As the plant hormone abscisic acid (ABA) is involved in responses to salinity stress. We tested its putative relationship with the degree of tolerance to this abiotic stress. For this purpose we have examined the responses of sensitive (IR29) and tolerant (IR651) varieties of indica rice (Oryza sativa L.) to a range of salinity (0 (control) and 100 mM NaCl). Shoot and root dry weight was reduced and leaf Na concentration increased in response to salinity for both cultivars with a bigger extent in sensitive. Tolerance of IR29 to saline stress was generally improved by ABA treatment and leaves Na content reduced to their respective control treatment. This ABA effect was evident in IR29 with low tolerance, as their ability to recover from stress increased up to seven fold. Independent of the saline treatment, the absolute endogenous leaf ABA content in sensitive variety was significantly more than tolerant one. However, upon stress, the increase in endogenous ABA synthesis was higher in tolerant than in sensitive varieties. These data together with those obtained by using Fluridone, an inhibitor of ABA synthesis, suggested first, there was differential sensitivity to ABA in the tolerant and sensitive leaves cultivars and enhanced concentrations at tolerant levels acted primarily to maintain root and shoot growth salt stress and second, the differences in the level of tolerance to saline stress is related to their different capacity of ABA synthesis under stress conditions.
Key words: Abscisic acid, fluridone, Oryza sativa L., salinity, stress tolerance.
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