Root Response to Water Stress in Rainfed Lowland Rice

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Abstract
The relation between soil mechanical impedance as a result of soil drying, and root system growth (mass and length density) of rice was investigated in greenhouse and field studies. In a greenhouse experiment, soil drying for 16 days increased mechanical impedance in the 0-20 cm soil layer from near 0 to 2.5 MPa, and decreased root growth by 47% compared to the continously flooded control. Root length density decreased with decreasing soil moisture and increasing soil mechanical impedance. In a lowland field experiment using a sprinkler irrigation gradient treatment for 19 days during the vegetative growth stages, soil mechanical impedance as low as 0.01 MPa inhibited root growth while values greater than 0.3-0.5 MPa decreased root growth and extension by 75%. The relative loss of potential root growth was continued after reflooding. Root length density, measured at flowering, was linearly related to yield.