Leaf photosynthesis and breathing

The leaf between photosynthesis, respiration and colored pigments

The leaf is the organ of the plant that contains in itself the highest number of regulatory functions of plant life: responsible for the synthesis of sugars, oxygen production, transpiration, guttation, the wonderful autumn colors of some species and important food source for animals and men.
Fundamental for all gardeners to know the leaf in depth: is it not in fact the first to reveal the malaise of plants, with its withering, drying out and signs of the most dangerous pathologies?
If the trunk speaks of the history of the plant and its growth over time, the leaf speaks of the present of the plant and its life at that time.
Without getting lost in the intricacies of botany, an analysis is proposed whose subject is the leaf organ in general and its functions.


A leaf is formed by a lamina furrowed by ribs, which flow into the petiole, which is a sort of connection with the hydraulic system of the branches.
Not all the leaves have the same structure, some have modified laminae or are presented without petioles and others have enlarged parts that make them very different from the classic leaflet that a child can draw on the school notebook.
In the leaf tegumental tissues are present, which protect it from external agents and also have the function of regulating gas exchange. Then there is a special tissue called the chlorophyll parenchyma in which photosynthesis takes place.


Chlorophyll is present in all the green parts of the plant and in particular in the leaf, a pigment capable of capturing light rays and exploiting its energy:
6 molecules of water + 6 molecules of carbon dioxide are transformed into a molecule of glucose with the release of 6 oxygen molecules, in the most famous chemical process in the world chlorophyll photosynthesis (from the Greek foto = light and synthesis = construction).
6 CO2 + 6 H2O → C6H12O6 + 6 O2
The plant is composed entirely of substances derived from glucose: cellulose, starch and everything that may be useful to the plant is created in the leaf.


It is the process by which water vapor is lost from the leaf surface. From the roots the water travels in conductive fabrics that carry it up to the leaves, which act as a kind of valve for the plumbing of the plant. The steam escapes through the stomata and the output quantity is directly proportional to the opening of the stomata.


It is the inverse process to photosynthesis, it uses sugars and oxygen to create energy, it happens in every part of the plant not exposed to light, not only in green ones. Breathing therefore also occurs in the leaves; consuming sugars produces the plant's energy.
Breathing consumes oxygen, so a plant at night or in general in low-light environments removes oxygen from the atmosphere; hence the scientific explanation of the popular warning that suggests not placing plants in the bedrooms.
Breathing, photosynthesis, transpiration and guttation regulate the life of our plants: the health of a plant and its growth depend on the plant-environment relationship, atmospheric humidity, precipitation, solar irradiation and radical condition influence the aforementioned processes determining the development values.
An example :
a tomato seedling, grown in pots has received abundant irrigation and nitrogen fertilization, consequently there has been an excessive development of the epigeal part and in particular of the leaf apparatus.
In the summer season following a period of drought, the plant closes the stomata to minimize water loss and consequently slows photosynthetic activity, decreasing glucose production.
Respiration on the other hand does not stop, because it is independent of photosynthetic activity and continues to consume the sugars of the plant. Summing up and simplifying, the plant consumes more than it produces, creating an imbalance that will result in less production of the fruit.
If the drought persists having a plant with a very luxuriant epigeal part, it will therefore be a disadvantage.


When perspiration fails to expel water vapor due to excessive atmospheric moisture, the leaves directly expel water drops with the guttation process. This happens even when the roots absorb too much water.

Leaf photosynthesis and respiration: Pigments

Why are the leaves green, in some red species, or variegated and change their color to the alternation of the seasons?
The coloring of the leaves is linked to the presence of 4 pigments that in addition to performing particular functions color the leaf surface.
chlorophyll = green
anthocyanins = red
carotenes = orange
xanthophylls = yellow
In the autumn season with the gradual disappearance of chlorophyll due to the decrease in photosynthetic activity, the other pigments prevail, hence the different shades of autumn foliage.