THE 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 pho tosynthate and sucrose-phosphate synthase (SPS) activity in maize (Zea mays L.) leaves was investigated. The ratio of partitioning of photos ynthate between sucrose and starch shifted from about 17:1 to 2:1 when the irradiance was increased from 180 to 1450 mu mol 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 affin ity for its substrates, UDPGlc and Fru6P, and was less inhibited by P- i than the enzyme from darkened leaves but the V-max was unaffected. S PS was fully light activated at an irradiance of 340 but not 180 mu mo l quanta m(-2) s(-1). Increasing the sucrose content of maize leaves m ore than 3-fold had little or no effect on the activation state of SPS which, together with the partitioning data, suggests that sucrose doe s not exert significant short-term feedback inhibition of its own synt hesis in this species.