Drought Drives Rapid Evolution of Flower Size and Nectar in Common Morning Glory

Yedra García and colleagues at the University of New Brunswick have discovered that drought conditions can drive the rapid evolution of flower size and nectar production in the Common Morning Glory plant. The findings, published in AoB PLANTS, suggest that water stress from climate change may alter the evolution of plant-pollinator interactions faster than declining pollinator populations.

Water Stress Reshapes Morning Glory Flowers and Selection

The researchers conducted experiments growing Common Morning Glory plants under different watering conditions to test the effects of drought stress. The plants were grown outside at the University of New Brunswick under natural rainfall and pollination. To impose drought, some plants were watered substantially less frequently than normal – just once per week instead of every other day.

The experiments demonstrated that drought stress led to changes in floral traits that mediate plant-pollinator interactions. Morning glory plants under drought conditions produced flowers that were, on average, 23% smaller in diameter compared to well-watered plants. Drought also caused a 53% reduction in the nectar volume produced in each flower. Nectar volume is an important reward that attracts pollinators. These significant reductions in flower size and nectar under drought stress point to large effects of water deficit on the flowering phenotype.

Not only did drought affect floral traits, it also changed patterns of natural selection acting on flower size and nectar volume. The researchers estimated selection by relating plant traits to seed production as a measure of fitness. Under drought conditions, they detected selection favouring larger morning glory flowers, even though water deficit already reduced flower size. This result suggests that producing larger flowers is beneficial and adaptive for plant reproduction when water is limited. For nectar volume, selection under drought favoured flowers with less nectar, likely because of the water costs of nectar production.

In contrast, the experiments showed declining pollinator access had relatively weak effects on floral traits and their evolution. Although restricting pollinators reduced plant seed production a little, it did not alter flower size or nectar volume compared to open-pollinated plants. And patterns of selection on floral traits with and without pollinators were similar. These results indicate abiotic factors like drought may drive faster evolutionary changes in plant-pollinator interactions than declines in pollinator populations.

Studying Drought, Pollinators and Floral Evolution

The researchers conducted these experiments to understand how climate change impacts the evolution of plant-pollinator interactions. Factors like drought and declining pollinators are altering environments worldwide. The researchers wanted to test how these changes affect the evolution of floral traits that mediate interactions between plants and pollinators.

The team performed the experiments outdoors at the University of New Brunswick in Canada using commercial seeds of Ipomoea purpurea, the Common Morning Glory. Growing the morning glory plants from commercial seeds provided genetic diversity to study variation among flowers.

Measuring floral traits and fitness under the different treatments quantified how drought and pollinator decline impact the selection of flowers. Using a controlled experimental set-up allowed direct comparisons to reveal the stronger evolutionary effect of drought stress compared to reduced pollination.

Experimental Methods Reveal Drought’s Evolutionary Impact

The researchers used several experimental techniques to tease apart the effects of drought and pollinator decline on morning glory evolution. Drought stress was imposed by reducing the watering frequency compared to normal conditions. Floral buds were covered with fine mesh bags to exclude pollinators.

The team precisely measured floral traits like flower size, nectar volume and petal colour across over 400 morning glory plants. They related these trait measurements to seed production as an estimate of fitness. Quantifying the spectral reflectance of petal colour helped analyse selection on flower colouration.

This rigorous experimental approach provided direct evidence that drought drives stronger and more rapid evolutionary changes in floral traits critical to plant-pollinator interactions compared to declining pollinator populations. The results highlight the significant evolutionary impact abiotic factors like water availability may have on flowering plants and their pollinators in the face of climate change. García and colleagues conclude:

Our study shows that drought could affect the evolution of floral signals and nectar rewards, which may further impact patterns of pollinator visitation. While our results suggest drought may lead to faster evolutionary changes in plant pollination systems than pollinators decline, further experimental work on pollen-limited and/or self-incompatible species is needed. Our study reinforces using experimental approaches to uncover the relative role of different agents of selection acting on multiple floral traits. As climate change comprises both biotic and abiotic factors that may have interactive effects on selection on floral traits, we call for future studies considering multiple agents of selection in factorial-crossed experiments.

García et al. 2023

READ THE ARTICLE
García, Y., Dow, B.S. and Parachnowitsch, A.L. (2023) “Water deficit changes patterns of selection on floral signals and nectar rewards in the common morning glory,” AoB PLANTS, 15(5), p. lad061. Available at: https://doi.org/10.1093/aobpla/plad061.

Cover: Ipomoea purpurea. Image: Canva.