Why One Ripe Banana Can Ripen a Whole Box of Fruit

Plants have no nerves, yet they bend towards light, drop old leaves on time and ripen fruit together. These changes are coordinated by tiny amounts of chemicals called plant growth regulators, or plant hormones.

This part covers their classification and sources, auxins and gibberellins, cytokinins, and ethylene and abscisic acid.

Plant growth regulators are grouped as promoters — auxins, gibberellins and cytokinins — and inhibitors — abscisic acid — with ethylene able to act either way, and each was identified by tracing a clear effect back to a chemical.

Chemical nature:

- Indole compounds — indole-3-acetic acid (IAA), an auxin
- Adenine derivatives — kinetin, a cytokinin
- Carotenoid derivatives — abscisic acid (ABA)
- Terpenes — gibberellic acid (**GA${}_3$)
- Gases — ethylene

Roles. Promoters drive cell division, enlargement, flowering, fruiting and seed formation. ABA drives dormancy, abscission and stress responses. Ethylene can promote or inhibit, but mostly inhibits.

The observations behind each:

- Auxin — canary grass coleoptiles bent towards one-sided light only when their tips were present; the active substance was isolated from oat coleoptile tips
- Gibberellin — the bakanae (foolish seedling) disease** of rice, caused by the fungus Gibberella fujikuroi, traced to gibberellic acid
- Cytokinin — autoclaved herring sperm DNA promoted cell division in tobacco tissue culture; the substance was named kinetin
- Ethylene — ripe oranges gave off a volatile substance that hastened ripening of stored bananas
- ABA — inhibitor-B, abscisin II and dormin proved chemically identical

An everyday example. Rice seedlings standing unusually tall and pale among their neighbours may be infected by the bakanae fungus.

The substance. A hormone's effect depends on its concentration and the tissue — a dose of auxin that promotes stem growth can inhibit root growth.

Auxins made at the shoot tip suppress side buds, help cuttings root, produce seedless fruit and, in synthetic forms, kill broad-leaved weeds; gibberellins lengthen stems and fruits, make rosette plants bolt before flowering, and speed up malting.

Auxins — natural IAA and IBA; synthetic NAA and 2,4-D:

- Apical dominance — the apical bud suppresses lateral buds; decapitation lets side branches grow, as in tea plantations and hedges
- Rooting — initiate roots in stem cuttings
- Flowers and fruit — promote flowering in pineapple; prevent early fruit and leaf drop
- Parthenocarpy — seedless fruit, as in tomato
- Weedicide — 2,4-D kills dicot weeds but not mature monocots

**Gibberellins — many kinds, GA${}_3$ best studied:

- Elongation — lengthen grape stalks; make apples elongate and improve their shape
- Delay senescence — fruit stays on the tree longer, extending the market period
- Malting — speed up malting in brewing
- Sugarcane — spraying increases stem length and yield
- Bolting — rosette plants such as cabbage and beet elongate internodes just before flowering

An everyday example. Pinching off the tip of a mehendi or hibiscus hedge makes it bushier by removing apical dominance.

The substance. Monocot crops tolerate 2,4-D**, so it clears broad-leaved weeds from wheat fields without harming the crop.

Cytokinins promote cytokinesis, help new leaves, chloroplasts and lateral shoots form, overcome apical dominance, and delay leaf senescence by drawing nutrients into the leaf.

Forms and sources. Kinetin, a modified adenine, does not occur naturally in plants; zeatin, found in corn kernels and coconut milk, is a natural cytokinin. Cytokinins are made where rapid cell division happens — root apices, developing shoot buds and young fruits.

Effects:

- Cell division — promote cytokinesis, especially together with auxin
- New organs — help produce new leaves, chloroplasts, lateral shoots and adventitious shoots
- Apical dominance — help overcome it, so lateral buds grow
- Senescence — promote nutrient mobilisation, which delays leaf ageing

The auxin-cytokinin balance. In tissue culture, more cytokinin than auxin favours shoot formation, while more auxin than cytokinin favours roots.

Worked example — predict the outcome. A callus placed on a medium with much more cytokinin than auxin will mainly form shoots.

An everyday example. Coconut water, rich in natural cytokinins, is often added to tissue culture media to help plant cells divide.

The substance. Auxin and cytokinin pull in opposite directions at the shoot — auxin enforces apical dominance, cytokinin weakens it.

Ethylene, a gas, speeds up fruit ripening, ageing and the shedding of leaves, flowers and fruits, while abscisic acid inhibits growth, closes stomata under water stress and keeps seeds dormant — earning it the name stress hormone.

Ethylene:

- Ripening — raises the rate of respiration during ripening, the respiratory climacteric
- Senescence and abscission — promotes ageing and shedding of leaves, flowers and fruits
- Dormancy — breaks seed and bud dormancy; starts germination in groundnut and sprouting of potato tubers
- Deep-water rice — rapid internode elongation keeps leaves above water
- Flowering — synchronises fruit-set in pineapple and induces flowering in mango
- Ethephon — releases ethylene slowly; hastens ripening of tomatoes and apples

Abscisic acid (ABA):

- General growth inhibitor — inhibits seed germination and metabolism
- Stomatal closure — raises tolerance to stresses such as drought
- Seed dormancy — helps seeds withstand drying until conditions suit germination
- Seed maturation — supports seed development

An everyday example. Keeping one ripe banana in a box of raw ones makes the whole box ripen faster, because the ripe fruit gives off ethylene.

The substance. ABA and gibberellins usually act as antagonists — ABA keeps seeds dormant, while gibberellins help break dormancy.