Daily changes in carbon-dioxide exchange and photosynthate translocation of leaves of Vicia faba.
Net carbon-dioxide exchange and loss of photosynthate-(14)C from leaves of Vicia faba L. were estimated throughout a day (16/8 h light/dark periods) by infrared gas analysis and Geiger-Müller-tube monitoring, respectively. Net photosynthesis reached 95% of the mean maximum rate within 1 h of the start of the photoperiod, and thereafter remained relatively constant before declining during the last 2-4 h of the photoperiod. Respiration was highest at the start of the dark period. The loss of photosynthate-(14)C from leaves exhibited two phases: a rapid phase which was fastest but shortest following (14)C incorporation in the middle of the photoperiod, and a slower phase of much longer duration. Absolute rates of (14)C loss exhibited diurnal trends reaching 95% of the maximum 6 h after the start of the photoperiod and remaining high until darkness, and then declined markedly. Of the total carbon fixed in one 24 h period, about 50% was lost during the current photoperiod, 14% during the dark period and 5% during the next photoperiod. The estimated amount of carbohydrate remaining within the leaf followed a diurnal pattern of accumulation and depletion. The daily course of translocation was not closely related to that of net photosynthesis or carbohydrate concentration.
School of Biological Sciences, Macquarie University, 2113, N.S.W., Australia.
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Pearson