Plant Growth and Mineral Nutrition

A seed weighs practically nothing. A few months later, it's a plant with stems, leaves, roots, flowers, and fruit — all built from air, water, and a handful of minerals. How does a plant know when to grow, where to grow, and when to stop? The answer involves a cast of hormonal characters, environmental signals, and internal clocks. This chapter is about plant growth and development — the processes that turn a dormant embryo into a complex, functioning organism. Growth in plants is fundamentally different from growth in animals. Animals grow until they reach a fixed adult size, then stop. Plants grow throughout their lives — they have meristems (regions of actively dividing cells) at their tips (apical meristems for length growth) and in rings (lateral meristems for girth growth). This is called open growth. A thousand-year-old tree is still growing new leaves and branches. Growth involves three phases: cell division (meristematic), cell enlargement (elongation), and cell differentiation (maturation). The rate of growth can be arithmetic (linear) or geometric (exponential). The grand period of growth curve is S-shaped (sigmoid) — slow at first, then rapid, then slowing again. Plant hormones (phytohormones) are the chemical messengers controlling growth and development. They're produced in tiny amounts but have dramatic effects. There are five major classes: Auxins, Gibberellins, Cytokinins, Abscisic acid (ABA), and Ethylene. Each has multiple functions and they often work together. Auxins (like IAA) promote cell elongation, apical dominance, phototropism (growth toward light), and root initiation. Auxin moves down the stem from the shoot tip. In phototropism, auxin distributes unevenly to the shaded side, causing cells there to elongate more and bending the stem toward light. In gravitropism, auxin accumulates on the lower side — inhibiting root cell growth (roots curve down) but stimulating shoot cell growth (shoots curve up). Gibberellins promote stem elongation, seed germination, fruit development (especially seedless fruits — parthenocarpy), and bolting. They were first discovered in the fungus Gibberella fujikuroi, causing 'foolish seedling' disease in rice. Cytokinins promote cell division, delay senescence, promote nutrient mobilization, and stimulate lateral bud growth (counteracting auxin's apical dominance). They're produced in roots and transported upward. Abscisic acid (ABA) is the stress hormone. It promotes stomatal closure during drought, induces seed dormancy, and promotes senescence. Ethylene is a gaseous hormone that promotes fruit ripening, senescence, and abscission of leaves and flowers. It's used commercially to ripen bananas and tomatoes. Photoperiodism is the response of plants to the relative lengths of day and night. Short-day plants (like chrysanthemums) flower when day length is shorter than a critical period. Long-day plants (like spinach) flower when day length is longer. The pigment phytochrome exists in two forms: Pr (red-absorbing, inactive) and Pfr (far-red-absorbing, active). Vernalization is the exposure of plants to prolonged cold to induce flowering. Seed germination requires water, oxygen, and suitable temperature. Epigeal germination (like beans) brings cotyledons above ground; hypogeal germination (like peas) keeps them below ground.