Studies on Avena sativa L. seedlings, with particular reference to the role of indole-3-acetic acid contained in the endosperm
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy (PhD)
The role of free indole-3-acetic acid (IAA) in the endosperm of Avena sativa L. cv. 'Terra' seedlings, was investigated to determine its contribution to free IAA in the shoot. IAA-2- 14c (in amounts equivalent to 177, 17.7, or 1.77 ng IAA), was injected into the endosperm of 63 h old etiolated seedlings, and transport and metabolism of the 14c-labelled compounds determined over 24 h. It was concluded that translocation of free IAA directly from the endosperm, is probably not a significant source of free IAA in the shoot; mainly because even small amounts of IAA-14c introduced into the endosperm, were rapidly metabolised. This suggested that in Avena, free IAA does not normally exist in the liquid endosperm. The two main hypotheses on the source of free IAA in cereal shoots (i.e. the grain, or synthesis in the coleoptile tip) are critically evaluated. The suggestion is made that both may serve as sources of IAA, with the relative importance of each possibly varying between species (since the auxin economies of Avena and Zea appear to differ). It is argued that IAA supplied from the grain may be derived from hydrolysis of high molecular weight IAA-esters at the endosperm-scutellar interface, rather than from direct transport of IAA-ester (cf Epstein, Cohen and Bandurski, 1980). A major metabolite in the grain was a mild alkali-labile IAA - conjugate with some properties similar to the IAA – glucoprotein reported by Percival and Bandurski (1976). Considerable destruction of IAA-14c (25% in 15 min) occurred on silica gel TLC plates, incubated in normal laboratory lighting prior to development. Breakdown was exacerbated by time and light. The effects on seedling extension of endosperm injections of unlabelled IAA, gibberellic acid (GA3), kinetin, or abscisic acid (ABA), with or without brief illumination, were determined; as were those of application to the coleoptile tip of IAA in agar. Histological studies were also conducted on treated seedlings.