Nutrient uptake patterns over the surface of germinating wheat seeds

Abstract

K⁺, Ca²⁺ and H⁺ flux profiles were studied around the surface of germinating wheat (Triticum aestivum L. cv. Machete) seeds using a non-invasive microelectrode technique. The seed surface was scanned every 500 µm along the longitudinal axis, and net ion fluxes were measured 2 min and 1, 4 and 24 h after seed rehydration com-menced. Functionally different seed zones exhibited different ion exchange patterns, which also changed differently with time. One h after seed rehydration, the peak of H⁺ extrusion was observed in the embryo-scutellum region. By 24 h this peak had shifted towards the region of radicle emergence, and its magnitude had increased 5-fold. By con-trast, H⁺ activity in the endosperm region decreased noticeably during this period. At the same time, Ca²⁺ flux shifted from initial net influx to efflux in the embryo and scutellum zones. Most striking were K⁺ flux profiles, with large K⁺ efflux observed at each end and almost zero flux near the geometrical center of the seed. Significant K⁺ extru-sion from the embryo end was observed for as long as 24 h after seed rehydration and was not related to the loss of membrane integrity. Seed treatment with metabolic inhibitors (vanadate and tetraethylammonium) caused signifi-cant changes in both H⁺ and K⁺ fluxes suggesting that at least part of the measured fluxes originated from activities of plasma membrane transporters. We discuss possible mechanisms responsible for the observed regularities and question whether the seed coat in wheat caryopses is as ion-impermeable as previously believed.