Question 1: Discuss Excellence

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.

Discuss the phytochrome system, and justify why New Zealand growers change the light and dark hours to make short-day plants such as poinsettia flower in 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 Start from the explain answer. How does phytochrome tie flowering to the season in each kind of plant?
P_fr made in daylight reverts during the dark, so the P_fr left at dawn tracks night length. Long winter nights leave little, which lets a short-day plant flower; short summer nights leave a lot, which lets a long-day plant flower. Each kind flowers in its own season from the same pigment.
2 What keeps a poinsettia from flowering in a New Zealand summer?
Its nights are shorter than its critical night length, so P_fr is still above the threshold at dawn, and a short-day plant cannot flower while P_fr is high.
3 First method: what can a grower change about the dark period itself?
Its length. Blackout covers or a dark room for part of each day give an unbroken night longer than the critical night length, about 14 hours, for several weeks. P_fr then falls below the threshold by morning, the leaves send the flowering signal, and the plant flowers as though it were winter.
4 Second method: what does far-red light do to P_fr?
It converts P_fr to P_r at once. So if the night is broken by red light, far-red straight afterwards returns the phytochrome to P_r, as in Figure 2’s red-then-far-red bar: the plant still reads one long, unbroken night and flowers.
5 Why does each method work, in one line?
Both leave P_fr low at the end of the night, the condition a short-day plant needs. The first does it slowly with more darkness, the second quickly with far-red light.
6 Why is it worth the grower’s trouble?
Poinsettia sells for Christmas, which in New Zealand falls in summer, when the plant would not flower by itself. Controlling the light and dark hours times flowering, and the bright bracts that come with it, to the market instead of to the season.

Answer Phytochrome links flowering to the season. P_fr made in daylight reverts to P_r in the dark, so the P_fr left at dawn tracks night length: long nights leave little, which a short-day plant needs to flower, and short nights leave a lot, which a long-day plant needs. In a New Zealand summer the nights are shorter than poinsettia’s critical night length, so its P_fr stays above the threshold and it does not flower. Growers can change that two ways. They can lengthen the dark period, with blackout covers or a dark room for part of each day, to an unbroken night longer than the critical length (about 14 hours) for several weeks, so P_fr falls below the threshold and the plant flowers as if it were winter. Or they can use far-red light, which converts P_fr to P_r at once: a night broken by red light and then far-red is still read as one long night, as Figure 2’s red-then-far-red bar shows, and the plant flowers. Both work because they leave P_fr low at the end of the night. Growers go to this trouble because poinsettia is sold in flower for a summer Christmas, when it would not flower by itself.

What the marker wanted

Achieved The describe points: photoperiodism, both kinds of plant, the phytochrome system and the critical night length.
Merit Those points explained: P_r and P_fr converting, why the night must be unbroken, how each kind of plant is brought into flower, and one grower’s method.
Excellence A detailed, linked discussion of two things. First, photoperiodic flowering in short-day plants AND in long-day ones, through the phytochrome system. Second, how growers bring poinsettia (or any short-day plant) into flower in a New Zealand summer: BOTH methods, a longer unbroken night and far-red after red light, each justified. One gives E7, both E8.

One answer, N0 to E8. Excellence points come last: one gives E7, both give E8.

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

Common mistake

Explaining why poinsettia is a short-day plant and stopping there. NZQA’s 2025 report says Merit answers often never reached long-day plants, or how growers make flowering happen out of season, and that is what the Excellence points ask for.

How did it go?

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