The role of nitrogen in photosynthetic acclimation to elevated [CO ₂ ] in tomatoes

Background: Although elevated [CO ₂ ] causes an increase of photosynthesis in the short-term, this increase is often attenuated over time due to decreased photosynthetic capacity of the leaf in a process called photosynthetic acclimation to elevated CO ₂ (PAC). PAC is often accompanied by N deficiency and sink:source imbalance. The aim of this study is to investigate mechanisms that lead to PAC, N deficiency and sink:source imbalance in tomato plants grown in elevated [CO ₂ ] and how they are affected by different levels of N treatments. Methods: Two long-term experiment and two short-term experiments were conducted in which tomato plants were grown in chambers with ambient [CO ₂ ] and elevated [CO ₂ ] combined with different levels of N nutrition. The following parameters were measured: 1) Biomass 2)Leaf N, P and K concentrations, 3) leaf NO ₃ ⁻ concentration, 4) Gas exchange 5) Rubisco expression and 6) Leaf starch concentration. Results: Plants grown at e[CO ₂ ] had increased biomass and starch, and decreased gas exchange, stomatal conductivity, Rubisco expression, V _cmax , NPK and leaf NO ₃ ⁻ . Increasing N fertilization counteracted many of the effects of elevated [CO ₂ ]. Conclusions: PAC was caused by decreased N uptake or transport coupled with increased growth which leads to N deficiency and a sink:source imbalance. Increased N fertilization counteracted the effect of e[CO ₂ ] on photosynthesis, N status, and sink:source imbalance. Furthermore, elevated [CO ₂ ] caused stomata to partially close, which accounted for some of the PAC observed.