The Effect of Certain Growth Regulators on Growth and Apical Dominance of Young Apple Trees

SummaryUnfeathered maiden trees of apple Stark Earliest (syn. Scarlet Pimpernel) were pruned back in winter to a height of 70 cm. The following spring five shoots were allowed to grow out, the others being removed. When the leading shoots were about 12 cm. long the trees were sprayed once with various concentrations of GA, TIBA, CCC and B.9 (see below), other trees being left unsprayed as controls. Subsequent effects on shoot growth were analysed in terms of leaf number and internode length. Apical dominance was destroyed or greatly reduced by GA (200 p.p.m.), TIBA (200 p.p.m.) and CCC (5,000 p.p.m.) and these substances all increased leaf number on the lower shoots. B.9 (2,000 p.p.m.) had no effect on apical dominance. TIBA, CCC and B.9 reduced mean internode length on all shoots, whereas GA increased internode length on the lower shoots only.CCC caused only a temporary check to growth, but the growth-retarding effects of B.9 and TIBA persisted throughout the growing season. The possible mode of action of these growth regulators is discussed in relation to current theories of shoot growth and apical dominance.