Alterations in the physiology of CO₂ exchange in tomato plants grown in CO₂-enriched atmospheres

Young, greenhouse-grown tomato plants were transferred to growth cabinets where they were maintained in normal air (0.03% CO 2 ) or in air enriched to 0.1 or 0.5% CO 2 . CO 2 enrichment increased net assimilation rate but decreased leaf area ratio. As a result, relative growth rate was greatest at 0.1% CO 2 and was less in 0.5% CO 2 than in 0.03% CO 2 . Gas exchange measurements were made on the third true leaf of plants from different CO 2 regimes. They indicated that growth under conditions of CO 2 enrichment affected photosynthesis at an early stage of leaf development (leaf plastochron index (PI) = 5) but not at a later stage (PI = 10.5). The effects were linked to changes in mesophyll resistance, not stomatal resistance. At PI = 5 and under equivalent test conditions of irradiance and CO 2 concentration, net photosynthesis tended to be increased following growth in 0.1% CO 2 but was decreased or unchanged by 0.5% CO 2 . Young leaves developed in 0.1% CO 2 were less subject to photosynthetic inhibition by atmospheric oxygen and had low CO 2 compensation points. CO 2 enrichment also affected the activities of the enzymes ribulose-1,5-bisphosphate carboxylase and glycolic acid oxidase and the enzyme responses corresponded well with the gas exchange responses. The results indicate that photosynthetic adaptations may occur in response to the concentration of CO 2 present during growth, and that enrichment to concentrations much above 0.1% CO 2 may be detrimental to net photosynthesis and growth rate.