Salinity strengthens plant-pollinator mutualism through leaf physiological status in grafted tomatoes

The interaction between rootstock-mediated leaf Na and nutrient concentrations, and the subsequent impact on Fv/Fm and unbalanced C/N are major traits associated with plant growth, yield, and bumblebees' pollinating activity under salinity. Soil and water salinization compromises crop yield by reducing plant growth and reproductive fitness through altering source-sink activities, which in turn, affects pollination ecosystem services. However, while pollination services are barely considered as crop breeding targets, little is known about how stress-induced changes in plants affect pollinators' nutritional landscape and their foraging decisions. To address this gap, visually assessed pollination activity of managed bumblebees (Bombus terrestris) was monitored in a tomato commercial cultivar (Solanum lycopersicum L. Mill) grafted onto 26 experimental rootstocks, and related to agronomic and leaf/flower physiological traits under optimal (control) and saline (75 mM NaCl) conditions. Rootstock and salinity generated variability in agronomic (biomass, fruit yield and quality) and physiological (leaf and flower ionomics, chlorophyll fluorescence) traits, as well as in pollinator foraging behavior (flowers visited and preference). Based on a correlation analysis, the environmental pressure imposed by salinity strengthened plant-pollinator mutualistic interactions since rootstock-mediated fruit yield and plant vigor were associated not only with the leaf nutrient (C, N, P, K, Ca, S, Zn) and Fv/Fm status, but also with bumblebees' activity. Leaf Na concentration emerged as the strongest negative factor associated with plant growth, physiology and yield as well as with pollinator activity, while flower abortion was more closely linked to leaf C, N and P status rather than Na levels in flowers. The results suggest that pollinator foraging decisions under salinity may reflect the plant physiological status and C/N balance, supporting a potential role for pollinators as ecosystem phenotypers for plant selection and breeding under environmental pressure.