Visualisation of the uptake of two model xenobiotics into bean leaves by confocal laser scanning microscopy: diffusion pathways and implication in phloem translocation.

The diffusion of two fluorescent dyes, Oregon Green 488 (Oregon Green) and Rhodamine B into the leaves of broad bean (Vicia faba L) plants was studied to simulate the foliar uptake process of pesticides. The uptake rate of these model xenobiotics into bean foliage was measured using a standard leaf surface wash-off method. Diffusion into leaf tissues was visualised in vivo by confocal laser scanning microscopy (CLSM). The moderately lipophilic dye (Rhodamine B) showed faster uptake than the hydrophilic one (Oregon Green), despite the former being a larger molecule. While no distinct channels or domains for preferential entry of any of the dyes could be detected in the cuticle layer by CLSM, two different diffusion patterns were identified for the movement of these two dyes after traversing the cuticle. Upon desorption from the cuticle, Rhodamine B diffused extensively into the vacuole of the epidermal cells. Further transport of this dye from the epidermal cells to the mesophyll cells was not observed. In contrast, Oregon Green was found in the epidermal cell walls and cytoplasm, and was also present in the mesophyll cells. Examination of the petioles of the treated leaves revealed that, once absorbed, Oregon Green moved readily out of the treated leaf, whereas Rhodamine B did not show any phloem translocation. It is proposed that these two different diffusion characters may be responsible for the contrasting phloem mobility of the two xenobiotics. The results are discussed in relation to the current knowledge on the uptake, translocation and efficacy of pesticides as influenced by their properties.