Exogenous Auxin andN-1 -Naphthylphthalamic Acid Effects onPopulus deltoidesXylogenesis

Abstract

A method was developed for evaluating the empirical alteration of xylem vessel differentiation in the central leaf trace of Populus deltoides, a species that exhibits helical phyllotaxis. Effects of experimental treatments for a period of six plastochrons were evaluated by vessel parameter ratios = 2.PT/ (PT+1 + PT−1), where P was either vessel number or mean transverse vessel area measured at mid-intern ode at Leaf Plastochron Indices of T − 1, T, and T + 1. Excising leaf laminae reduced vessel number and mean vessel area in the associated central leaf trace by 50% and 70%, respectively, compared to unexcised laminae controls. Replacing excised laminae with a concentration series of exogenous indoleacetic acid (IAA) resulted in a 5% increase per log mol m−3 of IAA in the number of vessels differentiating in the associated central leaf traces compared to excised controls. Mean vessel areas within these leaf traces were 50% of those of intact leaf traces. No significant effects of different concentrations of exogenously applied IAA on mean vessel area could be demonstrated. A lanolin paste ring of N-1 -naphthylphthalamic acid (NPA), an auxin transport inhibitor, around the petioles of intact leaves reduced the number of differentiating vessels by 7% and mean vessel area by 29% per log mol m−3 of NPA compared to central leaf traces of leaves ringed with plain lanolin paste. The results suggest that NPA treatments may be used to distinguish experimentally, at least in part, the cell division from the cell enlargement phases of primary xylogenesis within central leaf traces of P. deltoides stems.