Ultrastructure of the Small Vascular Bundles and Transfer Pathways for Photosynthate in the Leaves of Rice Plant

Abstract

The ultrastructure of small vascular bundles of the 8th leaf blades in rice seedlings was examined with the electron microscope. 1. In the parenchyma sheath cells the chloroplasts have a centrifugal position, whereas the mitochondria have a centripetal position. The phloem is bounded by a single layer of thick-walled parenchyma cells, which differentiate into the mestome sheath cells in the upper leaves. The thick-walled parenchyma cells have the plastids with starch granules, mitochondria and dictyosomes. The suberized lamellae are not detected in the walls of these sheath cells. 2. The protophloem sieve elements and companion cells locatcd abaxially within the vascular bundles show an apparent degeneration in the expanded leaf, but the degeneraion is not observed in the elongating zone of the folded leaf. Thus, the protophloem may degencrate during the leaf emergence and ceases to function after leaf expansion. 3. Late-formcd metaphloem sieve elements are narrow and thickwalled, but the associated companion cells are wider than the sieve elements. The metaphloem sieve elements have the P-type plastids and a small amount of mitochondria and endoplasmic reticulum. The rnetaphloem companion cells contain many mitochondria and endoplasmic reticulum, and often contain osmiophilic globules. Ultrastructural features of the metaphloem parenchyma cells are the same as the companion cells. 4. The distribution of plasmodesmata in transections of the small vascular bundles has been determined. The plasmodesmata occur in the outer and inner tangential walls of the parenchyma sheath cells. The late-formed metaphloem companion cells are connected by numerous plasmodesmata with adjacent parenchyma cells of the metaphloem and metaxylem, and also with the thick-walled parenchyma cells respectively. The companion cell and the sieve element are connected by plasmodcsmata which are branched on the companion cell side. The data support the view that photosynthate moves through a symplastic pathway from mesophyll to the metaphloem sievc elements. The following pathways are suggested.[table]