A study of translocation in cereal leaves using Cs-137 and C-14

Abstract

In the absence of natural carbohydrates a number of radioactive tracers undergo translocation in young rye leaves but only when external sugars are simultaneously applied. Sucrose and lactose are among such transport-activating sugars. The distribution of tracers down the leaf length when applied at the leaf apex shows the common exponential fall-off pattern with the distance, while the semi-logarithmic curves display upward concavity. When tracers are applied in media of low osmotic potential there is an absorption of water by the leaf with consequent flow of tracer downwards in the xylem; this unwanted effect can be overcome by raising the osmotic potential of the milieu. On a plausible 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. The slope of these curves is a function of the concentration of applied sugars, and varies as the 5th or 6th root of the latter. The slope is steeper for chloride than it is for potassium; while the slope for caesium is less steep than that for potassium. When lactose is administered more is absorbed by the leaves normally illuminated than by those emptied of their natural photosynthates by prior dark treatment, thus suggesting an active uptake. New and promising microextraction and colorometric methods are introduced with some initial success. In general the investigation supports a theory of mass-flow in the sieve tubes. Observations such as the relationship between sieve tube concentration and velocity, faster lateral leakage for potassium than for caesium and a faster cation mobility compared with that for anions suggest a preference for the electrokinetics theory against the pressure-flow hypothesis.