DEVELOPMENTAL MORPHOLOGY AND PHYSIOLOGY OF HETEROPHYLLOUS AQUATIC ANGIOSPERMS (ABSCISIC ACID, HALORAGACEAE, LEAF DEVELOPMENT, WATER STRESS, STOMATA)

Within the watermilfoil family, Haloragaceae, certain species of the amphibious genera Myriophyllum and Proserpinaca develop morphologically distinct juvenile (vegetative) and adult (flowering) growth phases. All species exhibit morphological and anatomical differences between leaves produced on submerged and aerial shoots (heterophylly). Laboratory experiments were designed to: (1) compare the organogenetic potential of whole leaf explants taken from the aerial juvenile and adult growth phases of Myriophyllum heterophyllum using plant tissue culture techniques and (2) study the environmental and hormonal factors regulating aerial leaf development in seven species of Myriophyllum and two of Proserpinaca. Whole leaf explants from both aerial juvenile and adult growth phases of Myriophyllum heterophyllum regenerated both roots and shoots when cultured on Gamborg B5 medium supplemented with combinations of cytokinins (benzyladenine BA , 6-gamma-gamma-dimethylallyl amino purine 2iP , or zeatin) and auxins (naphthalene acetic acid NAA or indole-3-acetic acid IAA ). Adult leaf explants consistently produced greater numbers of adventitious shoots and roots. The presence of cytokinin in the medium effectively promoted adventitious shoot production on all explants with zeatin \u3e 2iP \u3e BA. Maximum shoot formation occurred when both cytokinin and auxin were present in the medium. Shoots regenerated from both juvenile and adult leaf explants were juvenile as judged by leaf shape and the absence of flowering. Addition of auxin to the medium promoted adventitious root formation from both explant types with NAA being more effective than IAA. Adventitious shoot-buds originated from the leaf epidermal cells and root primordia originated within the perivascular tissue of the explants. Based upon the histological origin of these adventitious shoots and roots, quantitative differences in the capacity for organogenesis between juvenile and adult explants reflected differences in explant size and venation. Although the juvenile and adult growth phases of Myriophyllum heterophyllum are morphologically distinct, whole leaf explants derived from both growth phases exhibited a similar competency for organogenesis. Both the plant hormone abscisic acid (ABA) and water stress caused the development of aerial juvenile leaves on submerged juvenile shoots of Myriophyllum heterophyllum. These aerial-type leaves were characterized by reduced epidermal cell length, induction of stomatal differentiation, and increased thickness of the cuticle. Induction of aerial juvenile leaves by water stress was initiated by reduced cell turgor. (Abstract shortened with permission of author.