Question 1: Explain Merit

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.

Explain how the phytochrome system controls when a short-day plant and a long-day plant flower, and how growers could make poinsettia flower in a New Zealand summer.

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 The describe answer said P_fr builds up by day and reverts in the dark. How does that let the plant measure the night?
Sunlight holds more red than far-red light, so by dusk most of the phytochrome is P_fr. In darkness it reverts at a slow, steady rate, so the longer the night, the less P_fr is left by morning: the phytochrome is a timer for the dark period.
2 What level of P_fr does a short-day plant need to flower?
A low one. Only a night longer than the critical night length lets enough P_fr revert for its level to fall below the threshold by morning; then the leaves send a flowering signal to the buds and the plant flowers. After a short night P_fr is still high, and it does not.
3 Why must the darkness be unbroken?
P_fr only reverts in the dark. A flash of red light partway through a long night turns P_r straight back into P_fr, so a short-day plant reads two short nights and does not flower. Far-red light straight after the red reverses the flash: the last light the plant gets decides.
4 And a long-day plant?
It needs a high level of P_fr to flower. A night shorter than its critical night length leaves too little time for P_fr to revert, so P_fr is still high at dawn and the plant flowers: in New Zealand, in summer.
5 So why does poinsettia not flower in a New Zealand summer by itself?
Summer nights are shorter than its critical night length, so its P_fr is still too high at dawn, and flowering is not triggered.
6 What could a grower do about that?
Give it a longer night: cover the plants or put them in a dark room for part of each day, so the unbroken dark period is longer than the critical night length and P_fr falls low enough for flowering.

Answer Daylight converts phytochrome to P_fr, and in darkness P_fr slowly reverts to P_r, so the P_fr left at dawn measures the length of the night. A short-day plant needs low P_fr to flower: only a night longer than the critical night length lets P_fr fall below the threshold, and then a flowering signal passes from the leaves to the buds. The darkness must be unbroken, because a flash of red light converts P_r back to P_fr and stops a short-day plant flowering, unless far-red light follows and reverses it. A long-day plant needs high P_fr: a night shorter than its critical night length leaves P_fr high, so it flowers in summer. Poinsettia therefore does not flower in a New Zealand summer, when the nights are too short, unless growers lengthen its night by covering it or moving it into darkness for part of each day.

What the marker wanted

Achieved The describe points, each stated: photoperiodism, both kinds of plant, the phytochrome system and the critical night length.
Merit Each of those explained, not just named: how the two kinds of plant differ in terms of the critical night length; how P_r and P_fr convert (an annotated diagram counts if it explains); why the darkness must be unbroken for P_fr to revert to P_r; what makes a long-day plant flower; what makes a short-day plant flower; and one way growers get short-day plants to flower in a New Zealand summer. 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 2025 marking schedule.

Common mistake

Treating P_r as the form that switches flowering on or off. P_fr is the active form; P_r is inactive. NZQA’s 2025 report says many candidates did not know which was active.

How did it go?

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