Charlotte Noble of Balclutha Primary School, asks :-
My teacher brought some plum blossom to school. Some had pink centres and some yellow centres. Why was that?
Janice Lord, a botanist at Otago University, responded.
Coloured petals are a common form of advertising in plants that are pollinated by animals. The aim of this advertising is to attract the attention of a passing insect which is hopefully carrying pollen from a neighbouring tree. By fertilising the flowers, the insect is helping create a good crop of plums later in the year. Plum blossoms, like many coloured flowers, can have different colours in the centre of the flower. This is essentially a "sign-post" to the nectar that rewards the insect for its pollinating services.
In other flowers, such as orchids and foxgloves, the lower petals are patterned to indicate a "landing platform" for the visiting pollinator.
Differences in colour are caused by different plant pigments. Blue-purple-red-pink colours are usually due to a pigment called Anthocyanin. This is the same pigment that makes strawberries red and cornflowers blue. Yellow colours are usually due to Carotenoids, which make dandelions yellow, and carrots orange. Carotenoids can also reflect UV light.
Generally speaking, different types of pollinators are associated with different coloured flowers. Bees, for example, are particularly sensitive to blue-purple colours and can also "see" UV light, whereas humans can't. Yellow markings caused by UV-reflecting carotenoids are more likely to attract bees than are pink markings. However, as flowering plums are usually horticultural varieties, what you are really looking at are colours preferred by the plant breeder, rather than colours that have evolved naturally to attract either pink-loving or yellow-loving insects.