Seasonal variation in ribulose bisphosphate carboxylase activity in Pinus silvestris

Ribulose bisphosphate carboxylase activity was examined in Pinus silvestris L. during successive seasons. The enzyme activities were studied both in seedlings, kept under controlled conditions in a climate chamber, and in needles from a 15-year-old tree in a natural stand. The enzyme activities were analysed in cell-free extracts prepared with Tween 80 as protective agent. The carboxylase activity fluctuated periodically both in the seedlings and in the natural stand. In the seedlings, the weight-related activity in the older needles increased 50–100% (in the cotyledons c. 200%) in the beginning of the “summer”. It decreased as the new shoot developed. The specific activity increased c. 100%. With chlorophyll as base, the activity usually decreased during “summer”. In the developing current needles the carboxylase activity increased when expressed on a weight or on a protein basis. The decrease in weight-related carboxylase activity in the older needles was preceded by, or simultaneous with, loss of total protein. It is suggested that protein, including the carboxylase, is utilized as nitrogen reserve for the new shoot. During hardening by combined photoperiod and thermoperiod, the carboxylase activity decreased when expressed relative to dry weight and protein. Calculated on a chlorophyll basis, the activity was rather constant. In the natural stand the activity in the one- and two-year-old needles increased during spring and summer and decreased during autumn and winter. Even at severe winter stress substantial carboxylase activity remained in the needles. The activity of the enzyme in vivo is discussed with respect to electron transport and net photosynthesis.