Accumulation and conversion of sugars by developing wheat grains. 4. Effects of phosphate and potassium ions in endosperm slices

Transverse slices through developing grains of Triticum aestivum cv. SUN 9E 16 d after anthesis were incubated in simple defined media with various radioactive labels. In some enzymic assays slices were pretreated with 2.5% Triton X‐100 or with 5% butanol to remove cellular membranes and endogenous substrates. Endogenous potassium leaked from endosperm slices into 30mol m−3 sucrose while sucrose was converted partly into starch. Exogenous alkali‐ions, except Li+, stimulated conversion of sucrose to insoluble matter, specifically to starch with K+. Starch synthetase activity of Triton‐pretreated slices was stimulated by K+ at both high and low substrate ADPG concentration, but was not affected by phosphate (25 mol m−3). Phosphate in the medium had no effect on incorporation of sucrose or glucose into alcohol‐insoluble material or starch in fresh slices (internal inorganic phosphate (P,) concentration was about 11 mol m−3). Three‐ to four‐fold contrasts in internal Pi level, achieved by prolonged preincubations in different media, did not show an inhibition of starch synthesis by Pi. However, phosphate (25mol m−3) inhibited starch synthesis, that was mediated by ADPG pyrophosphorylase in butanol‐pretreated endosperm slices by 15–18%. It is concluded that starch synthesis in wheat endosperm is not regulated directly by apoplastic Pi; level.