Canopy position determines the photoprotective demand and antioxidant protection of leaves in salt-stressed Salvia officinalis L. plants

The effects of salt stress and leaf canopy position on mechanisms of photo- and antioxidant protection, including changes in photosynthetic pigments and low-molecular-weight chloroplastic antioxidants, were examined in sage (Salvia officinalis L.) plants exposed to two doses of NaCl (50 mM and 100 mM) for 5 weeks. Sage appeared to be a moderately salt-resistant glycophyte. The addition of 100 mM NaCl to the nutrient solution reduced total leaf biomass, the number of leaves, leaf water potential, net photosynthesis, stomatal conductance and chlorophyll levels. However, the malondialdehyde levels, which indicate the extent of lipid peroxidation, did not increase in plants treated with either 50 mM or 100 mM NaCl, relative to controls. In the plants treated with 100 mM NaCl, the accumulation of Na+ in the leaves occurred in parallel with a drastic reduction in the net CO2 assimilation rates, but also with the activation of mechanisms of photo- and antioxidant protection, including xanthophyll cycle de-epoxidation and the accumulation of tocopherols and phenolic diterpenes. Furthermore, we examined the extent to which canopy position determines the photoprotective demand and antioxidant protection of leaves, and how salinity affects this demand. The lower leaves showed a lower photoprotective demand than the upper leaves. However, the former showed higher lipid peroxidation than the latter under salt stress, which suggests that the lower, older leaves suffer from greater photo-oxidative stress than the upper, younger ones, despite being located in areas with a lower photoprotective demand within the canopy. We concluded that leaf position in the canopy should be carefully considered in studies aimed at unravelling mechanisms of salt-stress tolerance in plants.

► Salt tolerance is achieved through activation of photoprotection mechanisms. ► Leaf position in the canopy determines the photoprotective demand of leaves. ► Lower leaves suffer photo-oxidative stress despite receiving low light intensities. ► Leaf position should be considered in studies of salt stress tolerance in plants.