Why Mangrove Seeds Germinate While Still Hanging on the Tree

A single mango seed can grow into a tree taller than a house. On the way it germinates, its cells divide and specialise, it grows fast and then slows, and chemical messengers inside it decide when to lengthen, branch, flower and drop its leaves.

This lesson covers germination and differentiation, the phases and measurement of growth, plant growth regulators and their uses.

In hypogeal germination the cotyledons stay below the soil, in epigeal germination they are pushed above it, and in viviparous germination the seed germinates while still on the parent plant; differentiation is the maturing of cells for specific jobs, dedifferentiation is regaining the ability to divide, and redifferentiation is losing it again.

Types of germination:

- Hypogeal — the epicotyl lengthens, so the cotyledons stay underground; pea, gram, maize
- Epigeal — the hypocotyl lengthens, lifting the cotyledons above the soil; bean, castor, cotton
- Viviparous — the seed germinates inside the fruit on the parent plant; mangroves such as Rhizophora

Differentiation, dedifferentiation and redifferentiation:

- Differentiation — cells from meristems mature into specialised cells, such as xylem elements that lose their protoplasm and gain lignified walls
- Dedifferentiation — mature cells regain the power to divide, as when parenchyma forms interfascicular cambium or cork cambium
- Redifferentiation — cells made by these new meristems mature again into specialised cells, such as secondary xylem and cork

An everyday example. Moong sprouts soaked at home show epigeal germination as the hypocotyl lifts the seed leaves, while gram sprouts keep their cotyledons low.

The substance. Plant development is open-ended — meristems keep making new cells throughout life, and the same meristem can give different structures depending on its position.

Growth passes through a meristematic phase of division, a phase of elongation and a phase of maturation, and it is measured as an increase in weight, length, area, volume or cell number, with growth rate expressed as arithmetic or geometric growth.

Phases of growth:

- Meristematic — at root and shoot tips; cells divide constantly and have dense protoplasm and large nuclei
- Elongation — just behind the tip; cells enlarge, form vacuoles and lay down new wall
- Maturation — further back; cells reach full size and differentiate

Measuring growth. Plants are measured by increase in cell number, as in a maize root tip, surface area, as in a leaf, or length, as in a pollen tube.

Growth rates:

- Arithmetic — one daughter cell keeps dividing while the other differentiates; length rises at a constant rate, $L_t=L_0+rt$
- Geometric — both daughter cells keep dividing; growth is slow at first, then rapid, then slows as nutrients run short, giving a sigmoid curve with lag, log and stationary phases, $W_1=W_0{e}^{rt}$

An everyday example. Measuring a bean seedling every morning for a school project gives a curve that rises slowly, then quickly, then levels off.

The substance. Absolute and relative growth rates can rank plants differently — two leaves that each grow by 5 cm² in a day have the same absolute rate, but the smaller leaf has the higher relative rate.

The five main plant growth regulators — auxins, gibberellins, cytokinins, abscisic acid and ethylene — were each identified from a distinct source, and together they control growth, dormancy, flowering, ripening and ageing.

Auxins:

- Source — isolated from the growing tips of oat coleoptiles, which bend towards light
- Roles — apical dominance, cell elongation, rooting of stem cuttings, and preventing early fruit and leaf drop

Gibberellins:

- Source — the fungus Gibberella fujikuroi, which makes rice seedlings grow abnormally tall in bakanae or foolish seedling disease
- Roles — stem elongation, bolting in rosette plants such as cabbage, breaking seed dormancy, and larger fruit

Cytokinins:

- Source — kinetin, a modified form of adenine, from autoclaved herring sperm DNA; zeatin from maize kernels and coconut milk
- Roles — cell division, new leaves and shoots, overcoming apical dominance, and delaying leaf ageing

Abscisic acid (ABA):

- Source — identified as the inhibitor linked with abscission and bud dormancy
- Roles — inhibits growth, closes stomata during drought, induces seed dormancy; called the stress hormone

Ethylene:

- Source — a gas given off by ripening fruit
- Roles — fruit ripening, the triple response in seedlings, abscission of leaves and flowers, and sprouting of potato tubers

An everyday example. Fruit sellers keep unripe bananas beside ripe ones so that ethylene from the ripe fruit speeds up ripening of the rest.

The substance. Most responses are controlled by several regulators acting together — the balance of auxin and cytokinin, for example, decides whether a tissue forms roots or shoots.

Farmers and gardeners use growth regulators to root cuttings, kill weeds, produce seedless fruit, enlarge fruit, speed up malting and ripening, and control flowering.

Auxins:

- Rooting of stem cuttings for propagation
- Parthenocarpy — seedless fruit, as in tomatoes
- 2,4-D as a weedkiller of broad-leaved weeds in cereal fields

Gibberellins:

- Longer grape stalks and larger fruit; better-shaped apples
- Faster malting in the brewing industry
- Longer sugarcane stems, raising sugar yield

Cytokinins:

- Keeping cut flowers and leafy vegetables fresh for longer by delaying ageing

Ethylene (applied as ethephon):

- Hastens ripening of tomatoes and apples
- Promotes female flowers in cucumbers, increasing yield
- Brings on flowering together in pineapple crops

An everyday example. Pineapple growers in Kerala and the North East use ethylene-releasing sprays so that plants flower together and the fruit can be harvested at one time.

The substance. Artificial ripening must use safe methods — ethylene-releasing products are permitted, while ripening mangoes with calcium carbide is banned in India because of harmful impurities.