MINERAL ELEMENT ACQUISITION AND GROWTH-RESPONSE OF PLANTS GROWN IN SALINE ENVIRONMENTS

Plants acquire mineral nutrients from their native soil environments. Most crop plants are glycophytes and have evolved under conditions of low soil salinity. Consequently, they have developed mechanisms for absorbing mineral nutrients in non-saline soils. Under saline conditions, which are characterized by low nutrient-ion activities and extreme ratios of Na+/Ca2+, Na+/K+, Ca2+/Mg2+ and Cl-/NO3-, nutritional disorders can develop and crop growth may be reduced. Additions of N and P generally increase the growth of plants grown in N- and P-deficient environments, provided that the plant is not experiencing severe salt stress. When salinity and nutrient deficiency are both factors limiting growth, relief of the most limiting factor will promote growth more than the relief of the less limiting factor. Therefore, addition of a limiting nutrient can either increase, decrease or have no effect on relative plant tolerance to salinity, depending on the level of salt stress. Failure to account for the severity of salt stress when interpreting salinity X nutrient interactions has caused considerable confusion among researchers. Salinity disrupts mineral nutrient acquisition by plants in two ways. First, the ionic strength of the substrate, regardless of its composition, can influence nutrient uptake and translocation. Evidence for this is salinity-induced phosphate uptake in certain plants and cultivars. The second and more common mechanism by which salinity disrupts the mineral relations of plants is by reduction of nutrient availability by competition with major ions (i.e. Na+ and Cl-) in the substrate. These interactions often lead to Na+-induced Ca2+ and/or K+ deficiencies and Ca2+-induced Mg2+ deficiencies. Halophytes have not received the attention glycophytes have in the area of salinity-mineral nutrient relations. Nevertheless, some halophytes may show symptoms of mineral nutrient imbalances despite their remarkable ability to absorb nutrients selectively from soil solutions dominated by Na+ and Cl-.