1980 Volume 49 Issue 2 Pages 197-202
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.