title: Multi-temperature effects on Hill reaction activity of barley chloroplasts.
abstract: 1. The relationship between temperature and Hill reaction activity has been investigated in chloroplasts isolated from barley (Hordeum vulgare L. cv. Abyssinian). 2. An Arrhenius plot of the photoreduction of 2,6-dichlorophenolindophenol (DCIP) showed no change in slope over the temperature range 2--38degreesC. The apparent Arrhenius activation energy (Ea) for the reaction was 48.1 kJ/mol. 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 36degreesC. The Ea was highest below 9degreesC at 63.7 kJ/mol. Between 9 and 20degreesC the Ea decreased to 40.4 kJ/mol and again to 20.2 kJ/mol between 20 and 29degreesC. Between 29 and 36degreesC there was no further increase in activity with increasing temperature. The temperature-induced changes at 9, 20 and 29degreesC were reversible. At temperatures above 36degreesC (2 min) a thermal and largely irreversible inactivation of the Hill reaction occurred. 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. 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. The temperature-induced changes in Hill activity at 9 and 29degreesC 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.
mesh_terms: D015086:2,6-Dichloroindophenol / Q000494:pharmacology; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D004579:Electron Transport; D006096:Gramicidin / Q000494:pharmacology; D001467:Hordeum / Q000378:metabolism; D007700:Kinetics; D010785:Photophosphorylation / Q000187:drug effects; D010944:Plants / Q000378:metabolism; D013696:Temperature
chemical_list: D006096:Gramicidin; D015086:2,6-Dichloroindophenol