Question 3: Discuss Excellence

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.

Discuss reasons for the success of the dodder, through living off other plants and flowering at the same time as its short-day and long-day hosts.

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 Start from the explain answer: how does thigmotropism make parasitism possible?
Auxin building up on the side away from the touch makes that side grow faster, so the stem coils tightly round the host and its haustoria attach firmly and grow in. Without a firm grip there would be no feeding.
2 What does parasitism then give the dodder?
A supply of water and nutrients it has not had to collect or make, with no roots or leaves to build. More energy can go into growing, spreading to nearby hosts and reproducing, so it is more successful and more likely to survive.
3 Why can the dodder use both short-day and long-day hosts?
Because it flowers when its host flowers rather than on its own day-length timetable, it can live on either kind, and on a far wider range of host species.
4 Why does flowering in step with the host help it reproduce?
The host’s flowers draw pollinators. Flowering at the same time, the dodder takes advantage of those visitors, so its flowers are more likely to be pollinated and set seed, increasing its success and the survival of the species.

Answer Positive thigmotropism makes parasitism possible: auxin builds up on the side of the stem away from the touch, that side grows faster, and the stem coils tightly round the host so the haustoria attach firmly and grow in. Parasitism then gives the dodder water and nutrients it has not had to collect or make, with no roots or leaves to build, so more energy goes into growing, spreading to nearby hosts and reproducing. Because it flowers when its host does rather than on its own day-length timetable, the dodder can live on short-day and long-day hosts alike, a far wider range. And flowering in step with the host lets it take advantage of the pollinators the host’s flowers draw, so its flowers are more likely to be pollinated and set seed. Both routes increase the dodder’s reproductive success and the survival of the species.

What the marker wanted

Achieved The describe points: parasitism, positive thigmotropism and how it happens, short-day and long-day plants, an advantage to the dodder.
Merit Those explained: the dodder gaining at the host’s expense, firm attachment, what auxin does, flowering in step with pollinator visits, and short-day against long-day.
Excellence E7: a full discussion of how parasitism, made possible by positive thigmotropism (with its auxin mechanism), lets the dodder gain more energy and put more into reproduction, for greater success and survival. E8: that AND how flowering in step with both short-day and long-day hosts lets it use the pollinators drawn to its host, so pollination is more likely, for greater success and survival.

One answer, N0 to E8. In 2022 Question Three, E7 is the first Excellence bullet and E8 both.

In our words, from NZQA’s 2022 marking schedule.

Common mistake

Explaining parasitism and synchronised flowering well without linking either to survival and reproductive success. NZQA’s 2022 report says that is where Merit answers stopped.

How did it go?

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