Relation Between Nitrogen Source and the Effects on Flax of an Excess of Manganese or Molybdenum in the Nutrient Solution

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Abstract
The effects on flax of a given concn. of Mn or Mo in the nutrient soln. depended on the amt. and form of N supplied. The interactions that occurred between either of these 2 elements on the one hand, and N source on the other, were highly significant but opposite in effect. Thus, with a given excess of Mn, symptoms were most severe in plants grown in soln. supplied with N as nitrate or urea, and were absent in N-deficient solns. or those supplied with N as ammonium. In solns. supplied with ammonium nitrate, no Mn toxicity symptoms occurred or these were only slight. A concn. of 50 ppm. of Mn in the soln. caused a very significant decrease in growth in the presence of nitrate, had no harmful effect on growth in the presence of ammonium nitrate, and caused a significant increase in growth with ammonium. With urea, the reduction in growth was not as great as with nitrate. The presence of ammonium ions in the soln. markedly reduced the Mn concn. in the plants when compared with that of plants receiving nitrate only. By contrast, at a given excess of Mo in the nutrient soln., toxicity symptoms in the test plants were affected by N source in the following order of decreasing severity N-deficiency > ammonium > ammonium nitrate > urea > nitrate. In soins. supplied with nitrate, a given concn. of Mo was significantly more toxic to growth when applied as ammonium molybdate than as Na molybdate. In one expt. the addition to the nitrate soln. of 10 ppm. of Mo as Na molybdate resulted in a significant increase in the dry weight of the plants compared with the controls, even though such plants contained 400 ppm. of Mo on a dry basis. In the presence of ammonium or urea. no significant differences occurred between the effects of Na molybdate or ammonium molybdate on plant growth. In the nitrate nutrient soln. less Mo was absorbed by the plants when a given excess of this element was added to the soln. as Na molybdate than when it was added as ammonium molybdate. It is concluded that the toxicity to flax of a given concn. of Mo in the nutrient soln. is increased by the presence of ammonium ions. Where excess of Mn and Mo were added together to the nutrient soln., an interaction between the 2 elements occurred. Thus the severity of Mn-induced Fe-deficiency chlorosis and lower leaf necrosis was reduced. However, a golden-yellow chlorosis, not typical of Fe-deficiency chlorosis but similar to Mo toxicity symptoms, was induced in the plants in the presence of nitrate, ammonium nitrate, or urea, but not where ammonium was the sole source of N. The Mn-Mo interaction was facilitated by the presence of some ammonium ions in the soln. Thus, a given concn. of Mo was more effective as ammonium molybdate than as Na molybdate in reducing the severity of Mn toxicity symptoms. However, the addition to the nitrate soln. of ammonium in an amt. equivalent to ammonium molybdate had no effect on Mn toxicity symptoms. Excess B (2 ppm.) had no effect on Mn toxicity symptoms. When grown in nutrient solns. containing nitrate, flax plants affected with Mn toxicity symptoms had a higher nitrate content than normal. Mn-Mo antagonism (i.e. reduction in severity of Mn-induced Fe-deficiency chlorosis and lower leaf necrosis in the presence of excess Mo), was associated in this instance with a reduction in the nitrate content of the plants. However, when grown in nutrient solns. containing urea, no nitrate was detected in plants showing severe Mn toxicity symptoms, although, in this instance also, the severity of the leaf necrotic symptoms was reduced by adding Mo.