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Effects of Potassium Nutrition on Protein and Total Free Amino Acids in Zea mays¹

Corn (Zea mays L.) seedlings were grown in nutrient solutions that ranged in K content from none to sufficient. In comparison with K-sufficient plants, the shoots of K-deficient plants exhibited up to 40% increase in protein concentration while the amount of protein per shoot was much reduced. The concentration of free amino acids was increased by about threefold in extreme deficiency, but was hardly affected by moderate deficiency although growth in the latter case was still much retarded.

During the development of K deficiency, the ratio of free amino acids to protein in leaves of two different ages did not increase until growth was retarded; similarly the amount of protein per leaf in the maturing leaf was not affected until growth was substantially reduced. When K was added to deficient plants, to permit their recovery, the ratio of free amino acids to protein in the shoot did not decrease for many hours after K was detected in adequate amounts in the shoots. The ratio decreased only after the added K had induced an increase in growth. This suggests that there was no acceleration of polymerization of amino acids resulting directly from the added K.

The specific activities of aldolase and triosephosphate dehydrogenase were some 30% lower in deficient than in sufficient shoots. The level of nitrate reductase in the tissue, however, appeared to be related to the supply of nitrate and not to the K level.

These data do not support the contention of K level in deficient plants being low enough to directly limit the polylnerization of amino acids and of a reduced rate of protein synthesis being the primary factor limiting growth of K-deficient plants.

Key Words: Nitrate reductase • protein synthesis

² Formerly Wright Foundation and National Science Foundation Cooperative Fellow (now Associate Professor. Department of Water Science and Engineering, University of California, Davis, 95616) Professor of Plant Physiology, and Professor of Soil Fertility, respectively, University of Illinois, Urbana.

Received for publication July 30, 1969.