Sucrose synthase activity as a potential indicator of high rice grain yield

The Chinese rice cultivar Guichao2 is a potentially valuable resource for breeding rice (Oryza sativa L.) with many valuable characteristics including upright leaves, prolific tillering ability, and high yields. The objectives of this study were to compare grain yields and yield components between Guichao2 and U.S. rice lines and to determine if the enzyme sucrose synthase was associated with the observed differences in grain yield and yield components. Grain yield, grain dry matter accumulation, yield components, and endosperm sucrose synthase activity were measured in field and greenhouse experiments with Guichao2 and 'Lemont' rice, as a U.S. standard, and various other lines. In three of four field tests, Guichao2 had higher grain yields than Lemont and other U.S. rice cultivars. Guichao2 also usually had higher sucrose synthase activity per grain and per unit of protein, in three of four field tests. The cultivar Qiguizao, derived from a cross of Guichao2 and Qiyouzhan, also had higher sucrose synthase activity on a protein basis. Since the grains of Qiguizao weigh less by approximately one half of the other cultivars, the sucrose synthase activity per grain was somewhat lower. Greenhouse tests supported these findings. In one test, the F, hybrids of Guichao and Lemont had higher sucrose synthase activity than either Guichao2 or IR36. From this research, we conclude that sucrose synthase activity, and perhaps the genes coding for sucrose synthase, will be a valuable resource in transferring the yield characteristics of Guichao2 and Qiguizao into U.S. rice cultivars.