Physiological responses of orchid pseudobulbs to drought stress are related to their age and plant life form

Abstract

As storage organs for water and nutrients, pseudobulbs play an important role in the survival of orchids. However, the differences in morphological and physiological responses of pseudobulbs to drought stress between epiphytic and terrestrial orchids remain undefined, and little is known about the physiological imparity of different-aged pseudobulbs. We investigated the anatomy and changes in physiology of pseudobulbs in an epiphytic orchid (Cymbidium tracyanum) and a terrestrial orchid (C. sinense) and compared their responses and recovery during and after periods of drought stress. In particular, “ramets severance treatment” and “multiple leafless pseudobulbs treatment” were applied for C. tracyanum to verify the utilization strategy for the stored water. When compared with C. sinense, the pseudobulbs of C. tracyanum have larger water-storage cells and higher water content, and the enhanced water storage of C. tracyanum can be used and recovered more rapidly. And they had more flexibility in shifting stored nonstructural carbohydrates. The remarkably high concentration and different change trends of abscisic acid (ABA) between different-aged pseudobulbs under drought stress were only found in C. tracyanum. When pseudobulb age was considered, the stored water and carbohydrate changed more rapidly in the youngest pseudobulbs of both species in response to stress. Our results indicated that the pseudobulbs differ in their means for coping with drought conditions according to their life form and age. These findings contribute to our understanding about the functional diversification of pseudobulbs and their strategies for ecological adaptations.