A plant needs a variety of factors, both internal and external, to grow and develop, and this includes differentiating into shapes, originating a variety of cells, tissues and organs.
Examples of external factors affecting plant growth and development include: light (solar energy), carbon dioxide, Water and mineralsincluding atmospheric nitrogen (fixed by fixing and cyanophytic bacteria), temperature, day length and gravity.
Internal factors are basically chemical and will be discussed in this text. The main internal factors are the so-called plant hormones or phytohormones, chemicals that act on cell division, elongation and differentiation.
Plant hormones are organic substances that play an important role in regulating growth. In general, they are substances that act or not directly on the tissues and organs that produce them (there are hormones that are transported to other places, not acting on their synthesis sites), active in very small quantities, producing specific physiological responses (flowering, fruit growth, ripening etc).
The word hormone comes from the Greek term. hormanwhich means "to excite". However, there are inhibitory hormones. Therefore, it is more convenient to consider them as chemical regulators.
The role of chemical regulators depends not only on their chemical compositions, but also on how they are "perceived" by their target tissues, so that the same plant hormone can have different effects depending on where it is acting (different tissues and organs), the concentration of these hormones and the time of development of the same tissue.
Five groups or classes of plant hormones (or phytohormones) are recognized:
- Abscisic acid