ISSN:
1432-2048
Keywords:
Calvin cycle
;
Electrochromic shift
;
Light scattering
;
Ribulose-1,5-bisphosphate carboxylase
;
Spinacia
;
Temperature regulation
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Abstract Photosynthetic CO2 fixation rates in leaves and intact chloroplasts of spinach measured at 18°–20° C are substantially decreased by pretreatment at temperatures exceeding 20° C. Mild heating which causes 80% inhibition of CO2 fixation does not affect phosphoglyceroacid reduction and causes increases in the ATP/ADP ratio and the light-induced transthylakoid proton gradient. The inactivation of the CO2 fixation is completely reversible with half-times of recovery in the order of 15–20 min. Comparison of steady-state patterns of 14C labeled Calvin cycle intermediates of heat-treated and control samples reveals a large increase in the ribulose-1,5-bisphosphate/phosphoglyceroacid ratio and a large decrease in the phosphoglyceroacid/triosephosphate ratio. It is concluded that inactivation of CO2 fixation occurring at elevated temperatures is caused by inhibition of the ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). Measurements of light-induced light scattering changes of thylakoids and of the light-induced electrochromic absorption shift show that these signals are affected by mild heating in a way which is strictly correlated with the inactivation of the CO2 fixation. It is proposed that the function of the ribulose-1,5-bisphosphate carboxylase in vivo requires a form of activation that involves properties of the thylakoid membrane which are affected by the heat treatment. The fact that these changes in thylakoid membrane properties and of ribulose-1,5-bisphosphate carboxylase activity are already affected at elevated temperatures which can still be considered physiological, and the reversible nature of these changes, suggest that they may play a role in temperature regulation of the overall photosynthetic process.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00384234
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