Developing a Screening Tool for Osmotic Stress Tolerance Classification Based on in Vitro Seed Germination of Rice Cultivars.

The dry-seeded, delayed-flood culture with alternate wetting and drying weather irrigation management options to grow and sustain rice is favored in US Mid-south. Understanding the variation in response to soil moisture deficit among rice cultivars during seed germination would benefit in management and in breeding programs.  An in vitro experiment was conducted to study the impact of osmotic stress using polyethylene glycol (PEG) on germination properties of 15 rice cultivars commonly grown in US Mid-south production system. Time-series for seed germination was developed at wide range of osmotic potentials, 0 to -1.0 MPa, with -0.2 MPa increments. Cumulative seed germination over time was fit to a 3-parameter sigmoid function to deliver the parameters needed for further analysis. Seed germination rate (SGR), maximum seed germination (MSG), maximum osmotic potential when seed germination is zero (MSGOP_max), and maximum osmotic potential when seed germination rate is zero (GROP_max) were derived based on the appropriate regression techniques between these parameters and osmotic potential. Cultivars differed significantly for MSG, SGR, MSGOP_max, and GROP_max. MSG and SGR significantly decreases with decreasing osmotic potential. Seed weight for each cultivar was measured, but had poor correlation with MSG, and SGR. Cumulative drought response indices (CDRI) were developed by summing individual response indices of all cultivars. Rice cultivars were classified based on means and standard deviations of CDRI into low, moderately, and high drought-tolerant groups. Cheniere was identified as the least tolerant and RU1204122 as the most tolerant to drought based on the seed germination traits. The identified tolerance and the numerical scores among the rice cultivars will be helpful for the rice producers to select a variety best suited for a niche environment and rice breeders to develop drought tolerance among the cultivars for current and more variable future climatic condition.