Does growth under elevated CO2 moderate photoacclimation in rice?

Acclimation of plant photosynthesis to light irradiance (photoacclimation) involves adjustments in levels of pigments and proteins and larger scale changes in leaf morphology. To investigate the impact of rising atmospheric CO2 on crop physiology, we hypothesize that elevated CO2 interacts with photoacclimation in rice (Oryza sativa). Rice was grown under high light (HL: 700 mu molm2s1), low light (LL: 200 mu molm2s1), ambient CO2 (400 mu ll1) and elevated CO2 (1000 mu ll1). Leaf six was measured throughout. Obscuring meristem tissue during development did not alter leaf thickness indicating that mature leaves are responsible for sensing light during photoacclimation. Elevated CO2 raised growth chamber photosynthesis and increased tiller formation at both light levels, while it increased leaf length under LL but not under HL. Elevated CO2 always resulted in increased leaf growth rate and tiller production. Changes in leaf thickness, leaf area, Rubisco content, stem and leaf starch, sucrose and fructose content were all dominated by irradiance and unaffected by CO2. However, stomata responded differently; they were significantly smaller in LL grown plants compared to HL but this effect was significantly suppressed under elevated CO2. Stomatal density was lower under LL, but this required elevated CO2 and the magnitude was adaxial or abaxial surface-dependent. We conclude that photoacclimation in rice involves a systemic signal. Furthermore, extra carbohydrate produced under elevated CO2 is utilized in enhancing leaf and tiller growth and does not enhance or inhibit any feature of photoacclimation with the exception of stomatal morphology.