The chemistry and mode of action of naturally-occurring growth regulators from avena

Abstract

This thesis is concerned with the nature and function of endogenous chemicals in the maintenance of oat seed dormancy. The prime germination inhibitors within dormant Avena spp. Are characterised and their development pattern during seed maturation and after-ripening established. Their in vitro biological activities as inhibitors of germination and reserve starch degradation are also shown. Abscisic acid (ABA) and a group of saturated, medium-chain length carboxylic acids (MCFA) have been detected in dormant Avena seed. Levels of endogenous fatty acid and ABA were monitored in developing grain of dormant and non-dormant oat species. It is shown that the highest amounts of MCFA are present within dormant seed, these levels decreasing during, or prior to, that period of increasing seed germinability. Reduced amounts of MCFA are associated with non-dormant grain. It is shown that oat seed germination correlates well with V the endogenous level of MCFA, particularly nonanoic acid. It is speculated that volatile fatty acids are lost from dormant wild oat grain during dry storage by a natural physical process involving gradual volatilisation. ABA did not appear to be directly involved with oat seed dormancy. Large variations in ABA content only occurred in hydrated grain of both species, the level increasing markedly during grain filling. As desiccation on the parent plant proceeded the ABA content of seeds rapidly decreased. At harvest the levels of free ABA in common and wild oat were found to be similar, although their germinations were 65% and 0%, respectively. It is shown that ABA is not efficacious as an inhibitor of germination in the common oat. MCFA are detected in the 3 inhibitor mixtures isolated from a wide variety of plant material. It is proposed that they contribute to the inhibitory properties of this complex. Fatty acids in the series pentanoic to undecanoic are tested for their physiological activity in seed germination. MCFA are found to inhibit seed germination and suppress gibberellin-induced amylolysis in barley at concentrations below 10-3M. In general, nonanoic acid is the most effective treatment. At a concentration of about 2 x 10-3M nonanoic acid reduces the germination of non-dormant oat seed by 50%. MCFA-induced inhibition of lettuce seed germination is shown to be similar to natural thermodormancy, being reversed by treatment with red light or applications of cytokinin or gibberellin. It is speculated that MCFA impose their inhibitory action by modifying membrane function and/or enzyme activity within the plant cell.