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Thalli of the chlorophyte macroalga Ulva rotundata were acclimated to sun (100 % I,) and shade (9% I,) in outdoor tumble tanks under nutrient-sufficient conditions. Diurnal patterns were determined for area1 photosynthetic O2 exchange capacity (P,,) and quantum yleld (@), respiration (Rd) , and photosystem (PS) I1 chlorophyll fluorescence using 2 independent techniques. Net P, was maximal at midday in sun-grown thalli and at midmorning in shade-grown thalli exposed to 100 "L I,. A slight afternoon decline in net P, in shade-grown thalli exposed to 100 % I, could be explained entirely by a concurrent rise in Rd. Sun thalli exhibited a larger diurnal fluctuation of net P, which was inconsistent with photoinhibition or end product inhibition by accumulated photosynthate. This leaves endogenous control as the most likely cause, with slight modification by Rd Fluorescence data were consistent with simultaneous damage to PSI1 and photoprotection by thermal deexcitation, with sun thalli showing greater capacity for protection. Both of these processes reduced @ but apparently did not affect P,,. In that U. rotundata thalli were light-saturated in our tumble tanks for most of the day, we conclude that photoinhibition (E damage + protection) had little effect on ~ntegrated diurnal production under these conditions. Our results demonstrate the utility of fluorescence analysis and the importance of precise determination of P,, @ and Rd in ascribing mechanisms to diurnal fluctuations in photosynthetic performance. The validity of using photosynthetic light-response (P-I) curves determined at one time of day to predict integrated diurnal production will depend on the extent to which P,, @ and Rd vary. This in turn depends on species, irradiance history, and possibly other factors such as temperature and nutrition.

Diurnal responses of photosynthesis and fluorescence in Ulva rotundata acclimated to sun and shade in outdoor culture