Multi-temperature effects on Hill reaction activity of barley chloroplasts

Abstract

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 ( E a) for the reaction was 48.1 kJ/mol. 3. 3. In the presence of an uncoupler of photophosphorylation, methylamine, the E a 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 E a was highest below 9 °C at 63.7 kJ/mol. Between 9 and 20 °C the E a 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 E a 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 E a 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.