Plant Reactions, Hormones, Movements, and Seed Development
Item 12: The Function of Relation
The function of relation consists of the capacity that living things have to capture internal and external stimuli and prepare responses that ensure their survival. The reaction usually occurs with a movement or by secreting hormones. Plants detect stimuli because they have specialized cells that function as receptors, and these receptors are found in the epidermis of organs.
Types of Receptors
There are different types of receptors depending on the stimulus:
- Photoreceptors (detect light)
- Thermoreceptors (detect changes in temperature)
- Mechanoreceptors (sensitive to pressure)
The stimulus is captured by the plant’s receptors, inducing changes in cells and developing the first response (hormonal secretion and action).
Plant Hormones
Phytohormones are chemical substances of different compositions, produced in small quantities by different tissues, such as embryonic or meristematic tissues. From there, they travel to the target organs (organs where they will exert their action) to exercise the effect of the hormone. Hormonal transport is carried out by accelerating or slowing down, passing through the plant, either through the phloem or xylem vessels.
Phytohormones
Auxin:
- Place of synthesis: Meristematic tissue of the stem
- Hormonal action:
- Determines growth in the length of the stem.
- Favors the emergence of roots.
- Stimulates the development of the secondary xylem and increases the thickness of the plant.
- Inhibits the growth of secondary buds and promotes apical dominance of the stem.
- Regulates phototropism and geotropism.
- Chemical nature: Protein
Gibberellin:
- Place of synthesis: Meristematic tissue of stems and roots, young leaves, developing seeds, and fruits
- Hormonal action:
- Promotes flowering and the development of fruits.
- Stimulates pollen maturation and seed germination.
- Chemical nature: Lipid
Cytokinin:
- Place of synthesis: Meristematic tissue of the root
- Hormonal action:
- Stimulates cell division.
- Favors the growth of lateral buds, counteracting apical dominance.
- Stimulates chlorophyll synthesis.
- Induces flowering in plants that need long days and cold to flower.
- Chemical nature: Nucleic acid with a nitrogenous base
Abscisic Acid (ABA):
- Place of synthesis: Chloroplasts
- Hormonal action:
- Slows down or inhibits the growth of the stem.
- Stimulates the fall of leaves and fruits.
- Favors the storage of reserve substances in the seed.
- Induces the closure of stomata, lowering evapotranspiration.
- Chemical nature: Lipid
Ethylene:
- Place of synthesis: Fruits
- Hormonal action:
- Stimulates fruit ripening.
- Favors the wilting of flowers and stimulates the fall of leaves and fruits.
- Chemical nature: Hydrocarbon
Movements in Plants: Tropisms and Nasties
Plant movements can be classified as tropisms and nasties.
1. Tropisms
Tropisms are movements of a plant or its organs in response to an external stimulus that acts in a single direction. The movement is achieved through the growth of the plant towards the stimulus, and the process is permanent. For example, palm trees. If the organ moves in the direction of the stimulus, it is called orthotropic, and if it moves with a certain inclination, it is called plagiotropic. If the plant’s organs approach the stimulus, it is called positive tropism; if they move away, it is called negative tropism.
Principal Stimuli that Produce Tropisms
Light: The growth of a plant generated by light is called phototropism. Light is captured by photoreceptors, and auxins (hormones) are produced, causing cell changes in the stem (producing cell elongation). In response, the stem exhibits positive orthophototropism; branches exhibit positive plagiotropism, and roots exhibit negative phototropism.
Gravity: The growth of a plant induced by the force of gravity is called geotropism. Gravity is captured by georeceptors, producing auxins that cause negative orthogeotropism in the stem, negative plagiotropism in branches, positive geotropism in the main root, and positive plagiotropism in secondary roots. Geotropism is controlled by auxins (hormones) and cells located in the root cap and the apex of the stem called statocysts. Statocysts contain amyloplasts (cytoplasmic organelles loaded with starch) that are positioned in the cell influenced by the attraction of gravity, and this conditions the direction and sense of growth of stems, branches, and roots.
Contact with a Solid: The growth of a plant stimulated by contact with a solid is called thigmotropism. It is negative if it moves away from the solid and positive if it approaches. Contact with a solid is captured by thigmoreceptors, producing hormones (possibly auxins) that result in positive or negative thigmotropism. Hormones produce growth inhibition in areas in contact with the solid, while those not in contact continue to grow, resulting in the plant embracing the solid. For example, the vine.
Presence of Chemical Substances: The growth of a plant caused by the presence of chemical substances is called chemotropism. A notable example is the chemotropism of roots searching for mineral salts, water, or air (positive or negative). Aerotropism refers to the roots directing themselves towards airy areas of the soil.
2. Nasties
Nasties are rapid and reversible movements of a plant’s organs in response to the presence of an external factor, unlike tropisms, which are not influenced by the direction of the stimulus. For example, a plant that opens its leaves during the day and closes them at night.
Stimuli that Cause Nasties
Temperature: Nasties caused by temperature are called thermonasties. For example, flowers close in cold temperatures and open in high temperatures. Stomata close in high temperatures.
Light: Nasties induced by light are called photonasties. Increased light causes the opening or closing of flowers. If light and temperature increase, stomata close, locking in leaves.
Shaking of the Plant: Nasties caused by contact or shaking of the plant (e.g., by wind) are called seismonasties. They cause the closure of leaves and flowers. For example, the mimosa.
Parts of a Dicot Seed
Seeds are plant reproductive structures that ensure the perpetuation of the species because they protect the embryo against adverse environmental conditions. When environmental conditions are favorable (temperature, water, light, etc.), germination occurs, which is the development and growth of the embryo. The time period ranging from the formation of the seed until it germinates is called latency or dormancy.
Plant Development
We study development within the functions of relationship because it is directly related to changes occurring in the environment (changes in light intensity, temperature, photoperiod duration, etc.). The phases are:
1. Seed Germination
In seed germination, two hormones are mainly involved: abscisic acid (ABA) and gibberellins. ABA has two modes of operation: on one hand, it inhibits seed germination, but on the other hand, it favors the accumulation of nutrients in the seed.
During the winter, the plant produces a lot of ABA, but as winter ends, the amount decreases until it disappears. Then, other hormones can act, which occur when temperatures rise. These other hormones are gibberellins. Gibberellins promote the formation of hydrolytic enzymes (that break complex molecules into simpler molecules) in the seed. For example:
- Amylase: breaks down starch into monosaccharides.
- Lipase: breaks down fats into fatty acids.
- Proteases: break down proteins into amino acids.
The embryo uses monosaccharides, fatty acids, and amino acids to develop. Germination occurs, and a seedling is formed with a small stem, a small root (with the ability to absorb water and salts), and some small leaves (with chlorophyll that performs photosynthesis). This results in vegetative growth.
2. Vegetative Growth
Growth is the increase in plant mass and is determined by two distinct processes: cell division and elongation of cells by dilation. Meristematic tissue cells will increase in size until a critical moment when they can start cell division. Cytokinins are the cells that begin cell division. The plant grows (stems, roots, branches, etc.). Auxins appear, making the cell walls become less rigid and stretch because vacuoles store more water. This is called cellular elongation.
3. Differentiation and Organization of Tissues
Due to nutrients, light intensity, and other external factors, phytohormones appear that permit the specialization of cells and the organization of tissues. When a plant grows and develops, there is a balance between the size of the root and the branches. For example, if the root is large, it produces more cytokinins and has more branches. If the root is small, it produces fewer cytokinins and has fewer branches. Upon reaching certain development in phases 2 and 3, the plant reaches maturity and begins the flowering phase.
4. Flowering
Development is conditioned by the value of the photoperiod. Photoperiod is the number of daylight hours versus the number of hours of darkness in a day. There is a diversity of plants that need different photoperiods, classified as:
- Short-day plants (SDP): require days with few hours of light.
- Long-day plants (LDP): require days with many hours of light.
- Day-neutral plants (DNP): flourish regardless of the length of the photoperiod.
In equatorial latitudes, most plants are SDP. In temperate latitudes, LDP bloom in summer and SDP in spring/autumn. In subpolar latitudes (where a day has 24 hours of darkness and gradually reaches 24 hours of light), plants are usually LDP (in relation to temperature).
To produce flowering, the tips of the stems and leaves contain embryonic cells (or vegetative meristems). They are influenced by two substances produced in the leaf:
- Phytochrome (pigment)
- Florigen (hormone-like substance)
Buds from the stems and leaves, influenced by these substances, are concluded with meristems in floral buds. (Previously, they only served to grow; now, they can also give rise to flowers).
5. Formation of the Seed and Fruit
After fertilization (the union of male and female gametes, resulting in a seed and fruit development), the flower fades, the petals fall, the ovary transforms into the fruit, and the fertilized egg transforms into the seed. What hormones are involved and how do they work? The pollen begins to secrete auxin and gibberellin, which reach the ovary and cause it to become fruit (they make it get fat, etc.). There are other hormones, the cytokinins, that reach the egg and develop into a seed, which in turn produces more auxin and gibberellin, influencing the seed to produce the accumulation of reserve substances. In chloroplasts, ABA also promotes the accumulation of reserve substances (and prevents germination).
The ripe seeds will produce ethylene (another hormone) and reach the fruit. As a result, it promotes the conversion of starch (polysaccharide without a sweet taste) into monosaccharides (glucose or fructose) with a sweet flavor. The fruit becomes softer, sweeter, and more attractive to pollinating animals.
Definition: transport of pollen grains from the anther to the stigma of the same flower or to another flower of the same species.
6. Senescence or Aging
(Dropping of flowers, leaves, etc.) Two hormones influence this: ABA and ethylene. They favor the formation of the abscission zone (a thin layer of cells with digestive enzymes located near the binding site of flowers, fruits, and leaves with the stem).