Fixation patterns of (14)C within developing leaves of eastern cottonwood.

Individual leaves of eastern cottonwood (Populus deltoides), representing an ontogenetic series from leaf plastochron index 0.0 to 8.0, were fed (14)CO2 photosynthetically and then harvested at times ranging from 15 to 1440 min. The lamina of each fed leaf was sectioned from tip to base into 5 parts, and each part was quantitatively assayed for (14)C activity. In young leaves, the percentage of the total (14)C fixed (expressed in dpm/mg of dry leaf tissue) was high in the lamina tip and decreased almost linearly toward the base. With increasing leaf age, the percentage of (14)C fixed decreased in the lamina tip and increased in the base. The relative activity in mature leaves was almost uniform throughout the lamina. No differences were detected in the (14)C distribution patterns within leaves over the time series.On the basis of the data presented and of anatomical observations of developing cottonwood leaves, the hypothesis that the precociously mature lamina tip may provide photosynthates to the still-expanding lamina base was shown to be invalid. It is concluded that bidirectional transport in a developing cottonwood leaf results from simultaneous import to the immature basal region and export from the mature tip.