Indirect evidence for bulk water flow in root cortical cell walls of three dicotyledonous species

Water moves radially through the root in response to the tension generated by the transpiration stream. This movement occurs through both the cell walls and the protoplasts of the cells intervening between the soil solution and the lumena of the tracheary elements. The mechanism of movement is commonly believed to be diffusion in both these compartments. In the present study, we applied the apoplastic, fluorescent tracer, berberine, to roots of three dicotyledonous (Helianthus annuus L. cv. Mammoth Russian, Phaseolus vulgar is L. cv. Kinghorn wax, and Phaseolus aureus Roxb.) and four monocotyledonous species (Triticum aestivum L., Hordeum vulgare L., Zea mays L. cv. Seneca Chief, and Allium cepa L. cv. Ebeneezer). The tracer was precipitated in place by potassium thiocyanate. The entry of berberine into the main roots of the monocotyledonous species was limited, and no conclusions could be drawn about its movement. Tracer entered more readily into the main roots of dicotyledonous species and its movement by diffusion (in excised roots) was characterized by an evenly advancing diffusion ring in the cortex. However, when short treatment times were used for transpiring plants, some berberine was moved across the cortex by solvent drag, resulting in the formation of isolated crystals near the endodermis in advance of the diffusion ring. The phenomenon of solvent drag, in turn, is indirect evidence for movement of water by bulk flow in the cortical cell walls. Whether or not bulk how also occurred in lateral roots could not be determined since the narrow width of the cortex and the high permeability of the walls to berberine resulted in very fast progression of the diffusion ring.