Sites of entry of water into the symplast of maize roots

Following our publication of a new method of calculating rates of water uptake by roots from measurements of the rate of accumulation on the roots of a marker solute, this paper describes the sites of accumulation of the solute, which indicate the sites where the water entered the symplast. Sulphorhodamine G (SR) was supplied in aeroponic mist culture to large maize plants with fully developed root systems. Root samples were collected after 4 to 8 h of transpiration in the dye-mist from both axes and branches of the main roots, and from non-transpiring (detopped) controls, frozen rapidly, freeze-substituted, and embedded and sectioned by an anhydrous procedure that preserves the SR in place. Whole mounts and sections were examined by bright-field, polarizing and epifluorescence microscopy. Major accumulations of SR were all at the outer surface of the roots, on Epidermal or root hair cell walls, or, in older roots where the epidermal cells were separating or dead, on the outer wall of the hypodermis. On some branch roots, though not on any main axes, the accumulations of SR were conspicuously aligned in the grooves over anticlinal cell walls of the epidermis. Non-transpiring plants showed very slight accumulations. Diffusion of SR into the cell wall apoplast was limited by the suberized lamellae and Casparian bands of the hypodermis, except in some branch roots, where SR diffused throughout the cortical cell walls. In parts of roots where the epidermis and hypodermis had been damaged, SR diffused through cell walls of the cortex from the wound site. These patterns of accumulation show that water enters the symplast of roots at the outermost cell membranes of the root, whether they are epidermal or hypodermal cells. Water enters roots with fully developed hypodermises at high rates. The rote of the hypodermal suberization is to limit solute movement in the wall apoplast. A symplastic path for water throughout the cortex, endodermis and living cells of the stele is suggested.