Characterization of the heat-induced stimulation of Photosystem-I-mediated electron transport

Uncoupled Photosystem I (PS I) activity driven by either reduced 2,6-dichlorophenolindophenol (DCPIPH2) or N,N,N',N'-tetramethyl-p-phenylene diamine (TMPDH2) showed a stimulation following pre-treatment at 40-50°C, followed by inhibition at higher temperatures. The stimulation was more marked in thylakoids isolated from warm-grown plants than in those isolated from cool-grown plants. Approximately maximal rates of PS I activity were achieved at physiological temperatures (25-35°C) using highly lipophilic-reduced diaminodurene (DADH2) as an electron donor with little stimulation occurring as a result of high temperature pre-treatment. Electron transport driven by all three electron donors was markedly inhibited by KCN/HgCl2 following all pre-treatment temperatures, but relatively insensitive to ethyldimethylaminopropyl-carbodiimide (EDAC). It is suggested that the stimulation of PS I activity involves a phase change in the thylakoid membrane leading to an increase in permeability which allows enhanced access of DCPIPH2 and TMPDH2 to a common electron acceptor site located in the region of cytochrome f. The greater stimulation of electron transport in thylakoids isolated from warm-grown plants may be due to a more pronounced phase change occurring as a consequence of altered membrane composition as modified by growth temperature.