Solution Effects on the Thermostability of Bean Chloroplast Thylakoids
Knowledge of the physicochemical nature of the strains imposed by heat stress on biological membranes requires a definition of other stresses that impose similar strains. To that end, this paper defines several solution properties that alter the effects of heat on isolated chloroplast thylakoid membranes. Isolated bean (Phaseolus vulgaris L.) ‘Oregon 1604’ thylakoids in various milieus were exposed to 25 or 45 °C, and the repercussion(s) of heat were determined. Raffinose (0.4–0.5 M) was slightly more thermostabilizing than 2 M sucrose as evidenced by an amelioration of the: (i) heat-induced decline electron transport, (ii) heat-induced alteration in the susceptibility of thylakoid polypeptides to trypsin digestion, and (iii) heat-induced decline in the maximum fluorescence transient measured at 685 rim. Equimolar glucose + fructose appeared to be equivalent in effectiveness to sucrose. Solutions with a pH of 5.5 or below and 7.0 or above appeared to increase the susceptibility of thylakoid electron transport to heat. Thermoprotection of electron transport to heat was afforded by aspartate at pH 6 and glutamate at pH 7. Dimethylsulfoxide and ethylene glycol appeared to provide some thermostability at concentrations tested (1 M and 100 mL/L, respectively). Urea appeared to mimic heat-induced injury. Exposure of thylakoids to 45 °C for 2 min appeared to cause the same degree of impairment as ca. 0.8 M urea, while exposure for 4 min appeared to cause the same as ca. 0.9 M urea. Finally, low concentrations (0.05-0.1 M) of mercaptoethanol appeared to exacerbate heat-induced thylakoid malfunction.
Copyright © 1990.