Soluble Sugars as the Carbohydrate Reserve for CAM in Pineapple Leaves

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Abstract
Neutral ethanol-soluble sugar pools serve as carbohydrate reserves for Crassulacean acid metabolism (CAM) in pineapple (Ananas comosus (L.) Merr.) leaves. Levels of neutral soluble sugars and glucans fluctuated reciprocally with concentrations of malic acid. Hexose loss from neutral soluble-sugar pools was sufficient to account for malic acid accumulation with about 95% of the required hexose accounted for by turnover of fructose and glucose pools. Hexose loss from starch or starch plus lower molecular weight glucan pools was insufficient to account for nocturnal accumulation of malic acid. The apparent maximum catalytic capacity of pyrophosphate:6-phosphofructokinase (PPi-PFK) at 15.degree. C was about 16 times higher than the mean maximum rate of glycolysis that occurred to support malic acid accumulation in pineapple leaves at night and 12 times higher than the mean maximum rate of hexose turnover from all carbohydrate pools. The apparent maximum catalytic capacity of ATP-PFK at 15.degree. C was about 70% of the activity required to account for the mean maximal rate of hexose turnover from all carbohydrate pools if turnover were completely via glycolysis, and marginally sufficient to account for mean maximal rates of acidification. Therefore, at low night temperatures conducive to CAM and under subsaturating substrate concentrations, PPi-PFK activity, but not ATP-PFK activity, would be sufficient to support the rate of glycolytic carbohydrate processing required for acid accumulation. These data for pineapple establish that there are at least two types of CAM plants with respect to the nature of the carbohydrate reserve utilized to support nighttime CO2 accumulation. The data further indicate that the glycolytic carbohydrate processing that supports acidification proceeds in different subcellular compartments in plants utilizing different carbohydrate reserves.