Enhanced Salt Tolerance under Nitrate Nutrition is Associated with Apoplast Na+ Content in Canola (Brassica. napus L.) and Rice (Oryza sativa L.) Plants.

To analyze the effect of nitrogen form on salt stress, we studied the response of two different plant species, canola (Brassica napus L.), a dicotyledon which prefers NO3- nutrition, and rice (Oryza sativa L.), a monocotyledon which prefers NH4+ nutrition, to salt stress under NO3- (NN) and NH4+ (AN) nutrition. Salt stress was simulated by the addition of 150 and 100 mM NaCl to NN (NNS) and AN (ANS) in canola and rice seedlings, respectively. Salt stress induced reductions of shoot and root biomass that were more drastic under ANS. A higher Na+ content was obtained in NNS than in ANS. The impact of Na+ on the reduction of biomass (Δbiomass/Na+) was 162, 181, 230 and 245% higher in canola root, canola shoot, rice root and rice shoot in ANS than in NNS, respectively. In both canola and rice seedlings, the ratio of leaf Na+ content in apoplasts to symplasts ([Na+]apo/[Na+]sym) was higher in NNS than in ANS. Also, in canola seedlings, the ratio of apoplast Na+ in the leaf edge to the leaf center ([Na+]LE/[Na+]LC) was 18 times higher in NNS than in ANS. Our results illustrate that the confinement of Na+ in the canola leaf edge, as well as the restriction of Na+ in leaf apoplasts of canola and rice seedlings, protect cells from suffering Na+ stress and contribute to the higher tolerance of NO3--fed plants.