Sodium exclusion as a key trait for maize adaptation to salinity: Relationship with photosynthetic efficiency and membrane integrity

Maize is widely used for human consumption but its cultivation is hampered by salinity. Photosynthetic activity and hence, growth are adversely affected by salt stress due to salt accumulation in the plant tissues. Thus, it is of paramount significance to identify reliable salt-tolerant cultivars able to minimize the uptake of salt ions and preserve several physiological and metabolic processes under such circumstances. Here, the behaviour of two maize cultivars (‘SY Verdemax’ and ‘SY Sincero’, respectively Na+ includer and Na+ excluder) subjected to long term salinity stress was addressed. Salinity, especially at 150 mM NaCl inhibited several growth-related traits, chlorophyll concentrations, photosynthetic activity and leaf hydration in both cultivars, but ‘SY Verdemax’ which showed higher Na+ contents in leaves was more salt-sensitive. Salinity reduced proline content and led to oxidative damage reflected by disturbed membrane integrity and lower chlorophyll stability index, especially in ‘SY Verdemax’. We assume that the better behavior of ‘SY Sincero’ when salt-challenged would be partly related to its ability to restrict Na+ accumulation in leaves (as shown by lower Na+ translocation index) and its immobilization in roots, which allows to maintain the photosynthetic machinery functioning and sustain plant growth. Besides, limiting excessive Na+ buildup in shoot reduced water loss, preserved chlorophyll synthesis while minimizing oxidative stress damage to cell membranes. Taken together, our results further highlight the significance of sodium exclusion in the response of maize to salt stress, and strongly suggest that ‘SY Sincero’ would be a suitable candidate for maize cultivation in moderately salt-affected soils.