Irradiation of thylakoid membranes at 40 °C resulted in complete inhibition of photosystem (PS) 2 activity measured as 2,6-dichlorophenol indophenol (DCIP) photoreduction either in the absence or presence of 1,5-diphenylcarbazide (DPC). Concomitant with the inactivation of PS2 activity, several thylakoid proteins were lost and high molecular mass cross-linking products appeared that cross-reacted with antibodies against proteins of PS2 but not with antibodies against proteins of other three complexes PS1, ATP synthase, and cytochrome b6f. Irradiation of thylakoid membranes suspended in buffer of basic pH or high concentration of Tris at 25 °C resulted in the formation of cross-linking products similar to those in thylakoid membranes irradiated at 40 °C. Presence of radical scavengers and DPC during the high temperature treatment prevented the formation of cross-linking products. These results suggest the involvement of oxygen evolving co mplex (OEC) in the formation of cross-linking between PS2 proteins in thylakoid membrane irradiated at high temperature. and Abhay K. Singh, G. S. Singhal.
2-years-old cypress needles (A2) were physiologically most active with regard to net photosynthetic (PN) and electron transport rates. Variable to maximum fluorescence (Fv/Fm) ratios of dark-adapted needles were higher in A2 needles than in current year (A1) or senescent (A4) needles. Lower Fv/Fm values in these stages seemed to be caused not by photoinhibition but by a low photochemical capacity as suggested from the chlorophyll (Chl) a/b ratios. In isolated thylakoids, lower rates of whole chain and photosystem 2 (PS2) activities were observed in A4 needles, while higher rates were observed in A2 needles. A similar trend was noticed for contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPC) and total soluble proteins. The artificial exogenous electron donor Mn2+ failed to restore the loss of PS2 activity in 3-year-old (A3) and A4 needles, while diphenylcarbazide and NH2OH significantly restored the loss of PS2 activity. The marked loss of PS2 activity in A4 needles was primarily the result of the loss of 33, 28-25, 23, and 17 kDa polypeptides. A marked loss of RuBPC activity in A4 needles is mainly due to the loss of 15 (SSU) and 55 (LSU) kDa polypeptides. and N. La Porta ... [et al.].