GROWTH, WATER AND ION RELATIONSHIPS OF HALOPHYTIC MONOCOTYLEDONAE AND DICOTYLEDONAE - A UNIFIED CONCEPT

Halophytic angiosperms seem to have secondarily acquired adaptations to saline terrestrial environments, and have lost the requirement for salt of enzymes and membranes as is primarily present in aquatic marine bacteria, algae and fungi. Halophytic monocotyledonae differ in some respects from salt-adapted dicotyledonae: their growth rate is not stimulated by salt and is often lower than in dicotyledonae at increased salinity; they have a markedly lower internal sodium to potassium ratio and a lower water content. In addition, diurnal variation of leaf extension and leaf thickness differs significantly between monocotyledonae and dicotyledonae. This is related to the position of the growth meristem of expanding leaves. Monocotyledonous halophytes seem to be more efficient in the use of water than dicotyledons. The correlation that exists between values of the mean relative growth rate measured under saline conditions and leaf elongation and leaf thickness variation indicates that water stress rather than ion (toxicity) stress is the cause of growth reduction at increased salinity under these circumstances. A qualitative model is presented focusing on the different position and exposition of the growth meristem in monocotyledonae and dicotyledonae, which aims to provide a physiological and morphological explanation of the different growth, water and mineral economy of monocotyledonous and dicotyledonous halophytes.