Effects of Photoperiodic Induction and Debudding inXanthium pennsylvanicumand of Partial Defoliation inPhaseolus vulgarison Rates of Net Photosynthesis and Stomatal Conductances

Abstract

Xanthium pennsylvanicum plants received four treatments in the factorial experiment (a) debudded v. not debudded, and (b) long days υ. photoinductive short days. Rates of net photosynthesis, carbon dioxide compensation points (Γ), and stomatal conductances were assessed after 8 days and before leaf growth or stomatal size were appreciably affected. Leaf size, stomatal frequencies, and lengths of guard cells were estimated at this time and again 22 flays after treatment. Debudding alone slightly Increased stomatal conductance; induction alone had a similar but larger effect. Debudding and induction together caused a more than additive increase in net photosynthesis at 8 days, with marked decreases in Γ. At 22 days this combination caused more than additive increases in leaf size and guard-cell lengths while stomatal frequencies had decreased. Induction alone directly increased stomatal conductance and this may be responsible for the increase in photosynthesis; but debudding alone may directly affect photosynthesis by increasing the supply of cytokinins to the leaves. The positive interaction of these factors in photosynthesis could not be explained in terms of stomatal conductance and a synergism between cytokinins and a photoperiodically induced hormone is suggested. In Phaseolus vulgaris plants, 4 days after partial defoliation, stomatal conductances and rates of net photosynthesis increased greatly in the remaining leaflets.