Question 2: Explain Merit

Practises Photoperiodism: flowering triggered by day length

The kōwhai (Sophora spp.) is a native New Zealand tree whose bright flowers open in late winter or early spring. Auxins act at every stage of its life, from root development to growth and flowering. Once it flowers, animals such as the bumblebee (Bombus terrestris) pollinate it. In New Zealand the bumblebee forages by day, but most actively in the cooler parts of the day. The same species lives in the Arctic too, where summer daylight lasts all 24 hours, and an experiment there recorded a colony’s foraging under that constant light. Figure 1, double-plotted actograms of when two of its worker bees foraged, is greyed out in NZQA’s public copy; as the schedule reads it, the bees foraged in distinct daily bouts rather than around the clock.

Explain the evidence for kōwhai’s type of plant, the mechanism and the advantage of its shoots’ geotropic response, the advantage of the bumblebee’s response to the nectar, and whether both species have biological clocks entrained to environmental factors (Figure 1).

The 2024 paper, page 6 ↗ Marking schedule, page 4 ↗

Work it out

Try it on paper first. Stuck? Take one hint at a time.

1 The describe answer called kōwhai a short-day plant. Tie the evidence to the critical night length.
Kōwhai blooms from late winter into early spring, when nights have lengthened past its critical night length; in summer, with nights shorter than that, it does not flower. Long nights trigger it, which is what a short-day plant needs.
2 How does auxin turn a sideways shoot upwards?
Auxin made in the growing tip moves under gravity to the shoot’s lower side. There it makes the cells elongate, so the lower side grows faster than the upper side and the shoot bends up.
3 Why is growing up an advantage?
The shoot gets out of the soil and above competing plants fast, so the leaves capture light for photosynthesis, making the glucose the tree needs for the energy that growth, flowers and nectar cost.
4 Why is positive chemotaxis an advantage to the bee?
Following the scent gradient takes it straight to the nectar, so it spends less time and energy searching and gains more food per trip, energy it can put into keeping safe or reaching the next flower.
5 What do the actograms show, and how does entrainment fit?
Even in 24-hour daylight, the bees forage in daily bouts, not all the time: a circadian rhythm that continues without a light–dark cycle, so it is endogenous. Entrainment is the zeitgeber resetting that clock each day, so in New Zealand the rhythm runs exactly with the day.
6 So what do the two clocks achieve together?
The kōwhai’s clock, entrained by night length, sets when it flowers, and the bee’s, entrained by light, sets when it forages. Flowering therefore falls when the bees are active, so the flowers are pollinated, which increases kōwhai reproductive success.

Answer Kōwhai blooms from late winter into early spring, when nights have lengthened past its critical night length, and not in summer, when nights are shorter: it is a short-day plant. Its shoots grow up because auxin moves to the lower side, makes those cells elongate faster than the upper ones, and bends the shoot upwards; getting out of the soil and above competitors lets its leaves capture light for photosynthesis and the energy that flowers and nectar cost. Positive chemotaxis takes the bee straight to the nectar, saving time and energy. The actograms show the bees foraging in daily bouts even in 24-hour daylight, so the circadian rhythm is endogenous; entrainment is a zeitgeber resetting it each day, so in New Zealand it runs exactly with the day. Because the kōwhai’s clock is entrained by night length and the bee’s by light, flowering falls when bees are active, the flowers are pollinated, and kōwhai reproductive success increases.

What the marker wanted

Achieved The describe points: short-day plant and its evidence, positive chemotaxis, negative geotropism and its advantage, auxin, a named rhythm, the endogenous rhythm, clocks, entrainment and a zeitgeber.
Merit Explains: the short-day evidence, tied correctly to critical night (or day) length; negative geotropism and what it gains the tree; positive chemotaxis and what it gains the bee; how auxin brings the bend about; what entrainment does to the rhythm; the actograms as evidence of a named rhythm in the bee; and the two clocks making flowering coincide with active pollinators, so kōwhai is pollinated and its reproductive success increases. Three make M6.

One answer, N0 to E8. Merit points sit above the Achieved ones: two give M5, three M6.

In our words, from NZQA’s 2024 marking schedule.

Common mistake

Explaining phytochrome. NZQA’s 2024 report says some candidates wrote about phytochrome, which this question did not ask for; the evidence it wants for a short-day plant is the flowering season against the critical night length.

How did it go?

Got it moves it further out. Review brings it back tomorrow.