Leaf structure and translocation of dry matter in a C3 and a C 4 grass

Dry weight analyses and (14)CO2 were used to study translocation in leaves of the C3 grass Lolium temulentum L. and the C4 grass Panicum maximum Jacq. and the results related to the distribution and amount of phloem in the lamina. The rate of specific mass transfer rose from the tips to the bases of leaf blades, in both species high rates were recorded. Major veins were responsible for the bulk of longitudinal translocation and minor veins were important in collecting and loading photosynthate. Transverse veins stored (14)C-assimilate and may have coordinated the functioning of the longitudinal veins. The bearing of the results on the mechanism of translocation is discussed.

Translocation and incorporation of (14)C into the petiole from different regions within developing cottonwood leaves

The ability of a developing cottonwood (Populus deltoides Bartr.) leaf to export (14)C-labeled assimilates begins at the lamina tip and progresses basipetally with increasing LPI. This progression indicates that portions of leaves function quasi-independently in their ability to export (14)C-photosynthate. Although most of the exported radioactivity was recovered in the petiole as water-80% alcohol-soluble compounds, there was also substantial incorporation into the chloroform and insoluble fractions. This observation indicates that assimilates translocated from the lamina are used in structural development of the petiole. Freeze substitution and epoxy embedding were used to prepare microautoradiographs for localization of water-soluble compounds. Radioactivity was found in all cell types within specific subsidiary bundles of the petiole. However, radioactive assimilates appeared to move from the translocation pathway in the phloem toward active sinks in the walls of the expanding metaxylem cells. Translocation in the mature xylem vessels was not observed.

Distribution of imported (14)C in developing leaves of eastern cottonwood according to phyllotaxy

Individual leaves of eastern cottonwood (Populus deltoides Bartr.), representing an ontogenetic series from leaf plastochron index (LPI) 3.0 to 8.0, were fed (14)CO2 and harvested after 2-24 h. Importing leaves from LPI-1.0 through 8.0 on each plant were sectioned into 9 parts, and each part was quantitatively assayed for (14)C activity. The highest level of (14)C import was by leaves from LPI 1.0 to 3.0, irrespective of source-leaf age. (14)C was translocated preferentially to either the right or left lamina-half depending on the position of the importing leaf in the phyllotactic sequence and its stage of development. For example, import was high when the importing leaf and the source leaf had two vascular bundles in common, moderately high with one bundle in common, and low with no bundles in common. The distribution of (14)C within young importing leaves was highest in the lamina tip and decreased toward the base. With increasing leaf age, incorporation declined in the lamina tip and increased in the base.It may be concluded that each cottonwood leaf progresses through a continuum of importing and exporting stages as its lamina expands. The photosynthate imported by a given leaf is compartmentalized, with different exporting leaves supplying photosynthate to rather restricted regions of the lamina. Such localization within the importing leaf depends on its vascular connections with each of the exporting leaves, and these are predictable from a knowledge of the phyllotaxy.

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.