The physiological and molecular mechanisms of tolerance to osmotic and ionic components of salinity stress are reviewed at the cellular, organ, and whole-plant level. Plant growth responds to salinity in two phases: a rapid, osmotic phase that inhibits growth of young leaves, and a slower, ionic phase that accelerates senescence of mature leaves. Plant adaptations to salinity are of three distinct types: osmotic stress tolerance, Na + or Cl − exclusion, and the tolerance of tissue to accumulated Na + or Cl − . Our understanding of the role of the HKT gene family in Na + exclusion from leaves is increasing, as is the understanding of the molecular bases for many other transport processes at the cellular level. However, we have a limited molecular understanding of the overall control of Na + accumulation and of osmotic stress tolerance at the whole-plant level. Molecular genetics and functional genomics provide a new opportunity to synthesize molecular and physiological knowledge to improve the salinity tolerance of plants relevant to food production and environmental sustainability.

https://rodas5.us.es/file/c2a700e9-8b4a-1970-d343-4a269c302970/1/estres_salino_bibliografia_scorm.zip/files/estres_salino_bibliografia.pdf

Mechanisms of salinity tolerance

Plant responses to salt and water stress have much in common. Salinity reduces the ability of plants to take up water, and this quickly causes reductions in growth rate, along with a suite of metabolic changes identical to those caused by water stress. The initial reduction in shoot growth is probably due to hormonal signals generated by the roots. There may be salt-specific effects that later have an impact on growth; if excessive amounts of salt enter the plant, salt will eventually rise to toxic levels in the older transpiring leaves, causing premature senescence, and reduce the photosynthetic leaf area of the plant to a level that cannot sustain growth. These effects take time to develop. Salttolerant plants differ from salt-sensitive ones in having a low rate of Na + and Cl ‐ transport to leaves, and the ability to compartmentalize these ions in vacuoles to prevent their build-up in cytoplasm or cell walls and thus avoid salt toxicity. In order to understand the processes that give rise to tolerance of salt, as distinct from tolerance of osmotic stress, and to identify genes that control the transport of salt across membranes, it is important to avoid treatments that induce cell plasmolysis, and to design experiments that distinguish between tolerance of salt and tolerance of water stress.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.0016-8025.2001.00808.x

Comparative physiology of salt and water stress

Salinity tolerance comes from genes that limit the rate of salt uptake from the soil and the transport of salt throughout the plant, adjust the ionic and osmotic balance of cells in roots and shoots, and regulate leaf development and the onset of senescence. This review lists some candidate genes for salinity tolerance, and draws together hypotheses about the functions of these genes and the specific tissues in which they might operate. Little has been revealed by gene expression studies so far, perhaps because the studies are not tissue-specific, and because the treatments are often traumatic and unnatural. Suggestions are made to increase the value of molecular studies in identifying genes that are important for salinity tolerance.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2005.01487.x

Genes and salt tolerance: bringing them together.

INTRODUCTION 89 BACKGROUND: THE BASIC PHYSIOLOGY 91 Conclusions 9S ION RELATIONS 96' Ion Uptake and Osmotic Adjustment 96 Nutrient Uptake: NalK Specificity 99 Ion Distribution within the Plant 100 Ion Distribution within the Cell 100 Metabolic evidence 100 Physical measurements .... .... ......... ...... 104 Energetics 106 ROLE OF ORGANIC SOLUTES ........ 108 OrganiC Acids 108 Nitrogen Compounds 109 Carbohydrotes 110 CONCLUSIONS 114

The mechanism of salt tolerance in halophytes

This review describes physiological mechanisms and selectable indicators of gene action, with the aim of promoting new screening methods to identify genetic variation for increasing the salt tolerance of cereal crops. Physiological mechanisms that underlie traits for salt tolerance could be used to identify new genetic sources of salt tolerance. Important mechanisms of tolerance involve Na+ exclusion from the transpiration stream, sequestration of Na+ and Cl- in the vacuoles of root and leaf cells, and other processes that promote fast growth despite the osmotic stress of the salt outside the roots. Screening methods for these traits are discussed in relation to their use in breeding, particularly with respect to wheat. Precise phenotyping is the key to finding and introducing new genes for salt tolerance into crop plants.

Approaches to increasing the salt tolerance of wheat and other cereals

Anoxia can be one consequence of waterlogging and submergence of plants. Anoxia in plant tissues reduces the rate of energy production by 65-97% compared with the rate in air. Thus, adaptation to anoxia always includes coping with an energy crisis. Tolerance to anoxia is relevant to wetland species, rice cultivation and transient waterlogging of other agricultural and horticultural crops. This perspective, in two parts, examines mechanisms of anoxia tolerance in plants. Part 1 covers anoxia tolerance in terms of growth and survival, the interaction of anoxia tolerance with other environmental factors, and the development of anoxic cores within plant tissues. Equally importantly, Part 1 also examines anaerobic carbohydrate catabolism (principally ethanolic fermentation in plants) and its regulation. We put forward two modes of anoxia tolerance, one based on reduced rates of anaerobic carbohydrate catabolism and the other on accelerated rates (Pasteur effect). Further, Part 1 examines mechanisms of post-anoxic injury. In Part 2 (Greenway and Gibbs, manuscript in preparation) we consider flow of the limited amount of energy produced under anoxia to processes essential for cell survival. We show that acclimation to anoxia in plants involves integration of a set of sophisticated characteristics, as a consequence of which the habitat within the anoxic cell is a very different world to that of the aerobic cell.

Review: Mechanisms of anoxia tolerance in plants. I. Growth, survival and anaerobic catabolism

Enzymes which are affected by the addition of inorganic salts during in vitro assay were extracted from salt-sensitive Phaseolus vulgaris, salt-tolerant Atriplex spongiosa, and Salicornia australis and tested for sensitivity to NaCl. In each case malate dehydrogenase, aspartate transaminase, glucose 6-phosphate dehydrogenase, and isocitrate dehydrogenase showed NaCl responses similar to those found for commercially available crystalline enzymes from other organisms. Enzymes extracted from plants grown in saline cultures showed no important changes in specific activity or salt sensitivity. Interaction of pH optima and NaCl concentrations suggests that enzymes may differ in the way they respond to salt treatment.

https://www.jstor.org/stable/info/4262703

Salt responses of enzymes from species differing in salt tolerance

Anoxia in plant tissues results in an energy crisis (Gibbs and Greenway 2003). How anoxia-tolerant tissues cope with such an energy crisis is relevant not only to anoxia tolerance, but also to adverse conditions in air that cause an energy crisis.To survive an energy crisis, plant cells need to reduce their energy requirements for maintenance, and also direct the limited amount of energy produced during anaerobic catabolism to the energy-consuming processes that are critical to survival.We postulate that during anoxia, reductions in ion fluxes and protein turnover achieve economies in energy consumption. Processes receiving energy from the limited supply available under anoxia include synthesis of anaerobic proteins and energy-dependent substrate transport. Energy would also be required for maintenance of membrane integrity and for regulation of cytoplasmic pH (pHcyt). We suggest that a moderate decrease in the set point of pHcyt, from approximately 7.5 to approximately 7.0 is an acclimation to the energy crisis in anoxia-tolerant tissues. This decrease in the set point of pHcyt would favour metabolism of acclimative value, such as reduction in protein synthesis and stimulation of ethanolic fermentation. During anoxia lasting several days, a proportion of the scarce energy produced may need to be spent to mitigate the acidifying effect on pHcyt arising from fluxes of undissociated organic acids across the tonoplast as a consequence of high concentrations of organic acids in the vacuole. Increases in vacuolar pH (pHvac), with concomitant decreases in the vacuolar concentrations of undissociated acids, would mitigate such an 'acid load' on the cytoplasm. We present evidence that a preferential engagement of V-PPiases, over that of V-ATPases, may direct energy flow at the tonoplast to maintain pHcyt.We conclude that the likely causes of death under anoxia are firstly, a decrease in pHcyt below 7.0. Cytoplasmic acidosis occurs in several anoxia-intolerant tissues and may contribute to their death. Such adverse decreases in pHcyt can be mitigated by the biochemical pH stat. Secondly, deterioration in membrane selectivity culminating in loss of membrane integrity would be fatal. We suggest these two causes are not mutually exclusive but may act in concert.

Hank Greenway

Papers

Review: Mechanisms of anoxia tolerance in plants. I. Growth, survival and anaerobic catabolism

Salt responses of enzymes from species differing in salt tolerance

Review: Mechanisms of anoxia tolerance in plants. II. Energy requirements for maintenance and energy distribution to essential processes.

The Use of Agar Nutrient Solution to Simulate Lack of Convection in Waterlogged Soils

Conditions leading to high CO2 (>5 kPa) in waterlogged-flooded soils and possible effects on root growth and metabolism.