5.2.8  Solute recycling: scavenging cells

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Figure 5.10 Higher resolution view of the pine needle in Figure 5.3 showing suberin/lignin deposits in radial walls of the endodermis (arrowheads) that block diffusive movements through the cell wall apoplasm and confine fluxes to the symplasm.Transfusion tissue within the endodermis consists of two cell types, dead transfusion tracheids (dark), which contain the transpiration stream, and living transfusion parenchyma (dull red), which scavenge solutes from the stream and forward them through the endodermal symplasm to the mesophyll cells.The complicated cavities of the recessed stomata are fluorescing blue-white. Rhodamine B stain, fluorescence optics. Scale bar = 50 μm. (see Colour Plate 24) (Photograph courtesy M. McCully)

Recycling of solutes out of leaves can also be fed by a variety of special structures adapted for processing rather larger volumes of sap, and so suited for collecting solutes present in the stream at quite low concentrations. The transfusion tissue of conifer needles is one of these structures. As sap leaves the xylem of a vascular strand it moves through a bed of trach-ids mixed intimately with transfusion parenchyma cells (Figure 5.10). The endodermis acts as the ultrafiltration barrier, allowing water to pass through while leaving dilute solutes to accumulate in transfusion tracheids. Transfusion parenchyma cells have very active H+-ATPases in their cell membranes which accumulate selected solutes (certainly some amino acids) back into the symplasm for return to the phloem and re-export. Such actively accumulating cells are called scavenging cells.

A tissue that acts in the same way is a special layer of cells in the central plane of many legume leaves (extended bundle sheath system or paraveinal mesophyll). It consists of scavenging cells with active H+-ATPases and accumulates amino acids, stores them and forwards them to developing seeds via the phloem.

Jagged ‘teeth’ on the margins of many leaves also contain scavenging cells. Veins carry large volumes of the xylem sap to these points, where evaporation is especially rapid. Within a ‘tooth’, xylem strands end in a spray of small vessels among a bed of scavenging cells. Scavenging cells can thereby collect amino acids and load them into the phloem.