Question 2: Discuss Excellence

Practises Photoperiodism: flowering triggered by day length

The kōwhai (Sophora spp.) is a native New Zealand tree whose bright flowers open in late winter or early spring. Auxins act at every stage of its life, from root development to growth and flowering. Once it flowers, animals such as the bumblebee (Bombus terrestris) pollinate it. In New Zealand the bumblebee forages by day, but most actively in the cooler parts of the day. The same species lives in the Arctic too, where summer daylight lasts all 24 hours, and an experiment there recorded a colony’s foraging under that constant light. Figure 1, double-plotted actograms of when two of its worker bees foraged, is greyed out in NZQA’s public copy; as the schedule reads it, the bees foraged in distinct daily bouts rather than around the clock.

Discuss how the control of life processes through biological clocks results in kōwhai success.

The 2024 paper, page 6 ↗ Marking schedule, page 4 ↗

Work it out

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

1 Start from the explain answer: why does flowering at one fixed time of year matter to the kōwhai?
Flowering too early risks frost damaging the blooms; flowering in mid-summer risks water shortage. Either loses flowers, and with them a chance to reproduce. Flowering at the right time protects reproductive success.
2 What keeps flowering at that time each year?
A circannual rhythm run by a biological clock, entrained to the seasons by its zeitgeber, the photoperiod (night length), which is the same on a given date every year.
3 Why must flowering line up with the bees?
If the kōwhai flowers while bumblebees are foraging, they carry pollen between trees: cross-pollination, which increases genetic diversity. The clock makes the flowers open when pollination is most likely.
4 What does the bee’s own clock add?
The Arctic actograms show its foraging rhythm continues in constant light, so it is endogenous. That internal clock keeps the bee foraging, and so pollinating, at regular times every day.
5 In New Zealand the bee is busiest in the cooler parts of the day. What does that say about its zeitgebers?
That light, rising and falling in intensity, is not the only cue: temperature probably entrains its clock as well.
6 Put it together.
Clocks in both species make reproduction and other life processes happen at the time of year, and of day, that gives the best chance of success. The kōwhai flowers safely when the bees are foraging, is cross-pollinated, and sets varied seed: its reproductive success increases.

Answer Flowering at a fixed time protects the kōwhai: too early and frost can damage the flowers, too late in summer and water runs short, and either way a chance to reproduce is lost. That timing is a circannual rhythm, run by a biological clock and entrained to the seasons by the photoperiod (night length), which never varies on a given date. Flowering must also coincide with foraging bumblebees, which carry pollen between trees; cross-pollination increases genetic diversity, and the clock makes the flowers open when it is most likely. The bee has its own clock: the Arctic actograms show its foraging rhythm persisting in constant light, so it is endogenous, and it keeps the bee foraging at regular times. Its preference for the cooler parts of the day suggests temperature entrains its clock as well as light intensity. Together, clocks in both species place reproduction and other life processes at the time of year and day that gives the best chance of success: the kōwhai flowers safely while bees forage, is cross-pollinated, and its reproductive success increases.

What the marker wanted

Achieved The describe points: short-day plant, the two movement responses, auxin, named rhythms, the endogenous rhythm, clocks, entrainment and a zeitgeber.
Merit Those explained, ending on flowering coinciding with active pollinators and so kōwhai reproductive success.
Excellence Discusses biological clocks in both species, through six possible points: flowering at a set time of year avoiding frost or summer drought, which would lose flowers and reproductive chances; the circannual rhythm entrained to the seasons by photoperiod; flowering coinciding with active bees, so cross-pollination raises genetic diversity; the bee’s clock being endogenous (the Arctic actograms) and timing its foraging; cooler-time activity suggesting temperature as a zeitgeber as well as light intensity; and clocks placing reproduction in the season and at the hour that best favour reproductive success. Two give E7; three or more, E8.

One answer, N0 to E8. In 2024 the Excellence points are counted higher than usual: two give E7, three or more E8.

In our words, from NZQA’s 2024 marking schedule.

Common mistake

Explaining the parts well (the short-day evidence, negative gravitropism) without relating them to survival and reproductive success. NZQA’s 2024 report says this is where Merit answers to Question Two stopped.

How did it go?

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