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

Abstract

Thalli of Ulva rotundata were acclimated to sun (100% Io) and shade (9% Io) in outdoor tumble tanks under nutrient-sufficient conditions. Diurnal patterns were determined for areal photosynthetic O2 exchange capacity (Pm) and quantum yield, respiration (Rd), and photosystem (PS) II chlorophyll fluorescence. Net Pm was maximal at midday in sun-grown thalli and at midmorning in shade-grown thalli exposed to 100% Io. A slight afternoon decline in net Pm in shade-grown thalli exposed to 100% Io could be explained entirely by a concurrent rise in Rd. Sun thalli exhibited a larger diurnal fluctuation of net Pm 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. -from Authors