Hydration up to Threshold Levels as the Triggering Agent of the Processes Preparing Germination in Quiescent Seeds

Abstract

Water operates as a triggering agent in quiescent seeds: in the course of seed hydration to threshold levels the metabolic systems are activated step by step culminating in the initiation of cell elongation. Two experimental approaches were used: (1) close inspection of a particular process in dependence of water content and (2) incubation of imbibed seeds under the conditions preventing their further hydration (humid air or polyethylene glycol 6000). In the experiments with pea seeds the following threshold hydration levels (% fr wt) were shown: 45% — starch degradation, 45% — final activation of respiration, 52–55% degradation of reserve proteins. With the axial organs of broad bean seeds, the accumulation of osmotic solutes was evident at 65–68%) and activation of H+-ATPase in the plasmalemma resulting in acid growth at 68–70%). By close inspection of literature threshold hydration levels were found for other main metabolic systems. A general scheme of early germination is presented: hydration up to threshold water content levels triggers the main metabolic systems in the axial organs step by step, thus performing the first (metabolic activation) and the second (preparation for cell elongation) stages of early germination. In the cotyledons only metabolic activation occurs triggered by water inflow.