Fifty Shades of Gold

GOLD medal winning exhibit at the RHS Chelsea Flower Show 2018

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What gives flowers and leaves their colour? Colour in plants is created by plant pigments. They exist in a wide variety of forms and are found in flowers, leaves fruit and vegetables. Pigments selectively absorb and reflect light of specific wavelengths and it is the reflected light that we see as the flower or leaf colour. Pigments have important roles in photosynthesis, in pollination and in protection of plant tissues.

Yellow colour in flowers is complicated.
Unlike in leaves, yellow flower colour in petals is due to many hundreds of plant pigments which vary from species to species. Some are carotenoids e.g. carotene, xanthophyll and lutein. Others are flavonoids such as chalcones and aurones which are deep yellow and flavones, flavonols and flavanones which are light yellow or nearly colourless. In a study of Narcissus flowers, 10 carotenoids and 18 flavanoid compounds were found in 15 different cultivars.

plant pigments

Yellow colour can be influenced by several factors. In Gerbera manganese and calcium make the colour more intense whilst cold temperatures make them paler. Increasing fertiliser levels intensifies the orange colour of Marigolds (Calendula) by raising xanthophyll levels. Carotenoid production increases in response to heat and lots of sun, changing colours form yellow to orange to red.

Petals also have textural patterns on their surface which reflect back the light and maximise reception of low light. In Viola these look like tiny fingers called papillae. In buttercup (Ranunculus), the yellow colour is intensified by the shiny surface of the petals which is due to a special layer of reflective cells beneath the petal’s outer layer.

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The humble snapdragon donates its yellow genes to science.

Sanapdragons (Antirrhiunm majus) are playing a useful role in the quest to produce yellow flowers in plants which don’t normally display this colour naturally such as Cyclamen, Sweet Pea, Verbena, Saintpaulia, Impatiens, Hydrangea, Morning Glory and Pelargonium. These plants seem to have lost the ability to synthesise yellow pigment or maybe never had it. The bright yellow colour of Antirrhinum is due to aurones, water soluble group of flavonoid pigments also found in Cosmos, Dahlia, Linaria, Coreopsis grandiflora and Bidens pilosa. The name ‘Aurone’ comes from the Latin ‘aurum’ meaning gold.

Yellow and magenta are predominant colours in wild populations of Antirrhinum probably attracting bumble bees for pollination. Aurones also emit yellow green fluorescence in ultra violet light extending the range for pollinator’s who can detect this, like bees. Antirrhinum has been studied in order to work out the pathway of aurone synthesis and identify the genes responsible for yellow flower colour and also for patterns on the petals. Using genetic techniques, these genes have been transferred to blue flowered Torinia x hybrida ’Summer Blue Wave’ and at the same time production of the blue pigment anthocyanin has been switched off. The result is a plant with bright yellow flowers.

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Flower Colour and pollination

Fossils suggest that more than 100 million years ago there were no brightly coloured flowers. Instead scientists think they were dull, pale yellow or green, Insect colour vision appeared long before the emergence of flowering plants so I is thought that plants evolved flower colours to attract specific pollinators. Yellow flower colour for example, has been associated with flies and wasps. Many yellow flowers can also absorb or reflect ultra-violet wavelengths of light. Humans are sensitive to only three wavelengths in visible light and our brains interpret the ratio of red, green and blue light to give the range of colours we see. Bees can detect ultra violet light but not red light which means they see flowers very differently from us. Yellow flowers often have ultra-violet patterns on their petals called nectar guides, invisible to humans, which act like signposts to the nectar deep within the flower.

The association between specific flower colours and pollinator species is still very controversial. Many pollinators are not fussy about the flowers they visit and there are also functions other than colour which influence selection such as flower shape and size, scent and nectar provision.


Yellow pigments and photosynthesis

The leaf is the main site of photosynthesis which take splace in small structures within leaf cells called chloroplasts. These structures conatin igments which apture lights enery from sunlight to fuel theprocess. The yellow pigments in leaves are carotenoids (yellow carotenes ad orange xanthophylls) Their colour is normally hidden by the green pigment chlorophyll.

Carotenoids are an ancient class of pigments, thought to have evolved perhaps 3 billion years ago. They are found in the chloroplast membranes in clusters with chlorophyll pigments at the heart of the photosynthetic apparatus. Their role is twofold. In photosynthesis, light energy from the sun is absorbed by chlorophyll and is used to convert carbon dioxide and water into sugars. However, chlorophyll can only utilise a narrow range of wavelengths (in the red and blue range), so to make the best use of the sunlight available, Carotenoids act like a satellite dish, absorbing a wider range of wavelengths (in the light blue to green range) and transferring that extra light energy to the chlorophyll molecule close by.

Carotenoids also have a second protective function. If a leaf receives too much light, the extra energy can cause damage. For example, sunlight passing over a woodland creates transient ‘sunflecks’ on the woodland floor. Leaves adapted to shade could be injured by the sudden spike in light but the carotenoids absorb the extra energy and release it as heat. Leaves have a sophisticated mechanism to deal with this.
Their cells become more acidic in response to dangerous light levels and this triggers production of a specific carotenoid, violaxanthin, which vibrates rapidly and dissipates the heat, acting like a pressure valve. This prevents the photosynthetic apparatus from overloading and releasing highly damaging molecules which destroy cell membranes and ultimately the leaf itself.

Yellow leaved plants

Yellow leaved plants still contain some chlorophyll and can photosynthesise to some degree but their colour is dominated by carotenes and xanthophylls. Cultivars with entirely yellow leaves can be highly decorative in the garden but would not survive in the wild as they are less vigorous. They are propagated by cuttings to maintain their attractive leaf colour.

Variegation in plants
Many evergreen shrubs are prized for their yellow and green (variegated) leaves, for example Ilex x altaclarensis ‘Golden King’ and in Elaeagnus pungens ‘Maculata’. Yellow areas on leaves are due to absence of chlorophyll, revealing the carotenoids which remain. The commonest cause of variegation in leaves is mutation in the cells when they are first formed in the bud. This gives rise to two genetically different tissues which continue to grow side by side. Such genetically different tissues are called ‘chimaera. Variegation becomes apparent because plants have a layered structure. As new cells are formed in the growing tips they are organised into an outer (epidermal) layer and an inner ‘core’ in monocots (grasses, bulbous plants) with an additional layer between these in dicots( broad leaf plants). Conifers may or may not have layered meristems and are seldom variegated. These layers are maintained in the plant’s life with the cells of the outer layer forming the skin of the plant and the core cells forming the internal tissues. In a leaf, if the thin outer layer lacks chlorophyll it will have a pale or yellow edge where the reverse is true if the inner layer lacks chlorophyll the outer layer is so thin the yellow will show through.

Imagine a yellow plastic glove inside a larger clear glove. When both gloves are flattened, the outer edge will be clear whereas the middle will have a ‘sandwich filling’ of yellow. Variegated plants would not survive in the wild normally s the reduction in chlorophyll would make them less vigorous than their all- green neighbours. Also, they rarely come true from seed and have therefore to be propagated from cuttings to be maintained. Some variegated plants ‘revert’ that is they lose their variegation and become green again. In this situation it is important to prune out green reverted stems as they will be more vigorous and eventually will take over the plant completely.

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As you can see, what may seem simple on the surface is really a lot more complicated than you might think.