purpose – although the purpose or benefit might not be apparent to those of us who grow plants. A great example of a “why does the plant do that?” came up this week – the occurrence of “guttation”.
|The white spots on the leaves of this |
hydrangea occur when guttation evaporates,
leaving behind salts and sugars that
were dissolved in the guttation fluid.
Turns out that these spots are residue left behind after droplets of guttation fluid had evaporated from the hydrangea leaves. Guttation is water that is forced out of pores called hydathodes that are present in the leaves of some plant species. Different than stomata (structures on leaf surfaces that open and close to regulate air and water exchange), hydathodes are always open and are connected to the xylem of the plant (those tube-like structures in stems and leaves that move water and nutrients from the roots to the leaves).
|Guttation on grass leaves (often occurs along with dew) can|
encourage the occurrence of some diseases - so early morning
irrigation can wash it off and help reduce disease incidence.
Under nighttime conditions of high soil/media moisture levels, warm soil temperatures (think summer…or a warm greenhouse), high relative humidity, and cool air temperature (cooler than the soil or media in the container), roots will absorb water that travels upward in the plant to the leaves. Since stomata are mostly closed at night – which prevents water loss from the leaves – this water being forced upward into the leaves needs someplace to go. The hydathodes allow the water to exit the leaf, acting as a sort of pressure relief system. The result is that droplets of water form on the margins and tips of leaves during the early morning hours. After a few hours of sunlight, the water evaporates - sometimes leaving a whitish residue on the leaf.
Tomatoes (and many other vegetables) can form guttation. It is
thought that bacteria and viruses can be "sucked" inside the
plant leaf when guttation dries - providing a opportunity for
Guttation doesn’t occur with all plants. Because of physics (we won’t go there!), guttation occurs in shorter plants (less than about 3 feet tall). Guttation has not been observed with conifers, but is very common in grasses (like your lawn!). It is also common on some houseplants (including many succulents) – especially if you water your plants just before dark.
|The residue left behind when guttation evaporates is often|
mistaken for disease or an insect infestation. Look
closely to be sure before you attempt control of a