THE WATER AND THE PHOTOSYNTHESIS

The water is the main factor in the entire physiology of a plant, effectively participating in the nutrition and the growth of plants. A plant is formed mainly by water, representing from about 80% in its leaves and roots, to 50% in other parts such as the trunk. However, the water is not in a static form, but instead, it circulates through the interior of the plant.

IWater Circulation
The water is collected by the piliferous zones of the roots, which originate a difference in the osmotic potential that "pushes" the water towards the leaves. The leaves, when the stomas are open, evaporate water through them. This process is called perspiration. This water vapor disperses in the atmosphere. The absence of water in the leaves originates a lower internal pressure, which "pulls” the water that comes from the roots.
The main driver for the ascension of the water is the perspiration. The “buoyancy” from the roots has less importance as a cause for this circulation. Therefore, the major issue is in the open or closed stomas.

During the night, the stomas are closed and the leaf does not transpire. However, the roots proceed creating a pressure that "pushes" the water up and a little water leaves from the leaves to relief the excess pressure. If the atmosphere is humid, at the boundaries of the leaves, some droplets can be seen – this receives the name of "guttation", a process that is of secondary importance.
When not enough water reaches the leaves, these dry up (the internal water pressure drops), due to the loss of turgescence by the plant cells, and the stomas close totally or partially. This is an auto-defense mechanism of limited results. Is the plant lacks little water, it automatically protects itself against more losses, if it lacks a lot of water, it will suffer and ultimately die.
The high temperature, the wind and the low atmospheric humidity accelerate the evaporation of the water. On days like these, more attention shall be given to the water needs of the plants. Even though a terrain may contain enough water, it may not reach the leaves fast enough. In this way, a humidity micro-climate shall be created around the plant.
The circulating water has three purposes:

• Refrigeration of the leaves – For each gram of water to pass from the liquid state to the vapor state it is necessary to absorb at once 540 calories from the leaves, which by doing so cool down. If we detach, as an experiment, a leaf and we leave it exposed to the Summer sun and, a few minutes later, we put it next to an undetached leaf in the original plant, it will be noticed that there is a great difference in temperature, the detached leaf being hot while the undetached leaf in the plant will remain cool.
• Internal transport of the nutrients collected by the roots – the elements Nitrogen (N), Phosphorous (P) and Potassium (K), as well as others such as Iron (Fe), travel dissolved in the water that comes from the roots and reach all parts of the plant by means of the conducting veins of the plant.
• Photosynthesis – The water is capital in the process of photosynthesis, as it will be seen later. The amount of water that participates in the photosynthesis is of the order of 2% of the total used, the rest being used for cooling (perspiration).

The Process

Inputs
Coming from the atmosphere: Oxygen for breathing and carbon dioxide for the photosynthesis.
Coming from the roots: Water and mineral elements (micro and macro elements), this water is used for several purposes.

Outputs
To the atmosphere: Oxygen from the photosynthesis, carbon dioxide from the breathing and a large amount of water from the perspiration. A small fraction of the water is used as solvent for the products of the photosynthesis, being conducted to other parts of the plant.

Photosynthesis
The photosynthesis is a very complex phenomenon, and it is not necessary to describe it here in maximum details, even so because today that are still voids in the knowledge of all its characteristics. In synthesis, the most important information is now described:

With the capital aid of light energy and the chlorophyll, it takes place in the leaves and in the green parts of a plant, the synthesis of sugars (glucose), carbon hydrates (starch) and other substances from the CO2 (carbon dioxide), which comes from the atmospheric air and that penetrates in the plant through the stomas, and the H2O (water), which comes from the roots.

If the stomas are closed, the CO2 does not penetrate into the leaves and, therefore, there is no growth.
The wood, in this way, is no more than a carbon compound, being that the carbon does not come from the earth, but from the CO2 in the atmospheric air.
The growth of a plant depends on the optimization of the following parameters that are inter-related and that may have non-linear expressions:

Size of the plant – A well-developed plant may have larger growths because their production zone is larger (some substances produced in adult leaves are relocated and feed a new growth, without the need for consumption of external elements).

Use of the Water – The roots must absorb water as per the velocity required by the plant.
Opening of the Stomas – The stomas are opened if there is enough light and water.
Intensity of the Light – Each plant has a different limit for the intensity of light and wavelengths. On the other hand, excessive light causes a reduction in photosynthesis.

Temperature – Very high temperatures damage internal substances in the plant – as for the enzymes – needed for the photosynthesis. That is way many plants stop growing during summer.

Availability of CO2 – It shall not exceed nor be less than the amount needed.

As it was mentioned previously, these parameters are inter-related. If there is too much water and lack of light, or if the stomas are open and there is a lack of CO2, the plant will not reach its optimal conditions. Besides that, the curves for several parameters have the shape that is shown in the graph below, where it can be seen that there is a drop after the optimum point, after which the conditions are no longer satisfactory (excess).

 

UIn the photosynthesis, several stages may be separated:

• Photolysis of the water (the water is decomposed in hydrogen and oxygen)

• Combination of the hydrogen of the water with the CO2 from the air.

• Transformation in glucose (with the aid of the phosphate that comes from the soil)

In the following topics, the balances for the photosynthesis in the entire plant will be addressed.

Respiration
Besides the previous phenomena of perspiration and photosynthesis, all parts of the plant (including the roots) need to breath. For such, they absorb O2 (oxygen) from the air in the atmosphere e release CO2 and H2O. Because of that, it is capital that the roots be developed in a porous substrate, capable of allowing the penetration of air and the infiltration of the water, not remaining soaked for a long time.
The non-consideration of the breathing of the roots leads to many lacks of success. To cite an example, the porous ceramic vases have a better performance than those made of plastic, but are not recommended for situations where it is not possible to water the plant more frequently, except for xerophile plants.
Balances
In a leaf, penetrate from the stomas O2 and CO2 from the atmosphere, and H2O (with nutrients) through the roots. O2, CO2 and H2O, which go to the atmosphere, are released. A small amount of water is returned to the rest of the plant as solvent for the products of the synthesis that took place in the leaves.
It is being noticed that the O2 that gets into the plant is used for the breathing during 24 hours a day. The carbon and the oxygen of the CO2, on the other hand, are used for the photosynthesis only when there is sun light. The O2 released comes from the photolysis of the water caused by the photosynthesis. The CO2 expelled comes from the breathing. The H2O released comes from the perspiration and basically has no connection to the process of photosynthesis.
The sum of the volumes of O2 plus CO2 that come into the plant is basically equal to the sum of the volumes of the same gases that leave the plant, but with the characteristic that the volume of O2 is somewhat larger than the volume of CO2 that enters, and from this results the “purifying” effect of the air that the plants have.
Stomas
The stomas (stoma = mouth) are very small openings that are located mainly at the borders of the leaves. These porous open when there is enough light, to allow for the entrance of CO2 to the interior of the leaf and to allow the exit of the water vapor that is generated during the process of perspiration. The stomas close when there is not enough light or when the plant risks getting dried because not enough water is reaching its leaves. If they are closed, the growth mechanisms do not work, and the plant does not grow; if they remain open for a long time, the plant grows more. The stomas are not binary elements, because it is possible that they reach intermediate states during their opening processes.