Salt stress response and effect of hypoxia on sodium uptake in two Italian rice cultivars

Salt stress is one of the environmental constraints that affect crop cultivation worldwide, since more than 800 Mha of land throughout the world suffer from salinization problems. Soil salinity adversely damages plant growth in two ways: high concentrations of salts in the soil can reduce water uptake by plant roots, whereas high concentrations of salts within plants tissues may be toxic and alter the plant physiology. Plants species display different responses to high salinity conditions. Rice (Oryza sativa L.) is the cereal most sensitive to salt stress, although cultivars can differ in their response to salinity. European rice productivity is affected by abiotic and biotic stresses. In particular, due to scarce water availability and the rise in sea levels, there is a clear tendency toward salinization in the river deltas where rice is grown in Europe. In Italy, salt-affected soils in rice cultivation areas can be found in the eastern Po valley, along the Ionic and Tirrenic littorals, and in coastal areas of Sardinia. For this reason, the identification of rice cultivars tolerant to salt stress and the dissection of salt stress tolerance mechanisms are of high interest for rice breeding, also in Italy. The aim of this work is to evaluate the salt stress response of two Italian cultivars, Baldo and Vialone Nano, as well as the effect of hypoxia on root anatomy and sodium uptake. A physiological analysis, which was conducted on plants grown in hydroponic culture under a long–term (30 days) mild-salinity treatment (25 mM NaCl), highlighted that in hypoxic conditions Baldo showed lower Na+ contents than Vialone Nano in both shoots and roots, whereas in aerated conditions the difference in the Na+ content between the two cultivars was observed only in shoots, suggesting a different effect of hypoxia on Na+ uptake by the roots between genotypes, as well as a different Na+ translocation to the shoot in more aerated conditions. A preliminary histological analysis of adventitious roots of plants grown in hypoxic conditions showed in Baldo the presence of Casparian bands and suberin lamellae closer to the root tip than in Vialone Nano, mainly in the endodermis, suggesting a different development of root apoplastic barriers, that may alter Na+ uptake by the root. A further experiment was conducted to study the Na+ translocation to the shoot in the two genotypes. Rice plants were grown in hydroponic culture under a short-term (24 h) high-salinity treatment (100 mM NaCl), and the Na+ level in root, culm and in the first, the second and the third leaf of plants was separately measured. The existence in the cultivar Baldo of a Na+ gradient among the plant organs was observed, with the lowest Na+ content in the youngest leaf. Differently, in Vialone Nano the youngest leaf displayed a higher amount of Na+ than the oldest one, thus confirming a different mechanism of Na+ translocation between the two genotypes. These results are in accordance with literature data, indicating that some rice genotypes are able to restrict Na+ translocation to the youngest leaves, thus preventing toxicity in active photosynthetic tissues, and to keep the highest levels of Na+ in the old leaves. Further investigations are in progress to confirm these data. This work was supported by Progetto AGER, grant n° 2010-2369 (RISINNOVA - Integrated genetic and genomic approaches for new Italian rice breeding strategies).