AGE-RELATED PATTERNS OF METABOLISM AND BIOMASS IN SUBTERRANEAN TISSUES OF ZOSTERA-MARINA (EELGRASS)

Production and metabolic features of the rhizomatous marine angiosperm Zostera marina L. (eelgrass) from a subtidal meadow in Monterey Bay, Monterey, California, USA, were examined to assess the impact of the subterranean system on whole plant metabolism. Total subterranean biomass of eelgrass was correlated with above-ground (shoot) biomass. The subterranean system presents a gradient in tissue age, from young (proximal to shoot) to old (distal). While root biomass was constant, indices of metabolic capacity (respiration, soluble carbohydrate content, glutamine synthetase activity) decreased with increasing tissue age. Rhizome internodal biomass and carbohydrate levels were influenced by season and tissue age, and rates of respiration declined with increasing tissue age. The first 4 (youngest) root bundles along the rhizome accounted for >90 % of total plant NH4+ assimilatory potential, while O2 consumption increased linearly with increasing amount of subterranean tissue. A model of whole plant carbon balance predicted compensation depths (photosynthesis = respiration) for Monterey Bay eelgrass of 4.2 to 11.6 m depth, given instantaneous shoot P(net):R ratios of 11 to 4.5. The model also predicted that small changes in both P(net):R and depth (light availability) have the potential to effect large changes in the rate of new tissue production. Although the subterranean tissues constitute 20 to 26 % of plant biomass, carbon consumed by respiration in the subterranean tissue represented <15 % of gross photosynthetic production (P(g)) at depths < 10 m. At the deep edges of the eelgrass bed, the model predicts that total subterranean respiration increases to 25 % of P(g). Since respiration by subterranean tissues represents only 10 to 15 % of total plant respiration, eelgrass carbon balance is strongly controlled by shoot carbon metabolism.