Absorption and perspiration

The root is the organ responsible for sustaining our plants and supplying them with water and nutrients dissolved in the soil or substrate.
This function is carried out by absorbent hairs or root hairs, formed from epidermal cells.
Once the water reaches the central cylinder of the root, the transport along the plant is produced by the xylem (set of vascular bundles responsible for the flow of water and nutrients to the leaves for their transformation, through photosynthesis, into sugars and starch). In this transport act both the root pressure (positive pressure) and a negative pressure: transpiration, the loss of water that occurs in the leaves each time the plant opens the stomata (discontinuities existing in the epidermis of the underside of the leaves that result from the face-to-face arrangement of two cells different from those of the rest of the epidermis; the occlusive cells) for the uptake of CO2.
The soil-plant-atmosphere hydrodynamic system follows a flow of water in favor of potential gradient: the water potential of water in the soil is greater than that of water at the root; this in turn is greater than that of water in the xylem; that of water in the xylem is greater than that of water in the leaf and this in turn greater than that of water existing in the atmosphere.
Therefore, the loss of water by transpiration is a process that does not cost the plant energy; it is a «lesser evil» in the face of the need to open the stoma to capture CO2 and be able to perform photosynthesis. This water lost in the transpiration process constitutes up to 98% of the water absorbed by the root. Water consumption in metabolic reactions is derisory.
Stomata are not only the outlet pathway of water vapor but also the flow pathways of CO2 and O2. The opening mechanism of the stoma is known to influence the K+ cation; an accumulation of this in the occlusive cells decreases the osmotic potential which determines the entry of water and an increase in the turgor of the occlusives, causing the opening of this. But as we already know, there has to be an equilibrium of charge neutrality: this is compensated by the entry of chloride anion CL- simultaneously with K+ in a ratio 40% CL 60% K.
External factors affecting perspiration speed include:
  • Humidity of the atmosphere; the more moisture difference there is between the substomatic cavity and the outside, the greater the rate of transpiration.
  • Soil moisture; the higher the soil moisture, the greater the absorption of water and the greater the turgor of the occlusive cells, which implies a higher transpirative rate.
  • Concentration of CO2 in the atmosphere; the higher the concentration of CO2 in the atmosphere, the lower the stomatal opening and therefore the lower the rate of transpiration to the point that a high concentration of CO2 can even close the stoma. This is because the plant adjusts the opening of the stoma to the need for inputs for photosynthesis.
One would expect that with the increase that is occurring in the concentration of CO2 worldwide due to human activity, the plant would transpire less over the years, but this is not the case because this fact is opposed to another fact also derived from human activity such as the increase in temperatures of the planet.
  • Lighting; under normal conditions the stomata open and close rhythmically with a period of hours following the circadian rhythm of day and night, but an increase in illumination increases the photosynthetic rate with which the plant demands more CO2 producing an increase in the speed of transpiration.
  • Temperature; for a certain relative humidity of the atmosphere, as the temperature increases, the relative humidity rapidly decreases, which favors the opening of the stoma. But this is true in a temperature range of to 30º C; above 30º there is an increase in transpiration so strong that the plant chooses to close the stoma to avoid dehydration.
  • Wind speed; it does not directly influence the opening of the stoma but it does influence the difference in humidity between the stoma and the atmospheric air layer immediately close to it. The substomatic cavity is always looking for the balance of humidity with the atmosphere by giving it water vapor; if that air attached to the stoma is replaced due to the wind, the stoma must give water vapor proportionally to the wind speed with which the more wind the more transpiration.
  • Oxygen concentration; although oxygen is one of the gases that are exchanged in the stoma, as its atmospheric concentration is much higher than that of CO2, small changes in its concentration do not imply large imbalances. As a general rule, a high concentration of O2 favors the closure of the stoma.
It is estimated that most premature deaths of plants are due to their desiccation due to excessive transpiration. There are ways to act against perspiration such as spraying some plastic and waxy substances on the cuticle of the leaf. But this treatment, not being permeable to O2 and CO2, ends up acting on the photosynthetic rate decreasing it.
Guttation; so far we have seen how the plant loses water in the form of steam through the stoma but there is another form of water loss, this time in liquid form. It is the phenomenon of guttation and occurs when after an excess of water absorption through the root, it cannot be compensated by transpiration, and the plant eliminates it in the form of small droplets. This phenomenon is observed at dawn after a hot and humid night.