Divergence In Behavior Of Ethylene-Induced Leaf Abscission In Vascular Plants

Leaf abscission is a typical adaptation of plant to cold and drought stress in winter.Ethylene is without doubt the most important factor inducing leaf abscission.However,whether there is differentiation of deciduous behavior in autumn induced by ethylene from ferns and gymnosperms to angiosperms remains to be studied.This experiment confirmed the differentiation of deciduous behavior in different phylogenetic of vascular plants and the role of ethylene in leaf abscission process by measuring the deciduous behavior and ethylene evolution rate in autumn.By adding exogenous ethephon to different kinds of vascular plants,the mechanism of why different kinds of vascular plants are different in the leaf abscission process has been studied.The main results are :1.In the process of nature leaf abscission in autumn,the chlorophyll content of ferns decreased,and the ethylene evolution rate increased too,but the detachment force between leaf and petiole remained unchanged.In gymnosperms and angiosperms,the chlorophyll content of evergreen plants remained unchanged or slightly increased,while the ethylene evolution rate and the detachment force between leaves and petiole did not change significantly.In deciduous plants,it is showed that the chlorophyll content decreased,and the ethylene evolution rate increased and then decreased at last,the detachment force decreased gradually.The results showed that gymnosperms and angiosperms had already evolved the mechanism of ethylene dependent leaf abscission.2.Scanning electron microscopy(SEM)was used to observe the abscission zone of different species.Pteridophytes had no obvious abscission zone,the morphological structure of the cells at the base of the petiole was similar,and there was no obvious difference in cell size.Gymnosperms and angiosperms had distinct abscission zone in which the cell size was smaller than that of the surrounding cells.It is proved that the formation of abscission zone is the basis for the differentiation of deciduous behavior from ferns,gymnosperms to angiosperms.3.With the exogenous addition of ethephon,the ethylene evolution rate of ferns increased significantly and rapidly,and the leaves turned yellow in 6-8 days.However,there was no significant change in the detachment force between leaves and petioles,and no significant change in the hydrogen peroxide content and hydrolase activity at the base of the petiole.Both gymnosperms and angiosperms showed a large increase in ethylene evolution rate,companied with a decrease in detachment force to 0 within four to five days,and an increase in hydrogen peroxide content and hydrolase activity in petiole base.The results showed that the physiological response of plants to ethylene appeared in gymnosperms and angiosperms.4.The effect of ethylene and hydrogen peroxide which was one of the signals in ethylene pathway were further verified in vitro.After exogenously adding ethephon,there was no increase of hydrogen peroxide in the base of the petiole of ferns.Whether treated with ethephon or hydrogen peroxide could not induce leaf abscission.Exogenously adding ethephon increased the amount of hydrogen peroxide in the abscission zone of gymnosperms and angiosperms.Ethephon and hydrogen peroxide could accelerate the leaf abscission process.The addition of ethylene synthesis inhibitor or ethylene action inhibitor,hydrogen peroxide synthesis inhibitor or hydrogen peroxide scavenger can effectively delay the defoliation.hydrogen peroxide induced by ethylene is the physiological basis of plant leaf abscission,but this physiological basis was appeared from gymnosperms.The results showed that ferns had not evolved the ethylene-dependent leaf abscission,but the leaf abscission appeared in gymnosperms and angiosperms Both gymnosperms and angiosperms had ethylene dependent defoliation processes.It is presumed that,the ferns,especially the woody ferns,were extinct in the ice age as they could not adapt themselves to the environment because of lacking the mechanism of leaf abscission.For gymnosperms and angiosperms,some of them evolved the ethylene dependent leaf abscission process in autumn to adapt to low temperature stress in winter.Some others,such as evergreen plants adopted another strategy.They stayed evergreen so as to reduce the cost of leaf abscission by reducing the synthesis of ethylene.But they still retained the mechanism of ethylene dependent leaf abscission.To some extent,the results of this study explained why the distribution area of pteridophytes was limited,but the gymnosperms and angiosperm were widely distributed in the present day.