Growth Regulator Interactions in the Growth of the Shoot System ofAvena sativaSeedling

Studies have been made of the growth in culture medium of the component parts of compositesegments excised from 3 to 7-day-old Avena sativa seedlings and comprising portions of coleoptile and first leaf bases and various lengths of first internode tissue. The effects of various concentrations of gibberellic acid (GA) and indole-3- acetic acid (IAA) alone and in combination have been studied on the growth of these organs. Both GA and IAA stimulate the growth of coleoptile base tissue but in combination their joint effects are less than additive. No synergism occurs. The growth of the first-leaf base is greatly stimulated by GA but is inhibited by IAA. In combination, the stimulatory effect of GA (up to 1 0 p.p.m.) may be virtually eliminated by even low concentrations of IAA (0.01 p.p.m.). The inclusion of first internode tissue in the segments considerably increases the growth of first leaf base tissue but has no consistent effect on the growth of coleoptile base tissue. The presence of first internode tissue also greatly increases the degree of growth stimulation invoked by GA but does not influence the degree of IAA inhibition. It is postulated that the first internode tissue is the source of an unknown growth factor necessary for GA action in the first leaf and potentiating the action of endogenous gibberellin. Kinetin, adenine sulphate, glutarnine, glutarnic acid, asparagine, glycine, arginine, histidine, lysine, aneurin, and pyridoxine will not simulate the effects of this unknown growth factor in the growth of leaf tissue. Like IAA, kinetin virtually eliminates the GA stimulation of leaf growth. A study of extracts of internode tissue in various solvents, analysed by paper partition chromatography and assayed by the growth of the first leaf base, has indicated the presence of growth inhibitors and gibberellin-like substances but has failed to isolate the postulated endogenous GA-synergist. The implications of these results for growth correlations and the hormone control of shoot growth in Avena sativa seedlings is discussed.