1. 1. The relationship between temperature and Hill reaction activity has been investigated in chloroplasts isolated from barley (Hordeum vulgare L. cv. Abyssinian).
2. 2. An Arrhenius plot of the photoreduction of 2,6-dichlorophenolindophenol (DCIP) showed no change in slope over the temperature range 2–38 °C. The apparent Arrhenius activation energy (Ea) for the reaction was 48.1 kJ/mol.
3. 3. In the presence of an uncoupler of photophosphorylation, methylamine, the Ea for DCIP photoreduction went through a series of changes as the temperature was increased. Changes were found at 9, 20, 29 and 36 °C. The Ea was highest below 9 °C at 63.7 kJ/mol. Between 9 and 20 °C the Ea decreased to 40.4 kJ/mol and again to 20.2 kJ/mol between 20 and 29 °C. Between 29 and 36 °C there was no further increase in activity with increasing temperature. The temperature-induced changes at 9, 20 and 29 °C were reversible. At temperatures above 36 °C (2 min) a thermal and largely irreversible inactivation of the Hill reaction occurred.
4. 4. Temperature-induced changes in Ea were also found when ferricyanide was substituted for DCIP or gramicidin D for methylamine. The addition of an uncoupler of photophosphorylation was not required to demonstrate temperature-induced changes in DCIP photoreduction following the exposure of the chloroplasts to a low concentration of cations.
5. 5. The photoreduction of the lipophilic acceptor, oxidized 2, 3, 5, 6-tetramethyl-p-phenylenediamine, also showed changes in Ea in the absence of an uncoupler.
6. 6. The temperature-induced changes in Hill activity at 9 and 29 °C coincided with temperature-induced changes in the fluidity of chloroplast thylakoid membranes as detected by measurements of electron spin resonance spectra. It is suggested that the temperature-induced changes in the properties and activity of chloroplast membranes are part of a control mechanism for regulation of chloroplast development and photosynthesis by temperature.
Abbreviations: DADox, oxidized 2,3,5,6-tetramethyl-p-phenylenediamine; DCIP, 2,6-dichlorophenolindophenol; 16NS, 3-oxazolidenyloxy-2-(14-carbmethoxytetradecyl)-2-ethyl-4,4-dimethyl; Ea, Arrhenius activation energy
