As sugar beet source leaves lowered the CO2 concentration to compensation point in a closed atmosphere, leaf thickness and relative water content Dec.reased. Leaf water potential Dec.lined rapidly from −0.5 to −1.4 megapascals. At 340 microliters CO2 per liter, water potential and sucrose, glucose, and fructose contents were steady in photosynthesizing source leaves. Within 90 minutes after leaves were exposed to a CO2 concentration at the compensation point, leaf sucrose content Dec.lined to 60% of the preteatment level, rapidly in the first 30 minutes and then more slowly. During the subsequent 200 minutes, sucrose content increased to 180% of pretreatment level. Glucose and fructose remained unchanged during the treatment. Degradation of starch was sufficient to account for the additional sucrose that accumulated. Labeled carbon lost from starch appeared in sucrose and several other compounds that likely contributed to the recovery in leaf water content.
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