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.

Maximal sunlight slowly varies over a day due to earth’s rotation. However, it is unknown whether this predictable daily light pattern influences the short-term regulation of non-photochemical quenching (NPQ) responses across the day. Diurnal variation in NPQ at growth light intensity, NPQ capacity and kinetics during and after 30-minute high light period, were measured in tomato plants grown in diurnal constant (DC) or diurnal parabolic (DP) light intensity. We also measured leaf gas exchange and the violaxanthin de-epoxidation state (DES) during and after 30-minute high light period at different time points in the day. A clear diurnal pattern emerged in NPQ induction kinetics in DP-grown plants, which was characterized by longer time for NPQ to reach its maximal value at the end of the photoperiod compared to midday, but with the highest capacity. The observed diurnal variations in NPQ capacity were not explained by the DES, but were associated with slower relaxation of qE. These diurnal patterns in NPQ responses were less pronounced in DC-grown plants. Concurrently, CO 2 assimilation rate and stomatal conductance were lowest in the end of day but similar between light regimes. We conclude that the diurnal short-term responses of NPQ involves regulation of its ∆pH-dependent component, which in turn can be influenced by the partitioning of proton motive force.

Carbon (C) storage allows a plant to support growth whenever there is a temporal asynchrony between supply (source strength) and demand of carbon (sink strength). This asynchrony is strongly influenced by changes in light and temperature.Traditionally, C storage is considered as a passive process that occurs whenever there is an excess of C from photosynthesis compared with the demand of C for metabolism. However, the role of C storage may vary from being a passive overflow to being an active process as a strategy of the plant to buffer climate fluctuations and support long-term growth. Despite numerous experiments that have advanced our knowledge in the role of C storage in plants, the exact mechanisms and the consequences at whole-plant level are still limited. We propose that an active C pool needs to be included in simulation models for a better understanding of plant growth patterns under fluctuating environment. The insights gained here are important to optimize crop performance under fluctuating conditions and thus for developing more resource-efficient crop production systems.

Salt stress affects stomatal behavior and photosynthesis, by a combination of osmotic and ionic components, but it is unknown how these components affect photosynthesis dynamics under fluctuating light. Tomato (Solanum lycopersicum) plants were grown using a reference nutrient solution (Control, EC: 2.3 dS m-1), the reference containing extra macronutrients (only osmotic effect; EC: 12.6 dS m-1), or the reference containing an additional 100 mM NaCl (osmotic and ionic effects; EC: 12.8 dS m-1). Steady-state and dynamic photosynthesis along with leaf biochemistry were characterized throughout leaf development. Osmotic effects resulted in increased leaf chlorophyll content per unit leaf area, induced stomatal closure along with rapid stomatal responses to changes in light intensity, and limited dynamic but not steady-state photosynthesis. Ionic effects were barely observed in plant growth and dynamic photosynthesis, but led to a reduction in leaf chlorophyll content and photosynthetic capacity in old leaves. Steady-state and dynamic photosynthesis traits decreased with leaf age, due to increases in stomatal and non-stomatal limitations. With increasing leaf age, rates of light-triggered stomatal movement decreased across treatments, which is more strongly for stomatal opening rather than closure. We conclude that osmotic effect strongly impacts dynamic stomatal and photosynthetic behavior under salt stress.