Effect of the Osmotic Environment on the Balance between Uptake and Release of Sucrose and Amino Acids by the Seed Coat and Cotyledons of Developing Seeds ofPisum sativum

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Abstract
Excised seed-coat halves and cotyledons of developing seeds of Pisum sativum L. were incubated in a bathing medium (pH 5·5), in order to measure the release or uptake of sucrose and amino acids. Net efflux of sucrose and amino acids was reduced by a 250 mol m −3 mannitol solution and a 400 mol m −3 solution, in comparison with a 100 mol m−3 control. This effect could not be observed in the case of the amino acid analogue α-aminoisobutyric acid (AIB). Net uptake of labelled sucrose or valine by cotyledons and seed coats was enhanced by a high osmolality of the bathing medium. The data on AIB and the data from uptake experiments support the view that net efflux of assimilates is reduced by a high solute concentration in the apoplast (e.g. 400 mol m−3 mannitol), via a stimulation of carrier-mediated sucrose and amino acid uptake into cotyledonary and seed coat tissues. In experiments with attached empty ovules of pea in a very early stage of development, sugar release from the seed coat was enhanced by a low osmolality of the apoplast solution (e.g. 100 mol m−3 mannitol, in comparison with a 400 mol m −3 control). This paradoxical effect may be observed when the stimulatory effect on net assimilate efflux from seed coat tissues is exceeding the inhibitory effect on assimilate import into the seed coat.

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