Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. It is a passive process that requires no energy expense by the plant. Transpiration also cools plants, changes osmotic pressure of cells, and enables mass flow of mineral nutrients. When water uptake by the roots is less than the water lost to the atmosphere by evaporation plants close small pores called stomata to decrease water loss, which slows down nutrient uptake and decreases CO2 absorption from the atmosphere limiting metabolic processes, photosynthesis, and growth.
Carbon dioxide entry: When a plant is transpiring, its stomata are open, allowing gas exchange between the atmosphere and the leaf. Open stomata allow water vapor to leave the leaf but also allow carbon dioxide (CO2) to enter. Carbon dioxide is needed for photosynthesis to operate. Unfortunately, much more water leaves the leaf than CO2 enters for three reasons:
- H2O molecules are smaller than CO2 molecules and so they move to their destination faster.
- CO2 is only about 0.036% of the atmosphere (and rising!) so the gradient for its entry into the plant is much smaller than the gradient for H2O moving from a hydrated leaf into a dry atmosphere.
- CO2 has a much longer distance to travel to reach its destination in the chloroplast from the atmosphere compared to H2O which only has to move from the leaf cell surface to the atmosphere.
This disproportionate exchange of CO2 and H2O leads to a paradox. The larger the stomatal opening, the easier it is for carbon dioxide to enter the leaf to drive photosynthesis; however, this large opening will also allow the leaf to lose large quantities of water and face the risk of dehydration or water-deficit stress. Plants that are able to keep their stomata slightly open, will lose fewer water molecules for every CO2 molecule that enters and thus will have greater water use efficiency (water lost/CO2 gained). Plants with greater water use efficiencies are better able to withstand periods when water in the soil is low.
Water uptake: Although only less than 5% of the water taken up by roots remains in the plant, that water is vital for plant structure and function. The water is important for driving biochemical processes, but also it creates turgor so that the plant can stand without bones.
What path does water take to reach the leaf from the root hair?
Once water has entered a root hair, it must move across the cortex and endodermis before it reaches the xylem. Water will take the path of least resistance through a root to reach the xylem. Water can move across the root via three different pathways. One path is the apoplastic path where the water molecule stays between cells in the cell wall region, never crossing membranes or entering a cell. The other two routes, called cellular pathways, require the water molecule to actually move across a membrane.
The first cellular pathway is the transmembrane path where water moves from cell to cell across membranes; it will leave one cell by traversing its membrane and will re-enter another cell by crossing its membrane. The second cellular path is the symplastic path which takes the water molecule from cell to cell using the intercellular connections called the plasmodesmata which are membrane connections between adjacent cells. Regardless of the pathway, once the water molecule has traversed the cortex, it must now cross the endodermis. The endodermis is a layer of cells with a waxy inlay or mortar called the Casparian strip that stops water movement between cells.
At this point, water is forced to move through the membranes of endodermal cells, creating a sieving effect. Once in the endodermal cells, the water freely enters the xylem cells where it joins the fast moving column of water or transpiration stream, headed to the leaves.