Changes in freezing tolerance and polypeptide content of spinach and citrus at 5 °C

Abstract

The freezing tolerance of spinach leaf, petiole, hypocotyl, and root tissues and citrus leaf tissue was determined before and after 1 week of cold acclimation at 5 °C. Spinach leaf, petiole, and hypocotyl tissues increased in freezing tolerance upon exposure to 5 °C, while root tissue failed to show significant increased freezing tolerance. Citrus leaf tissue also increased in freezing tolerance following exposure to 5 °C. The protein composition of nonacclimated and cold-acclimated spinach leaf, hypocotyl, and root tissues and citrus leaf tissue was studied using two-dimensional gel electrophoresis and silver staining. Spinach leaf tissue exposed to 5 °C contained two cold-acclimation polypeptides (CAPs) ( M r 160,000 and 85,000) not found in nonacclimated leaf tissue. Numerous other changes in the polypeptide composition of spinach leaf tissue were observed following cold acclimation, but the appearance of CAPs 160 and 85 were the most consistent in all experiments. Cold-acclimated citrus leaf tissue also contained a polypeptide ( M r 160,000) not found in nonacclimated citrus leaf tissue. The similarity in molecular weight and isoelectric point between the CAP 160 of citrus and spinach suggests that these widely unrelated species may synthesize the same protein in response to exposure to low temperature. In contrast, both nonacclimated and cold-acclimated spinach hypocotyl tissue contained CAPs 85 and 160. However, cold-acclimated hypocotyls appeared to have concentrations of CAPs 85 and 160 higher than those of nonaccclimated hypocotyls. Protein analysis of nonacclimated and cold-acclimated spinach root tissue revealed no major qualitative differences. Furthermore, roots exposed to 5 °C did not synthesize proteins analogous to any of the CAPs synthesized in leaf tissue at low temperature. The inability of root tissue to synthesize CAPs at low temperature may be a factor in the lack of root freezing tolerance.