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Adaptability to Water Stress of Sitao (Vigna Sesquipedales Fruw.) Grown Before and After Rice
Dissertation Abstract:
This study sought to characterize the adaptability of string bean (Vigna sesquipedales Fruw.) under pre- and post-rice conditions. Morphological and physiological characteristics as well as earliness, germinability, and seedling establishment were determined.
Findings showed that string bean was generally not adaptable to pre-rice conditions. The pre-rice condition was characterized by drought during the early growth of the plant and three cycles of waterlogging from the peak of the vegetative growth until the plants flowered and formed pods. There was a considerable reduction in yield because of markedly reduced total leaf area, reduced total shoot dry matter, and degeneration of the tap roots as well as the inability of the plants to produce branched surface rooting.
The crop also exhibited poor germinability and non-uniform growth of seedlings under low levels of both soil moisture and osmotic potential. On the other hand, more conclusive evidence regarding the adaptability of string bean to post-rice conditions was obtained. The post-rice condition was characterized by water-saturated soil from germination and early seedling growth and thereafter a continuous depletion of soil moisture until the crop matured.
Fresh pod yields were comparable to the control.
The following mechanisms were responsible for the crop's adaptability: 1) reduction in total leaf area; 2) maintenance of high relative water content; 3) increased stomatal resistance; and 4) consequent decrease in the amount of water lost through transpiration.
Good germinability and early establishment of seedlings under high levels of soil moisture and high osmotic potentials as well as earliness also proved to be of adaptive significance to the crop. Based on ocular observations, development of long, deep-penetrating roots and changes in leaf orientation with respect to the incident solar radiation might have contributed to the overall crop's adaptation.
Both bush and pole types of string bean can be grown successfully after harvesting rice. However, among bush string bean varieties studied, UPBS-I was suggested to be highly adaptable to post-rice conditions regardless of the year of planting. Whereas the performance of the four pole string bean varieties, particularly their yield, was highly influenced by the pattern and degree of water stress that prevailed. Considering their overall performance, Acc 43 and Acc 44 was more suitable for post-rice planting. No single plant characteristic was found to correlate with fresh pod yield under the post-rice conditions. This indicated the significance of yield measurement when screening and selecting genotypes adaptable to this condition.