These tiny water droplets, (Figure 1) can be spotted on the edges of leaves early in the morning, but they are not dew! This phenomenon, called guttation, is a result of high water uptake and low transpiration, such as the conditions experienced on a cool, humid night after a warm day.

Figure 1. Guttation on strawberry leaves

Figure 1. Guttation on strawberry leaves.

In normal conditions plants absorb water from the soil and expel it as a vapour through tiny pores in the leaves called stomata. This expulsion of water vapour is called transpiration.

Guttation occurs when the rate of transpiration is reduced, such as on a humid night. Instead of being transpired through stomata, water accumulates in the plant. To get rid of the excess water, sap is expelled through special pores on the tips of leaves called hydathodes. It gathers on the margins of leaves, forming droplets in a very characteristic pattern. Older leaves may exhibit less guttation than younger leaves because over time their hydathodes become blocked with wax and solutes.

The droplets that form contain sugar, salts, potassium, and other solutes. The drop may roll off, be washed off by rain, be absorbed back into the leaf, or it may evaporate. If it evaporates, it may leave behind residues on the leaf, which can sometimes cause salt injury to leaves, seen as tip burn. There is some concern for the nutrients lost through guttation, but it is usually a negligible amount.

Otherwise, guttation is a good sign that tells us our plants have a good rate of transpiration, and want not for water.

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