Transport of Tritiated Water and ¹⁴C-Labelled Assimilate Into Grains of Wheat. I. Entry of Tho Through and in Association With the Stalk of the Grain

Abstract

Dry matter is transported through the stalk of the wheat grain, and during grain filling is accumulated within it with no change in the quantity of water in the grain, and with little or no net transfer of water through the stalk. Accumulation of dry matter is also independent of the magnitude and the direction of a water potential gradient imposed between the plant and the atmosphere surrounding the grain. A procedure is described, based on sealing the grain (minus its bracts) within an impermeable capsule, for measuring the total net influx of radioactive water into the grain (F_T). Tritiated water, THO, is used as a tracer for water. F_T is assumed to comprise transport in the liquid phase and transport in the vapour phase, by mass flow and by diffusion. Apoplastic flow makes a negligible contribution in the system described because the water potential gradient is minimized.

F_T was obtained by adding to the quantity of THO accumulating in the encapsulated assemblies an estimate of the amount of THO lost from the assemblies by leakage. Two types of ears were compared: ears on intact plants assimilating ¹⁴CO₂ and growing in small pots watered with THO, and detached ears cultured on [¹⁴C]sucrose in a solution of THO. Both types gave similar results but the detached ear system probably furnished the most reasonable estimate, a rate of 10.1 µl of THO per assembly per day.