Evaluation of root penetration ability in rice using the wax-layers and the soil cake methods

Root penetration ability (RPA) is an important character of rice against drought stress. The development of screening techniques is an initial step to determine the RPA of rice varieties before introducing RPA related genes to improve drought resistant rice in breeding. A 3-wax-layers method was developed to evaluate the penetration ability of seminal roots in rice under dry soil conditions. The wax layer with 3 mm thickness and 6cm in diameter was prepared by melting mixture of 60% paraffin and 40% vaseline, providing 1.5 MPa of hardness at 25°C. Three wax layers were installed tightly together in the tubes as the hardpans impeded the root growth. Rice seeds were sown in a randomized complete block design with three replications in a growth chamber at 25°C. Root penetration rate (RPR) refers to RPA was measured as the ratio of the number of the seminal roots (SRN) that penetrated the wax layer(s) (1, 2 or 3 wax layer(s)) to the total SRN. Simultaneously, another experiment using the soil cakes (9 mm thickness, 1.5 MPa of hardness, and 13% soil moisture content) was conducted to compare with the wax layer method. As a result, RPR through 3 wax layers (RPR3W) had significant positive correlation with that through the soil cake (r=0.68**). We suggest that, the use of 3-wax-layers could be more effective to estimate RPA than the common use of the 3 mm wax layer. Japonica upland rice showed greater RPR than the other groups. The impeded seminal root showed an increase in root diameter compare to the roots grown in non-impeded soil conditions. In response to the impedance, tend to increase its diameter linked with RPA.