Leaves alternate between periods of gradually changing photosynthesis and dark metabolism under the natural daily cycling of light, yet they provide a steady supply of sucrose to sustain the uninterrupted development of growing parts of the plant. To do this, leaves must utilize stored carbohydrate during periods of low light and distribute carbohydrate between export and renewal of storage during periods of high photosynthesis. Regulation of leaf carbohydrate metabolism and export from source leaves provides the means to meet these demands. Mutant plants which lack the ability to synthesize or degrade starch have markedly reduced seed production, which confirms the importance of export of stored carbohydrate during periods of low photosynthesis. This research project will study key aspects of processes by which leaves maintain this steady supply of sucrose for export. The program is aimed at identifying and understanding regulation of carbon allocation, translocation, and partitioning with a view to increasing basic knowledge needed for improving crop plants. These processes affect crop productivity directly by supplying carbon to sinks of economic importance. The PI's approach is to investigate the physiology and biochemistry of these processes and their integrated regulation needed to provide a basis for genetic and molecular manipulation of these processes. The response of leaf carbon metabolism to changes in photosynthesis rate during a sinusoidal light regime, which simulates a natural day, demonstrates the importance of multiple sources of carbon in maintaining export rate during the day. These sources will be studied to identify the biochemical and molecular mechanisms involved in their regulation and coordination. Plants that use either starch exclusively or sucrose and starch to maintain export will be compared to determine the basis of these differences. The coordination of starch synthesis and degradation with photosynthesis rate is important in maintaining a steady rate of sucrose synthesis throughout a sinusoidal light regime. The pathways by which newly- fixed carbon and carbon from starch are converted to sucrose will be investigated. A further program goal is to isolate and maintain plant mutants having altered starch metabolism for studying the regulation of starch synthesis and degradation.
