Predicting Phloem Loading Types with Electron Microscopy

Phloem loading is a critical step in the transport of photosynthetically produced sugars from leaves to sink tissues. The type of phloem loading-active apoplasmic, active symplasmic, or passive symplasmic-can be inferred from the density and structure of plasmodesmata (PD) along the phloem-loading pathway. This protocol describes a method to predict phloem loading types using transmission electron microscopy (TEM) to quantify PD density. The method follows established standards for sample preparation, imaging, and data analysis, enabling comparative studies across species.