Transport of Tritiated Water and ¹⁴c-Labelled Assimilate Into Grains of Wheat. III. Diffusion of Tho Through the Stalk

Abstract

Measuring the rate at which tritiated water is transported by diffusion through the stalk of the wheat grain is the main purpose of this paper. Diffusion of THO through the stalk is much slower than expected on the basis of the self-diffusion of THO in water, demonstrating that there are substantial resistances to diffusion of water in the stalk.

Entry of THO into the grain by physical processes alone (diffusion), as measured in assemblies in which the grain (stripped of its bracts) is sealed into an impermeable capsule, is fast enough to account for the observed rate of transport of THO into normally functioning grains. Metabolically linked transport of water through the stalk is not detectable. Either the volume of water involved in mass flow through the stalk is too low to be detectable - which would require the concentration of sucrose to be about 2 M, or the entry of assimilate is not accompanied by water. Short-distance transfer of solute (only) from the plant's vascular system to a separate system servicing the grain seems the simplest explanation on the basis of the available evidence.