Difference in Sodium Spatial Distribution in the Shoot of Two Canola Cultivars Under Saline Stress

Among different mechanisms of salt resistance, regulation of ion distribution among various tissues and intracellular compartmentation are of great importance. In this study, we investigated the effects of salt stress on growth, photosynthesis, and Na+ accumulation and distribution in leaf apoplast and symplast of two canola (Brassica napus L.) cultivars (NYY 1 and BZY 1). The results showed that the declines in shoot dry mass, leaf water potential and net photosynthetic rate of BZY 1 (salt sensitive) were higher than those of NYY 1 (salt resistant) in response to salt stress. Stomatal limitation to photosynthesis was mainly affected under moderate salinity, whereas the reduction in assimilation rate under severe salt stress was due to both stomatal and non-stomatal limitations. We also found that more Na+ was distributed to leaf veins in NYY 1 than in BZY 1; simultaneously, less Na+ accumulated in the leaf blade in NYY 1 than in BZY 1. The percentage of Na+ in the leaf symplast in NYY 1 was markedly lower than that in BZY 1. Also, Na+ diffusion in leaves through apoplastic and symplastic pathways of BZY 1 was stronger than that in NYY 1, and the transpiration rate in BZY 1, especially at the leaf edges, decreased more than in NYY 1. Our results showed that NYY 1 accumulated less Na+ in the shoot, especially in leaf blades, and confined Na+ to the apoplast to avoid leaf salt toxicity, which could be one reason for the higher resistance of NYY 1 than BZY 1 plants to salt stress.