Hormonal control of growth of freshwater aquatic plants

Abstract

1. The life cycle of this rosette aquatic plant was discussed in detail. 2. Anatomical differences between terrestrial and submerged forms of the rosettes were found. Terrestrial rosettes were induced very rapidly by exposure of previously submerged rosettes to air. Laminal development was greatly reduced in the leaves and roots of the terrestrial rosettes. 3. Two main aspects of the life cycle were considered in detail propagation of the rosettes; and leaf elongation. 4. Propagation: In nature the rosettes have the ability to propagate by (a) stolon formation or (b) by a less efficient means where the rosette elongates vertically. (a) Experimental evidence presented indicates that stolon initiation is controlled by gibberellic acid, although cytokinins may be involved. In nature, stolons from similar rosettes may be of varying lengths, and so the elongation phase of stolon development was considered. It was found that gibberellic acid may bring about a protraction of this elongation phase of growth of the stolon, at the expense of the development of the terminal plantlet. Ethylene supplied as ETHREL E inhibited further elongation of the stolons, and furthermore, promoted accumulation of starch. Starch grains in stolons of rosettes treated with gibberellic acid were limited to a central ring of cells. Some radial growth in response to supplied ETHREL E was noted. Stolons may persist in some cases, thus interconnecting many rosettes, but in some cases the stolons senesce. Evidence presented above suggests that gibbarellic acid and ethylene may control senescence. Where gibberellins levels are low in the stolons, accumulated ethylene will rapidly effect the senescence of the stolon, thus making the rosetts independent. (b) Where rosettes are partially and repeatedly buried by sediment, they have the ability to perennate themselves vertically by means of an elongated stem axis. Various rooting levels on the same rosette reflect the sequential nature of the deposition of the sediment. This was found experimentally where rosettes were grown under different conditions of deposition of sediment, and rosettes similar to those experimentally induced were found on a silt fan, near the inflow in Loch Drumore, near Glenshee, in Perthshire. The ability of the plant to propagate so rapidly by stolon formation on eroded shores, or by altering its rooting level and then forming stolons, explains to a great extent the ubiquity of this species throughout Great Britain, and thus implicitly explains the persistence of this species through many of the stages in hydrosere formation.