Salinity is an ever-present threat to crop yields, especially in countries where irrigation is an essential aid to agriculture. Although the tolerance of saline conditions by plants is variable, crop species are generally intolerant of one-third of the concentration of salts found in seawater. Attempts to improve the salt tolerance of crops through conventional breeding programmes have met with very limited success, due to the complexity of the trait: salt tolerance is complex genetically and physiologically. Tolerance often shows the characteristics of a multigenic trait, with quantitative trait loci (QTLs) associated with tolerance identified in barley, citrus, rice, and tomato and with ion transport under saline conditions in barley, citrus and rice. Physiologically salt tolerance is also complex, with halophytes and less tolerant plants showing a wide range of adaptations. Attempts to enhance tolerance have involved conventional breeding programmes, the use of in vitro selection, pooling physiological traits, interspecific hybridization, using halophytes as alternative crops, the use of marker-aided selection, and the use of transgenic plants. It is surprising that, in spite of the complexity of salt tolerance, there are commonly claims in the literature that the transfer of a single or a few genes can increase the tolerance of plants to saline conditions. Evaluation of such claims reveals that, of the 68 papers produced between 1993 and early 2003, only 19 report quantitative estimates of plant growth. Of these, four papers contain quantitative data on the response of transformants and wild-type of six species without and with salinity applied in an appropriate manner. About half of all the papers report data on experiments conducted under conditions where there is little or no transpiration: such experiments may provide insights into components of tolerance, but are not grounds for claims of enhanced tolerance at the whole plant level. Whether enhanced tolerance, where properly established, is due to the chance alteration of a factor that is limiting in a complex chain or an effect on signalling remains to be elucidated. After ten years of research using transgenic plants to alter salt tolerance, the value of this approach has yet to be established in the field.

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Improving crop salt tolerance

Tomato and salinity

The halobacteria are the only organisms that are tolerant of salinity at the molecular level. All other bacteria, all fungi, all plants, and all animals avoid the need for salt tolerance for most of their macromolecules by maintaining defined and conserved conditions in the cytoplasm. These conditions favour potassium over sodium, the limitation of total inorganic ion activity, and the supplementation of this where necessary with organic solutes which are metabolically neutral osmolytes that may also be osmoprotectant. The salt tolerance of an organism depends upon the range of external salinity over which it is able to sustain these conditions in the cytoplasm. There is substantial and increasing knowledge of the molecular biology and molecular genetics of the processes of ion and organic solute transport, solute synthesis, and compart-mentation that underpin cell-based tolerance. Much of recent research focuses on the identification of genes and gene products that affect cell-based tolerance, commonly derived from single-cell models. There is commonly the implicit or explicit assumption that incorporation of these genes will benefit the salt tolerance of food crop species. While this essential experimental approach is giving enormous insight there should not be rash or premature expectations. The unique and overriding consideration for the salinity tolerance of terrestrial plants is the net flux of water due to transpiration and so resides at a higher level of organization. Processes that are advantageous to a single cell in an aqueous medium may be lethal to a cell in a leaf in the air. The likely impact of single structural-gene changes in ion and solute transport upon co-ordinated plant response is probably overestimated, and recent views consider regulatory processes and multiple gene transfers. While the technical ability for plant transformation increases daily, the practicality of using transgenic plants in complex breeding programmes seems rarely to be given enough thought. If intervention at the molecular level is to lead to salt-tolerant crop plants then it will be essential to view this in the contexts of whole plants and of plant breeding. Recent indications that a relatively small number of quantitative trait loci (QTL) may govern complex physiological characters offer the most hope for the future.

Molecular biology of salt tolerance in the context of whole-plant physiology

In this paper, a number of strategies to overcome the deleterious effects of salinity on plants will be reviewed; these strategies include using molecular markers and genetic transformation as tools to develop salinity-tolerant genotypes, and some cultural techniques. For more than 12 years, QTL analysis has been attempted in order to understand the genetics of salt tolerance and to deal with component traits in breeding programmes. Despite innovations like better marker systems and improved genetic mapping strategies, the success of marker-assisted selection has been very limited because, in part, of inadequate experimental design. Since salinity is variable in time and space, experimental design must allow the study of genotype x environment interaction. Genetic transformation could become a powerful tool in plant breeding, but the growing knowledge from plant physiology must be integrated with molecular breeding techniques. It has been shown that the expression of several transgenes promotes a higher level of salt tolerance in some species. Despite this promising result, the development of a salt-tolerant cultivar by way of transgenesis has still not been achieved. Future directions in order to overcome the present limitations are proposed. Three cultural techniques have proved useful in tomato to overcome, in part, the effects of salinity: treatment of seedlings with drought or NaCl ameliorates the adaptation of adult plants to salinity; mist applied to tomato plants grown in Mediterranean conditions improves vegetative growth and yield in saline conditions; and grafting tomato cultivars onto appropriate rootstocks could reduce the effects of salinity.

Increasing salt tolerance in the tomato

Silicon alleviates the deleterious salt effect on tomato plant growth by improving plant water status

Attempts to enhance the salt-tolerance of the cultivated tomato using the tolerance of related wild species of Lycopersicon Mill. have been unsuccessful commercially. An alternative approach is to attempt to accumulate physiological characters that contribute to tolerance within a genotype. We have investigated the relationships between tolerance and certain physiological characteristics in accessions of L. esculentum Mill., L. cheesmanii (Hook) C. H. Mull., L. pennellii (Correll) D'Arcy, L. peruvianum (L.) Mill. and L. pimpinellifolium Mill, with this aim in view. At a salinity equivalent to 40 % of that in an artificial seawater, the growth of young plants was ranked both in absolute terms (fresh weight, dry weight or the growth in area of a leaf that developed during the salt treatment) and relative to growth in a salinity 2 % of that of seawater. In absolute terms, two cultivars of L. esculentum ('Moneymaker' and 'Edkawy') performed about as well as L. pimpinellifolium and better than L. pennellii and L. cheesmanii: L. peruvianum showed the poorest tolerance. In relative terms, L. pimpinellifolium remained the most tolerant of the species, and L. peruvianum the most sensitive. L. cheesmanii was little more tolerant than L. peruvianum. The five species differed in their succulence and the Na: K ratio in leaves that developed during the treatment with salinity. Succulence was higher at low salinity in L. pennellii and L. cheesmanii than in the other species, but increased less on salinization. L. pimpinellifolium showed the greatest increase in succulence on salinization, a consequence of its low water content in 2 % seawater. Sodium concentrations increased dramatically in all five species on salinization: L. pimpinellifolium had the highest and L. esculentum the lowest values. L. peruvianum was notable for maintaining a Na:K ratio of less than 3 throughout the period of the experiment.

Selection of donors for salt-tolerance in tomato using physiological traits

Sustainable strategies for irrigation in salt-prone Mediterranean: SALTMED

Variability for some physiological characters affecting salt tolerance in tomato

A major reason for the drought susceptibility of cultivated rice is the inability to regulate water loss as effectively as other cereals. We report studies of growth and gas exchange under well-watered and water-limiting conditions on selected species of Oryza and seek to relate this physiological information to the ecology of the genus. The immediate ancestor of the Oryza sativa cultigen is native to swamps and marshes, and wet habitats are typical for the genus as a whole. However, the ecological range of some species does extend to habitats that dry out seasonally. In all species studied growth was reduced by water deficit. In all species studied leaves had small absolute water contents and began to roll at a relative water content above 90%. However, there were species differences in leaf rolling and the response of stomatal conductance to an increase in vapour pressure deficit. Following re-watering, there were persistent reductions in stomatal conductance in most of the species tested, but the assimilation rate was not reduced in all of these cases. Where there was a persistant reduction in assimilation rate, there was also a reduction in carboxylation efficiency. It was a frequent observation that plants had a stomatal conductance greater than expected for their carbon assimilation rate; that is in the range where substantial changes in conductance have little effect upon photosynthesis. It is suggested that a reason for this may be the cooling of leaves which have a small thermal capacity in environments which often combine high temperature, humidity and irradiance. Large conductances combined with small water contents may be no disadvantage in the natural habitats of Oryza, but provide some reasons for the poor regulation of water loss in cultivated rice. Although there were significant differences in gas exchange amongst species the advantages that were observed over O. sativa were not of a magnitude likely to justify wide hybridisation. This implies that improvement in the drought resistance of rice is more likely to come from increasing water acquisition than from decreasing water loss.

Gas exchange, water loss and biomass production in rice and wild Oryza species in well-watered and water-limiting growth conditions

A field experiment was conducted in the middle area of the Nile Delta, Egypt, to study the effect of different salinities of irrigation water (055, 4 or 6 dS m(-1)) on yield, yield components and fruit quality of 27 tomato genotypes The results showed that salinity at 4 and 6 dS m(-1) decreased total yield, marketable yield, number of fruits and average fruit weight in all genotypes studied Cultivars "La Laguna" followed by "RETBA" (both known to be salt tolerant) showed the least reduction in yield under salinity treatments Cultivars "Floradade" and "Edkawy" and hybrid "Baraka" also showed low yield reductions This implies that such genotypes were relatively salt tolerant It was also observed that the reduction in yield in the salt tolerant genotypes, was mainly due to the reduction in fruit weight rather than fruit number Cultivar "Super Strain B" showed the highest reduction in yield Hybrids "Early Rock", "Shiva (95558)", "Confidence" and "HZ 8704" as well as cultivar "Pacmour", showed severe yield reduction under salinity conditions particularly at 6 dS m(-1) This suggests that these genotypes were salt sensitive However, irrigation with saline water up to 6 dS m(-1) increased fruit total soluble solids (TSS), vitamin C, and dry matter contents in all genotypes Fruit acidity was also enhanced by salinity although 4 dS m(-1) level gave the most acid fruits Salinity had no significant effect on fruit firmness and storage ability

J. Cuartero

Papers

Selection of donors for salt-tolerance in tomato using physiological traits

Sustainable strategies for irrigation in salt-prone Mediterranean: SALTMED

Variability for some physiological characters affecting salt tolerance in tomato

Gas exchange, water loss and biomass production in rice and wild Oryza species in well-watered and water-limiting growth conditions

Effect of irrigation water salinity on yield and fruit quality of tomato