ACCLIMATION OF PHOTOSYNTHESIS IN RELATION TO RUBISCO AND NONSTRUCTURAL 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 C-3 photosynthesis, had been exposed to normal ambient and elevated atmospheric CO2 concentrations (C-a) 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 C-a 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 C-a had a significantly higher (30-59%) net CO2 assimilation rate (A) than those grown at ambient C-a when measurements were performed on excised stems at the respective growth C-a. However, when measurements were made at normal ambient C-a, A was smaller (45-53%) in plants grown at elevated C-a than in those grown at ambient C-a. The reductions in A at normal ambient C-a, carboxylation efficiency and in situ carboxylase activity were caused by a decreased Rubisco concentration (30-58%) in plants grown at elevated C-a; 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 C-a. When measurements were made on intact stems in the field, the elevated C-a 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 C-a contained more non-structural carbohydrates (25-53%) than those grown at ambient C-a. Plants grown at elevated C-a 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 C-a leads to an increased carbohydrate concentration and the ensuing acclimation of the photosynthetic apparatus in C-3 plants results in a reduction in the protein complement, especially Rubisco, which reduces the photosynthetic capacity in plants grown at elevated C-a, relative to plants grown at normal ambient C-a. Nevertheless, when compared at their respective growth C-a, Scirpus olneyi plants grown at elevated C-a in their native habitat maintained a substantially higher rate of photosynthesis than those grown at normal ambient C-a even after 8 years of growth at elevated C-a.