Seed priming with ascorbic acid improves response of Medicago polymorpha L. seedlings to osmotic stress induced by NaCl and PEG solutions

This study assessed to which extent seed priming with ascorbic acid (0.2 mM) may improve Medicago polymorpha L. performance under water shortage induced by irrigation with either 50mM NaCl or 100g/L polyethylene glycol (PEG). Parameters related to plant morphology, CO2/H2O leaf gas exchanges, osmotic adjustment, pigment content, and proline accumulation were specifically determined. Both NaCl and PEG solutions induced osmotic stress and reduced plant biomass (-30% and -40%, respectively), number of leaves and ramifications, stem length, net CO2 assimilation (-31% and 63%, respectively), and leaf water content. However, both treatments and especially PEG led to increased root/shoot ratios and leaf proline content. Interestingly, seed priming using ascorbic acid improved CO2/H2O gas exchange and plant biomass production (+66%, +100%, and +92% in control, NaCl-, and PEG-treated plants, respectively). R also improved the water relations as reflected by the decrease of leaf osmotic potential and higher leaf proline accumulation (+67% and +120% in PEG- and NaCl-treated plants, respectively) and water content (especially under PEG treatment). Besides, seed priming with ascorbic acid increased leaf carotenoid and chlorophyll contents (+65 and +45%, respectively, for chlorophyll a and chlorophyll b), thereby contributing to the better photosynthetic activity, and hence plant performance under salinity. We conclude that seed priming with ascorbic acid is an easy, cost-effective and promising approach to mitigate the impact of osmotic stresses like drought and salinity, by especially improving plant water relations and photosynthetic activity.