Balance between Potassium and Phosphorus in the Nutrition of Barley

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Abstract
Potassium-deficient barley accumulates the amines agmatine and putrescine, the latter of which is known to produce necrotic spots on the leaves in some seasons. The severity of potassium deficiency symptoms is strongly influenced by the external level of phosphorus. An experiment was conducted in water culture, using two-week-old barley to discover whether amine accumulation was similarly affected. Four levels of potassium —1/64, 1/6, 1/4, and 1/1 of full supply—were combined factorially with the same levels of phosphorus, the diagonal in this design being a series of ‘balanced’ solutions in which potassium and phosphorus were present in the same ratio as in the full nutrient. The plants were sampled three times during the ten-week period of growth, and the form of the results was similar each time. Whenever potassium was reduced, putrescine content rose above normal, but for large amounts of putrescine to be accumulated, phosphorus supply had to be in excess of potassium. The highest levels of agmatine were also found where potassium was low and phosphorus in excess, but significant accumulations also developed whenever phosphorus was low relative to potassium, that is, where high levels of potassium were observed in the plants. Since agmatine and putrescine are very closely related biochemically and together make up 90 per cent of the amine content of barley, the influence of potassium and phosphorus on their summed content was considered. Potassium was found to have the dominant effect: at any one level of phosphorus supply, the summed content was minimal at the highest level of potassium, and reduction in potassium always increased it. The effect of phosphorus depended on the degree of imbalance between potassium and phosphorus in the nutrient. High summed contents were found where either potassium or phosphorus was present in excess, and minimum content was found where the levels were balanced. There were significant partial correlations between summed amine content and the potassium and phosphorus contents of the plants. The high significance of correlations between amine content and the proportion of dead and moribund shoot material suggested that amine accumulation might be partly responsible for the rapid death of the leaves of the deficient plants as well as for the production of necroses, which are only limited in area.