Results for the Indoor Halodule Experiments:

Halodule wrightii (shoalgrass) plants were grown under controlled light conditions for 16 weeks (Jul08 - Oct27, 2003), in a range of light intensities (Table 1). All but the greatest light treatment result in a deficit of light to saturate photosynthesis, ultimately resulting in mortality. Mortality in response to light-limitation occurred from the base up, with the entire leaf breaking off at the sheath. This observation may be important in signaling shoalgrass mortality under natural situations.

Table 1: Seven irradiance treatments used for indoor light experiment. Both instantaneous and integrated (12hr) irradiance fluxes are given, as well as integrated irradiance greater than the saturation irradiance (approx. 150uE) and compensation irradiance (approx. 10uE) for Halodule. Numbers in red indicate light deficits for photosynthesis.

Fig 1: Water temperatures in the indoor tank during the 16 week experiment. Water temperatures were recorded using Onset StowAway Tidbit loggers (www.onsetcomp.com) at half hour time intervals in the highest light (tank1) and lowest light (tank6).

 

Fig 2: Plants, starting with 2 shoots, were grown for the duration of the summer growth period. Branching or spatial expansion was observed in the highest irradiance treatment only. Shoot loss and/or mortality were observed at irradiances of 60uE or lower.

Fig 3: The length of the longest leaf on each surviving plant was measured as a proxy for canopy height. Leaf length was maintained only in the two highest light levels (irradiance = 95uE). Early mortality (leaf len = 0) was observed in the lowest light level, with evidence of plant stress by weeks 3-4, about half the time to mortality.

Fig 4: Leaf area (single surface) was calculated from maximum length x average width. Width was measured on alternative sampling dates on a random subset of 3-5 leaves, with linear interpolation for non-sampled dates. Area is primarily influenced by leaf length, as width remained approximately constant within a given light treatment..

Fig 5: Photosynthesis was measured with a Hansatech PEA (Plant Efficiency Analyzer). Photosynthetic yield, expressed as a ratio Fv/Fm ranged from 0.65-0.78 in healthy plants, and fell below 0.5 in plants that were dead or dying. No significant difference in Fv/Fm was observed in the apparently healthy (live) plants across all seven light treatments, indicating acclimation of the photosynthetic apparatus to the ambient light field. We conclude that the yield ratio (Fv/Fm) is not a sensitive measure of chronic light stress conditions to which the tropical seagrass, Halodule, and temperate eelgrass can photoacclimate.

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