Abscisic Acid-Regulated Growth Modulations and Its Application for Stress and Quality Management of Vegetable Transplants
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The goal of this study is to develop a management tool for producing high quality, more stress tolerant vegetable transplants and for prolonging transplant marketability. This study primarily involves physiological and morphological growth modulation by the stress hormone abscisic acid (ABA). The first part of this study evaluated the effects of ABA foliar spray on stress and quality management of vegetable transplants. In muskmelon seedlings subjected to water withholding, pre-stress treatment of ABA improved the maintenance of leaf relative water content by limiting transpirational water loss. Upon re-watering, the ABA-treated seedlings showed faster photosynthetic recovery and greater dry matter accumulation than the untreated seedlings. In jalape?o pepper, ABA applied at the cotyledon to 3-leaf stage improved transplant compactness with minimal negative side effects. Although this method induced undesirable growth modifications in bell pepper and watermelon, ABA applied immediately before the transplant maturity stage was effective in delaying excessive shoot growth of bell pepper seedlings. These results demonstrate three beneficial effects of ABA for vegetable transplants: stress control, height control, and extension of transplant marketability. The second part of this study examined the mechanisms of ABA-induced growth modulations in Arabidopsis: inhibition of leaf expansion, leaf chlorosis, and promotion of primary root elongation. Microscopic analysis of leaf epidermis revealed that ABA inhibits cell expansion, but not cell division or stomata formation, suggesting that the ABA-induced inhibition of leaf expansion is a mechanism to conserve water without limiting plant growth capacity. Leaf chlorosis induced by exogenous ABA occurred only in mature leaves and independently of ethylene synthesis. Tissue nitrogen (N) analysis with a ^(15)N-labeling technique indicated a role of ABA as a regulator of N distribution. A proposed new mechanism is that ABA limits distribution of N into non-growing mature leaves, thereby inducing leaf-age dependent chlorosis. Using scanning electron microscopy (SEM), dehydration-induced root damage was characterized by thickening and deformation of root tips. Although exogenous ABA did not alleviate this damage, it promoted primary elongation especially under water stress. These results suggest that the overall function of ABA in stress adaptation is to conserve water and nutrients to support new growth.