Sieve-element differentiation and fluoresceine translocation in wound-phloem of pea roots after complete severance of the stele.

Experimental interruption of the root stele of Pisum sativum L. induces in the cortex tissue the development of wound-sieve tubes which bridge the wound and reconnect the vascular stumps. Outside the stele, sieve plates arise from primary pit fields. This origin is confirmed by the distribution of future sieve pores over the original parenchyma cell wall and by remnants of the pitfield cavity in developing sieve plates. Differentiation of wound-sieve elements is similar to that of bundle-sieve elements and includes the chromatolytic disintegration of nuclei as well as the development of typical sieve pores arising from pit-field plasmodesmata. The completion of first woundsieve tubes (indicated by a continuous chain of anilin-blue-positive sieve plates by-passing the wound) was observed 55-62 h after wounding. However, effective translocation, visualized with fluoresceine as a phloem-mobile marker, was not found until 10 h (on average) later. It is suggested that this time delay corresponds to the maturing of the last link within a chain of wound-sieve-tube members. Presumably, enucleate sieve elements with widened pores are a prerequisite for effective phloem translocation.