Pit-field distribution, plasmodesmatal frequency, and assimilate flux in the mestome sheath cells of wheat leaves

Summary

The distribution of pit fields and plasmodesmata in the mestome-sheath cells of a wheat leaf has been determined by study of sections and partial macerates. Each bundle is approximately symmetrical about the sagittal plane and most plasmodesmata occur in the mestome sheath where its cells abut the metaphloem. Plasmodesmata are absent adjacent to xylem vessels, and the frequency of plasmodesmata declines sharply in cells that lie close to the sagittal plane. Calculations show that 1 cm² of leaf lamina has approximately 2x10⁸ plasmodesmatal connections to the phloem of the longitudinal veins, and that 85% of these connections are to the late-maturing intermediate bundles that do not complete their differentiation until leaf growth is nearly finished. The phloem area of inner tangential wall of the mestome sheath amounts to 0.26 cm² per cm² of leaf area and plasmodesmata occupy 1.5% of this area.

These anatomical facts are used to estimate the sugar flux across the inner-tangential wall of the sheath as 2x10² pmol s^-1 cm² _sheath. Further analysis strongly suggests that this flux must cross the sheath by diffusion through the plasmodesmata, creating there a flux of 1.5x10⁴ pmol s^-1 cm^-2 _plasmodesma. These results are compared with data recently obtained for the transfer-cell/sieve-element boundary in Vicia and are found to be about one tenth of the flux in that system, the transfer being adequately driven by a concentration gradient of 50 μg cm^-3 of sugar across the sheath. Such a concentration gradient could be achieved by the photosynthetic activity of about 50 chloroplasts acting for about 2 min. The transverse veins that lack a mestome sheath are unlikely to account for more than 10% of the fluxes calculated here and have been ignored in the calculations. It is concluded that the symplastic pathway is the only possible one for assimilate traffic across the mestome sheath in wheat, and that diffusion down a gradient of sugar concentration from the chloroplasts to the sheath acts as the driving force. This suggestion is reinforced by analysis of the contributions of the larger and smaller veins to the water flux from the same square centimetre of leaf. This analysis shows that 99% of the water flux must exit from the xylem of the 7 large bundles, presumably through the apoplast, securing an effective separation between the inwardly directed flow of sugar (laoded symplastically chiefly through the small longitudinal bundles) and the outwardly directed, very much larger flux of water.