Drought and salinity adaptive mechanisms in japonica and indica/japonica rice varieties: a bilateral comparative study

This work investigated physiological, biochemical, and molecular adaptive responses of two Egyptian rice cultivars representing two different subspecies, Giza 177 (japonica) and Giza 178 (indica/japonica), to osmo-equivalent concentrations of mannitol and NaCl to mimic drought and salinity stress, respectively. Phenotypically, Giza 178 was less sensitive to drought than Giza 177; however, salt stress generated comparable damage symptoms and sodium content in either variety. In Giza 178, more K+ were accumulated in shoots under drought compared to Giza 177 with a 1.48-fold increase, while Ca++ content was significantly higher in shoots under drought and salt treatments (2 and 1.7-fold increase, respectively). Carbon isotope discrimination (CID) implied a prolonged stomatal closure in Giza 177 under drought. Additionally, the oxidative damage marker malondialdehyde (MDA) was more severely accumulated in Giza 177 shoots with 1.67-fold increase. The antioxidative enzymatic activity of Glutathione Reductase (GR) increased in Giza 178 shoots under drought, indicating better antioxidative power. Interestingly, OsNHX1 gene expression (Na+/H+ antiporter) was significantly induced in Giza 178 shoots under drought stress, indicating better osmotic turgor pressure and less degree of nutrition deficiency. Furthermore, both phytohormone abscisic acid (ABA) and jasmonate iso-leucine (JA-Ile) were significantly elevated in Giza 177 shoots compared to Giza 178 under drought and salt stress. Generally, it is suggested that Giza 177 shoots displayed greater sensitivity to drought, mainly due to extensive an oxidative damage caused by a prolonged period of stomatal closure.