HETEROPHYLLY IN RANUNCULUS-FLABELLARIS - THE EFFECT OF ABSCISIC-ACID ON LEAF ULTRASTRUCTURE

Ranunculus flabellaris Rafin., an aquatic buttercup, exhibits heterophylly at the level of cellular ultrastructure. Compared to terrestrial leaves, underwater leaves have thinner epidermal cell walls and more numerous paramural bodies per epidermal and mesophyll cell cross-section. The number of chloroplasts and mitochondria in cell cross-sections also contrasts between the two leaf types. Despite within-and between-leaf variations, different patterns of organelle distribution for the two leaf forms were found using principal coordinates analysis. In addition, underwater leaf chloroplasts are smaller, have fewer grana, a greater number of thylakoids/granum, and less starch compared to chloroplasts from terrestrial leaves. At the ultrastructural level, submergence in ABA solution does not produce a leaf with as many characteristics of the terrestrial environment, as shown in previous studies of leaf morphology and anatomy. While number and distribution of organelles in ABA-treated leaves are similar to terrestrial leaves, some features of chloroplast internal structure and paramural body number and distribution resemble underwater leaves. It is postulated that ABA acts as a morphogen involved in guiding the irreversible processes of leaf development, but certain subcellular characteristics may be determined directly by the physical environment. Difficulties encountered in quantitative analyses of cellular ultrastructure are discussed. © 1990 Annals of Botany Company.