Diurnal and seasonal variations in activity of crassulacean acid metabolism and plant water status in a northern latitude population of Opuntia erinacea.

A northern latitude population of Opuntia erinacea in eastern Washington State, U.S.A. was investigated with regard to daily and seasonal activity of Crassulacean acid metabolism and water relations. Fresh samples were collected throughout the light and dark periods on 21 dates between October 1979 and December 1980. Daily activity of CAM varied seasonally with daily maximum titratable acidity values ranging from 190 μEq·g(-1) fresh weight in October 1979 to 84 μEq·g(-1) in January 1980, to 230 μEq· g(-1) fresh weight in June 1980. Daily patterns of acid fluctuation were generally typical of CAM plants. Afternoon increases in acid concentration were noticed occasionally, and morning peaks in acidity were frequently observed. Plant water potentials were generally more closely correlated with median cladophyll temperature than with precipitation patterns, exhibiting sharply increased negativity around 6C. The data suggest the importance of available moisture and temperature in regulation of CAM, the existence of a cryoprotection mechanism involving reduction of plant water content, and the necessity of spring recovery for post-winter resumption of CAM activity.