Photosynthesis and photoprotection in top leaves respond faster to
irradiance fluctuations than bottom leaves in a tomato canopy
Abstract
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 faster photosynthetic
induction (shorter time to reach 50% of full photosynthetic induction;
t50-A) 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. A large number of leaf
functional traits, including photosynthetic capacity, chlorophyll
a: b ratio, stomatal density and specific leaf area,
correlated strongly with the irradiance integral and the red:far-red
ratio, both of which showed strong gradients throughout the canopy. The
rate of stomatal movement after in- or decreases in irradiance was not
strongly affected by leaf layer, and neither was the rate of loss of
photosynthetic induction under low irradiance. Time-averaged
photosynthesis under fluctuating irradiance was strongly correlated to
steady-state photosynthesis under high irradiance, and to a lesser
extent to t50-A, suggesting that both
steady-state photosynthetic capacity and the rapidity of photosynthesis
response to a change in irradiance affect time-integrated dynamic
photosynthesis.