Mitochondrial Activity and Carbohydrate Levels in Tulip Bulbs in Relation to Cold Treatment

Changes in mitochondrial activity and in the levels of variouscarbohydrates were followed in tulip bulbs (cv. Apeldoorn) keptin the dark at low and high temperatures (9 and 18 ?C respectively)from the time of flower differentiation until anthesis. Afterthe first 10 weeks at either temperature, the activity of themitochondria, measured by their ability to oxidize succinate,malate, or 2-oxoglutarate, was at a minimum. Thereafter oxidationrates improved up to about the 26th week when mitochondria fromcooled (9 ?C) bulbs were usually more active than those fromuncooled (18 ?C) bulbs. Subsequently, rates for both treatmentsfell rapidly. In contrast, levels of total alcohol-soluble carbohydratesand of sucrose in the cooled bulbs were at a maximum after 10weeks and were falling rapidly by the 26th week. Statisticallysignificant inverse correlations between mitochondrial activity,typified by the succinate oxidation rate, and these carbohydrateswere established. No such relationships were found for uncooledbulbs, however, and carbohydrate levels in such bulbs tendedto increase slowly throughout the growing period. In addition,transfer of carbohydrates to the growing shoot was slow, andthe flowers either failed to develop normally or were of poorquality. The mechanism whereby the bulb controls the activityof its mitochondria is at present unknown, but the rise in sucroselevels and its subsequent redeployment to the shoot broughtabout by cold treatment appear essential for normal flowering.