Diurnal Variations in Non-Structural Carbohydrates, Leaf Extension, and Leaf Cavity Carbon Dioxide Concentrations in Allium cepa L.

Diurnal changes in non-structural carbohydrates, leaf extension,and leaf cavity CO₂ concentrations were determined. Before thestart of the photoperiod total non-structural carbohydrate beginsto accumulate due to an accumulation of glucose and fructose.After the start of the photoperiod total carbohydrate continuesto accumulate but at this time this is due to increasing sucroselevels. The pre- and early-photoperiod accumulation is finite,and a reduction of accumulated carbohydrate occurs before amore sustained increase commences. Utilization of accumulatedcarbohydrate commences at least 3 h before the end of the photoperiod.Leaf extension is constant throughout the dark and photoperiod.The CO₂ concentration within the leaf cavity is high at 0·3%,but declines before the start of the photoperiod and immediatelyafter ‘lights-on’ to about 0·06%, this concentrationbeing maintained throughout the day. A slow accumulation ofinternal CO₂ occurs after the end of the photoperiod.