Orientation in space: tropisms, taxes, kineses and migration

Four kinds of orientation in space

• Plants cannot move from place to place, so they orient by growing in a direction (tropisms) or by moving parts reversibly (nastic responses).
• Animals move the whole body: a taxis is movement steered by the direction of the stimulus, while a kinesis is a change in speed or turning rate set by how strong the stimulus is, not where it comes from.
• Name each response by its stimulus and direction: positive phototropism is growth towards light; negative phototaxis is moving away from light.

Tropisms

• A tropism is a growth response, so it is slow (hours to days) and permanent: the stem stays bent.
• Phototropism is a response to light, gravitropism (geotropism) to gravity, thigmotropism to touch, hydrotropism to water and chemotropism to chemicals.
• Shoots are positively phototropic and negatively gravitropic; roots are positively gravitropic, and pollen tubes grow chemotropically down the style to the ovule.
• Climbers such as native clematis (puawānanga) wrap their leaf stalks around a support: cells on the side away from the touch grow faster, a thigmotropism.

How auxin bends a shoot

• Auxin (IAA) is made in the shoot tip and moves down the shoot, cell to cell, through carrier proteins.
• Light from one side is detected by phototropin, a blue-light receptor in the tip, which redirects auxin carriers so auxin moves sideways to the shaded side.
• Auxin causes cells to pump hydrogen ions into their walls; the acid activates expansin proteins that loosen the links between cellulose fibres, and turgor pressure stretches the softened cells (acid growth).
• Cells on the shaded side elongate more than those on the lit side, so the shoot curves towards the light.

Roots and gravity

• Cells in the root cap contain statoliths, starch-filled amyloplasts that sink to the lower side of the cell when a root is turned on its side.
• Their position redirects auxin carriers so auxin collects on the lower side of the root.
• Root cells are far more sensitive to auxin than shoot cells: the high concentration on the lower side slows their elongation, the upper side grows faster, and the root curves down.
• So the same hormone causes opposite bending in shoots and roots because of the different concentrations each tissue responds to best.

Nastic responses

• A nastic response is a movement whose direction is fixed by the plant's structure, not by where the stimulus comes from, and it is usually quick and reversible.
• Touch Mimosa pudica and an electrical signal reaches the pulvinus at the base of each leaflet; potassium ions leave its motor cells, water follows by osmosis, the cells lose turgor and the leaflets fold within seconds.
• The Venus flytrap closes when two trigger hairs are touched within about 20 seconds, so a raindrop does not waste its energy; many daisies and oxalis close their flowers or leaves at night (nyctinasty).
• The advantage is linked to the niche: folding may startle or dislodge grazing insects and shows less leaf, and closed flowers keep pollen dry overnight.

Taxes and kineses

• In a taxis the animal compares the stimulus on its two sides, with paired receptors or by swinging its head, and steers towards (positive) or away from (negative) the source.
• Euglena swims towards light (positive phototaxis) to photosynthesise; mosquitoes follow rising carbon dioxide to a host (chemotaxis); fly larvae crawl away from light.
• In a kinesis the animal does not steer: in orthokinesis its speed depends on the stimulus intensity, and in klinokinesis its rate of turning does.
• Woodlice move fast and turn little in dry air, carrying them out of it, and slow down and turn more in damp air, so they end up gathered where it is damp.

Try it: name the response

Homing and migration

• Homing is returning to a home site, such as a nest, hive or breeding colony, after travelling away from it; migration is a regular, usually seasonal, journey between two areas.
• Kuaka (bar-tailed godwits) breed in Alaska and spend the southern summer on New Zealand estuaries; in 2007 a tagged female flew about 11,700 km from Alaska to the Firth of Thames in roughly eight days without landing.
• Tītī (sooty shearwaters) breed on islands around Stewart Island and loop around the whole Pacific in a figure of eight each southern winter.
• Longfin tuna (eels) live for decades in New Zealand rivers, then migrate thousands of kilometres to spawn once in the deep tropical Pacific and die.

The cues animals navigate by

• A sun compass gives direction from the Sun's position, but the Sun moves about 15° an hour, so the animal must use its internal clock to correct for the time of day (time compensation).
• Polarised light patterns in the sky still show the Sun's position through cloud; night migrants use the star pattern around the celestial pole.
• Magnetic receptors let birds, turtles and salmon read the direction and the angle of the Earth's field, giving a compass and a rough sense of latitude.
• Close to home, landmarks and smell take over: chinook salmon in Canterbury rivers return to the stream they hatched in by the smell of its water.

Try it: which cue?

Selective advantage in the niche

• A response is an advantage when individuals that have it survive and reproduce more than those without it, so the alleles behind it become more common.
• Link it to the niche: a shoot bending to light in a crowded forest floor captures more light for photosynthesis, makes more glucose, and grows taller and sets more seed than its shaded neighbours.
• Woodlice lose water easily through their thin cuticle, so a kinesis that keeps them in damp, dark places cuts deaths from drying out and from daytime predators such as birds.
• Kuaka gain from two summers a year: long Arctic days with an insect boom for raising chicks, and food-rich New Zealand mudflats while Alaska freezes; the cost is a non-stop flight that uses up most of the fat they store before leaving.

What Achieved, Merit and Excellence look like

• Achieved describes the response or its advantage; Merit explains how it happens and why it helps; Excellence links the mechanism to the advantage in this organism's niche, with comparisons or justification.