Johan Ehrlén tell us the story behind the cover image for our December issue:
This photo, taken in southern Sweden early May, shows inflorescences of Lathyrus vernus. Flowers at the bottom of the inflorescences have already started to wilt and changed colour from purple to light blue, while the uppermost flowers have just opened. This pattern of sequential opening of flowers within individuals is relatively common in species with many flowers.
When observing the timing of different yearly events in nature, we often focus on point measures, such as the arrival of migrating birds in spring or the leaf-out in trees. Similarly, plant flowering phenology of individuals is often described by the first flower opening date. However, in species with multiple flowers, like Lathyrus vernus, opening dates can vary considerably among flowers within the same individual, and the rate at which new flowers open influences both the appearance of the inflorescence at any given time and flowering duration. Flower opening rates could influence the climatic conditions experienced during flowering, as well as resource allocation to flowers and developing fruits. They might also influence how attractive the inflorescence is to pollinating insects and seed predators. This made us wonder if for plant individuals producing multiple flowers during a season, there is not a single optimal flowering time, but instead an optimal distribution of flower opening dates.
We tried to answer this question by following 5287 individual flowers of the perennial forest herb Lathyrus vernus in the field, and recording opening dates, fruit set, seed set and seed predation. We then used this data, representing 495 flowering events over three years, to investigate phenotypic selection on individual flowering schedules, in terms of mean, variance, skewness and kurtosis of flower opening dates within individuals.
Our analyses showed that variation in flower opening dates was mostly larger than the variation among individuals. In some years, selection favoured plant individuals with a higher variance in opening dates, and plants with a higher asymmetry of the flowering schedule. Overall, our study suggests that phenotypic selection on the spread and shape of flower opening date distributions within individuals might be at least as important as selection on the mean flowering date, and that considering only first or peak flowering dates might overlook important aspects of the evolution of flowering schedules.
Read the full article: Selection favours high spread and asymmetry of flower opening dates within plant individuals