Effects of Molybdenum Application on Plant Growth, Molybdoenzyme Activity and Mesophyll Cell Ultrastructure of Round Leaf Cassia in Red Soil

Round leaf cassia (Chamaecrista rotundifolia) cv. 'Wynn', a species of leguminous forage, was introduced into China from the Australian Centre for International Agricultural Research in 1989. It possesses strong resistance to adversity and is suited for planting in the red soil area of Southern China. The purpose of this study is to investigate the effects of molybdenum (Mo) application with different concentrations on plant growth, molybdoenzyme activity and mesophyll cell ultrastructure of C. rotundifolia in red soil. The results showed that, plant height, number of branches, and dry matter weight of C. rotundifolia under varying Mo applications (0.5, 1.0, 2.0, 4.0 mg kg(-1)) increased by 2.7%similar to 11.5%, 0.92%similar to 28.68%, and 52.9%similar to 115.2% compared with the control group, respectively, and they all increased with the increase ofMo concentration. Molybdenum contents in plants were also increased significantly with increasing the concentration of Mo, but Mo contents in plants at 4 mg kg(-1) Mo application were lower than that at 2 mg kg(-1) Mo application. The activities of nitrate reductase in leaf and nitrogenase in root nodule both reached the maximum at 1.0 mg/kg Mo, which increased by 57.77% and 113.1% than the control group respectively. However, their activities began to decrease at 2 and 4 mg kg(-1) Mo. The observation on mesophyll cell ultrastructure showed that, Mo application could increase the number of chloroplasts, enhance the stability of lipid bilayer of chloroplast membrane and increase the density of chloroplast lamellae and grana. Simultaneously, the total number of mitochondria and the density of inner cristae were decreased, but there was no significant influence on mitochondrial outer membrane. Moreover, the right amount of Mo fertilizer could enhance the stability of mesophyll cell wall and cell membrane. Although the molybdoenzyme activity decreased at the higher concentration (e. g., 2 and 4 mg kg(-1)), the application of Mo fertilizer in this test (0.5 similar to 4 mg kg(-1)) could promote the growth and development of C. rotundifolia.