Genotypic Variation in Osmotic Stress Tolerance Among Rice Cultivars and Its Association with L-Type Lateral Root Development

Abstract

The differences in osmotic stress tolerance and morphological characteristics of the root system of stress-tolerant and stress-sensitive rice cultivars were clarified by examining genetic variation. Fifty-four Rice Diversity Research Set cultivars, and Azucena, IRAT109, Dular (droughttolerant), IR64 (drought-sensitive), and IR28 (salt-sensitive) were used. Rice seedlings were cultivated for 14 days by water culture (control). Polyethylene glycol 6000 was dissolved in the culture medium as osmotic stress treatment on day 7 to adjust the water potential to –0.42 MPa (stress treatment). The stress treatment to control ratio (S/C ratio) was calculated to evaluate the degree of stress tolerance. The dry-weight S/C ratio in the shoot showed a significant varietal difference from 0.874 to 0.376 and that in the root from 0.931 to 0.342, and root, respectively, and these values were significantly correlated. Five osmotic stress-tolerant and 5 stress-sensitive cultivars were selected, and their root-system morphology was investigated in detail. The number of the L-type lateral roots, which were longer and thicker, increased 1.5 – 3.6 times that of the control with osmotic stress treatment in the tolerant cultivar group. It slightly decreased 0.8 – 0.9 times that of the control with osmotic stress in the sensitive cultivar group. The number of crown roots and S-type lateral roots, which were finer and shorter, decreased with stress in both cultivar groups; however, this decrease was significantly lower in the tolerant cultivar group. Thus it is suggested that the maintenance of root-system development, especially L-type lateral roots under osmotic stress, involves genetic variation in the genes responsible for the dry matter production.