The Influence of Varying Concentrations of Applied Sugar on the Transport of Tracers in Cereal Leaves1

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Abstract
When young leaves of rye which have been emptied of mobile carbohydrate by darkening have 137Cs applied to their apices, it is found that the tracer only moves, in the darkened plant, when sugar is applied at the same time. If a treated leaf is harvested after a suitable interval, cut into sections, and assayed for radioactivity, the usual pattern of exponential fall-off with distance is found. On a plausible mass-flow model the negative slope of the semi-logarithmic plot is simply related to the transport velocity and to the rate of lateral leakage out of the conducting channels. This slope has been investigated as a function of the concentration of the applied sucrose, and it is found to vary as the fifth or sixth root of the latter. When 30C1 is used as tracer the slope appears to have a consistently higher value than in the case of 137Cs. In both cases, and also when 14C lactose is used with lactose as the activating sugar, the semi-logarithmic plot is concave upwards. The significance of these findings is discussed.