Question 1: Describe Achieved

Practises Photoperiodism: flowering triggered by day length

Poinsettia (Euphorbia pulcherrima) is a short-day plant. Most New Zealand native plants are long-day plants, which flower in summer, when days are long and nights short. Growers use photoperiodism and the phytochrome system to bring poinsettia into flower during a New Zealand summer. The paper’s Figure 2, greyed out in NZQA’s public copy, compares a short-day plant under several 24-hour patterns of light and dark, among them a short night, a long night, and a long night broken by red light and then far-red light.

Describe photoperiodism, the difference between a short-day and a long-day plant (Figure 2), and the phytochrome system.

The 2025 paper, page 2 ↗ Marking schedule, page 2 ↗

Work it out

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

1 What is a plant responding to when it flowers at the same time every year?
The relative lengths of light and dark in each 24 hours. That response is photoperiodism, and it lets a plant time events such as flowering to the season.
2 What decides whether a short-day plant such as poinsettia flowers?
The night. A short-day plant flowers only when the unbroken dark period is longer than its critical night length, so it flowers as the nights lengthen in autumn and winter.
3 And a long-day plant, like most New Zealand natives?
It flowers once its night drops below the critical night length, which is why the natives flower in summer, when nights are short.
4 What exactly is the critical night length?
The dark period that acts as a switch for a given plant: on one side of it the plant flowers, on the other it does not. Each species has its own.
5 What in the plant detects light and dark?
Phytochrome, a light-sensitive pigment in the leaves. It has two forms that turn into each other: P_r, which absorbs red light, and P_fr, which absorbs far-red light and is the active form.
6 When does each form change into the other?
In daylight P_r is quickly converted to P_fr. In the dark P_fr slowly turns back into P_r, so how much P_fr is left by dawn depends on how long the night was.

Answer Photoperiodism is a response to the relative lengths of day and night, which plants use to flower at the right season. A short-day plant such as poinsettia flowers when the unbroken night is longer than its critical night length (autumn and winter); a long-day plant, like most New Zealand natives, flowers once the night drops below its critical night length (summer). The critical night length is the dark period that decides, for that plant, whether it flowers. Night length is detected by phytochrome, a leaf pigment with two interconverting forms: P_r, which absorbs red light, and P_fr, the active form, which absorbs far-red. Daylight converts P_r to P_fr quickly; in the dark P_fr slowly reverts to P_r.

What the marker wanted

Achieved Describes, one point each: photoperiodism; a short-day plant; a long-day plant; the phytochrome system, with its P_r and P_fr forms (a labelled diagram counts); and the critical night (or day) length. Four of the five 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, then one Excellence point E7 and two E8. The explain and discuss tiers show those points.

In our words, from NZQA’s 2025 marking schedule.

Common mistake

Pairing each light with the wrong form. P_r is the form that absorbs red light (and becomes P_fr); P_fr absorbs far-red. NZQA’s 2025 report says candidates often swapped these round.

How did it go?

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