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

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

Morpho-physiological responses of sugar beet (Beta vulgaris L.) genotypes to drought stress

The identification of morpho-physiological traits related to drought tolerance and high yield potential is a challenge when selecting sugar beet genotypes with greater tolerance to water stress. In this paper, root morphological parameters, antioxidant systems, leaf relative water content (RWC) and H⁺-ATPase activity as key morpho-physiological traits involved in drought tolerance/susceptibility of sugar beet were studied. Genotypes showing a different drought tolerance index (DTI) but a similar yield potential, under moderate (?0.6 Mpa) and severe (?1.2 MPa) water stress, were selected and their morpho-physiological traits were investigated. The results showed a wide genetic variation in morpho-physiological parameters which demonstrated the different adaptive strategies under moderate and severe drought conditions in sugar beet. In particular, an efficient antioxidant system and redox signalling made some sugar beet genotypes more tolerant to drought stress. The alternative strategy of other genotypes was the reduction of root tissue density, which produced a less dense root system improving the axial hydraulic conductivity. These results could be considered as interesting challenge for a better understanding of the drought tolerance mechanisms in sugar beet.     

Water-stressed maize, barley and rice seedlings show species diversity in mechanisms of leaf growth inhibition

The possible occurrence of species diversity in mechanisms underlying leaf-growth inhibition by water stress, was investigated in related cereal plants. Water stress was generated by additions of the osmoticum polyethylene glycol 6000 to the root medium. Effects of external water potentials ranging from 0 to -0.6MPa, on early growth parameters of emerging leaves were measured under controlled environment conditions, using pairs of maize, barley or rice genotypes with differing resistance to water stress under field conditions. Water potentials of -0.4 MPa for 24 h, similarly reduced leaf growth, comparative production rates of leaf epidermal cells and cell size in all genotypes. These reductions did not appear to be caused by reductions in the osmotic potential gradients between the expanding leaf cells and their external water source. However, growth inhibition in maize and barley, was accompanied by significant reductions in comparative leaf and cell wall extensibility. Moreover, regression plots revealed good linear correlations (r=0.83** for maize and r=0.77** for barley) between the reductions in leaf growth induced by a series of water potentials and associated reductions in leaf extensibility. In contrast, the reduction in growth of rice leaves, was not accompanied by any significant changes in leaf or cell wall extensibility. Similarly, regression plots revealed poor correlations between leaf growth and leaf extensibility in both paddy and upland rice (r=0.17 and r=0.07, respectively). Thus, despite numerous inter-species similarities, biophysical changes associated with stress-induced leaf growth inhibition in maize and barley, differed from those in rice.Key words: Cell walls, extensibility, water stress, cereal diversity, leaf growth.      

Giant reed genotypes from temperate and arid environments show different response mechanisms to drought

Studies at the root level and how the root–shoot interactions may influence the whole crop performance of giant reed (Arundo donax L.) under limited water conditions are largely missing. In the present study, we illustrate the effects of water stress on some phenotypic traits at the root–shoot levels of two giant reed genotypes (from Morocco and Northern Italy) that were reported to have different adaptive hydraulic stem conductivities despite the limited genetic variability of the species. The trial was carried out in 1 m³ rhizotrons (1 × 1 × 1 m) for two consecutive growing seasons. As expected, both genotypes showed an effective behavior to contrast water shortage; however, the Moroccan genotype showed a higher leaf water potential, a lower root length density (RLD) and thinner roots in the upper soil layer (0–20 cm), and similar to control RLD values at deep soil layers (40–60 cm). On the other hand the Italian genotype showed the opposite pattern; that is no drought (DR) effects in RLD and root diameter at upper soil layers and reduced RLD in deep layers, thus revealing different DR adaptation characteristics between two genotypes. This DR adaptation variability might bring new insights on DR tolerance of giant reed identifying potential traits aimed to improve the integral plant functioning, to a more efficient use of water resources, and to a more effective crop allocation to targeted stressful conditions under a climate change scenario that foresees the increase of DR periods.     

Evaluation of high yielding soybean germplasm under water limitation

Limited information is available for soybean root traits and their plasticity under drought stress. To date, no studies have focused on examining diverse soybean germplasm for regulation of shoot and root response under water limited conditions across varying soil types. In this study, 17 genetically diverse soybean germplasm lines were selected to study root response to water limited conditions in clay (trial 1) and sandy soil (trial 2) in two target environments. Physiological data on shoot traits was measured at multiple crop stages ranging from early vegetative to pod filling. The phenotypic root traits, and biomass accumulation data are collected at pod filling stage. In trial 1, the number of lateral roots and forks were positively correlated with plot yield under water limitation and in trial 2, lateral root thickness was positively correlated with the hill plot yield. Plant Introduction (PI) 578477A and 088444 were found to have higher later root number and forks in clay soil with higher yield under water limitation. In sandy soil, PI458020 was found to have a thicker lateral root system and higher yield under water limitation. The genotypes identified in this study could be used to enhance drought tolerance of elite soybean cultivars through improved root traits specific to target environments.     

干旱胁迫对花生根系生长发育和生理特性的影响

以花育17号和唐科8号两个花生品种为试验材料,在防雨棚栽培池内进行土柱栽培试验,研究了中度干旱胁迫和正常供水处理下花生生育后期根系形态发育特征和生理特性.结果表明: 唐科8号具有较发达的根系及较高的产量和抗旱系数,花育17号根系对干旱胁迫的适应性小于唐科8号.两品种根长密度、根系生物量均主要分布于0～40 cm土层中,但同一土层内两品种根系性状存在差异.与正常供水处理相比,干旱胁迫处理使花育17号各生育期总根长、根系总表面积和总体积均降低,而唐科8号除花针期显著降低外,其余生育期均明显升高;干旱胁迫增加了两品种20～40 cm土层内根系生物量、根系表面积和体积,而降低了40 cm以下土层内各根系性状;干旱胁迫处理使两品种饱果期40 cm以下土层内根系活力降低,且花育17号降低幅度高于唐科8号.干旱胁迫下两品种生育后期根系发育和生理特性的差异表明其根系在干旱胁迫下对水分吸收和利用存在差异.     

Water stress mediated root trait dynamics and identification of microsatellite markers associated with root traits in rice ( Oryza sativa L.)

To identify microsatellite markers associated with root traits for drought tolerance in rice (Oryza sativa L.) a study was conducted at Department of Plant Physiology, College of Agriculture, Trivandrum, Kerala Agricultural University. A set of thirty-five rice genotypes were exposed to water stress and evaluated for physio-morphological components as indices of water stress tolerance. Observations were made on leaf rolling score and root traits, especially the root length, root dry weight, root volume and root shoot ratio at booting stage. As of the data obtained, ten tolerant and ten susceptible varieties were selected for bulk line analysis to identify the DNA markers linked with target gene conferring drought tolerance. Out of 150 SSR primers screened, RM474 showed polymorphism between the tolerant and susceptible bulks. Individual genotypes of the bulks also showed the same product size of the respective tolerant and susceptible bulks.     

Effect of water stress mediated through agar on <Emphasis Type="Italic">in vitro</Emphasis> growth of potato

With the objective to develop a practical method of screening potato for drought tolerance, shoot and root growth in plantlets raised in vitro (from nodal cuttings drawn from in vivo as well as in vitro grown plantlets) were studied in three genotypes with known root mass production under field conditions. Different levels of water stress were induced using five concentrations of agar in MS (Murashige and Skoog in Physiol Plant 15:473–497, 1962) medium. Water potential of various media ranged from −0.70 MPa to −0.98 MPa. Water stress in culture adversely affected plantlet growth, and the responses varied with genotype and explant source. Genotype IWA-1 was less affected than Konafubuki and Norin-1. In the experiment with explants from in vivo grown plants, the time to rooting was considerably delayed in Konafubuki and Norin-1 by an increase in agar concentration, but no such effect was observed in IWA-1. In all media, the mean number of roots and root length was greater in IWA-1 than Konafubuki and Norin-1, and the latter two genotypes were at par. At 10 gl⁻¹ agar, IWA-1 had taller plantlets, heavier foliage dry weight, root volume, as well as root dry weight than Konafubuki and Norin-1, whereas the latter two genotypes were at par for all these characteristics. This pattern was similar to the reported pattern of these genotypes for root dry weight under field conditions. However, such similarity in the in vitro and field behavior of the tested genotypes was not observed when nodal cuttings drawn from in vitro plantlets were used as explants. It is concluded that in vitro screening of potato under specific and limited water stress conditions by raising plantlets from nodal cuttings drawn from in vivo grown plants may provide a system for effectively differentiating the genotypes for their expected root mass production under field conditions.     

DREB1A promotes root development in deep soil layers and increases water extraction under water stress in groundnut

Water deficit is a major yield‐limiting factor for many crops, and improving the root system has been proposed as a promising breeding strategy, although not in groundnut (Arachis hypogaea L.). The present work was carried out mainly to assess how root traits are influenced under water stress in groundnut, whether transgenics can alter root traits, and whether putative changes lead to water extraction differences. Several transgenic events, transformed with DREB1A driven by the rd29 promoter, along with wild‐type JL24, were tested in a lysimeter system that mimics field conditions under both water stress (WS) and well‐watered (WW) conditions. The WS treatment increased the maximum rooting depth, although the increase was limited to about 20% in JL24, compared to 50% in RD11. The root dry weight followed a similar trend. Consequently, the root dry weight and length density of transgenics was higher in layers below 100‐cm depth (Exp. 1) and below 30 cm (Exp. 2). The root diameter was unchanged under WS treatment, except a slight increase in the 60–90‐cm layer. The root diameter increased below 60 cm in both treatments. In the WW treatment, total water extraction of RD33 was higher than in JL24 and other transgenic events, and somewhat lower in RD11 than in JL24. In the WS treatment, water extraction of RD2, RD11 and RD33 was higher than in JL24. These water extraction differences were mostly apparent in the initial 21 days after treatment imposition and were well related to root length density in the 30–60‐cm layer (R² = 0.68), but not to average root length density. In conclusion, water stress promotes rooting growth more strongly in transgenic events than in the wild type, especially in deep soil layers, and this leads to increased water extraction. This opens an avenue for tapping these characteristics toward the improvement of drought adaptation in deep soil conditions, and toward a better understanding of genes involved in rooting in groundnut.     

Genotypic covariations of traits underlying sorghum stem biomass production and quality and their regulations by water availability: Insight from studies at organ and tissue levels

Sweet and biomass sorghum are expected to contribute increasingly to bioenergy production. Better understanding the impacts of the genotypic and environmental variabilities on biomass component traits and their properties is essential to optimize energy yields. This study aimed to evaluate whether traits contributing to stem biomass growth and biochemical composition at different biological scales (co)vary with the genotype and the water status in sorghum. Height genotypes were studied over two years in field conditions in southern France under two water treatments (well watered vs. 25 days’ dry down during stem elongation). Main stem internode number, size, (non)structural carbohydrate, and lignin contents were measured at the end of the stress period and/or at final harvest, together with biochemical and histological analyses of the youngest expanded internode. The tallest genotypes showed the highest stem dry weights and lignin contents. Stem (structural) biomass density was positively correlated with lignin content, particularly in internode parenchyma. Stem soluble sugar and lignin contents were inversely proportional across genotypes and water conditions. Genotypes contrasted for drought sensitivity and recovery capacity of stem growth and biochemical composition. The length and cell wall deposition of internodes expanding under water deficit were reduced and did not recover, these responses being weakly correlated. Genotypic variability was pointed out in the growth recovery of internodes expanding under re‐watered conditions. According to the observed genotypic variability and the absence of antagonistic correlations between the responses of the different traits to water availability, it is suggested that biomass sorghum varieties optimizing their responses to water availability in terms of growth and cell wall deposition can be developed for different bioenergy targets.     

Early water-deficit effects on seminal roots morphology in barley

In order to study seminal roots morphology in barley grown under different water treatments, experiments were carried out under glasshouse-controlled conditions. Eight genotypes were cultivated under four water treatments (100, 75, 50 and 25% of field capacity). Seminal root length and root-to-shoot dry matters' ratio were measured. Root volume was assessed at three soil depths. Results showed broad genotypic differences for all traits. The effect of low and moderate water deficit was slight. In contrast, the impact of severe water treatment was strongly marked on all traits. The impact of water deficit intensity on root traits at different soil depths is discussed.     

土壤水分胁迫与遮荫对生姜生长特性的影响

水分胁迫使生姜叶片叶绿素含量降低,叶片气孔密度及气孔面积减小,根系活力下降。根系伤流量减少,在相同土壤水分条件下,遮光使叶片叶绿素含量增加。气孔密度减小,叶片上表皮气孔变大,而下表皮气孔变小。根系活力增强,水分胁迫显著影响生姜的生长与产量,但不同水分条件下,光强对生姜生长与产量的影响不同,正常供水条件下,生姜在自然光照条件下生长较好,产量较高;水分胁迫条件下,生姜在遮荫条件下生长较好,产量较高。     

Induction of water deficit tolerance in wheat due to exogenous application of plant growth regulators: membrane stability,water relations and photosynthesis

Our experiment was carried out in order to explore effects of plant growth regulators (PGR; thidiazuron, paclobutrazol, and ascorbic acid) on physiological traits of wheat genotypes under water surplus and deficit conditions. Study revealed that relative water content, membrane stability index, chlorophyll content, photosynthetic rate (P_N), and maximal quantum yield of PSII improved with PGRs application across the genotypes both under irrigation and water stress. The response of HD 2733 genotype was more positive toward PGRs treatment as compared to other genotypes under water stress. Higher P_N and chlorophyll contents were observed in HD 2987 followed by C 306 genotype under water-stress conditions. Moreover, Rubisco small subunit (SSU) expression was lower in wheat genotypes under water stress as compared to irrigated conditions. Application of PGRs led to upregulation of SSU under water stress, while no significant change was found in Rubisco level and activity under irrigated condition in dependence on PGRs treatments. Yield-related traits showed also significant reduction under water-stress conditions, while application of PGRs enhanced the yield and its components. Results indicated that the PGRs exhibited a positive interaction and synergetic effect on water stressed wheat plants in terms of photosynthetic machinery and yield.     

Phenotypic plasticity and water flux rates of Citrus root orders under salinity

Knowledge about the root system structure and the uptake efficiency of root orders is critical to understand the adaptive plasticity of plants towards salt stress. Thus, this study describes the phenological and physiological plasticity of Citrus volkameriana rootstocks under severe NaCl stress on the level of root orders. Phenotypic root traits known to influence uptake processes, for example frequency of root orders, specific root area, cortical thickness, and xylem traits, did not change homogeneously throughout the root system, but changes after 6 months under 90 mM NaCl stress were root order specific. Chloride accumulation significantly increased with decreasing root order, and the Cl(-) concentration in lower root orders exceeded those in leaves. Water flux densities of first-order roots decreased to <20% under salinity and did not recover after stress release. The water flux densities of higher root orders changed marginally under salinity and increased 2- to 6-fold in second and third root orders after short-term stress release. Changes in root order frequency, morphology, and anatomy indicate rapid and major modification of C. volkameriana root systems under salt stress. Reduced water uptake under salinity was related to changes of water flux densities among root orders and to reduced root surface areas. The importance of root orders for water uptake changed under salinity from root tips towards higher root orders. The root order-specific changes reflect differences in vulnerability (indicated by the salt accumulation) and ontogenetic status, and point to functional differences among root orders under high salinity.     

Inoculation with arbuscular mycorrhizal fungi enhances growth of Litchi chinensis Sonn. trees after propagation by air-layering

This study investigated the patterns of root growth and water uptake of maize (Zea mays L.) and cowpea (Vigna unguiculata (L.) Walp) grown in a mixture under greenhouse conditions. The plants were grown in root boxes for 5 weeks under 2 watering regimes; fully irrigated and water stress conditions, followed by a 5-day drying cycle imposed during the 6th week of growth. Water uptake patterns were analysed during the drying cycle. The two-dimensional distribution of the roots of both plants in the boxes was determined immediately at the end of the drying cycle. Under well-irrigated conditions, the roots of the component plants grew profusely into all sections of the root box and intermingled considerably. Water stress resulted in the decline of root growth of maize and cowpea but the root:shoot ratios of maize and cowpea were not affected, suggesting that there was no significant effect of water stress on root:shoot partitioning. However, water stress affected the biomass distribution between fine and coarse roots in cowpea. About 64% by weight of cowpea roots under water stress were coarse whereas as against 48% under well-irrigated conditions. Furthermore, water stress generally restricted the lateral extent of the roots of both maize and cowpea with a tendency of clumping together of the root systems and a reduced degree of intermingling. Thus, the extent of mixing of the root systems was apparently controlled by the availability of soil water. Water uptake from the well-irrigated soil in the root boxes was initially restricted to the sections directly below the base of each plant. Although roots of both plants were present in almost all sections of the root box, all the sections did not contribute simultaneously to water uptake by each plant. Water uptake was delayed from the middle intermingled zones. In effect, uptake patterns did not relate generally to the root distribution. The tendency was for the component plants to initially `avoid' water uptake from zones of intense intermingling or competition.     

缺磷胁迫下的小麦根系形态特征研究

研究了缺磷条件下不同基因型小麦（Triticum aestivum L.)苗期根系形态学适应特征,以明确环境因子对根系不同组分（根轴和侧根）生长发育调控作用的强度和根系形态与磷营养效率关系。在缺P环境中,小麦根轴数量和侧根长度明显减小,同化物向根部的分配比例增加,根轴长度、侧根数量和根系长度等均有显著提高。供试基因型小麦的根轴数量及其长度的差异在每个供磷水平及不同供磷水平之间均呈显著,说明这两种性状的差异是由基因型和环境因素共同决定的;而侧根特征的差异只在不同供磷水平间显著,表明侧根性状主要受环境因素的控制。对6种基因型小麦的研究表明,根轴数量、根轴长度、根生长角度和根系长度根角之间存在着显著的基因型差异。相关分析表明,小麦的相对产量与缺磷条件下的小麦苗期根系形态指标的交互作用之间具有显著的线性关系。这种关系说明根系形态性状可作为早期有效地筛选磷高效小麦品种的指标。     

水分亏缺下玉米根系ZmPIP1亚族基因的表达

在PEG-6000胁迫条件下,以微管蛋白基因为内参基因、水通道蛋白基因ZmPIP1-1和ZmPIP1-2为检测基因,采用半定量逆转录聚合酶链式反应（RT-PCR）体系检测它们在玉米根系中的表达情况。实验结果是：胁迫条件下,ZmPIP1-1的表达量在杂交F,代‘户单4号’（抗旱）和母本‘天四’（抗旱）根系中增多,它的表达量与品种的抗旱性呈正相关,并且胁迫不同时间段它的表达量有差异;而ZmPIP1-2在3个玉米品种的不同水分处理条件下,表达量均没有明显变化。这提示,水分胁迫条件下根系中某些种类的水通道蛋白基因的表达量增多,并且与品种的抗旱性有关;而另一些水通道蛋白基因的表达不受水分亏缺的影响。     

Field phenotyping of potato to assess root and shoot characteristics associated with drought tolerance

Aims

Potatoes are a globally important source of food whose production requires large inputs of fertiliser and water. Recent research has highlighted the importance of the root system in acquiring resources. Here measurements, previously generated by field phenotyping, tested the effect of root size on maintenance of yield under drought (drought tolerance).

Methods

Twelve potato genotypes, including genotypes with extremes of root size, were grown to maturity in the field under a rain shelter and either irrigated or subjected to drought. Soil moisture, canopy growth, carbon isotope discrimination and final yields were measured. Destructively harvested field phenotype data were used as explanatory variables in a general linear model (GLM) to investigate yield under conditions of drought or irrigation.

Results

Drought severely affected the small rooted genotype Pentland Dell but not the large rooted genotype Cara. More plantlets, longer and more numerous stolons and stolon roots were associated with drought tolerance. Previously measured carbon isotope discrimination did not correlate with the effect of drought.

Conclusions

These data suggest that in-field phenotyping can be used to identify useful characteristics when known genotypes are subjected to an environmental stress. Stolon root traits were associated with drought tolerance in potato and could be used to select genotypes with resilience to drought.     

Germination profiling of lentil genotypes subjected to salinity stress

• Salinity is one of the most severe environmental stresses, negatively affecting productivity of salt‐sensitive crop species. Given that germination is the most critical phase in the plant life cycle, the present study aimed to determine seed germination potential and associated traits under salt stress conditions as a simple approach to identify salt‐tolerant lentil genotypes.
• The genetic material consisted of six lentil genotypes whose adaptation to various agroclimatic conditions is not well elucidated. Salinity stress was applied by addition of NaCl at three different levels of stress, while non‐stressed plants were included as controls. Evaluation of tolerance was performed on the basis of germination percentage, seed water absorbance, root and shoot length, seedling water content, seedling vigour index and number of seedlings with an abnormal phenotype.
• Overall, our findings revealed that salinity stress substantially affects all traits associated with germination and early seedling growth, with the effect of salinity being dependent on the level of stress applied. It is noteworthy, however, that genotypes responded differently to the varying salinity levels. In this context, Samos proved the most salt‐tolerant genotype, indicating its possible use for cultivation under stress conditions.
• In conclusion, the determination of seed germination and early growth potential may be exploited as an efficient strategy to reveal genetic variation in lentil germplasm of unknown tolerance to salinity stress. This approach allows selection of desirable genotypes at early growth stages, thus enabling more efficient application of various breeding methods to achieve stress‐tolerant lentil genotypes.

     

Yield response to water deficit in an upland rice mapping population: associations among traits and genetic markers

A population of recombinant inbred rice lines from a cross between the upland japonica cultivar Azucena and the upland indica cultivar Bala was evaluated in a series of upland field experiments. Water stress was imposed during the reproductive stage by managed irrigation during the dry season, while control treatments were maintained in aerobic, well-irrigated conditions. Water deficit resulted in a yield reduction of 17 to 50%. The genetic correlation between stress and control yields was quite high when stress was mild, and the heritability of yield was similar in stress and control treatments across both years of this study. Genetic correlations between secondary traits such as leaf rolling and drying and yield under stress varied from high (leaf drying) to insignificant (leaf rolling). Lines with superior yield tended to have fewer panicles and larger grain size than the high-yielding parent, Bala, even though the panicle number was positively correlated with yield and the thousand-grain weight was not associated with yield for the population as a whole. Analysis of quantitative trait loci (QTLs) for yield and yield components allowed the identification of 31 regions associated with growth or yield components. Superior alleles came from either parent. Several of the regions identified had also been reported for root mass at depth or maximum root length in this population in other studies made under controlled environments, and for leaf drying (LD) in field studies. However, the direction of the effect of QTLs was not consistent, which indicates that there was not necessarily a causal relationship between these secondary traits and performance. We conclude that mapping populations can provide novel insights on the actual relationships between yield components and secondary traits in stress and control environments and can allow identification of significant QTLs for yield components under drought stress.Abbreviations DAS Days after sowing - GPP Grains per panicle - QTL Quantitative trait locus - RWC Relative water content - SPP Spikelets per panicle - TGW Thousand-grain weight - VPD Vapor pressure deficit