CARBON PARTITIONING IN EELGRASS - REGULATION BY PHOTOSYNTHESIS AND THE RESPONSE TO DAILY LIGHT-DARK CYCLES

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 regulat ion of sucrose formation and partitioning in this ecologically importa nt species. Rates of C flux and SPS activity increased with leaf age, consistent with the ontogenic transition from sink to source status. R ates of C export and photosynthesis were low but quantitatively consis tent with those of many terrestrial plant species. The V-max activity of SPS approached that of maize, but substrate-limited rates were 20 t o 25% of V-max, indicating a large pool of inactive SPS. SPS was unres ponsive to the day/night transition or to a 3-fold increase in photosy nthesis generated by high [CO2] and showed little sensitivity to inorg anic phosphate. Consequently, regulation of eelgrass SPS appeared simi lar to starch- rather than to sugar-accumulating species even though e elgrass accumulates sucrose. Leaf [sucrose] was constant and high thro ughout 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 tempo ral stability of [sucrose] confers an ability for eelgrass to buffer t he effects of prolonged light limitation that may be key to its surviv al and ecological success in environments subject to periods of extrem e light limitation and chaotic daily variation in light availability.