PLANT PHYSIOLOGY , Vol 108, Issue 4 1665-1671, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Carbon Partitioning in Eelgrass (Regulation by Photosynthesis and the Response to Daily Light-Dark Cycles)
R. C. Zimmerman, D. G. Kohrs, D. L. Steller and R. S. Alberte
Department of Biology, University of California, Los Angeles, California 90024 (R.C.Z., D.G.K., D.L.S.)
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.
