Growth, carbon allocation and proteolytic activity in the seagrass Zostera noltii shaded by Ulva canopies.

The effects of light reduction [100%, 25%, 10% and 1% mean daily-integrated photon irradiance (I)] by Ulva rigida C. Agardh canopies on carbon balance, sugar-related enzymes and proteolytic activities of the seagrass Zostera noltii Hornem. were investigated. Shaded plants showed negative net growth and starch was mobilized in both above- and below-ground tissues. Sucrose declined in below-ground parts under severe light deprivation (10% and 1% I), but was accumulated in above-ground parts. Mobilization of the non-structural carbohydrates (sucrose and starch) was explained by changes in activities of sucrose synthase (SuSy, EC 2.4.1.13) and sucrose-phosphate synthase (SPS, EC 2.4.2.24). Under severe light reduction, the capacity of above-ground tissues for sucrose formation and export declined, indicated by the lowest SPS activity. In contrast, severe light reduction increased the 'sink strength' of below-ground tissues, demonstrated by the highest SuSy activities, and diminished the capacity for sucrose resynthesis from starch breakdown, as the lowest SPS activity was observed under low light. These results suggest a cessation of sucrose transport throughout the plant under extreme light limitation, the carbon supply being dependent on the starch breakdown in each tissue. The response of Z. noltii to gradual light reduction was co-ordinated at the whole-plant level, since an enhancement of proteolytic activities induced by carbon starvation in both above- and below-ground tissues was also recorded during prolonged light deprivation. Therefore, carbon mobilization was accompanied by enhanced protein turnover and changes in metabolic pathways.