Route of Substrate Movement Into Wheat Endosperm. II. Amino Acids

Abstract

Detached spikelets of wheat were cultured on solutions containing [³H]glutamine and [¹⁴C]sucrose in a series of experiments designed to define the pathways of amino acid movement in developing wheat grains and endosperm. Some functions measured on the spikelets provide confidence that the system is valid for studies on transport. Microdissection was used to determine time-dependent distributional patterns of [³H]amino acids and [¹⁴C]soluble carbohydrates along a radial axis from the vascular bundle at the base of the crease to the inner pericarp on the dorsal surface. Surgical treatments were used to disrupt putative transport pathways. Amino acids moved along the grain in the tissues at the base of the crease, into the endosperm cavity, and radially across the endosperm in an outward direction. Any longitudinal movement within the endosperm was small. Amino acids moved also into tissues surrounding the endosperm, but there was no evidence for an inward radial route to the endosperm. Hence, amino acids destined for deposition in the endosperm move within the grain and endosperm along the same pathways as shown previously for movement of soluble carbohydrates. Gradients of protein deposition across wheat endosperm, increasing in an outward direction, are not due to proximity or direction of substrate supply. Solutes moving to the inner pericarp did not rely on a route involving the main vascular bundle of the grain, but they moved into the inner pericarp from a basal point.