Resource allocation and sucrose mobilization in light-limited eelgrass Zostera marina

This study evaluated the ability of Zostera marina L. (eelgrass) to balance the daily photosynthetic deficit by mobilization of carbon reserves stored in below-ground tissues during a period of extreme winter light limitation . A quantitative understanding of the mobilization process and its limitati ons is essential to the development of robust models predicting minimum lig ht levels required to maintain healthy seagrass populations. Plants were gr own in running seawater tanks under 2 light regimes. One treatment was prov ided with 2 h irradiance-saturated photosynthesis (H-sat) to produce severe Light Limitation, while control plants were grown under 7 h H-sat, simulat ing the typical wintertime condition in Monterey Bay, California, USA. Alth ough plants maintained under 2 h H-sat were more severely carbon limited th an plants grown under 7 h H-sat, whole-plant carbon balance calculated from metabolic needs and growth rates was negative for both H-sat treatments. T he eelgrass studied here responded to negative carbon balances by suppressi ng the production of new roots, depleting sucrose reserves, and effecting a gradual decrease in growth rate and an increase in the activity of sucrose synthase (SS, E.C. 2.4.1.13) in sink tissues in the terminal stages of car bon stress. The 7 h H-sat plants survived the 45 d course of the experiment while the plants grown under 2 h H-sat died within 30 d, even though one-t hird of their carbon reserves remained immobilized in the rhizome. Thus. ex treme Light limitation can prevent full mobilization of carbon reserves sto red in below-ground tissues, probably through the effects of anoxia on tran slocation. Metabolic rates, particularly photosynthesis and respiration of the shoot, were unaffected by prolonged carbon limitation in both treatment s. The patterns observed here can provide useful indices for assessing the state and fate of seagrass ecosystems in advance of catastrophic declines.