Room 2, of Omaka School, asks :-

In spring our snowball tree develops flower balls which are green. After about a week these become white flowers and then the tips of some of the flowers turn pale pink. Why do these colour changes occur?

Janice Lord, a botanist at the University of Otago, responded.

Colour pigments in plants have a variety of functions and what you have so carefully observed most likely relates to three different ways in which pigments help plants.

Flowering is an expensive and sometimes risky process for a plant, as a lot of resources are diverted from growth into structures that are vulnerable to attack and may or may not bear fruit. The flowers of most plants are surrounded and protected by a structure called a calyx, when they are in bud. The calyx segments often look like petals but they are tougher and, in most plants, they are green. The green colour is due to chlorophyll, which is the main light-harvesting pigment that plants use to convert sunlight into energy. So before the flower has opened, the plant is using chlorophyll to make energy to help “pay” for the flowers.

As you observed, when the flower has fully opened the white petals become conspicuous and the green calyx is hidden. White flowers are attractive to a range of pollinating insects such as flies and bees, so here colour is helping the flower get pollinated and set seeds. I imagine your snowball tree is swarming with insects on a calm sunny day.

Damage from sunburn, and attack from flower-eating insects, can trigger the production of another important plant pigment: anthocyanin. Anthocyanins are responsible for most of the red, purple and blue colours we see in plants and have an important role as antioxidants, “mopping” up the effects of damage to plant tissues. So the changing colours you have observed on your tree represent just some of the ways that pigments help plants harvest energy, attract pollinators and protect themselves from damage.