Characteristics of root growth and water uptake from soil in upland rice and maize under water stress

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Abstract
A field experiment was conducted to characterize root growth and water uptake from soil layers with different N levels by upland rice (Oryza sativa L.) and maize (Zea mays L.) under severe water stress (no water supply) and mild water stress (intermittent water supply). Under mild stress conditions, medium N application led to the increase of the total root length but to the decrease of the root length/leaf area ratio compared with the no-N treatment in rice. Water was extracted from soil layers in a pattern approximately proportional to the root length distribution under mild stress. Both in rice and maize, a major portion of the water was extracted from the 0–20 cm depth. Application of N increased the total water uptake, which was attributed to the greater root length and higher specific water extraction rate per unit length of root. Specific water extraction rate can vary to some extent, depending on the transpiration demand of the shoots. In rice, water uptake, dry matter production in shoot, and root length were largely suppressed under severe stress compared with mild stress. In maize, water uptake under severe stress was maintained at a level similar to that under mild stress. This was attributed to the higher extraction from the deep soil below the 40 cm depth, which was supported by the increased root distribution and increased specific water extraction rate per unit root length at the depth under severe stress compared with mild stress. The lower water-capturing capacity of rice than that of maize under severe stress was characterized by a lower response to the enhancement of the morphological growth of the roots in deeper soil layers and the limited ability of water extraction per unit length of root, especially in the deeper layers.