When is it more appropriate to water the plants? It is evident that watering them at noon, and especially in summer, would be inefficient, since the heat will evaporate a good part of the water from the soil without passing through the plant, and it will be wasted. But eliminating the central hours of the day, what is the best time? Is it better to do it in the evening, when the heat is subsiding, during the night, or early in the morning?
To answer this question, it is first necessary to know how the water transport system works in a plant.
How does the water get to the plant?
The process by which plants transport water inside themselves, and by which they first absorb water from the ground with their roots, has the same principle as making juice rise through a straw. A suction effect, which was described in 1895 by the Irish botanist Henry H. Dixon, and which is described by the tension-cohesion model .
The description of the model was received with skepticism, and it was not until the 1990s, almost 100 years later, that experimental tests allowed this theory to be verified. Today it is widely accepted as the main mechanism for transporting water in the plant. In fact, the force generated by perspiration alone can generate enough tension to raise water up to 130 meters high . Above the height of Hyperion , a sequoia that at almost 116 meters is the tallest tree in the world.
Although other elements that help the process are recognized today, such as the presence of surfactants in the xylem, which prevents the formation of bubbles that cause embolisms in the water continuum.
The theory of tension-cohesion
The basic principle of this model is that the column of water found inside a plant forms a continuum, from the root, through the vascular bundles of the stem, to the leaf. Water is a fluid that has a high power of cohesion , its molecules are linked to each other through very resistant cohesive forces. This allows to maintain that column of water without breaking.
There is a large difference in water potential between the interior of the leaf, always saturated with water, and the surrounding atmosphere, which generates a continuous flow from the inside out . It is produced by the evaporation of the water inside the leaf, which is released into the atmosphere through the stomata, small valve-shaped openings that the leaves have in their epidermis. Evaporation generates a tension that moves water from the vascular bundles of the leaf towards its tissues.
Given the high cohesion of the water, the tension does not break the water column , and, therefore, all the tensile forces generated by the evaporation of the leaves are transmitted to the branches and the stem. Inside, the water flows upwards through the vascular bundles of the xylem, overcoming the gravitational pull thanks to the tension that comes from above and its own cohesion.
Finally, that tension, let’s remember, generated in the leaves, reaches the roots, where the water is absorbed from the soil.
The role of the stomata
For the proper functioning of the process, the stoma is an essential organ. Stomata are small valves that can be opened or closed depending on the needs of the plant. If the stomata are open, evaporation occurs, tension comes into play, and the root absorbs water from the soil. If the stomata are closed, evaporation falls to minimal levels, the tension in the vascular bundles falls, and water absorption becomes negligible.
But not all plants open their stomata at the same time. It depends on your metabolism.
The opening of the stomata, in addition to favoring the movement of water in the plant, also allows the exchange of gases for photosynthesis, and their behavior depends on the metabolism they use. Plants with C3 and C4 metabolism need to obtain CO₂ directly from the atmosphere for photosynthesis to take place. Since photosynthesis can only happen when there is light, these plants open their stomata during the day, and close them at night—although sometimes not completely. However, there are plants that do not do it that way.
There are plants that live in areas where water is scarce and it is very hot during the day; Under these climatic conditions, opening the stomata during the day would mean dying from dehydration.
These plants have developed a system, called crassulaceae acid metabolism (or CAM), with which they have adapted to opening their stomata at night, when it is colder and there is less evaporation. It is at this moment that they carry out the gas exchange, and store the CO₂ from the environment in malic acid molecules, which during the day, and with the light, they take advantage of to carry out photosynthesis with the stomata closed.
CAM plants are usually succulent, that is, they have leaves or other tissues with a high water content, generally swollen; cacti, crassulaceae, stone plants, aloe vera, pineapple, some euphorbiaceae or orchids are CAM plants.
So when should I water?
The best time to water cacti, aloes and other succulent plants is at night , when they open their stomata because they will absorb the water they need during the night and, during the day, the stomata will close and the flow of water will stop.
But these CAM plants are the exception. Most plants —both in the natural environment and in gardens— have a C3 or C4 metabolism, so they open their stomata during the day. If they are watered at night, they will absorb very little water and most of it will stay in the ground without being used for hours. If the night irrigation is repeated a lot over time, it can rot the finest roots.
In conclusion, for most plants, the ideal time to water is early in the morning and before it gets very hot, which is when the plants open their stomata and start their metabolism. It is at this time that they will begin to absorb and take advantage of the water in the soil, before it evaporates.
References:
Dixon, H. H. et al. 1895. On the Ascent of Sap. Philosophical Transactions of theRoyal Society of London. B, 186, 563-576.
Koch, G. W. et al. 2004. The limits to tree height. Nature, 428(6985), 851-854. DOI: 10.1038/nature02417
Schenk, H. J. et al. 2017. Xylem Surfactants Introduce a New Element to the Cohesion-Tension Theory. Plant Physiology, 173(2), 1177-1196. DOI: 10.1104/pp.16.01039
Solomon, EP et al. 2013. Biology ( 9th). Cengage Learning Publishers.
Steudle, E. 2001. The cohesion-tension mechanism and the acquisition of water by plant roots. Annual Review of Plant Physiology and Plant Molecular Biology, 52(1), 847-875. DOI: 10.1146/annurev.arplant.52.1.847
