A researcher working at the universities of Neuchâtel and Bern has bred a new line of petunia that produces significantly more seeds and less nectar than normal petunia. The downside of these positive changes is that pollinators spend less time visiting petunias that offer less nectar, which results in a lower seed production. This work, which was conducted by Anna Brandenburg with the support of the NCCR Plant Survival, showed that the cost-benefit ratio for the plant remains neutral.
She will present her results at the International Society of Chemical Ecology (ISCE) congress at the end of July in Tours (France).
In many relationships between plants and pollinators, nectar plays a central role as a reward to the pollinator for its efforts. However, some plant species, such as certain orchid, do not provide nectar in return. With this strategy, the plant saves energy resources that can instead be invested in growth or defence mechanisms. For her PhD thesis Anna Brandenburg tested this nectar reduction hypothesis on cultivated plants with the aim of increasing the plant's reproductive success. Her work was co-supervised by Redouan Bshary (University of Neuchâtel) and Cris Kuhlemeier (University of Bern).
She studied the impact of nectar reduction in petunia on its pollinators. Petunia is an interesting plant from an application perspective since it belongs to the same family, the Solanaceae, as the potato and tomato.
For her research, she first had to develop a new approach to obtain a petunia line that was clearly different in its nectar production than the original species Petunia axillaris. After multiple crosses between P. axillaris and P. integrifolia, the young biologist was able to produce a line that offered one third of the nectar volume of P. axillaris.
With the use of hand pollination, Anna Brandenburg observed two beneficial effects of nectar reduction. The petunias with this characteristic produced 20 to 30% more seeds than P. axillaris. Furthermore, still in comparison with the same species, the new petunia line emitted two times more methylbenzoate, an odorous substance that plays a role in attracting hawkmoth pollinators to the flower. A possible positive effect on pollinator attraction still needs to be verified since odours in greenhouses were saturated.
Everything changes, however, as soon as natural pollinators come into play. The hawkmoth Manduca sexta spends considerably less time feeding on flowers offering a smaller nectar reward, hence diminishing pollen transfer, which consequently results in a significant reduction in the quantity of seeds produced. This study shows that the hawkmoths are able to recognise plants that cheat, an innate ability, as demonstrated in subsequent tests.
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