The development and life cycle of a plant

The life of the plants is marked by some main developmental phases i.e. germination, flowering, fruit ripening, senescence. These phases are determined primarily by the seasons with the different availability of heat, water, and light. Internal substances, phytohormones instead regulate the internal coordination between the signals coming from the external environment and the responses of the plant.

Germination

Germination is the set of processes that lead to the development of a seedling from the seed. Before germinating the seed passes a period of quiescence (dormancy), more or less long according to the species, in which the metabolism is much reduced, and the seed undergoes initial dehydration, to prevent it from germinating. At the same time, it is still in the fruit. Dormancy is necessary to allow the breakdown of the tissues that surround the embryo and which prevent the passage of water and oxygen and the elimination of hormones that inhibit germination (post-maturation of the seed).

Post-maturation can take place:

For the passage of the seed in the digestive system of some animal, which "breaks" the integuments; due to temperature changes for the activity of microorganisms. In particular, a period of exposure to cold (vernalization) would favor the production of some activating hormones; by the action of the fire, which opens the woody casings of the fruit; by the action of water, water dilates the inhibiting substances of germination.

Only when the conditions of humidity, oxygen, light, and temperature are favorable, does germination begin. The water is absorbed avidly, the seed swells and the metabolic processes are activated which cause the radicle to stretch downwards. The growth of the stalk towards the light and the opening of the first leaflets occur that start the autonomous nourishment process (photosynthesis). In the monocotyledons, the cotyledon remains in the ground, and the stem pierces the ground protected by the coleoptile. In some dicotyledons (bean), the exit from the ground is forced by the hook-folded hypocotyl, and the cotyledons emerge from the ground (epigeal germination), while in others (pea) they remain in the ground (hypogeal germination).

The life cycle

The growth of the stem and leaves of the plant continues for a certain time, called the vegetative period, followed by the reproductive phase, in which the flowers, fruits, and seeds develop. According to the length of the vegetative period, annual, biennial, and perennial plants are distinguished. Annual plants develop, reproduce, and die before winter (for example, most herbaceous plants). In biennial plants, the vegetative phase lasts one year, and reproduction occurs in the second year before death (for example, carrots and other vegetables). In perennial plants each year, there is a vegetative and a reproductive period; plants live several years (for example, trees).

The beginning of the reproductive phase is flowering, the transformation of vegetative buds into flower buds, determined by the length of the daily light period, or photoperiod. Each species responds to a specific photoperiod so that we can distinguish:

- Long day plants, which bloom regardless of the length of the photoperiod; long-day plants, which bloom when the length of the photoperiod is greater than a specific critical value which depends on the species (usually in spring); short-day plants, which bloom when the length of the photoperiod is less than a particular critical value.

The organ sensitive to the photoperiod is the leaf, and the mediating substance of the biological clock of the plant is a pigment, the phytochrome. When the reproductive parts are mature in flower, several hormones induce the opening of the petals (anthesis), to expose the reproductive organs to pollinating agents. After fertilization, other hormones provide for the leaves to fall and the fruits to ripen.

The ripening of the fruits is more evident in the fleshy fruits.

The fruit initially green and with a sour taste, becomes more tender, colorful and sweet. The starch is converted into sugars, and they are volatile compound products that give the aroma; chlorophyll disappears, and other pigments such as carotenoids and anthocyanins appear. 

Senescence is the aging process of the plant, which manifests itself with the fall of the fruits and leaves and coincides with the beginning of a dormancy period of the plant, with slowed metabolism to face the winter period. The leaf has no change and therefore has limited growth. In evergreen plants, it dies in a few years, while in deciduous plants; it falls every year before the winter season.

The fall of the fruits and leaves is called abscission and is regulated by some hormones (including abscisic acid). It occurs due to the death of some cells, placed at the base of the petiole, which forms the so-called abscission layer.

Before the leaf falls, much of its organic material is demolished into simpler compounds, which are transferred to the roots for storage.

The change in color of the autumn leaves is due to the decrease in the production of green chlorophyll, which masked the other pigments (red and yellow).

In perennial plants also the buds enter a dormancy phase developing protective wraps that transform them into winter buds.

The Hormonal Balance of the Plant

In the plant structure the main elements that control and stimulate growth are the apical buds (present both at the end of the central branch and in the terminal portion of each branch) and the absorbent roots (or non-woody, which have a life cycle of 2, 3 years and the task of capturing the nutrients dissolved in the soil. They differ from those of anchoring or wood, which mainly perform static and support functions. The apical buds emit auxins, hormones that control and regulate the growth of the underlying buds (therefore of the branches); they dominate the underlying gems not only physically but also hierarchically, hence the definition of "apical dominance" attributed to them. 

The absorbent roots emit cytokinins that balance the production of auxins and keep the plant in hormonal balance. In gardening operations it is essential to maintain the right ratio of hormonal production of buds and roots. Therefore an excessive pruning or an incorrect transplant can move the needle of the hormonal balance putting the tree in crisis and forcing it to unwanted reactions (excessive spraying on the trunk and collar or desiccation of one or more branches). So here we understand why pruning is not always solvable with a single intervention, but sometimes, in order not to upset the balances described above, it may be necessary to operate by practicing only a part of the necessary cuts and wait for the following season to complete the work. In this way, we will allow the plant to rebalance itself gradually.