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In vivo measurements of the radioactivity in the root and shoot of single barley seedlings have been made in an attempt to determine the effect of environ· mental variables upon the penetration of the root by phosphorus. When a seedling was placed in a nutrient solution which contained 32P-Iabelled, 10-6M KH2P04 , radioactivity was absorbed at a constant rate, but its rate of movement into the shoot increased during the first 5 hr of absorption after which time it was constant. If the shoot was placed in darkness after translocation had attained a constant rate, there was an immediate fall in the rate of translocation, followed, in continuing darkness, by a very slow increase to its former constant rate. Experiments involving changes between labelled and unlabelled nutrient solutions and placing the shoot in darkness showed that the initial observation was compatible with the hypothesis, put forward in an earlier paper, that phosphorus entering the root mixes with only a small proportion of its total content before moving into the shoot. The results indicated that changes in the environment of the shoot could modify the turnover rate of the small phosphorus "pool" in the root which lay in the path of translocation. Possible mechanisms for the mediation of this response are discussed.

Effect of Light Upon the Translocation of Phosphorus by Seedlings of Hordeum Vulgare (L.)