Ammonium Injury to Poinsettia: Effects of NH4-N:N03-N Ratio and pH Control in Solution Culture on Growth, N Absorption, and N Utilization

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Abstract
Poinsettia (Euphorbia pulcherrima Willd. ex Klotz) was grown in unbuffered nutrient solutions varying in proportion of NH4-N:NO3-N at 50 ppm N. Reductions in shoot and root dry weight and subseqeunt NH4-N absorption, accompanied by the increase in solution acidity with initial NH4-N absorption, occurred with 75% and 100% NH4-N treatments, as compared to treatments of 50%, 25%, or 0% NH4-N. Depression of NH4-N absorption with 100% NH4-N treatment caused a reduction to the critical level of total reduced N in the upper leaves and the appearance of N-deficiency symptoms. Characteristic symptoms of NH4-N injury to plants—large increases in tissue NH4-N and soluble organic N, leaf interveinal chlorosis, and marginal necrosis—did not occur in any treatment. In a second experiment, CaCO3 added to the 100% NH4-N solution at 0, 88, 176, or 264 mg/liter resulted in varying degrees of pH control. Some increase in shoot and root dry weight occurred at 88 and 176 mg CaCO3/liter; CaCO3 at all rates greatly increased NH4-N absorption compared to 0 mg CaCO3/liter, and increased total reduced N in the leaves and roots. Lower leaves of plants with CaCO3 displayed symptoms of NH4-N injury and significant accumulations of NH4- N and soluble organic N. The detrimental effects of NH4-N on poinsettia appear to be dependent on the degree of pH control in the root medium and its effects on root growth and NH4-N absorption and utilization.

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