Plant Growth and Mineral Nutrition
Easy Overview
What makes a seed decide it's time to wake up and grow? How does a plant know to grow toward the sun? And what's with the whole 'roots go down, shoots go up' thing? This chapter is about plant growth — the hormones that control it, the nutrients it needs, and the weird things plants do to survive.
Plant hormones — the tiny managers
Auxins make plants grow toward light (phototropism) and roots grow downward (geotropism). Gibberellins make stems elongate and seeds germinate. Cytokinins promote cell division. Abscisic acid (ABA) is the stress hormone — it closes stomata during drought and puts seeds in hibernation. Ethylene ripens fruits. Five tiny chemicals running the whole show.
Photoperiodism — plants have calendars too
Plants measure day length to know when to flower. Short-day plants (like chrysanthemums) flower when nights are long. Long-day plants (like spinach) flower when nights are short. There's a pigment called phytochrome that senses light and darkness. It's basically the plant's internal alarm clock.
Essential mineral elements — plants need food too
Plants need 17 essential elements to survive. Macronutrients (C, H, O, N, P, K, Ca, Mg, S) are needed in large amounts. Micronutrients (Fe, Mn, Cu, Zn, B, Mo, Cl) are needed in trace amounts. Nitrogen deficiency shows as yellow leaves (chlorosis). Potassium deficiency gives scorched leaf edges. It's like vitamin deficiencies in humans.
Nitrogen fixation — turning air into fertilizer
The air is 78% nitrogen, but plants can't use that form. They need nitrogen fixed into ammonia (NH₃). Rhizobium bacteria live in root nodules of legumes and do this job for free. They get food from the plant, the plant gets nitrogen. Win-win. That's why farmers rotate crops with legumes.
Seed germination — wake up, little seed
A seed is a sleeping baby plant with a packed lunch. Give it water, oxygen, and the right temperature, and it wakes up. The radicle (baby root) comes out first, then the plumule (baby shoot). Some seeds germinate above ground (epigeal — like beans), some below (hypogeal — like peas).
Key Points
- •Auxin: cell elongation, phototropism, apical dominance; Gibberellin: stem elongation, seed germination
- •Cytokinin: cell division, delays senescence; ABA: stress response, stomatal closure, seed dormancy
- •Ethylene: fruit ripening, senescence; used artificially to ripen fruits
- •Photoperiodism: SDP (short day) vs LDP (long day) plants; phytochrome detects light duration
- •Essential elements: macronutrients (N, P, K, etc.) and micronutrients (Fe, Zn, Cu, etc.)
- •Nitrogen fixation: Rhizobium in legume root nodules converts N₂ to NH₃
- •Hydroponics: growing plants in nutrient solution without soil
- •Seed germination requires water, oxygen, and favorable temperature
Practice Questions
- Describe the roles of auxin, gibberellin, and cytokinin in plant growth.
- What is photoperiodism? Differentiate between short-day and long-day plants with examples.
- List the essential macronutrients and micronutrients. What happens if a plant lacks nitrogen?
- Explain how Rhizobium helps in nitrogen fixation. Why is the relationship mutualistic?
- Distinguish between epigeal and hypogeal germination with examples.