role of sucrose-phosphate synthase in the control of photosynthate partitioning in Zea mays leaves

The influence of light and leaf sucrose content on partitioning of photosynthate and sucrose-phosphate synthase (SPS) activity in maize (Zea mays L.) leaves was investigated. The ratio of partitioning of photosynthate between sucrose and starch shifted from about 17:1 to 2:1 when the irradiance was increased from 180 to 1450 µmol quanta m-2 s-1. Increasing the sucrose content of the leaves had little effect on the partitioning ratio. SPS from illuminated leaves had a higher affinity for its substrates, UDPGlc and Fru6P, and was less inhibited by Pi than the enzyme from darkened leaves but the Vmax was unaffected. SPS was fully light activated at an irradiance of 340 but not 180 µmol quanta m-2 s-1. Increasing the sucrose content of maize leaves more than 3-fold had little or no effect on the activation state of SPS which, together with the partitioning data, suggests that sucrose does not exert significant short-term feedback inhibition of its own synthesis in this species.