BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. PLANT TRANSPORT

13.3. Transpiration and Water Movement Through Leaves

13.3.3. Vacuolar Transport

In this pathway, Water moves from the vacuole of one Cell to the vacuole of an adjacent cell via the symplast and apoplast—and consequently across the tonoplasts and Plasmalemma—driven by osmosis (Fig. 13.8, B). The resulting water potential gradient is established as follows.

Water evaporates from The surface of mesophyll Cells into the intercellular spaces, predominantly into the relatively large substomatal cavities. Taking cell 1 in Fig. 13.8, A as an example, the water lost by this cell causes a drop in both its hydrostatic pressure and, consequently, its water potential. (For simplicity, let us assume that the entire system was initially in equilibrium, meaning this potential was uniform throughout.) Now, The water potential of cell 2 becomes relatively higher, causing water to flow down the gradient from cell 2 into cell 1. In turn, this lowers the WATER POTENTIAL OF cell 2 relative to cell 3. This is how a continuous water potential gradient is set up across the leaf, stretching all the way from the xylem vessels—which have a higher water potential—to the mesophyll cells, which have a much lower potential. Water moves through this system by osmosis. Although we describe this transport in a step-by-step manner, It is important to remember that the water potential gradient established within the leaf is actually continuous, and water flows smoothly down this gradient, much like liquid moving up a wick.

It is sometimes mistakenly assumed that water moves through the leaf along an osmotic potential gradient. However, while a water potential gradient certainly exists, there is no evidence to suggest significant variations in osmotic potential among the relevant cells. The Transpiration stream is driven primarily by differences in hydrostatic pressure: the loss of even a small amount of water by a cell has a far greater impact on turgor pressure than on solute concentration. The same holds true for the ROOT (see Section 13.5), which exhibits gradients of water potential and hydrostatic pressure, but not necessarily of osmotic potential.