Intrinsic water use efficiency at the pollination stage as a parameter for drought tolerance selection in Phaseolus vulgaris

Genotype differences in gas exchange during ontogeny and water stress responses at the vegetative and pollination stages were evaluated in four lines of Phaseolus vulgaris L. In the cultivar Carioca, net photosynthetic rate ( A ) and stomatal conductance ( g _s) were lower at the vegetative stage (20 days after sowing [DAS]) and maximum at the pollination stage (39 DAS), followed by a decrease at the flowering stage (46 DAS) and a dramatic fall at the grain-filling stage (60 DAS). Among the lines studied, the stomata of A320 closed faster than those of the other lines when water stress occurred at 20 or 39 DAS. The cultivar SC-90298823 had greater stomatal conductance at 39 DAS and a higher photosynthetic level than the other lines. Stomata of Ouro negro remained partially open during the water stress at the pollination stage (39 DAS) and supported a positive net photosynthetic rate ( A ). Differences were also found between lines in intrinsic water use efficiency (IWUE) at 39 DAS, but not at 20 DAS. The possibility of using IWUE at the pollination stage is discussed, in view of its use as one of the parameters for a drought tolerance breeding program in bean lines.     

Gas exchange and leaf water relations in two peanut cultivars of different drought tolerance

In summer seasons of 1991 and 1992 the gas exchange and leaf water relations were analysed in two peanut cultivars: drought tolerant cv. GG 2 (DT) and drought sensitive cv. JL 24 (DS). Soil moisture stress was imposed by withholding irrigation at pod development phase. The decrease in photosynthesis (P_N) under stress was associated with a decrease in stomatal conductance (g_s) and relative water content (RWC). The P_N and RWC were significantly higher under stress in DT than DS. On relief of stress the g_s and RWC recovered more quickly in DT than DS. The maintenance of higher RWC (>80 %), g_s and P_N under stress appears to be imparting drought tolerance in peanut.     

Leaf water relations and solute accumulation in two grain sorghum lines exhibiting contrasting drought tolerance

A drought-tolerant grain sorghum line (K886) maintained significantlyhigher relative water content (RWC), osmotic potential at fullturgor (     

Some important physiological selection criteria for salt tolerance in plants

Undoubtedly, plant breeders have made a significant achievement in the past few years, improving salinity tolerance in a number of potential crops using artificial selection and conventional breeding approaches, although molecular biology approaches are currently being intensively pursued for achieving this goal. However, most of the selection procedures used so far, were based merely on differences in agronomic characters. Agronomic characters represent the combined genetic and environmental effects on plant growth, and include the integration of the physiological phenomena conferring salinity tolerance. In fact, physiological criteria are able to supply more reliable information than agronomic characters. Although there are large numbers of reports in the literature mainly dealing with water relations, photosynthesis, and accumulation of various inorganic ions and organic metabolites in individual crops, there is little information available on the use of these attributes as selection criteria for improving salt tolerance through selection and breeding programs. In this review, the major adaptive components of salt tolerance such as osmotic adjustment, photosynthesis, water relations and ion relations are reviewed. In view of the complexity of salt tolerance and its great variation at intra-specific and inter-specific levels, it is difficult to identify single criteria, which could be used as effective selection targets. Rather it is most meaningful if physiological and biochemical indicators for individual species are determined rather than generic indicators.     

Defining selection criteria to improve yield under drought

The many selection criteria that have been proposed to increase drought resistance of our crops have had little, if any, impact on improving crop yields in dry environments. There are several likely reasons for this lack of success. Some of these are: (i) criteria proposed have been related more to survival mechanisms under drought than to productivity, (ii) criteria are inappropriate to the target environment, and (iii) criteria are temporal and are therefore likely to have minimal impact on growth and yield over the entire lifecycle. Another important reason is that breeders have not been convinced the proposed criteria will be successful as they are too difficult to measure. On the other hand, empirical breeding programmes to improve yield under drought have been successful. Surprisingly, some of the greatest successes have been achieved by breeding in environments where water is non-limiting. This paper reviews breeding approaches to improve yield under drought. It focuses on critical factors that must be considered to identify likely plant attributes that can be targeted. These factors, their link with yield, the nature of the target environment, the level of organisation where the trait is expressed are discussed. Three quite different examples are given to emphasize the above considerations and which show substantial promise in targeting traits to improve yield under drought. They are drought at flowering, improving transpiration efficiency and improving early leaf area development.     

Efficiency of physiological trait-based and empirical selection approaches for drought tolerance in groundnut

Drought is the major abiotic constraint affecting groundnut productivity and quality worldwide. Most breeding programmes in groundnut follow an empirical approach to drought resistance breeding, largely based on kernel yield and traits of local adaptation, resulting in slow progress. Recent advances in the use of easily measurable surrogates for complex physiological traits associated with drought tolerance encouraged breeders to integrate these in their selection schemes. However, there has been no direct comparison of the relative efficiency of a physiological trait‐based selection approach (Tr) vis‐à‐vis an empirical approach (E) to ascertain the benefits of the former. The genetic material used in the present study originated from three common crosses and one institute‐specific cross from four collaborating institutes in India (total seven crosses). Each institute contributed six genotypes and each followed both the Tr and E selection approaches in each cross. The field trial of all selections, consisting of 192 genotypes (96 each Tr and E selections), was grown in 2000/2001 in a 4 × 48 alpha design in 12 season × location environments in India. The selection efficiency of Tr relative to E, RE_Tr, was estimated using the genetic concept of response to selection. Based on all the 12 environments, the two selection methods performed more or less similarly (RE_Tr= 1.045). When the 12 environments were grouped into rainy season and post‐rainy season, the relative response to selection in Tr method was higher in the rainy than in the post‐rainy season (RE_Tr= 1.220 vs 0.657) due to a higher genetic variance, lower G × E, and high h². When the 12 environments were classified into four clusters based on plant extractable soil‐water availability, the selection method Tr was superior to E in three of the four clusters (RE_Tr= 1.495, 0.612, 1.308, and 1.144) due to an increase in genetic variance and h² under Tr in clustered environments. Although the crosses exhibited significant differences for kernel yield, the two methods of selection did not interact significantly with crosses. Both methods contributed more or less equally to the 10 highest‐yielding selections (six for E and four for Tr). The six E selections had a higher kernel yield, higher transpiration (T), and nearly equal transpiration efficiency (TE) and harvest index (HI) relative to four Tr selections. The yield advantage in E selections came largely from greater T, which would likely not be an advantage in water‐deficient environments. From the results of these multi‐environment studies, it is evident that Tr method did not show a consistent superiority over E method of drought resistance breeding in producing a higher kernel yield in groundnut. Nonetheless, the integration of physiological traits (or their surrogates) in the selection scheme would be advantageous in selecting genotypes which are more efficient water utilisers or partitioners of photosynthates into economic yield. New biotechnological tools are being explored to increase efficiency of physiological trait‐based drought resistance breeding in groundnut.     

Plant drought tolerance assessment for re-vegetation in heterogeneous karst landscapes of southwestern China

A better understanding of plant water relations is needed for evaluating the suitability of plant species to site-specific reforestation programs in the heterogeneous karst landscapes in southwestern China that are characterized by temporary water deficit. During both wet and dry periods, leaf water potentials of 65 plant species from five different growth forms were studied at three representative sites (forest, shrubland and grassland), to compare their adaptive strategies against water stress and assess their suitability for reforestation programs. Herbs showed the highest predawn and midday water potentials and smallest diurnal ranges of water potential values at all the three sites, indicating that they follow water stress avoidance strategies. During the dry period, evergreen shrubs showed low water potentials, the largest diurnal ranges and highest soluble sugar contents. This indicates that they have a tolerance strategy responding to water stress. Deciduous shrubs and trees still showed relatively large diurnal ranges of water potential values and high soluble sugar contents, and did not shed leaves when experiencing the lowest midday water potentials during the dry period. They shed leaves only later in the dry winter period when even more serious drought was experienced. Their strategies seem to include both tolerance and avoidance mechanisms. Evergreen trees revealed relatively low water potentials with smallest diurnal range water potentials at the shrubland site, especially during the dry period, which indicated their weak ability to tolerate severe water stress. Increasing degradation of the vegetation clearly impacts negatively plant water relations. Using the ranges of leaf water potentials, the relative suitability of the plants for reforestation could be evaluated.     

Mechanisms of selection for drought stress tolerance and avoidance in Impatiens capensis (Balsaminaceae)

For longer lived annual plants, high water-use efficiency (WUE) and low stomatal conductance are hypothesized to confer a fitness advantage under drought stress. To directly test the adaptive significance of WUE and stomatal conductance under drought stress, inbred lines of Impatiens capensis were grown in two field environments (watered and not-watered), in a year of unusual early-season drought. In contrast to the results from a previous study of late-season drought in the same system, selection was detected for lower WUE, increased stomatal conductance, and early flowering time. These findings suggest that early-season drought conditions may select for drought avoidance traits such as low WUE and early reproduction, whereas later drought selects for tolerance traits such as high WUE.     

园艺植物水分胁迫生理及耐旱机制研究进展

概述了园艺植物在水分胁迫下的生理生化,分子反应及耐旱机制研究进展,并指出尚需进一步研究的问题。     

Polyphenols as potential indicators for drought tolerance in tea (Camellia sinensis L.)

Plant polyphenols have gained prominence in quality of plant products and in human health. An experiment was conducted to determine the association of tea polyphenols with water stress and their suitability as indicators for drought tolerance. The experiment was conducted in a 'rain-out' shelter, and consisted of six tea clones (BBK 35, TRFK 6/8, TRFK 76/1, TRFK 395/2, TRFK 31/30, and TRFK 311/287) and four levels of soil water contents (38, 30, 22, and 14% v/v), which were maintained for a period of 12 weeks. The treatments were arranged in a completely randomized design and replicated three times. Plant growth was monitored over 6 weeks, and a water stress index was calculated to determine water-stress tolerant clones. Total polyphenols in tea shoots was analyzed and a regression analysis done. The results indicate that declining soil water content (SWC) reduced both growth and content of polyphenols in tea. Tolerant clones maintained a high polyphenol content at low SWC, and also showed less fluctuation in phenolics when subjected to changes in SWC. There was significant (P<0.001) correlation of total polyphenol content with shoot growth and WSI of tea, and a linear relationship (r2=0.97) between SWC for tea and both, water stress index and shoot polyphenol content. We report that there is a potential to use polyphenols as indicators for selection of drought-tolerant tea cultivars.     

Plant water relations at elevated CO2 -- implications for water-limited environments

Long-term exposure of plants to elevated [CO2] leads to a number of growth and physiological effects, many of which are interpreted in the context of ameliorating the negative impacts of drought. However, despite considerable study, a clear picture in terms of the influence of elevated [CO2] on plant water relations and the role that these effects play in determining the response of plants to elevated [CO2] under water-limited conditions has been slow to emerge. In this paper, four areas of research are examined that represent critical, yet uncertain, themes related to the response of plants to elevated [CO2] and drought. These include (1) fine-root proliferation and implications for whole-plant water uptake; (2) enhanced water-use efficiency and consequences for drought tolerance; (3) reductions in stomatal conductance and impacts on leaf water potential; and (4) solute accumulation, osmotic adjustment and dehydration tolerance of leaves. A survey of the literature indicates that the growth of plants at elevated [CO2] can lead to conditions whereby plants maintain higher (less negative) leaf water potentials. The mechanisms that contribute to this effect are not fully known, although CO2-induced reductions in stomatal conductance, increases in whole-plant hydraulic conductance and osmotic adjustment may be important. Less understood are the interactive effects of elevated [CO2] and drought on fine-root production and water-use efficiency, and the contribution of these processes to plant growth in water-limited environments. Increases in water-use efficiency and reductions in water use can contribute to enhanced soil water content under elevated [CO2]. Herbaceous crops and grasslands are most responsive in this regard. The conservation of soil water at elevated [CO2] in other systems has been less studied, but in terms of maintaining growth or carbon gain during drought, the benefits of CO2-induced improvements in soil water content appear relatively minor. Nonetheless, because even small effects of elevated [CO2] on plant and soil water relations can have important implications for ecosystems, we conclude that this area of research deserves continued investigation. Future studies that focus on cellular mechanisms of plant response to elevated [CO2] and drought are needed, as are whole-plant investigations that emphasize the integration of processes throughout the soil--plant--atmosphere continuum. We suggest that the hydraulic principles that govern water transport provide an integrating framework that would allow CO2-induced changes in stomatal conductance, leaf water potential, root growth and other processes to be uniquely evaluated within the context of whole-plant hydraulic conductance and water transport efficiency.     

Important Plant Areas: revised selection criteria for a global approach to plant conservation

Despite the severe threats to plant habitats and high levels of extinction risk for plant species in many parts of the world, plant conservation priorities are often poorly represented in national and global frameworks because of a lack of data in an accessible and consistent format to inform conservation decision making. The Important Plant Areas (IPAs) criteria system offers a pragmatic yet scientifically rigorous means of delivering these datasets, enabling informed national- or regional-scale conservation prioritisation, and contributing significantly towards global prioritisation systems including the International Union for Conservation of Nature Key Biodiversity Areas (KBAs) Standard. In this paper, we review the IPA rationale and progress on IPA identification to date, including the perceived limitations of the process and how these may be overcome. We then present a revised set of criteria for use globally, developed through the combined experiences of IPA identification over the past decade and a half and through a recent global consultation process. An overview of how the revised IPA criteria can work alongside the newly published KBA Standard is also provided. IPA criteria are based around a sound, scientific, global framework which acknowledges the practical problems of gathering plant and habitat data in many regions of the world, and recognises the role of peer reviewed expert opinion in the selection process. National and sub-national engagement in IPA identification is essential, providing a primary route towards long term conservation of key sites for plant diversity. The IPA criteria can be applied to the conservation of all organism groups within the plant and fungal kingdoms.     

Plant water relations and control of cell elongation at low water potentials

Recent developments in water status measurement techniques using the psychrometer, the pressure probe, the osmometer and pressure chamber are reviewed, and the process of cell elongation from the viewpoint of plant-water relations is discussed for plants subjected to various environmental stress conditions. Under water-deficient conditions, cell elongation of higher plants can be inhibited by interruption of water flow from the xylem to the surrounding elongating cells. The process of growth inhibition at low water potentials could be reversed by increasing the xylem water potential by means of pressure application in the root region, allowing water to flow from the xylem to the surrounding cells. This finding confirmed that a water potential field associated with growth process,i.e., the growth-induced water potential, is an important regulating factor for cell elongation other than metabolic factors. The concept of the growth-induced water potential was found to be applicable for growth retardation caused by cold stress, heat stress, nutrient deficiency and salinity stress conditions. In the present review, the fact that the cell elongation rate is primarily associated with how much water can be absorbed by elongating cells under water-deficiency, nutrient deficiency, salt stress, cold stress and heat stress conditions is suggested.     

Water relations and drought tolerance of young African tamarind (Tamarindus indica L.) trees

Tamarindus indica L. is an important multipurpose tree, indigenous to Africa, now introduced worldwide and known for its drought tolerance. The effects of drought on tamarinds, especially at seedling stage, are hardly investigated. However, this information is important for its conservation and domestication. In a growth chamber experiment we investigated the water relations of African tamarind seedlings under short-term soil drought stress. Initially tamarind seedlings can be considered as drought-tolerant at the expense of internal water storage reserves as they keep on transpiring (sap flow) and growing (diameter fluctuations). They finally spent 20% of their stem and root water storage reserves and experienced stem water potentials near − 3 MPa. Therefore, they can be classified as anisohydric. Their risk-taking behavior led to a high rate of seedling mortality (50%) because of whole plant hydraulic failure. They were not hydraulically efficient and they possessed low water storage capacity in stem and root (45%) due to high tissue density. When re-irrigated, remaining seedlings recovered slowly as a consequence of non-stomatal limitations and partial shoot dieback. Although tamarind seedlings show traits related to drought tolerance, we suggest that the species contains some water saving mechanisms. Contrasts with the co-occurring water-conserving tree species baobab (Adansonia digitata L.) are also discussed.