| Abstract: | Starch is an important agricultural product deposited in vegetative and reproductive storage organs (sinks) of various crop species. Starch yield may in some cases be limited by photosynthesis, i.e. source-limited. This is particularly true for starch synthesized in potato tubers. Here, the physiological sink is characterized by a symplastic phloem unloading path. In reproductive storage tissues (seeds), however, photosynthates must pass the apoplast on their path from phloem unloading to the storage cell. In cereal grains, phloem unloading of sucrose and poslunloading processes rather than photosynthesis may thus control starch synthesis (sink-limited). Various limiting steps along the path of photosynthate movement from the phloem to the storage cells are considered. The primary organic carbon for starch synthesis is sucrose. Sucrose delivered to the storage cell is metabolized to UDPglucose and fructose by means of sucrose synthase activity. Concerning sucrose breakdown the role of cell-wall bound invertase is not well defined. Competition for UDPglucose consumed for growth or storage may be a crucial process in photosynthate partitioning. High starch yields of crops require an undisturbed growth of the sink organ and an optimal filling of sink amyloplasts with starch. The most important form of organic carbon imported into amyloplasts of storage organs (cereal grain and potato tuber) and used for starch synthesis is glucose 1-phosphate. It is still to be clarified whether the rate of glucose 1-phosphate absorption has a direct impact on starch yield. In cereals, the total amount of starch accumulated depends significantly on the duration of grain filling. Ample nutrient and water supply at the post-anthesis stage prolongs the period of grain filling and hence favours starch production. High temperature reduces the activity of soluble starch synthase with negative consequences for starch accumulation. The biochemical and physiological implications of these stress factors are discussed. Recently, successful transgenic manipulations of starch synthesis in crop plants have increased starch yield. |
