STRESS PHYSIOLOGY IN TREES - SALINITY

1. an increase in the concentration of specific ions that may cause toxicity,
2. a reduction in the osmotic potential, consequently water potential of the soil water, and,
3. ionic imbalances.

If growth reduction can be correlated with the reduction in substrate water potential and similar responses occur with isosmotic concentrations of different salts, the salinity has an osmotic effect. If the response is more pronounced for one salt than for isosmotic concentrations of other salts, there is a specific (toxicity) effect (Meiri and Shalhavet, 1973).

Although salinity affects plants physiologically in many ways (Maas and Nieman, 1978) the predominant influence of salt stress is of growth suppression. Typically growth decreases more or less linerly as salinity increases beyond a threshold level and the effect is similar for a range of different salts ie. an osmotic effect. This response may be represented by the expression:

Relative Yield = 100 - B(Ge - A) (Maas and Hoffman, 1977) where knowledge of two parameters

1. the threshold salinity (A) beyond which yield loss commences, and,
2. the percentage yield decrease (B) per unit increase in salinity beyond the threshold value,

   is sufficient to describe the salinity-relative yield response of many plants. Over the range of economically acceptable yield loss, the percentage yield decrease is usually close to linear.

This model describes the response of many herbaceous species as well as certain growth parameters eg. lateral growth, leaf area development, of perennials. However, woody species also respond specifically to toxic accumulation of Na and/or Cl which cause leaf damage, dieback and eventual death.

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