Evidence for two pathways of phloem loading

The minor veins of small leaf discs, punched out of mature leaves and incubated in 14 C‐sucrose solution, appear labeled in macro‐ and microautoradiographs. Discs with a labeled vein pattern and with labeled sieve tubes in microautoradiographs were found in Beta vulgaris, Vicia faba, Gomphrena globosa and Antirrhinum majus . However, in several other plant species, minor veins appeared unlabeled in macroautoradiographs when the discs were incubated in 14 C‐sucrose. Mesophyll cells ( Acer pseudoplatanus, Juglans regia, Fagia, sylvatica, Syringa vulgaris, Laburnum anagyroides ), bundle‐sheath cells of major veins ( Salix viminalis, Robinia pseudoacacia, Commelina communis ) or epidermal layers ( Ginkgo biloba, Chlorophytum comosum ) appeared labeled. Lack of radioactivity in sieve tubes of this latter group was confirmed by microauto‐radiography. Using 14 C‐glucose instead of 14 C‐sucrose, leaf discs of Beta vulgaris showed no labeled vein pattern and in microautoradiographs the sieve tubes appeared unlabeled. In view of the by‐pass phloem loading, this study provides evidence for two pathways of phloem loading.