If plants want flowers that are coloured anywhere from pink and red to blue, via purple, their go-to pigments, most of the time, are anthocyanins. Think delphiniums at one extreme, pelargoniums at the other. But anthocyanins don’t just colour flowers – they also tend to occur throughout the plant, in stems and leaves too. Here they seem to have a range of functions, mostly involving protection from various stresses, including drought, ultraviolet light and fungal pathogens.
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- White flowers to plant
White foxgloves
Making anthocyanins is relatively hard work, so plants don’t always bother. For example, if you grow, say, foxgloves, you’ll have noticed that white-flowered forms, lacking anthocyanins, are common. All-white flowers can look a bit anaemic, but where the flowers are heavily spotted, this can be very attractive against a white or pale background, and you may even consciously select such forms in your garden. You may also have noticed that in foxgloves with white flowers, the whole plant is noticeably pale, which gives away that it’s going to have white flowers long before those flowers actually open. In normal purple-flowered forms, the whole plant is darker.
White-flowered, unpigmented plants, lacking anthocyanins, grew better in the well-watered treatment
In our gardens, we may prefer coloured or white versions of popular flowers, or we may like to grow a mixture of both kinds. But what happens in the wild? Over 20 years ago, researchers in Scotland investigated this by growing white-flowered and coloured forms of several species (including foxglove) under well-watered and droughted conditions. The results were clear: white-flowered, unpigmented plants, lacking anthocyanins, grew better in the well-watered treatment, while anthocyanin-containing plants were superior in the droughted conditions.
Those results show that anthocyanins are among the many things we might expect to respond to climate change. To get an idea of what might happen, American researchers recently used herbarium specimens to see how flower colour has responded to climate in the past. They looked at over 1900 herbarium records, the oldest from over a century ago, right up to the present day. The 12 North American species they chose to study aren’t all grown by gardeners, but they did include Hesperis matronalis (sweet rocket) and a couple of Michaelmas daisies. They then used climate records to see if plants with coloured flowers tended to be collected from times and places with drier conditions. Once again, the results were clear. White-flowered herbarium specimens of plants that normally have coloured flowers were always in the minority, but represented over a quarter of plants collected under the wettest conditions, declining to around 10 per cent under the driest conditions. In other words, wild plants behaved just like the ones in the Scottish experiment, with white-flowered forms doing best when they had plenty of water.
Pollinators often seem to favour coloured flowers, and plants lacking anthocyanins are less well defended against herbivores.
The past isn’t always a good guide to the future, but I think in this case it probably is. In 30 years’ time London is predicted to have a climate a bit like present-day Barcelona and, as the climate warms and droughts become both more frequent and intense, we can increasingly expect plants lacking anthocyanins to do less well than their more colourful relatives. Such plants might also face other problems; other research has shown that pollinators often seem to favour coloured flowers, and that plants lacking anthocyanins also seem to be less well defended against herbivores.
In our gardens, of course, we’ll no doubt continue to do what we always do, i.e. grow what we like. But those of us who like the white-flowered forms of plants that normally have blue or purple flowers might find that our favourites need just a bit more TLC in future. Those who prefer plants that can look after themselves might be best advised to stick to those with flowers in their original coloured form.
Discover a white garden in the city