Shedding light on the shifting shades of hydrangeas

Last updated: 12 Jan, 2021

A team of researchers from Nagoya University, Japan, has revealed the elements that come together to give hydrangea flowers their distinctive colour.

The parts of a hydrangea flower that look like petals are actually sepals, and they can exhibit a range of colours from deep purples and pinks to dark blue. Many gardeners will recognise that the colour of a hydangea's blooms can change when the plant is transplanted, depending on the pH of the soil. The research team, led by Professor Kumi Yoshida, found that the colour of hydangeas doesn't vary from plant to plant or sepal to sepal, but rather each cell in a hydrangea sepal has its own shade. Collectively the different colours of the sepal cells combine to produce the overall colour that we see in a single plant.

Purple hydrangea flowers, left, and a microscopic photo of a sepal consisting of cells of various colours, right (Courtesy of Professor Kumi Yoshida).

By studying the cells of a purple hydrangea sepal using a microscope, Yoshida's team found that the colour of each sepal was produced by a mosaic of blue, red and purple cells. The bluer the cell, the greater the concentration of aluminium ions and 5-O-acylquinic acids, as well as hydrogen ions. By manipulating the concentrations of these three elements in a test tube, the researchers were able to reproduce the range of colours found in hydangea sepals.

"We found that the colour of hydrangeas easily changes because at least three factors are involved compositely," Professor Yoshida said. "It's unclear why the colour differs among cells that have the same genetic information. We would like to clarify this."

Read the paper: Yoshida, K., Ito, D., Miki, N. and Kondo, T. (2021) Single‐cell analysis clarifies mosaic color development in purple hydrangea sepal. New Phytologist. DOI: 10.1111/nph.17099

Based on a press release and a news article in The Mainichi. Edited by Mike Whitfield.