Studies on the abscission of blue lupin leaves

Summary The responses of three types of explants of blue lupin leaves are considered: pulvinar explants, consisting of the pulvinar region alone, petiolar explants, consisting of the pulvinar region plus petiole and laminar explants consisting of the pulvinar region plus leaflets. Abscission is accelerated by removal of the leaflets; removal of the petiole has much less effect. Pulvinar explants fail to abscise in darkness but are the first to abscise in the light. This is in accordance with previous evidence of high light sensitivity of the pulvinar region. Kinetin applied directly to the pulvinar region delays abscission, as does kinetin supplied via the transpiration stream. As shown by experiment, this is probably due to transported kinetin reaching the abscission zones of the pulvinar region. The effects of photoperiodic treatments on explants or whole leaves are described. Abscission in the whole leaf is delayed by short daily photoperiods; the delay reaches a maximum with 8 hours light per day. However, abscission is more rapid in continuous light than in darkness. Removal of the leaflets greatly accelerates abscission even in darkness. The pulvinar explant fails to abscise with photoperiods of 4 hours or less; although it appears to have a long day response, preliminary attempts failed to demonstrate that this is a true photoperiodic response (replacement of a long day by a short day together with a light break). The complex responses of leaves and explants to day length lend further support to the hypothesis that light has effects on abscission other than in photosynthesis.