Herbicide translocation in the phloem is dependent on the production of transport sugar(s) in source tissues and their loading into the phloem. The movement of sugars (primarily sucrose) from mesophyll cells to the sieve element-companion cell complex has been the subject of much investigation in recent years. The current evidence suggests that there are at least two sucrose loading mechanisms in higher plants: one involves sucrose transfer via the apoplasm and subsequent loading into the phloem, while a second mechanism involves movement through continuous symplasmic connections between the mesophyll cells and the phloem. Similarly, both symplasmic and apoplasmic routes may exist at the site of unloading in sink tissue. It is postulated that the mechanism of sucrose loading may influence herbicide entry into the phloem and the likelihood of herbicide transfer to the xylem at the site of loading or unloading.
