SugarUptake andStarch Biosynthesis bySlices of Developing MaizeEndosperm

C-Sugar uptakeandincorporation intostarch byslices of developing maize(ZeamaysL.)endosperm wereexamined and compared withsugaruptake bymaizeendosperm-derived sus- pension cultures. Rates ofsucrose, fructose, andD-andL-glucose uptake byslices weresimilar, whereas uptakerates forthese sugarsdiffered greatly insuspension cultures. Concentration dependence ofsucrose, fructose, andD-glucose uptakewas biphasic (consisting oflinear plussaturable components) with suspension cultures butlinear withslices. Theseandother dif- ferences suggest that endosperm slices arefreely permeable to sugars. After diffusion into theslices, sugars weremetabolized andincorporated intostarch. Starch synthesis, butnotsugar accumulation, wasgreatly reduced by2.5millimolar p-chlorom- ercuribenzenesulfonic acidand0.1millimolar carbonyl cyanide m-chlorophenylhydrazone. Starch synthesis wasdependent on kernel ageandincubation temperature, butnotonextemal pH(5 through 8).Competing sugars generally didnotaffect thedistri- bution of14C amongthesoluble sugars extracted fromendosperm slices incubated in14C-sugars. Competing hexoses reduced the incorporation of14Cinto starch, butcompeting sucrose didnot, suggesting that sucrose isnotanecessary intermediate instarch biosynthesis. Thebidirectional permeability ofendosperm slices tosugarsmakesthecharacterization ofsugartransport into endosperm slices impossible, however themodelsystemisuse- fulforexperiments dealing withstarch biosynthesis which occurs inthemetabolically active tissue.