Essentiality and Toxicity for Plants

Essentiality and Toxicity for Plants It is generally accepted that various plant species, especially marine and shore plants and those adapted to saline soils (Australian Atriplex vesicaria), require small amounts of sodium for normal development. Sodium is important for plants, though definite beneficial effects on growth and development have been observed in only a few species. Na enhances the growth of some species if potassium is deficient. Sodium cannot generally perform the specific function of potassium in plants. It does so to a limited extent only, even in plants that respond to sodium fertilization (Saalbach 1973). Na is assumed to influence osmotic pressures in the vacuoles, and the water content of colloids in the plasma. In many species of plants it is reported to be involved in carbon dioxide assimilation. Salt fertilization always increases sodium yields of sugar beet, carrot and chard, even if potassium fertiliza- 

Sodium-deficient sugar beet and chard have unusually thin, metallic-green leaves with a purplish tinge on the underside. The leaf-stalks are limp, and in dry, warm weather the plants wilt more rapidly than those with an adequate salt status. The plants are stocky. In general, visual signs of sodium deficiency are unknown in practice because most soils contain enough sodium to prevent depletion (Bergmann 

Specific symptoms of sodium excess are not known. High levels of sodium reduced calcium uptake in some cases (Leh 1973). Citrus and stone fruit species are particularly susceptible to salt damage. The leaf damage sometimes starts with chlorotic blotches, which are followed by necrotic lesions at the leaf tips and margins or between the veins. The leaves of lucerne growing on saline soils turn reddish, and the leafstalks collapse, possibly indicating calcium deficiency (Bergmann 1992). 

Critical sodium levels in plants depend on species and on the age of the leaves or plant tissue. Sodium contents of l.Sgkg in wheat and lucerne are considered toxic. 

In general, the essentiality and toxicity of plants requires further studies to be conducted, and in greater detail. 

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