CHARACTERIZATION OF THE HEAT-INDUCED STIMULATION OF PHOTOSYSTEM-I-MEDIATED ELECTRON-TRANSPORT

Uncoupled Photosystem I (PS I) activity driven by either reduced 2,6-d ichlorophenolindophenol (DCPIPH2) or N,N,N',N'-tetramethyl-p-phenylene diamine (TMPDH2) showed a stimulation following pre-treatment at 40-5 0-degrees-C, followed by inhibition at higher temperatures. The stimul ation was more marked in thylakoids isolated from warm-grown plants th an in those isolated from cool-grown plants. Approximately maximal rat es of PS I activity were achieved at physiological temperatures (25-35 -degrees-C) using highly lipophilic-reduced diaminodurene (DADH-2) as an electron donor with little stimulation occurring as a result of hig h temperature pre-treatment. Electron transport driven by all three el ectron donors was markedly inhibited by KCN/HgCl2 following all pre-tr eatment temperatures, but relatively insensitive to ethyldimethylamino propyl-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 o f cytochrome f. The greater stimulation of electron transport in thyla koids isolated from warm-grown plants may be due to a more pronounced phase change occurring as a consequence of altered membrane compositio n as modified by growth temperature.