The Effects of Salinity on Cellular and Biochemical Processes Associated with Salt Tolerance in Tropical Plants

Tropical plant species exhibit a high degree of variability in salt tolerance, probably as a result of evolution in diverse ecosystems, intensive year-round selection pressures, and high temperatures. Salinity may cause major problems for agricultural production in semiarid tropics and areas subjected to seawater intrusion, and is also a concern in any irrigated region subjected to high temperatures. Mechanisms for salt tolerance vary among different species and range from the selective filtration systems of mangrove species to the more subtle ion accumulation and selective distribution mechanisms of wild tomato species. In crop species, sugarcane and date palm seem to exhibit high salt tolerance due to the accumulation of organic solutes in the form of sugars; whereas, some tropical legume species depend solely upon the restriction of salt uptake as a mechanism for tolerance. Abilities of particular plant genotypes to grow and produce yield under high salinity, well-drained environments are dependent upon restriction of salt from cytoplasmic compartments and maintenance of positive water balance. Temperature, humidity and light intensity have profound interactive effects with salinity at the upper limits of ionic and osmotic stress. Research efforts are needed to develop comprehensive plant models that integrate and interpret many of the known physiological and morphological responses to salt stress, initiate long-range breeding programs to select high salt tolerance in conventional crops and exploit agronomic suitability in salt-tolerant wild species. Aggressive research in the area of molecular biology is required to identify and transfer genes and gene systems that confer salt tolerance in wild species to either related or unrelated crop species. Purchased by USDA for Official Use Salt tolerance in tropical species has not been studied as extensively as in species of more temperate regions. Nonetheless, there are a number of-excellent reviews concerning salt tolerance physiology in plants that probably have general applicability to tropical crops (3, 14, 26, 27, 32, 55, 61, 87). This review will be limited to the effects of salt exposure through root zone salinity and will present examples of tropical species when applicable; the interested reader is directed to reviews by Maas (57, 58) for discussions concerning salt effects that occur through the leaf as the result of sea spray or irrigation. General Response of Higher Plants to Salinity Salinity concentrations that restrict plant growth vary widely among species and plants have adapted to a wide range of saline environments. Terrestrial halophytes survive root salt concentrations from 1.2 to 30 g L-l salt and vary markedly in the amounts of the various salts that they accumulate or restrict. Knowledge concerning variability in salt tolerance responses of tropical species is extremely limited, but the issue of salt tolerance is expected to become more serious as human population growth in the tropics begins to compete for finite water resources. Santo (72) indicated that over 42,000 acres of sugarcane are irrigated with saline water (over 0.75 dS m-l). Banana, Musa acuminata Colla, is usually grown in the humid tropical lowlands where salinity problems are not incurred, but efforts to grow them under irrigated conditions in Jordan indicate that salinity could be a potential problem if compeTable 1. Physiological responses of salt stress. tition for land or water force their production into other locations (43). Salt effects on plants are quantitative, qualitative, and dependent on variables such as salt concentration, duration of exposure, and specific salt composition. Plant-dependent variables of response include species and variety differences, growth stage and exposure site on the plant. Responses to salt stress differ among plant species, but common responses do occur even in species that are distantly related. At low concentrations, salt may have no effect on plant growth or may even stimulate growth to a small degree. At moderate salt concentrations, plant growth rate decreases and stunted plants may develop darker green leaves and/or succulence. At relatively high salt concentrations, more obvious visual symptoms such as leaf burn and wilting develop. Salts exert these effects upon plants through both osmotic and ionic influences. The osmotic effects of salinity are a result of increased ion concentrations at the root-soil water interface which decreases the water potential. Growth is reduced as a function of total electrolyte concentration, soil water content and soil matrix effect and is evidenced by reduced cell enlargement and changes in metabolism. Thus, isosmotic concentrations of different combinations of salts cause nearly equal growth reduction at moderate salinities, but single salts and extreme ion ratios can cause specific ion toxicities. A general scheme for plant responses to the various effects of salt stress includes the primary sensory responses, and subsequent responses designated herein as constitutive, inducible, and pathologic (Table 1). Salts may also interfere with plant nutrition in rather complex ways which will not be discussed in detail herein (see reviews by Grattan and Grieve (31), Page and Chang (65)). Sensory => osmotic effects on root => pressure-volume changes=> electrical potential changes => cystolic calcium changes => hormonal responses => reduced leaf elongation => reduced assimilate transport Constitutive => ion discrimination = > filtration => ion regulation => compartmentation => salt glands => morphology = > succulence => rosette growth Inducible => transcriptional changes => RNA and protein regulation => structural changes => membranes, cell walls => metabolic changes => increased respiration and energy demand => reduced transpiration (stomata1 closure) => increased mesophyll resistance => organic solute synthesis => decreased CO, fixation => CAM metabolism => reduced growth rate => morphological adaptations = > increased root/shoot ratio = > succulence => rosette growth => changes in reproductive initiation Pathological => salt loading => leaf burn, scorching => reduced photosynthesis