The Greenhouse Effect: Acclimation of Tomato Plants Growing in High CO2, Photosynthesis and Ribulose-1, 5-Bisphosphate Carboxylase Protein

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Abstract
Tomato plants were grown in solution culture in a controlled environment at 20 °C with a 12 h photoperiod of 400 μmol quanta m−2 s−1PAR with either normal ambient CO2, approximately 340 vpm, or with 1000 vpm CO2. The short- and long-term effects of CO2 enrichment on photosynthesis were determined together with the levels of ribulose-1, 5-bisphosphate carboxylase (RuBPco) E.C. 4.1.1.39 protein and activity throughout leaf development of the unshaded 5th leaf above the cotyledons. The high CO2 concentration during growth did not appreciably affect the rate of leaf expansion or final leaf area but did increase the fresh weight per unit area of leaf. With short-term CO2 enrichment, i.e. only during the photosynthesis measurements, the light-saturated photosynthetic rate (Pmax) of young leaves did not increase while those 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. While the CO2 concentration during growth did not affect the peak in Pmax measured in 300 vpm CO2 or Pmax in 1000 vpm CO2, RuBPco protein or its activity, the subsequent ontogenetic decline in these parameters was greatly accelerated by the high CO2 treatment. Compared with plants grown in normal ambient CO2 the high CO2 grown leaves, when almost fully expanded, contained only approximately half as much RuBPco protein and Pmax in 300 vpm CO2 and Pmaxin 1000 vpm CO2 were similarly reduced. The loss of RuBPco protein may be a major factor associated with the accelerated fall in Pmax since it was close to that predicted from the amount and kinetics of RuBPco assuming RuBP saturation. In the oldest leaves examined grown in high CO2 additional factors may be limiting photosynthesis since RuBPco kinetics marginally overestimated Pmax in 300 vpm CO2 and the initial slope of photosynthesis in response to intercellular CO2 was also less than expected from the extractable RuBPco.