describe the following processes in the plant life cycle and why they are important for the plant to complete, listing all of the plant hormones involved and describing their functions:
1) elongation of the plant shoot.
2) the process by which seeds orient themselves.
3) photoperiodism.
1) Elongation of the plant shoot:
The elongation process in the plant shoot is crucial for the plant's growth. It involves the extension of cells in the stem, allowing the plant to reach greater heights and access sunlight. This process is primarily regulated by a class of plant hormones called auxins, with the primary auxin being indole-3-acetic acid (IAA). The auxin hormone stimulates cell elongation by promoting the expansion of cell walls, increasing cell size, and enabling the plant to grow taller.
Auxins are synthesized in the apical meristem, which is the growing tip of the stem, and they move downwards towards the base of the plant by a process called polar auxin transport. Phototropins, which are light-sensitive proteins, play a role in this process by redistributing auxin towards the shaded side of the plant shoot, causing elongation to occur on that side. This bending towards light is known as phototropism.
2) Orienting of seeds:
The process by which seeds orient themselves, also known as seed orientation or positioning, is essential for successful germination and growth of the plant. It ensures that the roots of the developing plant grow downwards into the soil for anchorage and absorption, while the shoot grows upwards towards light for photosynthesis.
Seed orientation is influenced by several factors, including gravity and the hormone auxin. Gravitropism is the plant's response to the gravitational force, and it helps in determining the proper direction for the root growth. The hormone auxin plays a vital role in this process. In seeds, auxin is mainly present in the embryo, and when the seed orients itself with gravity, auxin redistributes towards the lower side of the seed, triggering cell elongation and root development in that direction. This hormone-controlled growth response is known as gravitropism.
3) Photoperiodism:
Photoperiodism refers to the plant's response to the length of day and night, which affects various aspects of its growth and development, such as flowering, dormancy, and leaf shedding. The perception of day length happens in specialized cells called photoreceptors in the leaves, primarily through two phytochromes: Pfr (active form) and Pr (inactive form). These phytochromes are light-sensitive pigments that regulate the plant's response to changes in day length.
Depending on the plant species and its characteristics, three categories of photoperiodic responses can occur:
- Short-day plants (long-night plants): These plants require longer dark periods (night) than a critical duration to initiate flowering and certain physiological changes. The phytochrome Pfr inhibits flowering in these plants, and during long nights, Pr converts to Pfr, which promotes flowering.
- Long-day plants (short-night plants): These plants require shorter dark periods (night) than a critical duration to induce flowering. In these plants, Pfr promotes flowering, and longer nights lead to the conversion of Pfr to Pr, inhibiting flowering.
- Day-neutral plants: These plants flower regardless of day length.
In addition to phytochromes, other plant hormones like gibberellins (GA) and florigen (the flowering hormone) also play a role in controlling the photoperiodic response and flowering time of plants. The intricate interaction between various hormones and photoreceptors enables plants to sense and respond appropriately to changes in day length, ensuring their reproductive success.