The C4 <i>Atriplex halimus</i> vs. the C3 <i>Atriplex hortensis</i>: Similarities and Differences in the Salinity Stress Response

Soil properties and the ability to sustain agricultural production are seriously impaired by salinity. The cultivation of halophytes is seen as a solution to cope with the problem. In this framework, a greenhouse pot experiment was set up to assess salinity response in the perennial C4 species <i>Atriplex halimus</i>, and in the following three cultivars of the annual C3 <i>Atriplex hortensis</i>: green, red, and scarlet. The four genotypes were grown for 35 days with water salinity (WS) ranging from 0 to 360 mM NaCl. Plant height and fresh weight (FW) increased at 360 vs. 0 WS. The stomatal conductance (GS) and transpiration rate (E) were more severely affected by salinity in the C4 <i>A. halimus</i> than in the C3 species <i>A. hortensis</i>. This was reflected in a lower leaf water potential indicating stronger osmotic adjustment, and a higher relative water content associated with more turgid leaves, in <i>A. halimus</i> than <i>A. hortensis</i>. In a PCA including all the studied traits, the GS and E negatively correlated to the FW, which, in turn, positively correlated with Na concentration and intrinsic water use efficiency (iWUE), indicating that reduced gas exchange associated with Na accumulation contributed to sustain iWUE under salinity. Finally, FTIR spectroscopy showed a reduced amount of pectin, lignin, and cellulose under salinity, indicating a weakened cell wall structure. Overall, both species were remarkably adapted to salinity: From an agronomic perspective, the opposite strategies of longer vs. faster soil coverage, involved by the perennial <i>A. halimus</i> vs. the annual <i>A. hortensis</i> cv. scarlet, are viable natural remedies for revegetating marginal saline soils and increasing soil organic carbon.