Biochemical versus stomatal acclimation of dynamic photosynthetic gas exchange to elevated CO 2 in three horticultural species with contrasting stomatal morphology.

Understanding photosynthetic acclimation to elevated CO 2 (eCO 2 ) is important for predicting plant physiology and optimizing management decisions under global climate change, but is underexplored in important horticultural crops. We grew three crops differing in stomatal density-namely chrysanthemum, tomato, and cucumber-at near-ambient CO 2 (450 μmol mol -1 ) and eCO 2 (900 μmol mol -1 ) for 6 weeks. Steady-state and dynamic photosynthetic and stomatal conductance (g s ) responses were quantified by gas exchange measurements. Opening and closure of individual stomata were imaged in situ, using a novel custom-made microscope. The three crop species acclimated to eCO 2 with very different strategies: Cucumber (with the highest stomatal density) acclimated to eCO 2 mostly via dynamic g s responses, whereas chrysanthemum (with the lowest stomatal density) acclimated to eCO 2 mostly via photosynthetic biochemistry. Tomato exhibited acclimation in both photosynthesis and g s kinetics. eCO 2 acclimation in individual stomatal pore movement increased rates of pore aperture changes in chrysanthemum, but such acclimation responses resulted in no changes in g s responses. Although eCO 2 acclimation occurred in all three crops, photosynthesis under fluctuating irradiance was hardly affected. Our study stresses the importance of quantifying eCO 2 acclimatory responses at different integration levels to understand photosynthetic performance under future eCO 2 environments.