Restoration of Photosynthesis in the Senescent Primary Leaves by the Removal of Foliage Leaves in Snap Bean Plants

摘要

The photosynthetic rates of the primary leaves of snap beans (Phaseolus vulgais L. cv. Yamashiro kuro sanndo) were enhanced by defoliation of trifoliate leaves at various growing stages. In the plants defoliated at about 20 days, a linear rapid increase in photosynthetic rates of the primary leaves was observed during 5 days after defoliation. Since this phenomenon was not accompanied by changes in leaf morphology, the effect of defoliation appeared to be caused by some physiological mechanisms. After the period of the rapid increase, photosynthetic rates expressed on leaf area basis increased at slower rates and those expressed on leaf weight basis became constant or decreased, suggesting that the effect of defoliation was caused by morphological changes in the leaves. In the primary leaves of the plants defoliated at later stages, the increase in leaf thickness and smaller enhancement in photosynthetic rates expressed on leaf area basis were observed. From these results, it was suggested that, iftrifoliate leaves were removed, photosynthetic ability of the primary leaves would be enhanced by physiological and/or morphological mechanisms to restore photosynthetic ability of the individual plant. Partial defoliation of foliage leaves increased photosynthetic rates of the unremoved foliage leaves as well as the primary leaves and also enhanced extension growth of the unremoved foliage leaves, resulting in the increase in photosynthesis of the unremoved foliage leaves. When one leaflet was removedfrom a trifoliate foliage leaf, the photosynthetic rate of an individual foliage leaf on the 1 st or 2nd node showed the comparable rate to the respective ones of control plants. These results suggest that photosynthetic ability of the plant, in the case of defoliation of a small amount of leaf area, can be recovered by increasing photosynthesis of the unremoved leaflets of foliage leaves. In the plant defoliated heavily, with two leaflets from one trifoliate leaf removed, photosynthetic ability of a foliage leaf can not recover, because the increase in photosynthetic rates was not accompanied by the enhancement of extension growth of the foliage leaf. In this case, however, photosynthetic ability of an individual plant could be recovered through theenhancing effects on the photosynthesis of the primary leaves.