TRANSFUSION TISSUE OF PINE NEEDLES AS A SITE OF RETRIEVAL OF SOLUTES FROM THE TRANSPIRATION STREAM

The fluorescent tracers sulphorhodamine G (SR) and pyrene trisulphonate (PTS) were used to explore the functions of cells and tissues within the pine needle, following their progress after feeding through the transpiration stream. The distributions of tracer in the needles were stabilized for fluorescence microscopy by rapid freezing and freeze-substitution, and anhydrous embedding and sectioning. After short pulse + chase times (up to 2 h), SR and PTS accumulated at higher-than-xylem concentrations in the transfusion tracheids on the flanks of the vascular strand, but did not pass out through the endodermis. The accumulations occurred locally where the transpiration water was separated from the tracers and passed into the symplast of the transfusion parenchyma and endodermis. After a 24 h water chase, SR had entered the symplast through the transfusion parenchyma, and spread through the endodermis and palisade. It is argued that this is evidence of active H+-ATPase systems lowering the external pH of the transfusion parenchyma, and characterizes these cells as scavenging cells similar to those found in the bundle sheath systems of legume leaves. The transport of SR through the endodermis and palisade is the first clear evidence that this tracer can also function as a symplastic tracer. The hypothesis that the transfusion parenchyma acts to scavenge solutes from the transpiration stream was tested by loading the stream with [C-14]aspartate and examining the subsequent distribution of C-14 by dry autoradiography. After a pulse + chase of (0.75 + 3) h, C-14 was found concentrated in the transfusion parenchyma, and at even higher levels in the Strasburger cells. It is proposed that major functions of the transfusion tissue of gymnosperms are (a) the concentrating of solutes from the transpiration stream and (b) the retrieval from the stream of selected solutes that are returned to the phloem through the Strasburger cells, or forwarded through the endodermis to the palisade.