Heavy Metal Toxicities in Vegetable Crops

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

Abstract

Solution culture experiments were conducted in order to investigate the effect of form of N supplied and pH level of the nutrient solution on Mn toxicities in vegetable crops. Materials tested were cucumber, tomato, spinach, cabbage, lettuce, and celery. Manganese was supplied at levels of 0.5, 30, and 100ppm. At each Mn level, NO₃, NO₃+NH₄(1:1), and NH₄ were supplied, with the total N concentration being held constant at 12me/l. The pH of the nutrient solution was adjusted to two levels, 4 and 6. Plants were grown under different treatments for about three weeks.
1. At the normal Mn level (0.5ppm), plants supplied with NH₄ developed various injury symptoms, which were more severe at pH4 than at pH₆. Excess Mn associated with NO₃ and NO₃+NH₄ induced interveinal chlorosis on upper leaves in tomato and spinach, and marginal chlorosis on lower leaves in cabbage, lettuce, and celery. Both types of Mn-induced chlorosis were generally less severe with NO₃+NH₄ than with NO₃, and in some cases these symptoms were less severe at pH4 than at pH 6. The incidence of necrotic brown spots due to Mn excess was reduced by increasing proportion of NH₄ supply.
2. At the normal Mn level, the growth of plants supplied with NO₃ and NO₃+NH₄ was almost equally good, but NH₄ plants produced much more inhibited growth. With increasing Mn level in the nutrient solution the growth of plants supplied with NO₃ and NO₃+NH₄ was inhibited, but in most cases NO₃+NH₄ produced better growth than NO₃. In general, the growth of plants supplied with NH₄ was almost constantly poor regardless of the Mn level in the nutrient solution. At the normal Mn level, plant growth at pH4 was generally inferior to that at pH6. At excess Mn levels, however, the difference of growth between two pH levels was reduced as compared with the normal Mn level, and pH4 often produced better growth than pH6 in NO₃ and NO₃+NH₄ treatments.
3. Increasing Mn level in the nutrient solution increased leaf Mn concentration, but increasing proportion of NH₄ supply inhibited excess accumulation of Mn in leaves. The leaf Mn concentration was generally lower at pH4 than at pH6.
4. In general, the growth of plants supplied with NH₄ was almost constantly poor regardless of leaf Mn concentration. On the other hand, in NO₃ and NO₃+NH₄ treatments, the more the accumulation of Mn in leaves increased, the more the growth was inhibited regardless of N treatments. In spinach, however, the growth of plants supplied with NO₃ and NO₃+NH₄ at pH 4 was more strongly affected by low pH than by leaf Mn concentration.
5. It may be concluded that NH₄⁺ and H⁺, unless their own levels are injurious, exert marked effects in reducing Mn uptake and Mn toxicities in vegetable crops.