Strategies providing success in a variable habitat: III. Dynamic control of photosynthesis in Cladophora glomerata

Diurnal patterns of photosynthesis were studied in July and April populatio ns of Cladophora glomerata (L.) Kutz. from open and from shaded sites. Summ er samples exposed to full sunlight showed decreased efficiency of open pho tosystem II at noon, and only slight differences were found between samples that had grown at open or at shaded sites. Electron transport rate was lim ited at highest fluence rates in shade plants, and non-photochemical quench ing (NPQ) revealed faster regulation in samples from open sites. Daily cour se of de-epoxidation was not linearly correlated with the course of NPQ. Th e comparison of samples from open and from shaded sites revealed a higher c apacity of thermal energy dissipation and an increase in the total amount o f xanthophyll-cycle pigments (21%) in samples from open sites. In April, do wn-regulation of the efficiency of open photosystem II was related to lower water temperature, and hence, increased excitation pressure. In April the pool size of xanthophyll-cycle pigments was increased by 21% in comparison with summer and suggested higher levels of thermal energy dissipation via d e-epoxidized xanthophylls. In both, summer and spring the amount of xanthop hyll-cycle pigments was 20% higher in samples from open sites. Acclimation of C. glomerata to growth light conditions was further shown by experimenta l induction of NPQ, indicating NPQ increases of 23%, and increases of 77% i n the reversible component of NPQ in open site samples. The effect of tempe rature on photosynthetic rate was non-linear, and different optimum tempera tures of electron transport rate and oxygen evolution were exhibited.