Evaluation of Root Penetration Ability in Rice Using the Wax-Layers and the Soil Cake Methods

Abstract

Root penetration ability (RPA) is an important character of rice against drought stress. The devel- opment 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 6 cm 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.