The plant uses the water and nutrients absorbed by the root of its supporting medium, and the CO2 captured from the atmosphere through the stomata, to transform them into organic matter (mainly hexose, sucrose and starch) and oxygen, using the light energy of the sun.
• CO2 is the substrate of photosynthesis; a change in its concentration is more influential than a change in oxygen concentration.
• Water acts as a transporter of nutrients and the basis of metabolic reactions.
• Nutrients are used as constituents of organic matter and as catalysts for metabolic reactions.
• Sunlight is used to transform it into chemical energy.
Relationship between transpiration and photosynthesis: as we have mentioned before, transpiration is a “lesser evil” for the plant. The opening of the stoma is necessary for the capture of atmospheric CO2, substrate of photosynthesis. In a normal plant (C3), for every molecule of CO2 that is captured, up to 300 molecules of water are lost to the atmosphere. In C4 plants this ratio is lower.
Chloroplasts: they are green organs located in the mesophile of the leaf (layer behind the epidermis of the leaf) of photosynthetic eukaryotic vegetables, and in which the transformation of the light energy of the sun into chemical energy usable by plants is carried out. In them, photosynthetic pigments are organized into a sac-like structure called thylakoids.
Photosynthetic pigments: if a molecule of any substance, when illuminated with a white sunlight reflects a certain color, it means that, from the electromagnetic spectrum (set of all the types of light and radiation that exist) the color we appreciate is the one that does not absorb.
That is why green plants contain molecules capable of absorbing light from different areas of the spectrum except the green zone. These molecules are known as photosynthetic pigments.
Most of the radiation that reaches the Earth’s surface is between 300 and 900 nanometers (nm=billionth of a meter); of this range the upper floors take advantage of the light between 400 and 500 nm (presenting maximum peaks at 440 nm) and between 650 and 700 nm (presenting maximum peaks at 680 nm). The zone between 500 and 600 nm is a low efficiency zone.
• Chlorophyll; are pigments that have their maximum absorption between 420 and 663, depending on whether they are type a, b or c.
• Carotenoids; pigments that have their maximum absorption between 450 and 490 nm.
• Phycobilines; they absorb in variable areas between 480 and 670 nm, capturing wavelengths where chlorophyll does not act.
PHASES OF PHOTOSYNTHESIS.
Photosynthesis is divided into two phases: the dark phase and the light phase. The concept should not confuse us; both phases occur in light, but one of them experimentally, can be performed in darkness.
Luminous phase; it is an oxide-reduction reaction in which water gives electrons that pass to the carbon of CO2. The action of light is necessary at some point in this stage, not at all. The result of this process is energy (ATP) and reducing power (NADPH) and is produced in the thylakoids of chloroplasts.
Dark phase; is the process by which CO2 is transformed into carbohydrates using atp and NADPH obtained in the light phase. It takes place in the stroma of the chloroplast (internal cavity). It does not need the excitation power of light photons, but is intimately linked to the light phase by ATP and NADPH. Basically there are three stages:
Fixation of CO2, that is, its incorporation into some organic compound (Calvin Cycle).
Reduction of intermediate metabolites.
Reorganization of new products.
We are going to look at the beginning of the first phase of the Calvin Cycle, a reaction catalyzed by the enzyme ribulose-1,5-diphosphate carboxylase (RUBISCO), since it will give way to another process that occurs in vegetables and that for growers is as important as photosynthesis: photorespiration.
It has been known since the early twentieth century that photosynthesis, measured as assimilated CO2, is inhibited by high concentrations of oxygen (remember that the concentration of CO2 in the atmosphere is 0.03% although in 2019 concentrations of up to 0.04% have been measured, while that of oxygen is 21%); the inhibition of photosynthesis increases greatly by increasing the proportion of oxygen above 21%. Cellular processes are known to consume oxygen and excrete carbon dioxide, such as cellular respiration, but this process is saturated at normal oxygen concentrations in the atmosphere. However, there must be another light-stimulated respiratory process, independent of cellular respiration. This process is PHOTORESPIRATION and occurs simultaneously with cellular respiration.
A respiratory process is the reverse of photosynthesis: oxygen is consumed and carbon dioxide is produced. Photorespiration is produced by an affinity for both CO2 and O2 of the rubisco enzyme, which means that when a certain CO2/O2 ratio is reached (higher in C3 plants than in C4 plants) the rubisco fixes oxygen instead of carbon dioxide in the first stage of the dark phase of photosynthesis. This process leads the plant to a high consumption of ATP and NADPH, an oxygen consumption, a production of carbon dioxide and a destruction of starch reserves. In reality, this process is a “self-destruction” of the plant itself.
The photorespiration process is triggered by a low CO2/O2 ratio, which in turn is triggered by the closure of the stomata due to unfavorable climatic conditions, such as lack of water, high transpiration speed, high temperatures, low photoperiod.
Factors that regulate photosynthesis.
Not all plants are equally efficient at transforming carbon dioxide into atmospheric matter. Those that do not photorespirate or that have low photorespiration values are the most efficient.
Among the set of factors that affect photosynthesis we have two groups: those that depend on the plant, in which we can influence very little, and those linked to the environment, which are those that have the greatest power of influence. The latter include:
Water; in order for the plant to capture carbon dioxide, the stomata have to be open. If the level of hydration of the plant decreases, the occlusive cells lose turgor and the stoma closes, which will decrease photosynthesis.
Temperature; in normal upper plants, the optimum photosynthesis between 25 and 35ºC is reached. Between 15 and 30ºC the temperature has no influence on the photosynthetic rate. But above this temperature cellular respiration increases and also the stoma tends to close due to an increase in the speed of perspiration, which leads to an increase in photorespiration and a decrease in the efficiency of photosynthesis.
Nutrients; with photosynthesis, the plant generates 90 to 95% of its dry weight; 5-10% is ash and nitrogen. A mineral deficiency greatly affects the functioning of photosynthesis; of all the nutrients studied, it is nitrogen that has the most marked influence, by decreasing with it the protein content of the chloroplast.
Light; influences both its intensity and duration (photoperiod). The longer the duration and intensity, the higher the photosynthetic rate. But an uncontrolled increase in intensity can lead to the closure of the stoma by an increase in the rate of perspiration.
CO2; normal plants (C3) are not saturated at the current concentration of carbon dioxide in the atmosphere; but doubling its current concentration would inhibit RUBISCO’s affinity for oxygen and photorespiration would decrease by half; it would reduce stomatal conductance and increase water use efficiency.
O2; as we have mentioned, a decrease in the CO2/O2 ratio leads to an increase in photorespiration and a decrease in photosynthetic efficiency.
Transport of carbohydrates; it is proven that, when eliminating the fruits in a pepper bush, a few days later a decrease in the photosynthetic rate is detected in the leaves closest to the fruit since the fruit has acted as a sugar sink, as there is no such sink, the level of leaf sugars increases, which leads to a slowdown in photosynthesis.
Leaf age; the old leaves lose chlorophyll and therefore photosynthetic rate.
Genetic factors; in certain plants, heterosis or hybrid vigor leads to an increase in the photosynthetic rate.