This paper details experiments performed to investigate the short-term stress effects of both high and low-light regimes on laboratory-cultured Halophila ovalis using chlorophyll fluorescence. Increasing irradiance up to 400 μmol quanta m −2 s −1 on laboratory-cultured H. ovalis caused a range of photoprotective (PSII downregulation) responses, depending on intensity and the exposure period. The 400 μmol quanta m −2 s −1 treatment appeared to inhibit the photosynthetic rate, although this was not permanent, as the tissue rapidly recovered. Low-light conditions over periods of up to 10 days did not adversely affect H. ovalis . The efficiency of photon capture increased, and the proportion of open PSII reaction centres was greater under lower PPFD conditions. The rate of onset and the degree of impact of the stress as identified by chlorophyll fluorescence was noticeably faster (within 1 h) at elevated PPFD, as opposed to reduced light conditions. Recovery from high-light stress was rapid, as demonstrated by the qP and quantum yield responses; however, the qN coefficient was much slower to adjust to altered ambient light regimes. The range of PPFD investigated indicated the plants' capacity to adjust to modified light regimes, and also showed the significance of chlorophyll fluorescence response to environmentally relevant PPFD levels. Pigment analysis supported the conclusions drawn from the chlorophyll fluorescence results. The 400 μmol quanta m −2 s −1 treatment resulted in a decrease in chlorophyll concentrations, with chlorophyll b suffering a greater decline. The 200 μmol quanta m −2 s −1 treatment was similar to the control with respect to pigment concentration. Carotenoids were not affected by elevated light. Reduced light caused an increase in both the chlorophyll and carotenoid pigments.
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