Question 2: Describe Achieved

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.

Describe whether kōwhai is a short-day or a long-day plant, with evidence; the geotropic response of its shoots and the part auxin plays; the bumblebee’s movement response to the nectar; and the biological clocks of both species.

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 Kōwhai blooms in late winter or early spring. What are the nights like then, and what kind of plant does that make it?
Long. A short-day plant flowers once nights grow longer than its critical night length, and in late winter they are, so kōwhai is a short-day plant.
2 Which way do kōwhai shoots grow in response to gravity, and what does that do for the tree?
Upwards, against gravity: negative geotropism. It takes the shoot up out of the soil and towards light for photosynthesis.
3 What does auxin do in a shoot?
It is a plant growth hormone that makes cells elongate. It collects on the lower side of a shoot, so that side grows faster.
4 What kind of movement takes a bumblebee to the nectar?
Positive chemotaxis (or positive trophotaxis): a directed movement towards a chemical, here the sweet scent of the nectar.
5 What is a biological clock, and which rhythms do these two organisms have?
A biological clock is a group of cells that keeps time on its own. The bee forages on a circadian rhythm, about 24 hours; the kōwhai flowers on a circannual one, once a year.
6 What does foraging in constant Arctic daylight tell you, and what keeps the clocks in time?
The bees still forage in daily bouts with no dark at all, so the rhythm is endogenous. Entrainment is the clock being reset by an outside cue, a zeitgeber: light for the bee, night length for the kōwhai.

Answer Kōwhai blooms in late winter or early spring, when nights are longer than its critical night length, so it is a short-day plant. Its shoots show negative geotropism, growing up against gravity, which takes them out of the soil towards light. Auxin, a growth hormone, collects on the lower side of the shoot and makes those cells elongate. A bumblebee reaches the nectar by positive chemotaxis (or trophotaxis), a directed movement towards its scent. A biological clock is a group of cells that keeps time; the bee forages on a circadian rhythm of about 24 hours and the kōwhai flowers on a circannual rhythm. The Arctic bees still foraged in daily bouts in constant light, so the rhythm is endogenous. Entrainment is the resetting of a clock by a zeitgeber: light for the bee, night length for the kōwhai.

What the marker wanted

Achieved Describes, one point each: a short-day plant; evidence that kōwhai is one; positive chemotaxis (or trophotaxis); negative geotropism; what either does for the kōwhai; what auxin does; a rhythm, named and defined; the bee’s rhythm as endogenous, since it carries on when conditions do not change; what a biological clock is; entrainment; a named zeitgeber. Four make A4.

The schedule marks the question as one answer from N0 to E8. Achieved points count first: one or two give N1 or N2, three A3, four A4. Above that, two Merit points give M5 and three M6. In 2024 Excellence needs more than usual: TWO Excellence points give E7, three or more E8.

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

Common mistake

Writing “chemotaxis” without “positive”. NZQA’s 2024 report gives this as its example of an answer that was not precise enough.

How did it go?

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