PHOTOSYNTHETIC ACCLIMATION IN TOMATO PLANTS GROWN IN HIGH CO2

The effects of prolonged CO2 enrichment of tomato plants on photosynthetic performance and Calvin cycle enzymes, including the amount and activity of ribulose-1,5-bisphosphate carboxylase (RuBPco), were determined. Also the light-saturated rate of photosynthesis (P(max)) of the 5th leaf throughout leaf development was predicted based on the amount and kinetics of RuBPco. With short-term CO2 enrichment, i.e. only during the photosynthesis measurements, P(max) of the young leaves did not increase while the leaves reaching full expansion more than doubled their net rate of CO2 fixation. However, with longer-term CO2 enrichment, i.e. growing the crop in high CO2, the plants did not maintain this photosynthetic gain. Compared with leaves of plants grown in normal ambient CO2 the high CO2-grown leaves, when almost fully expanded, contained only about half as much RuBPco protein and P(max) in 300 and 1000 vpm CO2 was similarly reduced. The loss of RuBPco protein may be a factor associated with the accelerated fall in P(max) since P(max) was close to that predicted from the amount and kinetics of RuBPco assuming RuBP saturation. Acclimation to high CO2 is fundamentally different from acclimation to high light. In contrast to acclimation to high light, acclimation to high CO2 does not usually involve an increase in photosynthetic machinery so the synthesis and maintenance costs (as indicated by the dark respiration rate) are generally lower.