Physiological and growth responses of Thalassia testudinum to environmentally-relevant periods of low irradiance

Turtle grass (Thalassia testudinum) was exposed to either Control (6 weeks of unscreened irradiance), Long Period (3 weeks of screened [8% of unscreened] followed by 3 weeks of unscreened irradiance), or Short Period (alternating 7-day periods of 8% and 100% unscreened irradiance for a total of 6 weeks) treatment. Six-week experiments were conducted during late winter (February-March) and summer (May-July), and indices of carbon (C) and nitrogen (N) metabolism and growth were monitored. The uptake of inorganic N (NH4+, [NO2- + NO3-]) and phosphorus (PO43-) by seagrasses and epiphytes was greater during the unscreened phases than during the screened phases of the treatments. Soluble carbohydrate content of leaf tissue of T. testudinum declined during the screened phases of the experimental treatments and increased under the unscreened phases while levels in rhizome tissue did not vary significantly. Leaf elongation rates of the Control and Short treatments followed similar patterns during both experiments, while rates under the Long treatment were depressed by an average of 18%. Shoot glutamine synthetase (GS) activity also responded to light availability, decreasing immediately when the plants were screened and then increasing upon removal of the screens. The magnitude of the decrease in GS activity depended on season (greater during summer). Only GS activity differed significantly as a function of treatment on the last (42nd) day of the experiments. Overall, the physiology of T. testudinum was resistant to change and resilient following severe, periodic light limitation on ecologically relevant, intermediate time scales. (C) 2000 Elsevier Science B.V. All rights reserved.