Differences In Sucrose Metabolism And Transport Of Pod Wall And Seeds Between Two Asparagus Bean Cultivars And Their Response To High Temperature

Seeds and fruits are the main product organs of most crops and thus play important roles in agricultural production since they contribute about75%of the total crop yield worldwide. Thus, studies on seed and fruit development, and their interaction will provide us a novel chance to further facilitate crop production. There are large differences in developmental vigor of seed and fruit among germplasm resources of asparagus bean (Vigna unguiculata ssp. sesquipedalis (L.) Verd.). Many cultivars excellent in agronomic traits (e.g. yield, early maturity and stress tolenrance) have serious bulging-pod syndrome resulted from rapid and enchanced early seed development, which decreases the pod yield and quality dramatically. Thus, asparagus bean is an ideal model studying seed and fruit development, and especially their interaction. In the thesis, we investigated the effects of genotype and high temperature on seed and fruit development of asparagus bean, sucrose meatabolism and transport pathway in seeds and fruits to elucidated the underlying biochemical mechanism for bulging-pod phenotype, which resulted from the imbanlanced seed and pod wall development. The key results are as follows:(1) Investigation on seed and pod wall growth of two asparagus genotypes dfferent in bulging degree showed that there was significantly positive correlation between bulging-pod degree and seed to pod ratio (R2=0.775, P<0.01), which implies enhanced seed development or relatively weak pod wall growth or both of them resulted in bulging-pod phenotype. Compared to non-bulging cultivar ’Zhi121’, bulging cultivar ’Zhijiang282’ had more and larger cells in embryos at the early stage of seed development, and hence bigger seeds. Furthermore, cell division and expansion phase in pod walls of ’Zhijiang282’ both terminated earlier than ’Zhi121’, which retarded pod wall growth of ’Zhijiang282’ at late stage. Thus, it can be concluded that the bulging phenotype resulted from a combination of enhanced seed and depressed pod wall growth at early and late stage, respectively.(2) Compared to ’Zhi121’,’Zhijiang282’ showed higher CWIN activities in seed coat and hexose to sucrose ratio in embryos at early stage which contributed to the production of more cells in embryos. Later on, the CWIN activities and hexose to sucrose ratio decreased more rapidly than ’Zhi121’, which resulted in earlier initiation of cell expansion in embryo, and consequently larger cells. At early stage, apoplasmic pathway dominated assimilate transport of seed coat in both genotypes. At late stage, apoplasmic pathway persisted in seed coat of ’Zhijiang282’. In ’Zhi121’, however, there was a transition from apoplasmic to symplasmic pathway. It is postulated that the transition may be related to the regulation of seed development at late stage and further investigation is needed to clarify the possible underlying mechanisms.(3) Compared to ’Zhi121’, INV and Sus activities in ’282’ pod wall was lower at late stage which may be responsible for the earlier termination of cell division and expansion in pod wall, and consequently the slowed pod wall growth. In addition, the assimilate transport in ’Zhijiang282’ pod wall shifted from symplamic pathway of early stage to apoplasmic pathway of late stage. However, in ’Zhi121’ pod wall, apoplasmic pathway was always the main assimilate transport pathway.(4) Under high temperature, there were increases in bulging degree of pod and seed to pod ratio in ’Zhijiang282’, which also resulted from enhanced seed development of early stage and depressed pod wall growth of late stage. High temperature increased CWIN activity in seed coat and hexose to sucrose ratio in embryo at early stage which stimulated cell division and produced more cells in embryos. At late stage, CWIN activity and hexose to sucrose ratio decreased more rapidly than that under normal temperature, which contributed to earlier activation of cell expansion in embryo. Collectively, more and larger cells in embryo were the cellular basis for enhanced seed growth at early stage under high temperature. It was not surprising that the growth of pod wall decreased at late stage under high temperature since high temperature reduced the activities of Sus and VIN, two main sucrose degrading enzymes in pod wall, during the period.