Question 3: Explain Merit

The golden dodder (Cuscuta campestris) has no leaves and no roots and lives off other plants. A growth response lets it wind up and around a host, branching into a tangle that can spread from the first host to the plants around it. It fixes itself to the host with a special organ, the haustorium, which grows into the host’s tissues and draws water and nutrients from them. When it flowers is critical to its reproduction. It picks up environmental cues, night length above all, but little is known about what starts it flowering; it is known that its hosts include both short-day and long-day plants, and that it seems to flower when its host does.

Explain how auxin lets the dodder grow up and wind around its host, how the dodder benefits from its host, and what flowering at the same time as its host does for it.

The 2022 paper, page 8 ↗ Marking schedule, page 5 ↗

Work it out

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

1 The describe answer named parasitism. How exactly does the dodder gain while the host loses?
Its haustoria grow into the host’s tissues and draw off water and nutrients that the host made or took up for itself. The dodder gets them without photosynthesising; the host is left with less for its own growth and seeds.
2 How does auxin make the stem wind around the host?
Auxin moves down from the shoot tip and builds up on the side of the stem away from the touch. There it loosens the cell walls, the cells take in water and elongate, so the far side grows faster than the touching side and the stem curls round the host.
3 Why does winding around the host matter?
Coiled tightly against the host, the stem holds the haustoria firmly in place, so they can grow in and keep feeding.
4 How do short-day and long-day plants differ?
By how they respond to the critical night length: a short-day plant flowers once nights are longer than it, a long-day plant once they are shorter.
5 Why is flowering when the host flowers an advantage?
A flowering host attracts pollinators. Flowering at the same time, whether the host is short-day or long-day, puts the dodder’s flowers on a plant pollinators are already visiting, whatever the season.

Answer The dodder’s haustoria grow into the host and draw off water and nutrients the host made or took up, so the dodder gains without photosynthesising while the host has less for its own growth and seeds. Auxin moves down from the shoot tip and builds up on the side of the stem away from the touch, loosening cell walls so those cells take in water and elongate; the far side outgrows the touching side and the stem curls round the host, holding the haustoria firmly in place. A short-day plant flowers once nights exceed its critical night length, a long-day plant once they fall below it. By flowering when its host does, short-day or long-day, the dodder flowers on a plant that pollinators are already visiting, whatever the time of year.

What the marker wanted

Achieved The describe points: parasitism, positive thigmotropism and how it happens, short-day and long-day plants, and an advantage to the dodder.
Merit Explains: how the dodder benefits while the host is harmed; how positive thigmotropism lets the haustoria attach firmly; what auxin does to wrap the stem around the host; why synchronised flowering puts the dodder on a host being visited by pollinators, whatever the season; and the difference between short-day and long-day plants, using the critical night length. 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 2022 marking schedule.

Common mistake

Writing out the phytochrome system. NZQA’s 2022 report says many candidates wrote all they knew about how plants time flowering, when the question was about the dodder flowering in step with its host.

How did it go?

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