CHANGING KINETICS OF L-VALINE UPTAKE BY IMMATURE PEA COTYLEDONS DURING DEVELOPMENT : AN UNSATURABLE PATHWAY IS SUPPLEMENTED BY A SATURABLE SYSTEM

The influx of L-[14C]valine was measured over a wide concentration range (1 μM to 100 mM) in immature, isolated pea (Pisum sativum L. cv. Marzia) cotyledons at two developmental stages, both in the absence and in the presence of 0.4 M mannitol. At an early developmental stage (water content of the cotyledons ≈80%) the valine influx was strictly proportional to the external amino-acid concentration over the whole concentration range (so-called linear component). This system renders the plasmalemma of the cotyledonary cells approximately 1000-fold more permeable to valine than the presumed basic permeability of the membrane for valine. At a later stage of development (water content of the cotyledons ∼55%) this transport pathway was supplemented by a saturable system (Km = 5mM; Vmax = 9.5 μmol · gFW−1). The saturable system emerged when the water content of the cotyledons was about 65%, and its activity increased steadily up to the latest developmental stage examined (water content of the cotyledons =50%). As a result, uptake rates of l-valine at low concentrations, expressed on a fresh-weight basis, increased about 15-fold in this phase of development. Low osmolarity of the bathing medium (0 mM mannitol) had no effect on the linear component, and reduced the uptake by the saturable component only slightly by increasing its Km from 3.8 mM to 5.6 mM. The possible role of the saturable system in the acquisition of amino acids by the developing embryo is discussed.