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Hormones appear to be important in controlling plant growth in soils of low water potential, particularly in changing the root:shoot ratio as the soil dries or becomes saline, and in communicating soil conditions to the leaves. This review has necessarily focused on abscisic acid (ABA), as there is little information about the role of other hormones in controlling growth in dry or saline soils. ABA is partly responsible for the differential response of root and shoot growth to dry soils. In dry soil it maintains root growth and inhibits shoot growth. However, when applied to well-watered plants, it usually inhibits root and shoot growth, showing that plants in dry soil respond quite differently from well-watered plants. ABA affects the rate of cell expansion in plants in dry soils: it maintains cell expansion in roots and inhibits that in leaves. It may also affect the rate of cell production, but little is known about this. The role of ABA as a long-distance signal in controlling growth by root-to-shoot communication is unclear: the concentrations found in xylem sap can affect stomatal conductance, but seem too low to affect leaf expansion. Yet drought and salinity generally affect leaf expansion before they affect leaf conductance. A possible solution to this puzzle is that ABA is transported in xylem sap in a complexed form, or that another compound in xylem sap stimulates the synthesis or activity of ABA in leaves, or affects leaf expansion independently of ABA.
Munns et al. (Fri,) studied this question.