Growth and photosynthetic responses of two subtropical seagrasses, Thalassia testudinum and Halodule wrightii to in situ manipulations of irradiance




Czerny, Andrew Barthel

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The growth and photosynthetic responses of two species of subtropical seagrasses, Thalassia testudinum and Halodule wrightii, were examined in relation to reductions in underwater light. Shade screens reduced irradiance to roughly 30% and 20% of in situ ambient (ISA). Short shoots of T. testudinum had a greater ability to tolerate a ten month period of light reduction as demonstrated by the longer survival of plants exposed to 30% ISA. Evidence for photoadaptation was observed in plants from shaded treatments compared to unshaded controls (plants receiving 100% ISA). Leaf elongation rates were lower in T. testudinum plants exposed to 20% ISA for one month (October 1992 to November 1992) compared to plants receiving 100% ISA, but there were no differences between treatments for the months of January, February, March, and May. However, by July both levels of shaded T. testudinum had significantly lower growth rates compared to plants receiving 100% ISA. Water temperatures above 25°C and the depletion of stored reserves by spring growth most likely contributed to the disappearance of all short shoots of T. testudinum shaded at 20% ISA and both shaded plots of H. wrightii by August 1993. Photosynthesis versus irradiance (P vs. I) parameters revealed significant differences in the rates of light saturated photosynthesis (P [subscript max]), respiration, saturation irradiance (I [subscript k]), and relative quantum efficiency (α) between the two species. In particular, the significantly lower respiration rates of Thalassia testudinum (89.8 ± 8.4 μmol O₂ gdw⁻¹ h⁻¹ at 30°C) compared to Halodule wrightii (186.0 ± 20.0 μmol O₂ gdw⁻¹ h⁻¹ at 30°C ) may contribute to its survival in light-limited environments. Respiration rates and P [subscript max] for both species significantly increased with seasonal increases in temperature. No significant differences were noted in shaded versus unshaded H. wrightii at 20°C or 30°C; however, in T. testudinum, after four months of shading at 20% ISA, P [subscript max] was significantly lower and α significantly higher relative to unshaded controls. The lower growth rates and increased light harvesting capability of T. testudinum in response to reduced irradiance reflects the K-selected strategy of this species compared to H. wrightii