In nature, soil salinity and fluctuating light (FL) often occur concomitantly. However, it is unknown whether salt stress interacts with FL on leaf photosynthesis, architecture, biochemistry, pigmentation, mineral concentrations, as well as whole-plant biomass. To elucidate this, tomato (Solanum lycopersicum) seedlings were grown under constant light (C, 200 μmol m −2 s −1) or FL (5-650 μmol m −2 s −1), in combination with no (0 mM NaCl) or moderate (80 mM NaCl) salinity, for 14 days, at identical photoperiods and daily light integrals. FL and salt stress had separate effects on leaf anatomy, biochemistry and photosynthetic capacity: FL reduced leaf thickness as well as nitrogen, chlorophyll and carotenoid contents per unit leaf area, but rarely affected steady-state and dynamic photosynthetic properties along with abundance of key proteins in the electron transport chain. Salt stress, meanwhile, mainly disorganized chloroplast grana stacking, reduced stomatal density, size and aperture as well as photosynthetic capacity. Plant biomass was affected interactively by light regime and salt stress: FL reduced biomass in salt stressed plants by 17%, but it did not affect biomass of non-stressed plants. Our results stress the importance of considering FL when inferring effects of salt-stress on photosynthesis and productivity under fluctuating light intensities.

NaCl stress affects stomatal behavior and photosynthesis, by a combination of osmotic and ionic components, but it is unknown how these components affect stomatal and photosynthetic dynamics. Tomato (Solanum lycopersicum) plants were grown using a reference nutrient solution (Control, EC: 2.3 dS m -1), a solution containing additional macronutrients (osmotic effect; EC: 12.6 dS m -1), or a solution with 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. The osmotic effect decreased steady-state stomatal conductance, while speeding up stomatal responses to light intensity shifts. After 19 days of treatment, photosynthetic induction was reduced by the osmotic effect, which was attributable to lower initial stomatal conductance due to faster stomatal closing rate under low light. Ionic effects of NaCl were barely observed in dynamic stomatal and photosynthetic behavior, but led to a reduction in leaf photosynthetic capacity, CO2 carboxylation rate and stomatal conductance in old leaves after 26 days of treatment. With increasing leaf age, rates of light-triggered stomatal movement and photosynthetic induction decreased across treatments. We conclude that NaCl impacts dynamic stomatal and photosynthetic kinetics by osmotic effects and reduces photosynthetic capacity by ionic effects.

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.