Chemical Coordination in Plants

Chapter in a nutshell: Plants have no nervous system or sense organs, yet they sense and respond to light, gravity, water, touch and chemicals. This coordination is chemical, achieved through tiny amounts of organic substances called plant hormones (phytohormones) that are produced in one region, travel to another, and there regulate growth — either promoting it or inhibiting it. The visible responses are slow growth movements called tropisms.

1. Why Plants Need Coordination

Growth is one of the most fundamental characteristics of living organisms. In plants, growth happens by cell division and cell enlargement, chiefly at the meristems located just behind the shoot tips and root tips. Movement in plants appears as bending, twisting and elongation of parts.

Plants do not need fast responses, but they do need to respond to light, gravity, water and seasonal change, and to coordinate the growth of cells in different parts of the body. Since there is no nervous system or sense organs, this sensitivity and control is exercised entirely by chemical means — plant hormones. The main way plants respond to a stimulus is by growth: growth is stimulated or inhibited.

The term phytohormone (phyton = plant) was coined to distinguish plant hormones from animal hormones. The term hormone itself was given by William Bayliss and Ernest Starling.

2. Plant Growth Regulators (Phytohormones)

Plant growth regulators are specific chemical substances that plants produce, which can move from one organ to another and are active in very small amounts to produce their effect on growth.

Hormones are divided into five major classes, which fall into two functional groups:

3. Discovery of Plant Hormones

The first indication of a plant hormone came from Charles Darwin (1880). Studying the bending of Canary grass coleoptiles towards light, he found that light falling on the tip produced some influence that was transmitted downward and caused the coleoptile to bend towards light. When the tip was removed, no bending occurred — the phototropic response did not happen.

In 1928, the Dutch botanist Fritz W. Went isolated this substance — auxin — by letting it diffuse out from the tip of oat (Avena sativa) coleoptiles into a gelatin (agar) block. Auxins are thus the first group of plant hormones discovered.

4. The Five Plant Hormones

4.1 Auxins

• Natural auxins: Indole acetic acid (IAA) and Indole butyric acid (IBA).
• Synthetic auxins: 2,4-dichlorophenoxy acetic acid (2,4-D) and Naphthalene acetic acid (NAA).

Physiological effects: promote cell elongation and growth of stems and roots; enlarge fruits; cause apical dominance (the apical bud suppresses lower axillary buds — removal of the apical bud releases the lower buds); destroy weeds when sprayed; induce rooting in stem cuttings (rose, Bougainvillea); cause parthenocarpy (fruit without fertilization in apple, tomato, banana); promote flowering in pineapple but delay it in lettuce; and increase cotton seed production.

4.2 Gibberellins

Physiological effects: promote seed germination in cereals and lettuce; cause broader, elongated leaves (pea, bean, tomato, cabbage); are more potent parthenocarpic agents than auxins; cause stem elongation and leaf expansion but have no effect on roots; cause bolting (sudden internode elongation just before flowering, e.g. cabbage); increase fruit size and bunch in grapes; break dormancy of buds and tubers. Used in the brewing/malting industry and in horticulture.

4.3 Cytokinins

Physiological effects: with auxin, stimulate cell division; delay ageing (senescence) of plant organs; break seed dormancy and promote germination; help translocation of solutes in phloem; give resistance to high/low temperature injury; cause expansion of cotyledons; inhibit apical dominance; and promote chlorophyll synthesis and organ formation.

4.4 Ethylene

Physiological effects: retards flowering in most plants but induces it in pineapple and mango; stimulates ripening of fruits (banana, apple, mango, citrus); inhibits stem/root elongation and causes swelling of nodes; breaks dormancy of buds and seeds; initiates germination in peanut seeds and sprouting of potato tubers.

4.5 Abscisic Acid (ABA)

Physiological effects: inhibits cell division and elongation (general inhibitor); increases cold resistance; under salinity/drought, closes stomata to reduce transpiration; induces bud dormancy; promotes rooting of cuttings in ivy/bean; inhibits seed germination and helps seeds withstand desiccation.

5. Tissue Culture — The Auxin : Cytokinin Ratio

In plant tissue culture, the ratio of these two hormones decides what forms:

6. Movement in Plants

Plants respond to light, water, gravity, wind and touch. Movements fall into two categories:

• (A) Tropic movements (Tropisms) — directional, depend on the direction of the stimulus.
• (B) Nastic movements — non-directional (not in syllabus).

A tropism is movement of a plant part towards the stimulus (positive) or away from it (negative).

Geotropism: the primary root grows vertically downward (positive); shoots grow upward (negative). Roots of cycas that do not respond are called ageotropic.

Chemotropism examples: pollen tube down the style to the ovary; movement of tentacles in Drosera; closing of the lid of Nepenthes to nitrogenous food; fungal hyphae towards sugars and peptones; haustoria of a parasite into the host.

7. Some Special Facts

• Lateral meristems lie below the bark and are responsible for increase in diameter (girth) of the stem.
• Florigen is the proposed universal flowering hormone — still not isolated.
• Auxins were initially isolated from the urine of humans suffering from the disease pellagra.
• Leaf fall (abscission): in autumn, auxin decreases in the lamina while ethylene increases, causing the leaf to drop at the base of the petiole.
• Senescence (ageing) is accelerated by ethylene and ABA; advantages of senescence include replacing old organs and withdrawing nutrients back into the trunk.

Worked / Structured Examples

1. How does chemical coordination occur in plants? Coordination occurs through fluids secreted in plants known as phytohormones (plant hormones), which regulate the plant's growth. There is no nervous system; growth promotors and inhibitors together control all activities.

2. Explain why a sunflower head points towards the sun. In the presence of light, auxin becomes concentrated on the shaded side of the stem. This causes greater growth on that side, producing a curvature towards the lighted side (the sun) — positive phototropism.

3. Experiment to prove hydrotropism. Take a conical flask with water; cover its neck with wire mesh; place a few germinated bean seeds on the mesh. After a few days the primary roots grow towards the water in the flask, while the shoots grow towards light — proving roots are positively hydrotropic.

4. Why does one rotten orange spoil the whole basket? Ripening of an orange is caused by the gaseous hormone ethylene. Being a gas, it diffuses from one orange to the next, spreading ripening (and then spoilage) through the entire basket.

5. The clinostat — eliminating gravity. A clinostat slowly rotates a potted plant so that gravity acts equally on all sides. Auxin is then distributed equally, growth is equal on all sides, and no curvature (no geotropic bending) takes place — the plant grows straight.

6. Labelled process — chemotropism of the pollen tube. A = Pollen grain, B = Pollen tube, C = Female gamete, D = Ovary. Sugary chemicals in the stigma stimulate the pollen tube to grow down the style towards the ovule so that fertilization can occur — a chemotropic growth movement.

Key Terms — Quick Glossary

Common Mistakes to Avoid

• Confusing tropic and nastic movements. A tropism depends on the direction of the stimulus; nastic movements (Mimosa leaf folding) do not — and nastic movements are not in the ICSE syllabus.
• Mixing up positive/negative tropism. Roots are positively geotropic and hydrotropic but negatively phototropic; shoots are the reverse.
• Saying gibberellins act on roots. Gibberellins cause stem elongation and leaf expansion but have no effect on roots.
• Calling ethylene a liquid. Ethylene is a gas — that is exactly why it spreads ripening through a basket.
• Crediting Went with discovery instead of Darwin. Darwin (1880) first showed the tip effect; Went (1928) isolated auxin using agar blocks.
• Treating apical dominance and parthenocarpy as the same auxin effect. Apical dominance = suppression of lateral buds; parthenocarpy = fruit without fertilization. Different effects.

Likely Exam Questions (with crisp answers)

1. What are plant hormones called? Phytohormones.

2. Name the five classes of plant hormones. Auxins, Gibberellins, Cytokinins, Abscisic acid, Ethylene.

3. Which hormones inhibit growth? Abscisic acid and ethylene.

4. Who discovered auxins, and from where were they first isolated? Darwin first demonstrated the effect; F. W. Went isolated auxin (1928). Auxins were initially isolated from human urine of patients with pellagra.

5. Give full forms of IAA, NAA, IBA. IAA – Indole acetic acid; NAA – Naphthalene acetic acid; IBA – Indole butyric acid.

6. Which hormone promotes ripening of fruits and is a gas? Ethylene.

7. What is parthenocarpy? Give an example. Development of fruit without fertilization, e.g. apple, banana, tomato.

8. What is bolting? Give an example. Sudden enormous internode elongation just before flowering, caused by gibberellins, e.g. cabbage.

9. Define phototropism and hydrotropism. Phototropism – growth movement of plant organs towards light (e.g. stem bending to light). Hydrotropism – growth of roots towards water.

10. Distinguish thigmotropism and geotropism. Thigmotropism is growth in response to touch (tendrils of pea); geotropism is growth in response to gravity (roots towards gravity).

11. Explain apical dominance and name the hormone. Auxin from the apical bud suppresses the lower axillary buds; removing the apical bud lets them grow. Hormone: auxin.

12. Why does leaf fall (abscission) occur? In ageing leaves, auxin decreases and ethylene increases in the lamina, so the leaf is shed at the base of the petiole. (ABA also promotes abscission.)

13. Which hormone regulates stomatal closure and is the stress hormone? Abscisic acid (ABA) — it closes stomata during drought to cut transpiration.

14. Growth of pollen tube towards the ovule is an example of which tropism? Chemotropism.

15. In tissue culture, what does a high auxin : cytokinin ratio favour? Root development (high cytokinin : auxin favours shoot development).

16. State two physiological effects of cytokinins. Stimulate cell division and delay ageing (also break seed dormancy, expand cotyledons).

17. Name the universal flowering hormone not yet isolated. Florigen.