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Study on the yield and grain starch synthesis of NaN3-induced mutant of Tainung 67
Rice is the most important crop in Taiwan. Yield and quality are the main goal for rice cultivation. Starch is the major constitutes in rice grains, therefore, the enzyme activities engaged in starch accumulation are closely related to the rice grain dry matter accumulation and grain quality, especially to amylose accumulation. The cultivar Tainung 67, one of high yield cultivar, is widely distributed in Taiwan, but the grain quality and the yield of second crop season are always needed to be improved. Recently a mutant SA419, mutated by sodium azide from Tainung 67 which contained 20% amylose content in rice grains, was isolated. The mutant SA419 with lower amylose content(8%)in grains has different grain quality in comparison with Tainung 67. In the present study, We compared rice cultivar Tainung 67 and mutant SA419 in the agronomic characteristics, and the relationship between grain dry mater accumulation or grain quality and sugar-starch conversion enzyme activities.
The results indicated that the grains located on different branches of panicle contributed different yield, but had the same contribution in mutant SA419. The mutant SA419 also had more concentration distribution of grain weight than cultivar Tainung 67. The mutant SA419, with less panicle number in the first crop, produced less yields than cultivar Tainung 67, but produced higher yield in the second crop season, the panicle number difference between two varieties decreased. The factor of growing less panicle number could be improved by cultural changes. The changes in the rate of starch accumulation correlated well with the changes in the activities of sucrose synthase, invertase. hexokinase, AGPglucose pyrophosphorylase, UDPglucose pyrophosphorylase, phosphoglucoisomerase, phosphoglucomutase, soluble starch synthase, granule-bound starch synthase, starch branching enzyme and starch debranching enzyme, during the grain filling period. The rapid grain-fill and shorter period of grain filling in mutant SA419, as compared with Tainung 67, were associated with its higher activity of starch synthesizing at the early phase of grain growth. The lower amylose content of SA419 grains (8%) in comparison with Tainung 67 grains (20%) was due possibly to their inferiority to synthesize amylose through granule bound starch synthase and starch debranching enzyme. The tested Tainung 67 and SA419 showed different starch and amylose accumulation patterns in different grain positions. In cultivar Tainung 67, the reduced activities of grain enzymes involved in starch biosynthesis, particularly during the early phase of grain filling, were related to the decreased starch accumulation for the grains located on proximal secondary branches as compared to the grains located on distal primary branches. The decreased amylose content in the grains of Tainung 67 located on proximal secondary branches was attributable to their lower granule bound starch synthase and starch debranching enzyme activities at early and mid phases of grain filling. In mutant SA419, significant changes were not observed in the activities of grain enzymes involved in sucrose to starch conversion between the grains located on distal primary and proximal secondary branches of panicle. As a result, little position effects were found in the contents of starch and amylose for these developing grains located on different branches. We found that the technology of enzymes activity in situ staining by NBT would apply to the enzyme involved to the rice grain sugar-starch conversion. The NBT in situ staining results showed that the enzyme activities expressed mainly on the endosperm. NBT staining was expressed at all the endosperm cell at the early phase of grain growth, and gradually reduced from central part of endosperm as grains growing, the faster grains growing, the faster reduction. The results of in situ enzyme activity staining could offer some suggestions on the biochemical analysis for physiological investigation.
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