Physiological response and agronomic performance of drought tolerance mutants of Aus rice cultivar Nagina 22 (Oryza sativa L)

Rice (Oryza sativa L) is a staple food crop for more than half of the world population, however its yield is adversely affected by drought stress. Traditional rice varieties have significant variability in morphological, physiological and molecular adaptation to drought stress but modern high-yielding cultivars are quite sensitive to drought. Here, we screened a set of 2000 EMS-mutants of a drought tolerant 'Aus' rice cultivar 'Nagina 22 ' (N22) using standard evaluation system of drought sensitivity (DRS) score and identified 73 mutants with either higher or lower DRS score than wild type (WT) N22. The mutants were also screened for epicuticular wax (EW) content to identify 25 mutants with altered levels of EW content, all of which were common to the DRS mutants. The remaining DRS mutants with EW content similar to WT N22 must have other mechanisms of drought tolerance. The identified mutants were evaluated for drought related physiological traits, drought sensitivity index (DSI) and agronomic performance for four consecutive years to understand the role of EW in drought tolerance. All the EW mutants showed altered levels of DSI, biomass and grain yield. The mutants also showed changes in chlorophyll content, relative water content, cell membrane stability and oxidative stress enzyme activities. The high-EW mutants showed lower DRS scores but higher RWC, biomass and grain yield under drought than WT N22, while low-EW mutants showed the opposite trends. The gain and loss of drought tolerance mutants identified here would help gain further insights into the mechanism of drought tolerance and gene discovery for the trait in N22. A high-EW low-DSI mutant viz., 'M59 ' with substantially higher grain yield under drought will serve as useful donor for breeding drought tolerant rice cultivars for rainfed upland agroecological regions.