Morning glory flowers grew larger over a decade

Morning glory flowers at Matthaei Botanical Gardens. (Credit: Malia Santos)

Wild populations of the common morning glory in the southeastern United States increased the size of their flowers between 2003 and 2012, according to a new study.

There’s been a well-documented shift toward earlier springtime flowering in many plants as the world warms. The trend alarms biologists because it has the potential to disrupt carefully choreographed interactions between plants and the creatures—butterflies, bees, birds, bats, and others—that pollinate them.

But much less attention has been paid to changes in other floral traits, such as flower size, that can also affect plant-pollinator interactions, at a time when many insect pollinators are in global decline.

Increased flower size suggests a greater investment by the plants in pollinator attraction, according to the researchers. The changes they discovered were most pronounced at more northern latitudes, in line with a broad range of previous work showing that northern plant populations tend to show more dramatic evolutionary responses to climate change.

A shift to earlier flowering was also observed among those morning glory populations. In addition, there were tantalizing indications that the plants have increased their investment in floral rewards—the nectar and pollen obtained by the bees, syrphid flies, and wasps that pollinate the white, pink, and blue morning glory flowers.

“There is a major gap in our understanding of how traits that are crucial for plant-pollinator interactions may be evolving over time as a response to a changing climate,” says lead author Sasha Bishop, a doctoral student in the University of Michigan ecology and evolutionary biology department.

“We show that—in addition to well-documented shifts to earlier flowering—floral architecture and rewards can also play significant roles in the evolutionary response to contemporary environmental change.”

Morning glory ‘resurrection’

The common morning glory is an annual weedy vine found across the eastern, midwestern, and southern United States. It is frequently seen along roadsides and crop fields.

For the study, researchers used a “resurrection” approach that involved germinating morning glory seeds collected from the edges of agricultural soy and corn fields in Tennessee, North Carolina, and South Carolina in two years: 2003 and 2012.

During that nine-year span, the region experienced rising temperatures—particularly rising minimum and nighttime temperatures—and an increase in the number of extreme rainfall events interspersed with more extreme drought.

To look for changes in floral morphology, the researchers planted field-collected seeds from both years in a greenhouse at the University of Michigan’s Matthaei Botanical Gardens. When the flowers bloomed, various floral traits were measured with digital calipers.

Measurements showed that morning glory corollas became significantly wider during the nine-year interval—4.5 centimeters (1.8 inches) in diameter in 2003 and 4.8 centimeters (1.9 inches) in 2012, and the change in corolla width was greatest in populations at more northern latitudes. The petals of a flower are collectively known as the corolla.

Bigger flowers, bigger rewards

The study also revealed a shift to earlier flowering times between 2003 and 2012, driven primarily by populations at more northern latitudes. The start of flowering occurred an average of four days earlier for the plants grown from seeds collected in 2012.

Interestingly, the researchers also observed a latitude-influenced trend toward greater investment in floral rewards (pollen and nectar) over time. On average, morning glory flowers grown from 2012-collected seeds produced more pollen grains and more nectar sucrose than the flowers from the 2003-collected seeds.

However, the pollen and nectar analyses involved only four populations of morning glory plants. Due to the low number of populations examined, the floral rewards findings were not included in a statistical test to look for evidence that adaptation through natural selection is occurring in the plants.

“Nonetheless, it appears likely that there is a temporal increase in investment in pollinator attraction and that this result is driven by populations at northern latitudes,” says study senior author Regina Baucom, an associate professor in the ecology and evolutionary biology department.

The study found no evidence that morning glories are increasing the rate at which they self-pollinate. Evidence from some previous studies pointed to increased “selfing” as a possible response to climate change and/or pollinator declines associated with land-use change.

“This is the first article to use the resurrection approach to examine the potential that traits responsible for plant-pollinator interactions may be evolving over time, concomitant to decreases in pollinator abundance and dramatic environmental changes due to changing climate and land-use regimes,” Bishop says.

Fifteen morning glory populations were included in the resurrection experiment looking at changes in floral morphology. Twenty-three populations were included in the study of earlier springtime flowering. In total, 2,836 flowers were measured from 456 plants.

Additional coauthors are from the University of Georgia. The US Department of Agriculture and graduate research funds from the University of Michigan ecology and evolutionary biology department supported the work.

Source: University of Michigan