Effect of Photoperiodic Induction on the Transpiration Rate and Stomatal Behaviour of Debudded Xanthium Plants

Abstract

In controlled-environment studies with debudded Xanthium plants, appreciable changes in stomatal activity and attendant rates of transpiration were found to be associated with photoperiodic induction. Leaves of plants kept on a 10-h inductive photoperiod after removal of the apical and axillary buds grew at rates comparable to those of similarly debudded plants kept on a 10-h-interrupted non-inductive photoperiod (consisting of an 8-h photoperiod and a 2-h interruption of the dark period). There was a large effect of leaf age on stomatal behaviour: the minimum stomatal resistance of leaves of non-induced plants decreased from about 5 s cm⁻¹ when the leaf was 25 per cent of its final size to 1.6 s cm⁻¹ when almost fully expanded. Thereafter, it slowly increased with time. Superimposed on this age response was a marked effect of photoperiodic induction. The stomata on induced leaves opened more widely than those on non-induced leaves, the response being greatest with the leaves which were youngest at the time induction commenced. However, this ‘opening’ tendency was maintained for only a few weeks; thereafter, the stomata failed to open as widely. This later ‘closing’ tendency of stomata on leaves of induced plants progressed rapidly and in a basipetal sequence and presaged a necrotic form of leaf senescence which developed in the same sequence. The closing tendency on leaves of non-induced plants progressed slowly in an acropetal direction; leaves senesced in the same sequence with the familiar yellowing symptoms. It is suggested that flower induction sets in train a sequence of events which influence stomatal movement (and other processes) and inevitably leads to the death of the induced axis. Transpiration rates calculated from measurements of the physical environments and stomatal resistances agreed well with those measured.