Salinity and drought

Summary

Water deficit (water stress — WS) and excess salt (salt stress — SS) evoke similar plant responses, yet clear differences have been observed. The effect of the two forms of stress applied consecutively to cotton (Gossypium hirsutum) and pepper (Capsicum annuum) was studied in a growth chamber (29/20°C day/night temperature, 50% RH, 12-h photoperiod) in 2.5-liter containers packed with a silt loam soil.

Leaf water potential (Ψ) under increasing WS [soil water potential decrease from −0.16 to −1.10 MPa] of transpiring cotton and pepper plants declined to lower levels than under equivalent SS. The decline of leaf solute potential ψ₀ on the other hand, was less under WS than under SS, resulting in reduced turgor potential (ψ _p ), in contrast with turgor maintenance under SS. Predawn turgor potential of WS plants was maintained at all levels of soil water potential. Transpiration, CO₂ assimilation and light period leaf extension rate were higher under low soil water potential produced by salinity than an equivalent value produced by water deficit.

The more severe effect of WS was attributed to incomplete osmotic adjustment — the reduction in solute potential did not keep pace with the reduction in leaf water potential, and to increased root interface resistance in the dry soil.

The leaf sap of cotton under WS had a higher proportion of sugars (65%) than electrolytes, compared to SS. When WS was converted to SS and plant solute potential decreased, electrolytes were taken up at the expense of a reduction in the sugar concentration. Water stress and salt stress may have an additive effect in depressing growth. But at equivalent levels, the relative magnitude of the effect of low soil matric potential (WS) on plant growth was twice as great as that of low soil solute potential (SS).