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

Salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification (i.e., damage control and repair) response pathways, and pathways for growth regulation. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. Osmotic stress activates several protein kinases including mitogen-activated kinases, which may mediate osmotic homeostasis and/or detoxification responses. A number of phospholipid systems are activated by osmotic stress, generating a diverse array of messenger molecules, some of which may function upstream of the osmotic stress-activated protein kinases. Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.

/pdf/salt-and-drought-stress-signal-transduction-in-plants-4bj87ki2ww.pdf

Salt and drought stress signal transduction in plants

1. Plant Nutrients. 2. The Soil as a Plant Nutrient Medium. 3. Nutrient Uptake and Assimilation. 4. Plant Water Relationships. 5. Plant Growth and Crop Production. 6. Fertilizer Application. 7. Nitrogen. 8. Sulphur. 9. Phosphorus. 10. Potassium. 11. Calcium. 12. Magnesium. 13. Iron. 14. Manganese. 15. Zinc. 16. Copper. 17. Molybdenum. 18. Boron. 19. Further Elements of Importance. 20. Elements with More Toxic Effects. General Readings. References. Index.

/pdf/principles-of-plant-nutrition-2k4wl46ou6.pdf

Principles of plant nutrition

Roles of glycine betaine and proline in improving plant abiotic stress resistance

Physiological and ecological constraints play key roles in the evolution of plant growth patterns, especially in relation to defenses against herbivores. Phenotypic and life history theories are unified within the growth-differentiation balance (GDB) framework, forming an integrated system of theories explaining and predicting patterns of plant defense and competitive interactions in ecological and evolutionary time. Plant activity at the cellular level can be classified as growth (cell division and enlargement) of differentiation (chemical and morphological changes leading to cell maturation and specialization). The GDB hypothesis of plant defense is premised upon a physiological trade-off between growth and differentiation processes. The trade-off between growth and defense exists because secondary metabolism and structural reinforcement are physiologically constrained in dividing and enlarging cells, and because they divert resources from the production of new leaf area. Hence the dilemma of plants: Th...

https://www.jstor.org/stable/pdfplus/2830650.pdf

The dilemma of plants: To grow or defend.

An analysis of major U.S. crops shows that there is a large genetic potential for yield that is unrealized because of the need for better adaptation of the plants to the environments in which they are grown. Evidence from native populations suggests that high productivity can occur in these environments and that opportunities for improving production in unfavorable environments are substantial. Genotypic selection for adaptation to such environments has already played an important role in agriculture, but the fundamental mechanisms are poorly understood. Recent scientific advances make exploration of these mechanisms more feasible and could result in large gains in productivity.

http://science.sciencemag.org/content/sci/218/4571/443.full.pdf

Plant Productivity and Environment

Everyone who grows plants, whether a single geranium in a flower pot or hundreds of acres of corn or cotton, is aware of the importance of water for successful growth. Water supply not only affects the yield of gardens and field crops, but also controls the distribution of plants over the earth's surface, ranging from deserts and grasslands to rain forests, depending on the amount and seasonal distribution of precipitation. However, few people understand 'fully why water is so important for plant growth. This book attempts to explain its importance by showing how water affects the physiological processes that control the quantity and quality of growth. It is a useful introduction for students, teachers, and investigators in both basic and applied plant science, including botanists, crop scientists, foresters, horticulturists, soil scientists, and even gardeners and farmers who desire a better understanding of how their plants grow. An attempt has been made to present the information in terms intelligible to readers with various backgrounds. If the treatment of some topics seems inadequate to specialists in certain fields, they are reminded that the book was not written for specialists, but as an introduction to the broad field of plant water relations. As an aid in this respect, a laboratory manual is available with detailed instructions for some of the more complex methods (J. S. Boyer in "Measuring the Water Status of Plants and Soils," Academic Press, San Diego, 1995).

Water Relations of Plants and Soils

Sugar Input, Metabolism, and Signaling Mediated by Invertase: Roles in Development, Yield Potential, and Response to Drought and Heat

Rates of photosynthesis, dark respiration, and leaf enlargement were studied in soil-grown corn (Zea mays), soybean (Glycine max), and sunflower (Helianthus annuus) plants at various leaf water potentials. As leaf water potentials decreased, leaf enlargement was inhibited earlier and more severely than photosynthesis or respiration. Except for low rates of enlargement, inhibition of leaf enlargement was similar in all three species, and was large when leaf water potentials dropped to about -4 bars.Intact sunflower leaves were held for 4 days at leaf water potentials which permitted maximal photosynthesis and respiration, but which inhibited leaf enlargement. Although leaf enlargement did not occur initially, enlargement resumed toward the end of the desiccation period. However, the rate of enlargement was not as rapid as in the well watered control, nor did it return to the control rate when the plant was rewatered.

/pdf/leaf-enlargement-and-metabolic-rates-in-corn-soybean-and-20hpsj3efi.pdf

Leaf enlargement and metabolic rates in corn, soybean, and sunflower at various leaf water potentials.

Plant reproduction is sensitive to water deficits, especially during the early phases when development may cease irreversibly even though the parent remains alive. Grain numbers decrease because of several developmental changes, especially ovary abortion in maize (Zea mays L.) or pollen sterility in small grains. In maize, the water deficits inhibit photosynthesis, and the decrease in photosynthate flux to the developing organs appears to trigger abortion. Abscisic acid also increases in the parent and may play a role, perhaps by inhibiting photosynthesis through stomatal closure. Recent work indicates that invertase activity is inhibited and starch is diminished in the ovaries or affected pollen. Also, sucrose fed to the stems rescues many of the ovaries otherwise destined to abort. The feeding restores some of the ovary starch and invertase activity. These studies implicate invertase as a limiting enzyme step for grain yields during a water deficit, and transcript profiling with microarrays has identified genes that are up- or down-regulated during water deficit-induced abortion in maize. However, profiling studies to date have not reported changes in invertase or starch synthesizing genes in water-deficient ovaries, perhaps because there were too few sampling times. The ovary rescue with sucrose feeding indicates either that the changes identified in the profiling are of no consequence for inhibiting ovary development or that gene expression reverts to control levels when the sugar stream recovers. Careful documentation of tissue- and developmentally specific gene expression are needed to resolve these issues and link metabolic changes to the decreased sugar flux affecting the reproductive organs.

John S. Boyer

Papers

Plant Productivity and Environment

Water Relations of Plants and Soils

Sugar Input, Metabolism, and Signaling Mediated by Invertase: Roles in Development, Yield Potential, and Response to Drought and Heat

Leaf enlargement and metabolic rates in corn, soybean, and sunflower at various leaf water potentials.

Grain yields with limited water