Morphological and physiological responses to cyclic drought in two contrasting genotypes of Catalpa bungei

Abstract

In natural environments, cyclic drought which consists of repeated drought-recovery cycles is considerably more common than a prolonged drought event. Two genotypes of Catalpa bungei with contrasting growth performances under semi-arid area were both exposed to cyclic drought and control (well-watered) conditions, and their morphological and physiological responses related to drought resistance were investigated. In both genotypes, the major growth traits were significantly suppressed by cyclic drought. In addition, the intrinsic water use efficiency (WUEi) of each genotype was significantly elevated upon re-watering and contributed to rapid growth recovery after severe drought. Both genotypes exhibited leaf anatomy adaptation and gas exchange adjustment to alleviate water losses under cyclic drought. Meanwhile, elevated abscisic acid (ABA) and jasmonic acid (JA) were detected. The contents of free proline, soluble sugars, and glutathione were also elevated at drought condition. These adaptations interactively promoted the drought resistance of each genotype. Most physiological responses related to drought resistance were more active in genotype 9-1, which exhibited better growth performance than genotype 1-3 under cyclic drought. Genotype 9-1 showed larger extent of adaptive changes in its leaf anatomy and root system compared with 1-3. At the early stage of cyclic drought, the inducement of ABA was more sensitive in genotype 9-1, whereas genotype-specific coordination among ABA, salicylic acid, and indole-3-acetic acid at the biosynthesis level was detected in 9-1 but not in 1-3. Such coordination contributed to the better stoma adjustment and drought avoidance in genotype 9-1. Leaf δ13C was higher in 9-1 than that in 1-3, indicating that the former had larger long-term WUE than the latter. The soluble sugar content was also high in genotype 9-1 under severe drought. These anatomical and physiological features of genotype 9-1 contributed to its enhanced drought avoidance and growth performance under cyclic drought, which are better than those of 1-3.