Phloem unloading in developing walnut fruit is symplasmic in the seed pericarp, and apoplasmic in the fleshy pericarp

The sieve element-companion cell (SE-CC) complex of the sepal bundles feeding the fleshy pericarp of developing walnut (Juglans regia L.) fruit is structurally symplasmically isolated, but the SE-CC complex of the minor ventral carpellary bundles located in the seed pericarp and feeding the seed is structurally symplasmically connected to its adjacent parenchyma cells. C-14-autoradiography indicated that the phloem of both the sepal and carpellary bundles was functional for unloading. Confocal laser scanning microscopy imaging of carboxyfluorescein unloading showed that the dye is confined to the phloem strands of the sepal bundles in the fleshy pericarp, but released from the phloem strands of the minor ventral carpellary bundles into the surrounding parenchyma cells in the seed pericarp. A 60-kDa acid invertase was immunolocalized to the cell wall of SE-CC complex and parenchyma cells in both the fleshy and seed pericarp. These data provide clear evidence for an apoplasmic phloem unloading pathway in the fleshy pericarp and a predominant symplasmic phloem unloading pathway parallel with a possible apoplasmic path as suggested by the presence of the extracellular invertase in the seed pericarp. A model of complex phloem unloading pathways in developing walnut fruit has been proposed.