Diel variations in rates of C export,
sucrose-phosphate synthase (SPS) and
sucrose synthase (SS) activity, and C reserves were investigated in Zostera marina L. (eelgrass) to elucidate the environmental regulation of
sucrose formation and partitioning in this ecologically important species. Rates of C flux and SPS activity increased with leaf age, consistent with the ontogenic transition from sink to source status. Rates of C export and photosynthesis were low but quantitatively consistent with those of many terrestrial plant species. The Vmax activity of SPS approached that of maize, but substrate-limited rates were 20 to 25% of Vmax, indicating a large pool of inactive SPS. SPS was unresponsive to the day/night transition or to a 3-fold increase in photosynthesis generated by high [CO2] and showed little sensitivity to
inorganic phosphate. Consequently, regulation of eelgrass SPS appeared similar to
starch- rather than to
sugar-accumulating species even though eelgrass accumulates
sucrose. Leaf [
sucrose] was constant and high throughout the diel cycle, which may contribute to the down-regulation of SPS. Root
sucrose synthase activity was high but showed no response to nocturnal
anoxia. Root [
sucrose] also showed no diel cycle. The temporal stability of [
sucrose] confers an ability for eelgrass to
buffer the effects of prolonged light limitation that may be key to its survival and ecological success in environments subject to periods of extreme light limitation and chaotic daily variation in light availability.