CarbonPartitioning andExport fromMature CottonLeaves

Thepartitioning ofcarbon inintact, mature cotton (Gossypium hirsutum L.)leaves wasexamined bysteady-state 14CO2 labeling. Plants wereexposed todarkperiods ofvarying lengths, followed bysimilar illuminated labeling periods. Thesetreatments producedleaves witharangeofstarch andsoluble sugarcontents, carbonexchange, andcarbonexport rates. Export during the illuminated periods wasneither highly correlated withphotosynthesis norwasexport during theilluminated periods significantly different amongthetreatments. Incontrast, therateofsubsequentnocturnal carbon export fromtheseleaves varied widely andwasfoundtobehighly correlated withleaf starch content at theendoftheillumination period (r= 0.934) andwithnoctumal leaf respiration (r= 0.954). Leaveswhichhadaccumulated the highest levels ofstarch (about 275micrograms persquare centimeter) bytheendoftheillumination period exhibited nocturnal export ratesverysimilar tothoseduring thedaylight hours. Leaveswhichaccumulated starch toonly50to75micrograms persquare centimeter virtually ceasednoctumal carbon export. Forleaves withstarch accumulations ofbetween50and275 micrograms persquare centimeter, noctumal export wasdirectly proportional toleaf starch attheendoftheillumination period. After thenoctumal export rate wasestablished, itcontinued ata constant ratethroughout thenight eventhough leaf starch and sucrose contents declined.