Photosynthesis and photoprotection in top leaves respond faster to irradiance fluctuations than bottom leaves in a tomato canopy

Accounting for the dynamic responses of photosynthesis and photoprotection to naturally fluctuating irradiance can improve predictions of plant performance in the field, but the variation of these dynamics within crop canopies is poorly understood. We conducted a detailed study of dynamic and steady-state photosynthesis, photoprotection, leaf pigmentation, and stomatal anatomy in four leaf layers (100, 150, 200 and 250 cm from the floor) of a fully-grown tomato canopy in the greenhouse. We found that leaves at the top of the canopy exhibited higher photosynthetic capacity and slightly faster photosynthetic induction compared to lower-canopy leaves, accompanied by higher stomatal conductance and a faster activation of carboxylation and linear electron transport capacities. In upper-canopy leaves, non-photochemical quenching showed faster induction and relaxation after in- and decreases in irradiance, allowing for more effective photoprotection in these leaves. Despite these observed differences in transient responses between leaf layers, steady-state rather than dynamic photosynthesis traits were more influential for predicting photosynthesis under fluctuating irradiance. Also, a model analysis revealed that time-averaged photosynthesis under fluctuating irradiance could be accurately predicted by one set of Rubisco activation/deactivation parameters across all four leaf layers, thereby greatly simplifying future modelling efforts of whole-canopy photosynthesis.