physiological traits and leaf structures of Leymus chinensis and Agropyron cristatum grown under drought stress

Aims To investigate the effects of dew on plants, we conducted the experiment to determine the physiological characteristics and leaf structures of Leymus chinensis and Agropyron cristatum in response to increasing dew under drought stress.Methods Four treatments (no dew, three times dew and five times dew per week under drought stress, and well-watering) were designed to examine leaf relative water content, water potential, net photosynthetic rate, water use efficiency, biomass, and leaf structures of L. chinensis and A. cristatum. Important findings There was a significant increase in the relative water content and water potential by simulated dew increase for two plants species under drought stress (p < 0.05). For A. cristatum, simulated dew increase significantly enhanced the net photosynthetic rate, stomatal conductance, and transpiration rate of plants under drought stress (p < 0.05). On the other hand, there was no significant difference in the stomatal conductance and transpiration rate for L. chinensis among treatments. Simulated dew increase improved the aboveground biomass and root biomass of two species. The ratio of yellow leaves to the total leaves was decreased by simulated dew increase for two species. Dew increase also protected leaf structures against the drought stress, suggesting that the dew increase can slow down the death process of leaves resulted from drought stress. Therefore, the study demonstrated that dew increased the available water for the leaves of L. chinensis and A. cristatum grown in the drought stress and thus had positive effects on the photosynthesis, water physiology and plant development.     

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。     

Effects of drought on photosynthesis in Mediterranean plants grown under enhanced UV-B radiation

The effects of drought on the photosynthetic characteristics of three Mediterranean plants (olive, Olea europea L.; rosemary, Rosmarinus officinalis L.; lavender, Lavandula stoechas L.) exposed to elevated UV-B irradiation in a glasshouse were investigated over a period of weeks. Drought conditions were imposed on 2-year-old plants by withholding water. During the onset of water stress, analyses of the response of net carbon assimilation of leaves to their intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bisphosphate on photosynthesis. Measurements of chlorophyll fluorescence were used to determine changes in the efficiency of light utilization for electron transport, the occurrence of photoinhibition of photosystem II photochemistry and the possibility of stomatal patchiness across leaves. The first stages of water stress produced decreases in the light-saturated rate of CO2 assimilation which were accompanied by decreases in the maximum carboxylation velocity and the capacity for regeneration of ribulose 1,5-bisphosphate in the absence of any significant photodamage to photosystem II. Leaves of rosemary and lavender were more sensitive than those of olive during the first stages of the drought treatment and also exhibited increases in stomatal limitation. With increasing water stress, significant decreases in the maximum quantum efficiency of photosystem II photochemistry occurred in lavender and rosemary, and stomatal limitation was increased in olive. No indication of any heterogeneity of photosynthesis was found in any leaves. Drought treatment significantly decreased leaf area in all species, an important factor in drought-induced decreases in photosynthetic productivity. Exposure of plants to elevated UV-B radiation (0.47 W m(-2)) prior to and during the drought treatment had no significant effects on the growth or photosynthetic activities of the plants. Consequently, it is predicted that increasing UV-B due to future stratospheric ozone depletion is unlikely to have any significant impact on the photosynthetic productivity of olive, lavender and rosemary in the field.     

五种高速公路边坡绿化植物的生理特性及抗旱性综合评价

采用盆栽控制土壤水分,测定不同程度水分胁迫梯度下5种高速公路边坡绿化植物的叶水势、光合作用参数及叶绿素荧光参数,并以水分胁迫下各生理指标的平均变化速率为原始数据进行抗旱性综合评价。结果表明:(1)随着水分胁迫的加强,5种植物的ΨL、Pn、Tr、GS、Fv、Fm、Fv/Fm、Fv/Fo均逐渐下降,Fo逐渐升高,WUE先升后降,Ci先降后升,不同生理指标的变化幅度及拐点有一定差异;平均变化速率相对较小的是八宝景天和马蔺,较大的是肥皂草和太行菊;(2)根据水分胁迫下Gs和Ci变化方向的差异判断八宝景天、马蔺以W2作为Pn降低由气孔限制转为非气孔因素的分界线,五叶地锦、肥皂草和太行菊以W1为分界线,八宝景天、马蔺在W2时WUE最高,五叶地锦、肥皂草和太行菊在W1时最高;(3)5种植物抗旱性大小排序为:八宝景天>马蔺>五叶地锦>太行菊>肥皂草,聚类分析结果为:八宝景天、马蔺为强抗旱植物,五叶地锦为中抗旱植物,太行菊、肥皂草为弱抗旱植物。     

Risk-taking plants: Anisohydric behavior as a stress-resistance trait

Water scarcity is a critical limitation for agricultural systems. Two different water management strategies have evolved in plants: an isohydric strategy and an anisohydric strategy. Isohydric plants maintain a constant midday leaf water potential (Ψ_leaf) when water is abundant, as well as under drought conditions, by reducing stomatal conductance as necessary to limit transpiration. Anisohydric plants have more variable Ψ_leaf and keep their stomata open and photosynthetic rates high for longer periods, even in the presence of decreasing leaf water potential. This risk-taking behavior of anisohydric plants might be beneficial when water is abundant, as well as under moderately stressful conditions. However, under conditions of intense drought, this behavior might endanger the plant. We will discuss the advantages and disadvantages of these two water-usage strategies and their effects on the plant’s ability to tolerate abiotic and biotic stress. The involvement of plant tonoplast AQPs in this process will also be discussed.     

北京山区干旱胁迫下侧柏叶片水分吸收策略

干旱与半干旱地区,水分是限制树木生长的重要影响因子。由于降水稀缺且分配不均,叶片吸收水分是此地区树木吸收和利用小量级降水和凝结水的主要方式。北京山区处于易旱少雨的生态脆弱地带,森林植被经常遭受干旱胁迫,所以对该地区的森林系统而言,叶片直接吸收利用截留的降雨是干旱时期树木获得水分的重要途径。基于野外对比控制试验和室内盆栽模拟试验,选取北京山区的主要造林树种侧柏为研究对象,进行利用天然降雨与模拟降雨试验,研究降雨前后侧柏叶片吸水特征,探究侧柏在干旱环境下如何通过叶片吸水缓解干旱胁迫。结果表明：当侧柏长期处于干旱胁迫状态时,叶片可以利用降雨,从中获益用来缓解树木的干旱胁迫状态;叶片的吸水能力与降雨强度呈正相关关系,与土壤含水率呈负相关关系;重度干旱下侧柏植株在降雨强度为15 mm/h时叶片吸水现象最明显,叶水势变化最大为(1.18±0.17) MPa,叶片含水率变化最大为(8.47±1.00)mg/cm~2;当土壤水率高于20.8%时,基本不发生叶片吸水现象。试验结果说明在干旱地区叶片吸水是树木除根系吸水外的重要水分来源方式,并且对干旱地区有效利用短缺水资源,减轻植物水分亏缺具有重要意义。     

Contribution of arbuscular mycorrhizal fungi (AMF) to the adaptations exhibited by the deciduous shrub Anthyllis cytisoides L. under water deficit

Anthyllis cytisoides L. is highly colonized by arbuscular mycorrhizal fungi (AMF) and behaves as a drought-avoider species in the field. Our objectives were: (1) to study the response of A. cytisoides when exposed to moderate (acclimation) or severe (peak) drought and subsequent rewatering under nursery conditions; and (2) to verify if AMF improved the adaptation of A. cytisoides to stress. The soil compactness in drought-acclimated treatments increased four times compared with that of well-watered controls, which could reinforce the effects of water deficit on plant physiology. Photosynthetic rates decreased by around 50% and 70% and leaf conductance decreased by 40% and 50% in drought-acclimated non-mycorrhizal and mycorrhizal plants, respectively. Peak drought limited plant growth, accelerated leaf senescence and induced the conversion of starch into soluble sugars in the leaves of stressed plants. The accumulation of sugars could contribute to a decrease in water potential in order to achieve the required tension to let water move from soil to shoot. Mycorrhizal plants showed a two-fold higher chlorotic leaf biomass than non-mycorrhizal plants under severe drought. Moreover, mycorrhizal A. cytisoides showed enhanced epicuticular waxes on the surfaces of the remaining green leaves. Increased leaf senescence, together with wax deposition, could reduce whole plant transpiration, thus allowing mycorrhizal plants to maintain a higher leaf relative water content (50%) than non-mycorrhizal plants (35%). After drought recovery, leaf abscission in stressed mycorrhizal plants was 10 times greater than that in non-mycorrhizal plants. The results suggest that AMF conferred greater responsiveness of A. cytisoides to drought. Enhanced wax deposition and leaf senescence could be an ecological adaptation to cope with severe water deficit.     

Development of Water Stress under Increased Atmospheric CO2 Concentration

The increase in water use efficiency (the ratio of photosynthetic to transpiration rates) is likely to be the commonest positive effect of long-term elevation in CO₂ concentration (CE). This may not necessarily lead to decrease in long-term water use owing to increased leaf area. However, some plant species seem to cope better with drought stress under CE, because increased production of photosynthates might enhance osmotic adjustment and decreased stomatal conductance and transpiration rate under CE enable plants to maintain a higher leaf water potential during drought. In addition, at the same stomatal conductance, internal CO₂ concentration might be higher under CE which results in higher photosynthetic rate. Therefore plants under CE of the future atmosphere will probably survive eventual higher drought stress and some species may even be able to extend their biotope into less favourable sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Responses of Papaya Seedlings (Carica papaya L.) to Water Stress and Re-Hydration: Growth, Photosynthesis and Mineral Nutrient Imbalance

The effects of water stress and subsequent re-hydration on growth, leaf abscission, photosynthetic activity, leaf water potential and ion content were investigated in papaya seedlings (Carica papaya L.) cv. “Baixinho de Santa Amalia”. Water stress was imposed by suspending irrigation during 34 days. Thereafter, plants were regularly re-watered. Drought arrested plant growth, induced leaf abscission and drastically decreased photosynthetic rate. However, leaf water potential was hardly reduced. Water deficit also induced sodium, potassium and chloride accumulation in leaves and roots, and did not modify nitrogen levels in both organs. Re-hydration stimulated growth, promoted emergence of new leaves, reactivated photosynthetic machinery function and reduced ion content to control levels. The results indicated that the ability of papaya plants to improve drought tolerance is not mediated through the reduction of leaf abscission, the detention of growth or the decrease of net CO₂ assimilation. In contrast, the data suggested that under water stress conditions these plants appear to posses a certain capacity to increase ion content, which might contribute to osmotic adjustment.     

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.     

盐旱复合胁迫对小麦幼苗生长和水分吸收的影响

为明确盐害、干旱及盐旱复合胁迫对小麦幼苗生长和水分吸收的影响,从而为盐害和干旱胁迫下栽培调控提供理论依据。以2个抗旱性不同的小麦品种(扬麦16和耐旱型洛旱7号)为材料,采用水培试验,以NaCl和PEG模拟盐旱复合胁迫,研究了盐旱复合胁迫下小麦幼苗生长、根系形态、光合特性及水分吸收特性的变化。结果表明,盐、旱及复合胁迫下小麦幼苗的生物量、叶面积、总根长与根系表面积、叶绿素荧光和净光合速率均显著下降,但是复合胁迫处理的降幅却显著低于单一胁迫。盐旱复合胁迫下根系水导速率和根系伤流液强度显著大于单一胁迫,从而提高了小麦幼苗叶片水势和相对含水量。盐胁迫下小麦幼苗Na~+/K~+显著大于复合胁迫,但复合胁迫下ABA含量却显著小于单一的盐害和干旱胁迫。因此,盐旱复合胁迫可以通过增强根系水分吸收及降低根叶中ABA含量以维持较高光合能力,这是盐旱复合胁迫提高小麦适应性的重要原因。洛旱7号和扬麦16对盐及盐旱复合胁迫的响应基本一致,但在干旱胁迫下洛旱7号表现出明显的耐性。     

水胁迫下多角度幼龄檀香图像颜色变化分析及含水率反演

干旱与水淹胁迫是植物遭受的主要非生物胁迫,对植物的生理活动造成严重影响.本研究基于单反相机获取幼龄檀香的纵向和冠层叶片图像,使用分割算法提取叶片和颜色特征,然后讨论两种胁迫条件下多角度檀香叶片颜色变化及含水率反演.结果表明:干旱组在胁迫前期(前6d)叶片亮度降低,绿色分量增加,之后叶片亮度增加,绿色分量降低;水淹组叶片在整个胁迫周期亮度持续降低,黄色分量增加;对照组则与干旱组的变化趋势类似,但拐点出现的时间较晚.当叶片含水率在50%~70%时,随着含水率的增加,彩色图像的红(R)、绿(G)、蓝(B)通道值均会减小;但当叶片含水率小于40%时,会出现R通道值大于G通道值的现象.在使用极限学习机反演含水率时,校正后的颜色分量对拟合优度及预测精度均有所提高.纵向图像更适合用来反演叶片的含水量,决定系数和平均绝对百分比误差分别为0.8352和2.3%;而冠层图像对叶片等效水厚度的表达更准确,上述指标分别为0.7924和9.3%.     

Comparison of proline accumulation in two mediterranean shrubs subjected to natural and experimental water deficit

The effect of water stress on proline accumulation was tested in two contrasted species of Mediterranean scrub: Halimium halimifolium (L.) Willk and Pistacia lentiscus L. Leaf water potential, stomatal resistance and proline content have been measured both in experimental and in natural water stress conditions. Both species accumulated proline in their leaves when leaf water potential dropped below a threshold value of –3.0 MPa, under natural as well as under experimental conditions. In the field, however, a time-lag between decrease of leaf water potential and proline accumulation could be observed. In Halimium halimifolium, proline accumulation appeared to be associated with severe stress conditions as most plants with high proline contents suffered irreversible wilting, especially in the greenhouse. P. lentiscus showed a different pattern, accumulating proline at two different times of the year, as a response to cold or to drought. The results of our study indicated that the role of proline in this species, rather than an osmotic agent, seems to be more related to a protective action in cases of severe stress conditions.     

Modification of Drought Resistance by Water Stress Conditioning in Acacia and Eucalyptus

Plants of Acacia and Eucalyptus species were grown under differentlevels of shading, nutrition, and irrigation to assess the effectof these factors on plant water use. Water use per unit of leaf(phyllode) area was affected only by the irrigation treatment,control plants that had received water daily using appreciablymore water than plants that had been repeatedly subjected towater stress. Water stress conditioning had little or no effecton plant height, leaf (phyllode) area, or minimum stomatal resistancein any of the species. Detailed study of the water stress conditioningof Eucalyptus robusta showed that controls used 46% more waterthan conditioned plants. Leaf area and plant height were unaffectedby conditioning. Control of transpiration was not due to stomatalfunctioning, both sets of plants operating with the same leafdiffusive resistance under conditions of ready water availability.Hydraulic conductivity of the intact root system was loweredby conditioning and it is suggested that this was due, at leastin part, to the effect that conditioning had on root xylem conductivity.Specific conductivity of stem sections was lowered by waterstress conditioning. Water stress avoidance was also associatedwith a more pronounced tendency for stomata to close prior towilting and with a higher level of leaf resistance which couldbe maintained at a low leaf water potential. Conditioned plantsexhibited drought tolerance in their ability to control lossof water from the leaf at lower leaf water potentials than thecontrols.     

Effects of water stress on the water relations of Phaseolus vulgaris and the drought resistant Phaseolus acutifolius

The tepary bean ( Phaseolus acutifolius Gray var. latifolius ), a drought resistant species, was compared under water stress conditions with the more drought susceptible P. vulgaris L. cvs Pinto and White Half Runner (WHR). In order to better understand the basis for the superior drought resistance of tepary, this study was designed to determine the relationships among leaf water potential, osmotic potential, turgor potential, and relative water content (RWC).
Plants were prestressed by withholding irrigation water. These stress pretreatments changed the relation between leaf water potential and relative water content of both species so that prestressed plants had lower water potentials than controls at the same leaf RWC. Tepary had lower water potentials at given RWC levels than Pinto or WHR; this can account for part of the superior resistance of tepary. In all genotypes, prestressed plants maintained osmotic potentials approximately 0.2 MPa lower than controls. Tepary reached osmotic potentials that were significantly lower (0.15 to 0.25 MPa) than Pinto or WHR. Both control and prestressed tepary plants had 0.05 to 0.25 MPa more turgor than Pinto or WHR at RWC values between 65 and 80%. Both prestressed and control tepary plants had greater elasticity (a lower elastic modulus) than Pinto or WHR. This greater turgor of tepary at low RWC values could be caused by several factors including greater tissue elasticity, active accumulation of solutes, or greater solute concentration.
Tepary had significantly lower osmotic potentials than the P. vulgaris cultivars, but there was little difference in osmotic potential between Pinto and WHR. Knowledge of differences in osmotic and turgor potentials among and within species could be useful in breeding for drought resistance in Phaseolus.     

四种不同生活型植物幼苗对喀斯特生境干旱的生理生态适应性

喀斯特石漠化是我国西南喀斯特地区最严重的生态环境问题, 生境干旱是限制该地区植物生长的主要因素之一, 掌握喀斯特植被不同演替阶段不同生活型植物对干旱胁迫的适应策略有助于提高植被恢复的成功率。通过人工模拟4种干旱强度, 测定叶片水势、气体交换、叶绿素荧光、光合色素含量、渗透调节物质浓度、抗氧化酶活性以及生物量, 研究了喀斯特地区4种不同生活型植物幼苗对干旱胁迫的适应策略。这4种植物为常绿灌木火棘(Pyracantha fortuneana)、落叶灌木小果蔷薇(Rosa cymosa)、常绿乔木猴樟(Cinnamomum bodinieri)和落叶乔木圆果化香树(Platycarya longipes)。结果表明: 随着干旱程度的加深, 4种植物幼苗的叶片水势、光合能力、叶绿素含量、生物量增长、叶重比(LMR)、叶面积比(LAR)和比叶面积(SLA)逐渐下降, 而热耗散(NPQ)、类胡萝卜素与叶绿素含量比值、丙二醛含量和根重比(RMR)逐渐上升; 圆果化香树和猴樟的水分利用效率(A_n/g_s)、渗透调节物质浓度和抗氧化酶活性呈先升高后降低的趋势, 而火棘和小果蔷薇的A_n/g_s、脯氨酸含量和超氧化物歧化酶活性呈上升趋势。严重干旱下, 火棘和小果蔷薇幼苗的叶片水势和叶绿素含量下降较少, 具有较高的光合能力和生物量增长, 这主要是由于它们具有较低的SLA和LAR、较高的NPQ和A_n/g_s以及较高的渗透调节能力和抗氧化保护能力。中度干旱下, 猴樟幼苗叶片水势下降很少, LMR和LAR也较高, 脯氨酸含量和抗氧化酶活性非常高。但在严重干旱下, 其叶片水势、LMR、LAR和生物量增长大幅度下降, 最大光化学效率和光合速率也非常低, 渗透调节能力与抗氧化酶活性大幅度下降至正常水平以下。水分好的条件下, 圆果化香树幼苗具有较高的RMR以吸收充足的水分, 具有较高的LAR和叶绿素含量, 保证了生物量的大量积累。然而, 干旱胁迫致使其生物量大幅度下降, 主要是由于LMR、LAR、气体交换和叶绿素含量的大量下降以减少蒸腾面积、水分散失和对光能的吸收。研究结果表明, 火棘、小果蔷薇和猴樟幼苗主要采用耐旱策略, 其中猴樟抗严重干旱的能力较弱; 圆果化香树幼苗对干旱胁迫更为敏感, 主要采取避旱策略。