Publisher Summary The subject of salt tolerance has received a tremendous amount of attention during the past two decades. The concept of developing salt-tolerant plants, even to the degree that they can be grown with seawater, captures the imagination of both the scientific and the public sector. There is a wide range of salt tolerance in vascular plants. Several genera include species that are extreme halophytes and are ecologically competitive only under saline conditions. Crop species primarily are glycophytes, salt-sensitive, and grow well only under cultivated conditions. Domesticated species are provided optimum conditions to obtain high yields. There is currently a need to develop new direction and cohesive impetus in the area of salt-tolerant crops development. Realistic short- and long-range goals need to be established and continually pursued. The information that is known is extensive but fragmented. This chapter outlines some of the issues and strategies concerning selection and breeding for plant salt tolerance, highlights crucial advances in knowledge in recent years, and identifies some of the fundamental gaps in our understanding.

Adaptation of Plants to Salinity

Seeds are central to crop production, human nutrition, and food security. A key component of the performance of crop seeds is the complex trait of seed vigour. Crop yield and resource use efficiency depend on successful plant establishment in the field, and it is the vigour of seeds that defines their ability to germinate and establish seedlings rapidly, uniformly, and robustly across diverse environmental conditions. Improving vigour to enhance the critical and yield-defining stage of crop establishment remains a primary objective of the agricultural industry and the seed/breeding companies that support it. Our knowledge of the regulation of seed germination has developed greatly in recent times, yet understanding of the basis of variation in vigour and therefore seed performance during the establishment of crops remains limited. Here we consider seed vigour at an ecophysiological, molecular, and biomechanical level. We discuss how some seed characteristics that serve as adaptive responses to the natural environment are not suitable for agriculture. Past domestication has provided incremental improvements, but further actively directed change is required to produce seeds with the characteristics required both now and in the future. We discuss ways in which basic plant science could be applied to enhance seed performance in crop production.

Seed vigour and crop establishment: extending performance beyond adaptation

Seeds of pearl millet were germinated on wet filter paper at temperatures up to 50 °C. In one experiment, the temperature was held at 50 °C during imbibition and was then lowered to 32 °C or 25 °C. Germination rate and the maximum fraction of seeds germinating (Gm) both decreased as the time of exposure to 50 °C increased. In contrast, exposure to 50 °C after imbibition for 8 h slowed germination but did not significantly reduce Gm. When the 'high' temperature imposed after imbibition was reduced from 50 °C to 45 °C, there was a small reduction in the rate of germination but not in Gm. The responses have implications for the optimum time of sowing in the tropics when maximum daytime soil temperature at the depth of sowing is in the range of 45-50 °C.

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.) II. ALTERNATING TEMPERATURE

Current interest in seed hydration treatments for the improvement of level, rate and uniformity of germination or field emergence has revealed how little is known of the physiology of germination control under water stress. This review surveys briefly the responses of seeds held at different hydration levels, from normal germination in a free water supply to seed activation without germination under slight moisture stress, seed deterioration at greater moisture stress and the damage that can be caused to seeds in very dry conditions, as well as the responses to subsequent dehydration. Inhibition of germination, though not of seed activation, at certain levels of water stress is likened to various forms of dormancy, and the mechanism governing the initiation of cell elongation is suggested as the possible key to control over germination. Several lines of evidence on cell membrane integrity and action, and their responses to external factors, point to the role that the membrane may play in cell elongation (and hence germination) control, and membrane integrity may also be associated with the transition between seed deterioration at one hydration level and seed activation and repair at slightly higher hydration levels. Seed activation without germination is also considered in an ecological context.

The physiology of seed hydration and dehydration, and the relation between water stress and the control of germination: a review

Seed reserve utilization and seedling growth of wheat as affected by drought and salinity

SUMMARY
Seeds of calabrese and carrot were held at 10°C or 20°C for periods of up to 21 days in water or polyethylene glycol ‘6000’ solutions of from −5 to −20 bars. Seeds were subsequently placed to germinate in a favourable environment or were first dehydrated before transfer to this environment. The seeds’ ability to germinate was virtually unaffected by any of the temperature, water potential or duration treatments. Increasing water stress progressively reduced the levels of water content and oxygen uptake rates after initial imbibition, and delayed the onset of germination with the accompanying increase in these factors. At −15 and −20 bars, water content remained approximately constant throughout the experiment but oxygen uptake rates fell after day 6. The results have a direct bearing both on the seed ‘priming’ treatment used to accelerate seed germination and on the tolerance to drought of seeds in the field.

Seed activation and seed germination under moisture stress

Sensitivity of Seed Germination and Seedling Radicle Growth to Moisture Stress in Some Vegetable Crop Species

Summary
Seeds of two cultivars of lettuce (Lactuca sativa L.), untreated or soaked in acetone containing kinetin and gibberellic acid (K + GA3), were germinated in light or dark at a range of (high) temperatures and at a range of water potentials. In addition, untreated and treated seeds were germinated at 20°C in the light and then transferred to the same high temperatures or to low water potentials. In both cvs light and K + GA3 promoted germination at high temperatures and at low water potentials, although the cvs differed quantitatively in their responses. The upper temperature limit of germination in the light, and of radicle growth after germination, were similar in treated and untreated seeds respectively, suggesting that light and K + GA3 in combination may have completely removed the specific high-temperature block to germination. Although light and K + GA3 increased the tolerance of germination to water stress, germinated, untreated seeds were able to grow at even lower water potentials. The K + GA3 treatment, whilst promoting germination under high temperature or water stress, reduced radicle growth subsequent to germination, thus providing support for the possibility that, in germination, the initiation of cell elongation has a degree of metabolic independence of the other processes associated with growth.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1979.tb07559.x

Effects of light and growth regulators on germination and radicle growth of lettuce seeds held under high-temperature stress and water stress

SUMMARY Lucerne seeds (Medicago sativa L.) were germinated in a range of polyethylene glycol (PEG) and sodium chloride (NaCi) solutions in the presence and absence of kinetin with ethrel. In addition, seeds were germinated in water and transferred to PEG and NaCl solutions, and root growth was measured. The water potentials at which there was 50 % germination inhibition and at which 50 % of the germinated seeds failed to grow after transfer were determined. Kinetin and ethrel together completely removed the block to germination through osmotic stress, allowing germination to proceed at the same water potential as growth. The action of kinetin and ethrel was studied by following the processes of cell expansion and division during germination and early growth. The control of germination by water stress was through cell expansion, and the initiation of this process was shown to be stimulated in the presence of the growth regulators. Cell division although initially permitted in their presence, was later inhibited causing a decline and finally cessation of seedling growth.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1980.tb00764.x

Action of growth regulators on lucerne germination and growth under water stress

Use of Growth Regulators to Remove the Differential Sensitivity to Moisture Stress of Seed Germination and Seedling Growth in Red Clover (Trifolium pratense L.)

T. W. Hegarty

Papers

Seed activation and seed germination under moisture stress

Sensitivity of Seed Germination and Seedling Radicle Growth to Moisture Stress in Some Vegetable Crop Species

Effects of light and growth regulators on germination and radicle growth of lettuce seeds held under high-temperature stress and water stress

Action of growth regulators on lucerne germination and growth under water stress

Use of Growth Regulators to Remove the Differential Sensitivity to Moisture Stress of Seed Germination and Seedling Growth in Red Clover (Trifolium pratense L.)