Short-term salt stress strongly affects dynamic photosynthesis, but not steady-state photosynthesis, in tomato (Solanum lycopersicum)

Salt stress occurs worldwide due to widespread soil salinization. Also, plants are often subjected to rapidly alternating periods of sun and shade (sunflecks). Despite this combined occurrence of salt stress and sunflecks, dynamic photosynthetic responses to sunflecks under salt stress remain unknown. This study addresses this discrepancy by exploring photosynthetic gas exchange and chlorophyll fluorescence, both after dark-light transitions and during artificial light fluctuations (lightflecks and shadeflecks), in salt stressed leaves. Three weeks old growth-chamber grown tomato (Solanum lycopersicum Mill 'Beijing Cherry Tomato') plants were exposed to 0, 70 or 140 mM of sodium chloride (NaCl), for 7-9 days. Photosynthetic induction after dark-light transitions was strongly inhibited in salt-stressed leaves, due to increased transient stomatal limitation and slower apparent Rubisco activation. During photosynthetic induction, non-photochemical quenching (NPQ) and intrinsic water use efficiency (WUEi) were positively correlated with [NaCl]. Under periods of low light (shadeflecks), the longer the shadefleck lasted, the more strongly photosynthesis after re-illumination was downregulated by salt stress, and this downregulation in photosynthesis was positively correlated with the severity of salt stress. Under regularly applied lightflecks, salt stress decreased photosynthesis by 12-42%, which was mainly caused by decreased stomatal conductance. Salt-stressed leaves also displayed significantly lower stomatal pore area and stomatal index. Crucially, salt stress did not affect steady-state photosynthetic capacity as indicated by similar light and CO2 response curves of photosynthesis. We conclude that a short-term salt stress strongly affects dynamic leaf photosynthesis in tomato while its effect on steady state photosynthesis is negligible.