Acclimation of photosynthesis in relation to Rubisco and non-structural carbohydrate contents and in situ carboxylase activity in Scirpus olneyi grown at elevated CO2 in the field

Stands of Scirpus olneyi, a native saltmarsh sedge with C3 photosynthesis, had been exposed to normal ambient and elevated atmospheric CO2 concentrations (C.) in their native habitat since 1987. The objective of this investigation was to characterize the acclimation of photosynthesis of Scirpus olneyi stems, the photosynthesizing organs of this species, to long-term elevated Ca treatment in relation to the concentrations of Rubisco and non-structural carbohydrates. Measurements were made on intact stems in the field under existing natural conditions and in the laboratory under controlled conditions on stems excised in the field early in the morning. Plants grown at elevated Ca had a significantly higher (30-59%) net CO2 assimilation rate (A) than those grown at ambient Ca when measurements were performed on excised stems at the respective growth Ca. However, when measurements were made at normal ambient Ca, A was smaller (45-53%) in plants grown at elevated Ca than in those grown at ambient Ca. The reductions in A at normal ambient Ca, carboxylation efficiency and in situ carboxylase activity were caused by a decreased Rubisco concentration (30-58%) in plants grown at elevated Ca; these plants also contained less soluble protein (39-52%). The Rubisco content was 43 to 58% of soluble protein, and this relationship was not significantly altered by the growth CO2 concentrations. The Rubisco activation state increased slightly, but the in situ carboxylase activity decreased substantially in plants grown at elevated Ca. When measurements were made on intact stems in the field, the elevated Ca treatment caused a greater stimulation of A (100%) and a smaller reduction in carboxylation efficiency (which was not statistically significant) than when measurements were made on excised stems in the laboratory. The possible reasons for this are discussed. Plants grown at elevated Ca contained more non-structural carbohydrates (25-53%) than those grown at ambient Ca. Plants grown at elevated Ca appear to have sufficient sink capacity to utilize the additional carbohydrates formed during photosynthesis. Overall, our results are in agreement with the hypothesis that elevated Ca leads to an increased carbohydrate concentration and the ensuing acclimation of the photosynthetic apparatus in C3 plants results in a reduction in the protein complement, especially Rubisco, which reduces the photosynthetic capacity in plants grown at elevated Ca, relative to plants grown at normal ambient Ca. Nevertheless, when compared at their respective growth Ca, Scirpus olneyi plants grown at elevated Ca in their native habitat maintained a substantially higher rate of photosynthesis than those grown at normal ambient Ca even after 8 years of growth at elevated Ca.