Photosynthetic electron transport and electrochromic effects at sub-zero temperatures.

Spinach chloroplasts, suspended in a liquid medium containing ethyleneglycol, showed reversible absorbance changes near 700 and 518 nm due to P-700 and "P-518" in the region from -35 to -50 degrees C upon illumination. The kinetics were the same at both wavelengths, provided absorbance changes due to Photosystem II were suppressed. At both wavelengths, the decay was slowed down considerably, not only by the System I electron acceptor methyl viologen, but also by silicomolybdate. The effect of the latter compound is probably not due to the oxidation of the reduced acceptor of Photosystem I by silicomolybdate, but to the enhanced accessibility of the acceptor to some other oxidant. In the presence of both an electron donor and acceptor for System I, a strong stimulation of the extent of the light-induced absorbance increase at 518 nm was observed. The most effective donor tested was reduced N-methylphenazonium methosulphate (PMS). The light-induced difference spectrum was similar to spectra obtained earlier at room temperature, and indicated electrochromic band shifts of chlorophylls a and b and carotenoid, due to a large potential over the thylakoid membrane, caused by sustained electron transport. It was estimated that steady-state potentials of up to nearly 500 mV were obtained in this way; the potentials reversed only slowly in the dark, indicating a low conductance of the membrane. This decay was accelerated by gramicidin D. The absorbance changes were linearly proportional to the membrane potential.