Early expression of apical control regulates length and crotch angle of sylleptic shoots in peach and nectarine
Syllepsis is the predominant mode of branching in young peach and nectarine trees [Prunus persica (L.) Batsch]. Cultivars differ considerably in expression of apical control of sylleptic shoots. This has practical implications regarding tree training. Four cultivars were selected for increasing apical control by the central shoot axis, viz., 'Zaigina', 'Mayglo', 'Fiesta Red' (all nectarines), and 'Oom Sarel' (clingstone peach), respectively. Young, actively growing shoots were harvested when ≃300 mm in length, at a time when development of sylleptic shoots (laterals) had begun. Length, crotch angle, and position (as distance from the apex) of the laterals were recorded. When length of the laterals was plotted against their position, two zones were observed. The gradient of length vs. position was shallower in the distal than in the proximal zone. Autonomy in lateral shoots can be described as their ability to grow independently of apical control by the apex of the branch. Autonomy of laterals near the branch apex increased with their length. In 'Zaigina' this was established via a more distal start of the second zone, and in 'Mayglo' via an increased gradient in the second zone. The early loss or maintenance of apical control regulates architecture in sylleptically branched peach and nectarine shoots. Crotch angle widening of laterals appears to be largely dependent on position, but in some cultivars, such as 'Mayglo', other factors are also involved. The data provide evidence of correlative phenomena between actively growing shoots.