Root box experiments were conducted to evaluate the effects of N and NK levels in the root zone on shoot photosynthesis and root growth of hybrid and cultivar of rice (Oryza sativa L.) on two paddy soils (clayey and silty). The results showed that dry matter yields in the hybrid and the cultivar were considerably increased by NK supply, but the effect was greater for the hybrid. Supply of NK in the root zone significantly increased photosynthetic rate of the lower position leaf and the active green leaf area per plant, in which the effects were much more obvious in the hybrid rice than in the cultivar. High NK supply in the root zone stimulated the root growth, and decreased PH and increased the oxidation zone in the rhizosphere in both entries, but to a greater extent in the hybrid. The results indicated that higher NK levels were needed to maintain higher root activity and shoot photosynthetic capacity in rice, particularly in hybrid rice.
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