The Effect of Form of Nitrogen Supplied and pH Level of the Nutrient Solution on Copper Toxicities in Vegetable Crops

Abstract

Bean, spinach, lettuce, and radish were grown in solution culture to investigate the effect of form of N supplied and pH level of the nutrient solution on Cu toxicities. Copper was supplied at levels of 0.02, 0.3, and 1ppm. At each Cu level, NO₃, NO₃+NH₄ (1:1), and NH₄ were supplied, with the total N concentration being held constant at 12me/liter. The pH of the nutrient solution was adjusted to two levels, 4 and 6.
1. At the normal Cu level (0.02ppm), the growth of plants supplied with NO₃ and NO₃+NH₄ was excellent in any vegetable crops, but NH₄ plants produced inhibited growth except for lettuce. In vegetable crops except for bean, pH 6 produced better growth than pH 4. The growth reduction due to excess Cu in the nutrient solution was more marked at pH 4 than at pH 6. At pH 4, NO₃+NH₄ was more advantageous than NO₃ for reducing the Cu toxicity, while NH₄ treatment did not show the same effect because of NH₄ toxicity.
2. In any vegetable crops, roots were found to accumulate a larger concentration of Cu than leaves, the difference being more marked where excess Cu was supplied. The Cu accumulation in roots was generally larger at pH 4 than at pH 6. This was considered to be due to the markedly higher concentration of Cu²⁺ in the nutrient solution at pH 4 than at pH 6, as the result of the influence of pH on the extent of chelate formation of Cu with EDTA supplied as Fe-EDTA. The application of NH₄ had effects to suppress the Cu accumulation in roots. In NO₃ and NO₃+NH₄ treatments, there was a close relationship between the Cu accumulation in roots and the reduction of plant growth. The root Cu-plant growth relationship in NH₄ treatment was quite different from that in NO₃ and NO₃+NH₄ treatments; especially in spinach and radish, the plant growth was almost constantly poor regardless of the root Cu concentration.