Arguments for the use of physiological criteria for improving the salt tolerance in crops

Efforts to develop new crop varieties with improved salt tolerance have been intensified over the past 15–20 years. Despite the existence of genetic variation for salt tolerance within species, and many methods available for expanding the source of genetic variation, there is only a limited number of varieties that have been developed with improved tolerance. These new varieties have all been based upon selection for agronomic characters such as yield or survival in saline conditions. That is, based upon characters that integrate the various physiological mechanisms responsible for tolerance. Yet over the same time period, knowledge of physiological salt responses has increased substantially.Selection and breeding to increase salt tolerance might be more successful if selection is based directly on the physiological mechanisms or characters conferring tolerance. Basic questions associated with using physiological selection criteria are discussed in the paper. These are centred around the need for genetic variation, the importance of the targeted mechanism, the ease of detection of the physiological mechanism (including the analytical requirements) and the breeding strategy. Many mechanisms, including ion exclusion, ion accumulation, compatible solute production and osmotic adjustment have been associated with genetic variation in salt tolerance. Yet their successful use in improving salt tolerance, via physiological selection criteria, is largely non-existent. Consideration is given to the role of physiological criteria in the short and long term in improving salt tolerance. In several glycophytic species, particularly legumes, physiological selection based on ion exclusion from the shoots shows promise. Recent results for white clover indicate the potential for using a broad physiological selection criterion of restricted Cl accumulation in the shoots, with scope for future refinement based upon the specific physiological characters that combined result in ion exclusion.     

Assessing the Suitability of Various Physiological Traits to Screen Wheat Genotypes for Salt Tolerance

Success of improving the salt tolerance of genotypes requires effective and reliable screening traits in breeding programs.The objective was to assess the suitability of various physiological traits to screen wheat genotypes for salt tolerance.Thirteen wheat genotypes from Egypt,Germany,Australia and India were grown in soil with two salinity levels(control and150mmol/L NaCl)in a greenhouse.The physiological traits(ion contents in leaves and stems,i.e.Na~ ,Cl~-,K~ ,Ca~(2 )),theratios of K~ /Na~ and Ca~ /Na~ in the leaves and stems,net photosynthesis rate,stomatal conductance,transpiration rate,chlorophyll content(SPAD value),and leaf water relations,were measured at different growth stages.The physiologicaltraits except for Na~ and Cl~-in stems and the leaf transpiration rate at 150 mmol/L NaCI showed a significant genotypicvariation,indicating that the traits that have a significant genotypic variation may be possibly used as screening criteria.According to the analysis of linear regression of the scores of the physiological traits against those of grain yield,however,the physiological traits of Ca~(2 )and Ca~(2 )/Na~ at 45 d and final harvest with the greatest genotypic variation were ranked atthe top.From a practical and economic point of view,SPAD value should be considered to be used as screening criteriaand/or there is a need to develop a quick and practical approach to determine Ca~(2 )in plant tissues.     

Assessing the Suitability of Various Physiological Traits to Screen Wheat Genotypes for Salt Tolerance

Success of improving the salt tolerance of genotypes requires effective and reliable screening traits in breeding programs. The objective was to assess the suitability of various physiological traits to screen wheat genotypes for salt tolerance. Thirteen wheat genotypes from Egypt, Germany, Australia and India were grown in soil with two salinity levels （control and 150 mmol/L NaCI） in a greenhouse. The physiological traits （ion contents in leaves and stems, i.e. Na^＋, Cl^-, K^＋, Ca^2＋）, the ratios of K^＋/Na^＋ and Ca^＋/Na^＋ in the leaves and stems, net photosynthesis rate, stomatal conductance, transpiration rate, chlorophyll content （SPAD value）, and leaf water relations, were measured at different growth stages. The physiological traits except for Na^＋ and Cl^- in stems and the leaf transpiration rate at 150 mmol/L NaCI showed a significant genotypic variation, indicating that the traits that have a significant genotypic variation may be possibly used as screening criteria. According to the analysis of linear regression of the scores of the physiological traits against those of grain yield, however, the physiological traits of Ca^2＋ and Ca2^＋/Na^＋ at 45 d and final harvest with the greatest genotypic variation were ranked at the top. From a practical and economic point of view, SPAD value should be considered to be used as screening criteria and/or there is a need to develop a quick and practical approach to determine Ca^2＋ in plant tissues.     

Halophyte Improvement for a Salinized World

It is more important to improve the salt tolerance of crops in a salinized world with the situations of increasing populations, declining crop yields, and a decrease in agricultural lands. Attempts to produce salt-tolerant crops have involved the manipulation of existing crops through conventional breeding, genetic engineering and marker-assisted selection (MAS). However, these have, so far, not produced lines growing on highly saline water. Hence, the domestication of wild halophytes as crops appears to be a feasible way to develop agriculture in highly saline environments. In this review, at first, the assessment criteria of salt tolerance for halophytes are discussed. The traditional criteria for the classification of salinity in crops are less applicable to strong halophytes with cubic growth curves at higher salinities. Thus, realistic assessment criteria for halophytes should be evaluated at low and high salinity levels. Moreover, absolute growth rather than relative growth in fields during a crop's life cycle should be considered. Secondly, the use of metabolomics to understand the mechanisms by which halophytes respond to salt tolerance is highlighted as is the potential for metabolomics-assisted breeding of this group of plants. Metabolomics provides a better understanding of the changes in cellular metabolism induced by salt stress. Identification of metabolic quantitative trait loci (QTL) associated with salt tolerance might provide a new method to aid the selection of halophyte improvement. Thirdly, the identification of germplasm-regression-combined (GRC) marker-trait association and its potential to identifying markers associated with salt tolerance is outlined. Results of MAS/linkage map-QTL have been modest because of the absence of QTLs with tight linkage, the non-availability of mapping populations and the substantial time needed to develop such populations. To overcome these limitations, identification by GRC-based marker-trait association has been successfully applied to many plant traits, including salt tolerance. Finally, we provide a prospect on the challenges and opportunities for halophyte improvement, especially in the integration of metabolomics- and GRC-marker-assisted selection towards new or unstudied halophyte breeding, for which no other genetic information, such as linkage maps and QTL, are available.     

马铃薯耐盐性的研究进展

土壤盐渍化已是影响全球农业生产和土地资源可持续利用的主要问题之一。近些年马铃薯的种植面积不断扩大,比重逐年上升,而马铃薯却是一种对盐中度敏感的作物,盐渍化对其造成严重的危害,因此研究马铃薯的耐盐性势在必行。本文对国内外马铃薯耐盐性筛选、生长发育、生理生化特性、耐盐途径、转基因耐盐马铃薯的研究及其二倍体马铃薯种质资源的利用和展望进行了综述。     

A review on transporters in salt tolerant mangroves

Key message

The role of transporters in imparting salt tolerance to mangroves is not yet understood. Identification of the role of transporters in halophytes is promising, as far as the development of genetically engineered salt tolerant crops is concerned.

Abstract

Mangroves are models for stress tolerance and they provide a reservoir for some of the novel genes and proteins, involved in salt tolerance. Biochemical or physiological mechanisms contribute to salt tolerance depending on variations in the environment. A great deal of research on salinity tolerance of plants, probes into water relations, photosynthesis, and accumulation of various in-organic ions and organic metabolites. The ability of the plant to react to high salinity depends on the genes that are expressed during stress. The mechanism of salinity tolerance becomes complicated when the responses of plants varies with salinity and environmental conditions. During the onset and development of salt stress within a plant, major processes such as photosynthesis, protein synthesis and lipid metabolisms are affected. The present review attempts to dissect out the role of transporters in salt tolerance of mangroves.     

Improving Salinity Tolerance in Cereals

Cereals are grown in almost every region of the world and are exposed to a variety of environmental stresses that severely affect their growth and grain yield. Of various abiotic stresses, salinity is one of the more significant threats to cereal crops. To ensure food security, there is a need to adopt strategies to overcome this specific threat. Undoubtedly, plant scientists have been exploiting a variety of approaches to achieve enhanced crop productivity on salt affected soils. Of the various biotic approaches, conventional breeding, marker-assisted selection and genetic engineering to develop salt-tolerant lines/cultivars of cereals all seem plausible. Some success stories have been reported for improvement in salt tolerance of wheat and rice, but are scarce for other cereals. A number of barriers to the development of salt-tolerant cultivars/lines have been identified and include a lack of knowledge about the genetics of crops, their physiological and biochemical behavior, wide variation in environmental conditions, and the complex polygenic nature of the salt tolerance character. This review focuses on how improvements have been made in salt tolerance in cereals through different biotic means, such as conventional breeding, marker assisted selection and genetic engineering.     

Hyperspectral imaging combined with machine learning as a tool to obtain high‐throughput plant salt‐stress phenotyping

The rapid selection of salinity‐tolerant crops to increase food production in salinized lands is important for sustainable agriculture. Recently, high‐throughput plant phenotyping technologies have been adopted that use plant morphological and physiological measurements in a non‐destructive manner to accelerate plant breeding processes. Here, a hyperspectral imaging (HSI) technique was implemented to monitor the plant phenotypes of 13 okra (Abelmoschus esculentus L.) genotypes after 2 and 7 days of salt treatment. Physiological and biochemical traits, such as fresh weight, SPAD, elemental contents and photosynthesis‐related parameters, which require laborious, time‐consuming measurements, were also investigated. Traditional laboratory‐based methods indicated the diverse performance levels of different okra genotypes in response to salinity stress. We introduced improved plant and leaf segmentation approaches to RGB images extracted from HSI imaging based on deep learning. The state‐of‐the‐art performance of the deep‐learning approach for segmentation resulted in an intersection over union score of 0.94 for plant segmentation and a symmetric best dice score of 85.4 for leaf segmentation. Moreover, deleterious effects of salinity affected the physiological and biochemical processes of okra, which resulted in substantial changes in the spectral information. Four sample predictions were constructed based on the spectral data, with correlation coefficients of 0.835, 0.704, 0.609 and 0.588 for SPAD, sodium concentration, photosynthetic rate and transpiration rate, respectively. The results confirmed the usefulness of high‐throughput phenotyping for studying plant salinity stress using a combination of HSI and deep‐learning approaches.     

Salinity Stress in Wheat: Effects,Mechanisms and Management Strategies

Salinity stress is a major threat to global food production and its intensity is continuously increasing because of anthropogenic activities. Wheat is a staple food and a source of carbohydrates and calories for the majority of people across the globe. However, wheat productivity is adversely affected by salt stress, which is associated with a reduction in germination, growth, altered reproductive behavior and enzymatic activity, disrupted photosynthesis, hormonal imbalance, oxidative stress, and yield reductions. Thus, a better understanding of wheat (plant) behavior to salinity stress has essential implications to devise counter and alleviation measures to cope with salt stress. Different approaches including the selection of suitable cultivars, conventional breeding, and molecular techniques can be used for facing salt stress tolerance. However, these techniques are tedious, costly, and labor-intensive. Management practices are still helpful to improve the wheat performance under salinity stress. Use of arbuscular mycorrhizal fungi, plant growth-promoting rhizobacteria, and exogenous application of phytohormones, seed priming, and nutrient management are important tools to improve wheat performance under salinity stress. In this paper, we discussed the effect of salinity stress on the wheat crop, possible mechanisms to deal with salinity stress, and management options to improve wheat performance under salinity conditions.