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

Understanding photosynthetic acclimation to elevated CO (eCO) 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 (450 μmol mol) and eCO (900 μmol mol) for 6 weeks. Steady-state and dynamic photosynthetic and stomatal conductance (g) 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 with very different strategies: Cucumber (with the highest stomatal density) acclimated to eCO mostly via dynamic g responses, whereas chrysanthemum (with the lowest stomatal density) acclimated to eCO mostly via photosynthetic biochemistry. Tomato exhibited acclimation in both photosynthesis and g kinetics. eCO acclimation in individual stomatal pore movement increased rates of pore aperture changes in chrysanthemum, but such acclimation responses resulted in no changes in g responses. Although eCO acclimation occurred in all three crops, photosynthesis under fluctuating irradiance was hardly affected. Our study stresses the importance of quantifying eCO acclimatory responses at different integration levels to understand photosynthetic performance under future eCO environments.