Longitudinal Transport of Sucrose and Amino Acids in the Wheat Grain

Abstract

The idea that sucrose and amino acids are transported along the furrow of the grain by dissimilar mechanisms has been tested by culturing detached ears of wheat on mixtures of [14C]sucrose and [3H]-labelled amino acid for short periods. The grain was cut transversely into slices 1 mm thick and radioactivity in the soluble fraction was measured. After 4-12 min of transport there was a steep downward gradient of radioactivity from the proximal to the distal end of the grain; and the logarithm of the amounts of both isotopes in the slices was related to distance from the proximal end. Apart from differences in the steepness of the gradients among the amino acids, and between sucrose and some amino acids, qualitative features of the distribution of sucrose and amino acids were indistinguishable from each other. Sucrose was not metabolized during its movement along the grain, but amino acids were metabolized before and/or after they reached the grain. We conclude: (1) there is no substantive evidence for the operation of different mechanisms for the transport of sucrose and amino acids along the furrow; (2) both classes of solute are transported in the phloem; (3) metabolism of amino acids occurs before and/or after transport in the phloem; (4) differences among the slopes of the profiles of radioactivity may be attributable to variation in diffusion coefficient of the transported solutes.