Abstract: | Excised seed-coat halves and cotyledons of developing seedsof Pisum sativum L. were incubated in a bathing medium (pH 5·5),in order to measure the release or uptake of sucrose and aminoacids. Net efflux of sucrose and amino acids was reduced bya 250 mol m 3 mannitol solution and a 400 mol m 3solution, in comparison with a 100 mol m3 control. Thiseffect could not be observed in the case of the amino acid analogue -aminoisobutyric acid (AIB). Net uptake of labelled sucroseor valine by cotyledons and seed coats was enhanced by a highosmolality of the bathing medium. The data on AIB and the datafrom uptake experiments support the view that net efflux ofassimilates is reduced by a high solute concentration in theapoplast (e.g. 400 mol m3 mannitol), via a stimulationof carrier-mediated sucrose and amino acid uptake into cotyledonaryand seed coat tissues. In experiments with attached empty ovulesof pea in a very early stage of development, sugar release fromthe seed coat was enhanced by a low osmolality of the apoplastsolution (e.g. 100 mol m3 mannitol, in comparison witha 400 mol m 3 control). This paradoxical effect may beobserved when the stimulatory effect on net assimilate effluxfrom seed coat tissues is exceeding the inhibitory effect onassimilate import into the seed coat. Key words: Seed development, turgor-sensitive transport, assimilate transport |