Physiological, environmental and genetic variation in apical dominance as determined by decapitation in Triplochiton scleroxylon

Patterns of lateral shoot growth following decapitation in 1-meter tall, rooled Triplochiton scleroxylon K. Schum. cuttings varied with clone and in response to it range of environmental conditions and growth regulator treatments. Two phases of bud activity were identified, the Sprouting Phase, in which many buds were released front correlative inhibition, and the Dominance Phase (stalling 3-4 weeks after decapitation) in which uppermost laterals began to dominate and suppress growth, and sometimes cause apical abscission of lower lateral shoots. Except in non-erect plants, the most distal lateral to elongate became the new leading shoot. During the Sprouting Phase the proportion of active buds was increased by removing leaves from the upper part of the stem, and by reducing the photoperiod from 13-15 h to 11 h, particularly at 20 degrees C rather than 25 degrees C. Conversely, the proportion of sprouting buds was decreased by injecting plant stems with NAA (250 mu g/plant). During the Dominance Phase, suppression of lateral shoot growth was hastened by stem injection with GA(3) (200 mu g/plant), especially when applied to the uppermost shoot at the end of the Sprouting Phase. Reimposition of dominance was delayed, however, by (1) high rates of N:P:K fertilization. (2) low temperature (20 versus 25 degrees C) under relatively long days (13 and 15 h), (3) low photon flux density (160 mu mol m(-2) s(-1)) and (4) severe defoliation. Plant orientation had no effect on bud activity of decapitated plants, but affected the relative vigor and orientation of new lateral shoots. High temperature (25 versus 20 degrees C) and injection with GA(3) increased the erectness of newly developing lateral shoots.