Water uptake by dryland rice root system during soil drying cycle

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Abstract
A dryland rice variety, OS4, was grown in the field during the dry season to study evapotranspiration, root distribution, and water extraction patterns by roots after irrigation was discontinued. Roorts were observed up to 80 cm, and 90% of the total root length was distributed within the top 50 cm. The high root density in the surface layer is a characteristic of the rice root system. Potential evapotranspiration continued for 6 days in the rice field. Soil water potentials on the sixth day in the rice field were about -10 bar in the 10-15 cm soil layer and -0.7 in the 40-50 cm soil layer. When the whole profile was wet, roots extracted almost all the water from the shallow layers. As the soil water potential in the soil surface decreased, water retained in deeper layers made a larger contribution to evapotranspiration. Water uptake rate of rice roots in the 10-15 cm soil layer decreased with the decrease in soil layer decreased with the decrease in soil water potential. In the 40-50 cm soil layer, however, the water uptake rate increased with the decrease in soil water potential. These results can be explained if axial root resistance is assumed to be significant Maximum water uptake rates per unit of length increased as the root density decreased. Water uptake rates calculated for potential evapotranspiration period are 0.0027-0.0034 cm3/cm per day at root densities of 7-14 cm/cm3 and 0.0063-0.014 cm3/cm per dat at root densities of 0.23-3 cm/cm3. These values are comparable to those of sorghum or cotton. The water uptake rate combined with high root density suggests that rice crop extracts soil water faster than other crps and consequently that it is susceptible to drought.