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I notice that certain (wild) flowers have the same colour, although they are not closely related. For example, the yellows of the dandelion (Taraxacum) and the buttercup (Ranunculus) are, at least to my eyes, identical (my observations are in the Netherlands).
I know that the colour is intended to attract insects; is this parallel evolution or are there certain biochemicals which provide colour and are easy for the plant to synthesize? Or something else?
Edit: I used the buttercup and dandelion because that was the first pair I found physically close enough for a photo; the cowslip, celandine and charlock mustard are all (to my eyes) the same yellow. I have also seen similar blues, reds and violets, and shall try to collect examples next time I go cycling on the dyke.
When I look at colour tables like https://en.wikipedia.org/wiki/Web_colors#Extended_colors I see numerous colours which I do not recall ever seeing in wild flowers.
Edit2: I retract my remarks about blues, reds and violets; it's hard to judge colours when cycling past and when I pluck the flowers and compare directly there's a definite difference, for example:
However I still find the yellows very similar; here the charlock (left) is lighter but the buttercup, celandine and dandelion look identical to my eyes.
Comments seem to suggest a biochemical cause; can anyone confirm (or disprove) it?
I think parallel evolution is likely due to the cost to produce the pigments in plants.
Xanthophylls and carotenes are made from only carbon, oxygen and hydrogen. This provides most of the yellow colours without using up nutrients.
Anthocyanins and betalains also need nitrogen, which can be a limiting nutrient for plants.
Wikipedia has a section on plant pigments at:
The surprises of color evolution
Nature is full of colour. For flowers, displaying colour is primarily a means to attract pollinators. Insects use their colour vision not only to locate the right flowers to feed on but also to find mates. The evolutionary interaction between insects and plants has created complex dependencies that can have surprising outcomes. Casper van der Kooi, a biologist at the University of Groningen, uses an interdisciplinary approach to analyse the interaction between pollinators and flowers. In January, he was the first author of two review articles on this topic.
Bees and other insects visit flowers to feed on nectar and pollen. In exchange for these goodies, they assist the reproduction of these plants by pollinating their flowers. That is the simple and slightly romantic view of pollination. The reality, however, is full of deception, chemical warfare and biomechanical trickery. 'The combination of chemistry and physics with evolutionary biology has broadened our view of pollination,' says Van der Kooi.
He is the first author of a review article on the evolution of colour vision in insects, which was published in the January 2021 volume of Annual Review of Entomology, and of a second review on the 'arms race' between plants and pollinators, which appeared on 25 January in Current Biology.
'For many insect families, we know very little about how they see colours,' says Van der Kooi. Bees have been studied in great detail but much less is known about colour vision in flies, even though many of their families, such as hoverflies, are very important pollinators. 'They are difficult to study and to keep in the lab and the anatomy of their eyes is more complicated,' explains Van der Kooi. 'Furthermore, some long-standing ideas on fly vision have recently been overturned.'
Van der Kooi and his co-authors tabulated which wavelengths can be seen by different insect species. 'Basically, insect colour vision occurs at wavelengths between 300 and 700 nanometres. Most photoreceptors in insect eyes detect ultraviolet, blue and green light but there is great diversity.' Insects evolved colour vision before the first flowers appeared. 'The pigments in flowers appear to be fine-tuned to be visible to pollinators. But of course, insects have subsequently co-evolved.'
Apart from colour, plants use scent to attract insects to the food that they provide. As production of nectar and pollen is costly, plants need to protect themselves from robbers, which eat the food but do not pollinate the flowers. This is the topic of the second review paper. 'This paper shows a huge diversity in the relationship between plants and pollinators, from real mutualism to outright abuse.' Some plants do not provide any food at all. 'Others have pollen or nectar that is toxic to most bee species. Only specific species can actually digest this food.'
Pollinators also have their own agenda. 'One particular plant is pollinated by moths in early spring. The moth also lays eggs on the plant and later in the year, the caterpillars will eat parts of it. Around that time, the main pollinators for this plant are flies.' This is one example of the complex relationship between plants and pollinators. 'There can be seasonal differences but the relationship can also be different in different locations -- there is variation in time and space and through different biological interactions,' says Van der Kooi.
The review focuses on different aspects of the complex relationship using views from chemical biology (e.g. the nutrient content of nectar or pollen), biomechanics (e.g. the barriers that flowers use to ward off unwanted insects or to make sure that pollen are dispersed by them) and sensory biology (e.g. the ways in which insects detect and recognize flowers).
Some plants, for example, many species in the potato family, have evolved the method of 'buzz-pollination', where the pollen are stored in tubes and insects need to vibrate on the flowers to release them. 'Honeybees, flies and butterflies cannot get to them but other bees such as bumblebees can shake the pollen free using their strong flight muscles.' The stiffness of the tubes, the stickiness of the pollen and the vibration frequency of the buzzing bees all play a part in this process. 'You really need tools from physics to understand their relationship.' The interdisciplinary study of insect-plant interactions is what Van der Kooi loves. He started his career using optics techniques. 'That is in part because I really like physics. But every new approach will show us new aspects of this complex relationship.'
A recent development in the field is the realization that plants are different in different geographic locations. 'A cornflower in the Netherlands is not necessarily the same as a cornflower in Italy. For example, the chemical composition of the pollen or the nectar may be different, which affects the interaction with insects.'
This has serious ramifications for attempts to boost insect numbers by creating insect havens, explains Van der Kooi: 'Sometimes, the seed mixtures for flowering strips are not sourced locally but from other countries. In that case, there may be a mismatch with the local insects, which may even harm insect numbers.' Insect havens are therefore best created using local seeds.
Both review articles stress how complicated the relationship between plants and pollinators can be. So why do plants bother? Why are they not all using wind dispersal of their pollen? 'Those are good questions,' says Van der Kooi. 'The efficiency of wind pollination is low but that is also true for animal pollination. Yet, roughly 90 per cent of plant species use the latter method, so it is a huge success.' But even this is complicated: 'Grasses use wind pollination, and in some ways, they are successful groups too. Like nearly everything in biology, the answer so often is "it depends. "'
What are Sepals
Sepals are the green color, leaf-like structures, enclosing the flower bud. They are responsible for protecting the flower bud from injuries and weather during its growth. Moreover, they support petals when blooming. Therefore, sepals form the outermost whorl of a flower and it is known as the calyx. The form and the development of sepals vary among plants. Some sepals occur free (polysepalous) or are fused together (gamosepalous).
Figure 1: Perianth of a Mature Flower
Furthermore, some sepals fuse together to form a calyx tube. Some flowers have sepals and petals with the same colour and are indistinguishable. These are known as tepals. However, after flowering, sepals are useless and they wither or become vestigial. But, some sepals keep alive to protect the fruit.
The blooms’ varied hues trace back to both soil pH and the right additives. Could different combinations produce new tints never seen in nature?
Biology Chemistry Botany
This Article From Issue
Volume 102, Number 6
One of the world’s most popular ornamental flowers conceals a bouquet of biological and biochemical surprises. The iconic “snowball” shaped blooms of Hydrangea macrophylla (big-leafed hydrangea) are a common staple of backyard gardens.
Many other, closely related hydrangea species are likewise known for their bountiful, showy, long-lasting blossoms, making them popular for both landscaping and the cut flower market. And their popularity continues to grow: Every year, gardening catalogs add new cultivars of these attractive plants. Hydrangeas are ubiquitous—but they are not what they seem.
Hydrangea blossoms are widespread and popular, but the chemical pathways behind their pink-to-blue color change (bottom left) are numerous and byzantine. The iconic species is Hydrangea macrophylla (top), but other species such as H. quercifolia (bottom middle) and H. paniculata (bottom right) also have large, showy blooms.
For starters, the bloom of the hydrangea is not a true flower, but an inflorescence: Sepals, or modified leaves, make up most of the bloom and overshadow the small, almost unnoticeable fertile floral portions at the center.
The bloom colors are what really make the hydrangea stand out: They range from pink to blue, including all shades of lavender to violet to purple, as well as green and white. Color intensities run the gamut from vibrant to pastel. Noticeably absent from the kaleidoscope of possible hydrangea colors are yellows and oranges.
Hydrangea colors are not what they seem, either they are not the result of a variety of different pigments, as is the case for flowers such as roses or tulips. They are more akin to the colors seen in litmus paper, the chemically treated strips classically used to determine whether solutions are acidic or basic. At the molecular level, acids are proton (or hydrogen ion) donators and bases are proton acceptors in chemical reactions. When one dips blue litmus paper into an acidic solution (pH < 7, where pH is a measure of the concentration of hydrogen ions), the paper turns red, whereas red litmus paper changes to blue in the presence of a basic solution (pH > 7).
In a similar fashion, the color of many hydrangea blooms acts as a natural pH indicator for the soil in which the plant grows. Such blooms have blue sepals when the shrub grows in acidic soil, but develop red or pink sepals when grown in neutral to basic soils. The hydrangea’s bloom color reveals the pH of the soil, but with its distinguishing colors being the reverse of those for litmus paper. The hydrangea is unique among plants in this ability to indicate soil acidity.
Because of this trait, gardeners can chemically manipulate hydrangea bloom colors using soil additives. In fact, a hydrangea can have different bloom colors on the same bush if the roots of the plant sample soils of differing pH. Homespun recipes abound for changing the pink blooms of a hydrangea to blue: pouring vinegar or lemon juice on the soil mulching the plant with coffee grounds, citrus fruit rinds, or pine tree needles or burying rusty nails, old tin cans, or copper pennies next to the bush. All these strategies tend to turn soil more acidic, and eventually transform the bloom color to blue.
Hydrangea colors turn out to be even more complicated that that, however. Soil acidity actually is not the underlying chemical mechanism behind the color change. The answer goes even deeper into the connection between soil composition and sepal color— a connection that has inspired our ongoing research into the biochemistry of these flowering plants.
A Metal Key
Hydrangea colors ultimately depend on the availability of aluminum ions (Al 3+ ) within the soil. The role of aluminum has been known since the 1940s, but it did not reach the mainstream horticultural literature until about the past two decades, and the exact mechanism was only recently defined. Aluminum ions are mobile in acidic soil because of the ready availability of other ions they can react with, which can be taken up into the hydrangea to the bloom where they interact with the normally red pigment. But in neutral to basic soil, the ions combine with hydroxide ions (OH - ) to form immobile aluminum hydroxide, Al(OH)3. Consequently, for the bluing of hydrangea blooms, one needs both aluminum ions and acidic soil. The best soil additive for bluing is one that contributes both, such as commercially available aluminum sulfate, Al2(SO4)3. Conversely, if one wishes to change blue-blooming hydrangea to red-blooming, adding lime (calcium hydroxide, Ca(OH)2) results in basic soil and the desired color transition.
However, such imposed red-to-blue or blue-to-red color changes do not happen instantaneously it often takes one or two growing seasons to instill the desired color on shrubs within one’s flower gardens. Growers of hydrangeas with blue blooms must regularly water with aluminum sulfate drenches onto the potted medium to maintain the needed levels to force the desired blue coloration (but they cannot water too often or the excess Al 3+ will kill the plant).
The chemistry of aluminum in soil establishes its different properties under acidic and basic conditions. In acidic soils, aluminum occurs in what are called coordination complexes, with Al 3+ ions at the center, surrounded by bonded strings of other molecules. These aluminum ions can travel from soil into the plant. But at neutral to basic pH, aluminum precipitates as aluminum hydroxide, making it unavailable for incorporation into the shrub. Lavenders, magentas, violets, and purples appear as bloom colors in transitional soil pHs, with aluminum ions only somewhat available to the hydrangea roots.
The color of the cultivar called “Endless Summer” of H. macrophylla depends on soil pH. Red or pink blooms result from neutral or basic soil (pH 7 and above), whereas blue blooms indicate acidic conditions (pH less than 7). However, the underlying mechanism of the color shift turns out to be aluminum ions (Al 3+ ), which are only mobile and available under acidic conditions.
Photograph courtesy of the author.
At extremely high pHs or very basic conditions, such as in hydroponic systems where plants are grown in nutrient water without soil, aluminum ions such as Al(OH)4 - , called the tetrahydroxyaluminate ion, become stable, so they no longer precipitate and are available as aluminum ions once again to the hydrangea. Indeed, at these extremely high pHs, the hydrangea bloom becomes blue, shortly before the plant dies from the extreme basicity, which causes cellular damage. On the other hand, because aluminum phosphate has limited solubility, it is also possible to block aluminum ion availability, even in acidic soils, through the use of high-phosphate fertilizers.
The amount of pigment within a bloom, regardless of color, is the same. Here, the “Blue Danube” cultivar always has 140 to 180 micrograms of pigment per gram of fresh sepal. The only change between blooms of different colors is their aluminum content. As shown in the graph (data point shapes indicate different cultivars), the sepals seem to need a threshold of 40 micrograms of aluminum per gram of fresh sepal for bluing.
Illustration by Tom Dunne. Photograph courtesy of the author.
Data on the sepal’s aluminum content (see figure above) show that red sepals possess essentially no aluminum. But a little aluminum goes a long way toward bluing the bloom. At a threshold of only about 40 micrograms of aluminum per gram of fresh sepal, hydrangea sepals turn blue, but they don’t become bluer with yet more aluminum. Intermediate sepal colors of lavenders to purples have aluminum contents lower than this threshold.
Thus, it’s all about the availability of aluminum ions in the soil for the generation of the blue sepal color in hydrangea blooms, with the soil pH just being a necessary facilitator of this aluminum mobility and availability.
A Single Pigment
In other cases where a plant has a flower that can be different colors, it is usually because the underlying pigments are likewise different, or the proportion of its pigments changes. However, the hydrangea is additionally unique because the color comes from only a sole pigment, delphinidin-3-glucoside (which is in the anthocyanin family, the same group that turns leaves red in autumn and gives berries their color). The underlying chemical system is thus, in a sense, relatively simple.
The color of the delphinidin-3- glucoside, as well as other anthocyanins, is a function of its molecular structure, which determines what wavelengths of light it absorbs. These molecules consist of a central three-ring carbon chain with one oxygen substitution, called a flavylium cation at low pH, to which various sugars are connected. The anthocyanin loses one or more hydrogen ions as the pH environment changes, which alters its absorbance spectra.
What goes on at the pigment level inside the cell is actually further proof that the soil pH is not directly responsible for the color switch, but rather mostly an indicator of aluminum ion availability. The internal cell pH remains constant for both red and blue sepals. The flavylium cation is red and stable at low pH, the opposite of the overall bloom color under acidic conditions. But under neutral conditions it transforms to the purple form of what is called a quinoidal base, meaning the molecule has lost a hydrogen ion and rearranged its double bonds. At basic pHs, the quinoidal base anion forms with a blue structure upon loss of another hydrogen ion and further rearrangement of the double bonds in the core delphinidin component of the pigment.
There are two methods for turning the hydrangea pigment blue. The core of the pigment, which is called a flavylium ion and is red in its acidic state (top left), under an increase in pH, loses two hydrogen ions and rearranges some of its double bonds to form a quinoidal base anion that is blue (top right). When the same flavylium ion, in a molecule that forms the hydrangea’s pigment, called delphinidin-3-glucoside, is exposed to aluminum ions when the cell’s internal conditions are acidic, it forms a complex with the aluminum that also leads to the blue-colored anion.
Illustration by Tom Dunne.
On the other hand, studies have shown that there is a way to stabilize this blue quinoidal base anion in an acidic cell medium. Aluminum ions will complex with the normally red pigment, as also shown in the figure above, for delphindin-3-glucoside, and result in additional bluing. Once again, the presence of Al 3+ becomes the key for the bluing of hydrangea sepals—both at the molecular level and in the field. Its presence circumvents the need for a high pH inside the cells to create the blue structure.
To ascertain the exact nature of the Al 3+ -anthocyanin complex, my research group undertook chemical modeling studies using acidic ethanol as a solvent. (Anthocyanins react with water to form yellowish to colorless structures called chalcones, which don’t behave chemically the same way as the pigments, so water can’t be used readily as a solvent.) We added aluminum chloride, which will break apart under the acidic conditions into aluminum ions, to a constant concentration of delphinidin, or delphinidin-3-glucoside. (The sugar substituent on the core delphinidin did not affect color appreciably. We also repeated this experiment with a direct extract from hydrangeas, with analogous results.)
Solutions of delphinidin pigments at similar concentrations (except in the final vial, which is lower) show the color shift that occurs when aluminum is added, from zero levels at left. The colors are intense, even though the level of aluminum added is relatively low. At bottom, an overall absorbance spectrum for one sample (black line) can be broken down into a large peak for the pigment’s central flavylium cation (red line) and the blue quinoidal base anion it turns into in the presence of aluminum (blue line).
Photograph courtesy of the author. Illustration by Tom Dunne.
The figure above illustrates a series of samples in which ever-increasing amounts of Al 3+ are added to delphinidin in the solvent. The color systematically changes from red to blue, through varying shades of purple, with the increasing Al 3+ . Once blue, the intensity of the blue plateaus much like in the natural system the color does not become any bluer with even more Al 3+ . We used a type of spectroscopy, which in this case excites the molecules with a high-energy visible light, so they absorb a wavelength characteristic of their structure. These data let us resolve the mechanism by which the Al 3+ complexed with the delphinidin. A peak at a wavelength of about 620 nanometers is characteristic of the blue quinoidal base anion, the structure complexed with the aluminum. As the amount of Al 3+ increases, the intensity of this peak (or the amount of the complex) increases, but finally plateaus.
The second peak we found, at a lower wavelength, is characteristic of the flavylium cation. As the amount of Al 3+ increases, its intensity tends to decrease, but the position of the peak steadily shifts to higher wavelengths until it too reaches a constant value—that is, the original red color of the flavylium cation transitions to a blue color. Thus, we discovered two contributions to the bluing of the solution: the previously acknowledged formation of the blue quinoidal base anion complexed with the Al 3+ , and the steady transition of the red to the blue flavylium cation.
To develop a complete picture of the Al 3+ -delphinidin complex, we pondered why the flavylium cation also went through a color transition. We collected other pertinent evidence showing that only about half of the available delphinidin molecules would form complexes (and create the blue quinoidal base anion structures) with the Al 3+ , regardless of how much of the latter was added. Evidently, each mechanism produces half of the final blue product. This behavior is often characteristic of stacking, when two molecules sit one on top of another like two slices of bread, but it turns out what’s happening is more complex than a simple stack.
The Al 3+ -delphinidin’s quinoidal base anion constitutes the primary blue complex. A second part of the complex, leading to enhanced bluing, is the stacking of a flavylium cation on this primary complex. The flavylium cation and quinoidal base anion are kept together not only by the electrostatic attraction that results from their opposite charges, but also, because their cyclic structures are similar, the molecules’ electron orbitals can align themselves for further stabilization. Thus, we have generated both a chemical mechanism and a model for the bluing.
Note that the Al 3+ serves as an anchor for this complex, probably attached to a phosphate network within the sepals’ cells, and not as a central ion for the complex. Indeed, we are finding that the aluminum ion is not material in the color generation, only for its stabilization, so that replacement of this metal by other metal complexing agents should not change the color. Our tests have shown that scandium (Sc 3+ , a common surrogate for Al 3+ ), gallium (Ga 3+ , in the same periodic family as Al 3+ ), tin, molybdenum, uranium, and other metal ions behaved analogously with delphinidin and formed blue complexes, albeit not as effectively as Al 3+ . That is, the chemical mechanism for bluing was the same, but the effectiveness of the specific metal ions in creating the resulting stacked complex was not.
Others have further characterized the Al 3+ -delphinidin complex and have shown the stacked flavylium cation is skewed at an angle from the underlying quinoidal base anion. The naturally forming complex inside the cellular environment of the hydrangea sepals has additional stacking and stabilizing with other co-pigments in the system. Such co-pigments, likely a unique mix of which exist in each cultivar, are a bit misnamed because they only serve to help stabilize the blue complex and do not contribute to the color. But the result is a complex probably in the shape of a helical spiral, rather than a simple stack.
A key step in the bluing of hydrangea sepals relies on getting Al 3+ into the plant and transporting it to the sepals, but as seems to be a theme with hydrangeas, it turns out that there’s another step in the process of aluminum transport. Al 3+ is mobile under acidic soil conditions and, in response to its stimulus, the roots of the hydrangea exude citric acid (C6H8O7). Consequently, a solution of citrate ions (C6H5O7 3- ) and citric acid forms around the roots at relative concentrations that are specific to the soil pH. Al 3+ then establishes a stable complex with the citrate ions, which is absorbable into the roots of the hydrangea. The plant transports Al 3+ throughout as this citrate complex. Other Al 3+ -tolerant plants, such as buckwheat and rye, likewise exude simple organic acids to detoxify aluminum. In fact, such strategies are becoming quite important in cultivating crops that are being both bred and genetically engineered to survive in acidic Al 3+ -rich soils.
This citrate complex is crucial for not only the incorporation of Al 3+ into the roots but also the constant circulation of Al 3+ throughout the plant, as shown in the figure at right. The hydrangea sepals actually do not concentrate the Al 3+ , as all leaves on the hydrangea possess about the same concentration of Al 3+ as the sepals (but only the sepals have the correct pigments to react with the ions). Because sepals are simply modified leaves, such behavior might be expected.
Citrate’s central role in transporting aluminum is demonstrated in the top row of blooms. A control bloom (top left) differs little from one infused with aluminum ions alone (top right). An aqueous solution of aluminum ions in a citrate–citric acid buffer turned the middle bloom blue. Field trials with this spray on pink blooms (bottom left) showed results after two (bottom middle) and four days (bottom right).
Photographs courtesy of the author.
In fact, it is possible to change a hydrangea’s color without affecting the soil chemistry at all. We have been able to circumvent the need for Al 3+ assimilation through the roots and subsequently transport to the sepals by developing a spray that introduces Al 3+ directly into the sepals. By dissolving appropriate amounts of Al 3+ in a buffered citrate–citric acid solution, we have changed red sepals to blue in a period of several days, as shown in the figure at above right. This outcome is further evidence that the bluing depends on the presence of Al 3+ in the sepals. The reverse spray to change blue sepals back to red has proven to be a more challenging chemical problem. That is, the Al 3+ -delphinidin complex, once formed, is difficult to break apart inside the plant.
The Color Process
New research has allowed the development of a comprehensive model for the chemical incorporation of aluminum ions (Al 3+ ) into the hydrangea, resulting in the bluing of sepals. First, the plant requires acidic soil so that Al 3+ is both available and mobile. In response to the presence of Al 3+ , the roots exude citric acid, resulting in a citric acid–citrate equilibrium in the soil (a). The subsequent generation of an Al 3+ -citrate complex allows incorporation of detoxified Al 3+ into the roots and transport throughout the shrub, including the sepals (b). Therein, the Al 3+ –citrate complex shuttles into the vacuoles of the cells where the pigment delphinidin-3-glucoside resides. The color occurs near the top surface of the sepals. In a sense, red is the default coloration of the pigment because of the delphinidin-3- glucoside’s flavylium cation. In the cellular environment of the sepal, Al 3+ not only exchanges the citrate ion for the delphinidin-3-glucoside in its complex but also catalyzes the formation of the blue quinoidal base anion of the delphinidin-3-glucoside (c). The flavylium cation of another delphinidin-3-glucoside stacks on top of this complex and enhances the blue coloration, whereas the Al 3+ anchors this blue complex onto phosphate groups within the vacuoles. Shades of lavender, magenta, and purple in the sepals have different proportions of these two end members of red and blue.
Illustration by Tom Dunne.
The illustration above provides a comprehensive model for the chemical incorporation of aluminum ions into the hydrangea, resulting in the bluing of sepals. The ease of bluing in hydrangea sepals depends on the relative concentrations of the delphinidin-3-glucoside and Al 3+ , as the number of Al 3+ molecules has to be in excess of about 3 to 10 times the number of pigment molecules. Thus, the less intense the coloration, the less the pigment concentration, and the less Al 3+ needed to attain the molecular excess. In addition, not all hydrangeas are equal in their ability to exude citric acid and incorporate Al 3+ into the plant.
Watch the chemistry behind hydrangea colors in action:
The biochemistry of hydrangeas opens up the possibility of developing new colors that are both interesting scientific experiments and potentially beautiful additions to the garden. The genetics of a particular hydrangea cultivar controls the amount of delphinidin-3- glucoside in the sepals. This content varies from zero for white cultivars to greater than 700 micrograms of delphinidin-3- glucoside per gram of fresh sepal in the most intensely colored breeds, as shown in the figure below. The concentration of pigment indicates the intensity of the coloration, from pastel to vivid, not whether the color is red or blue.
Not all cultivars of hydrangeas are equal, at least when it comes to color. The intensity of the red (or blue) color depends on the concentration of the anthocyanin pigment, which varies by cultivar.
Photograph courtesy of the author. Illustration by Tom Dunne.
Hydrangea cultivars that bloom on old wood, or the previous year’s growth, are susceptible to the loss of their blooms for an entire growing season upon a cold winter or a late frost, as the buds from the previous year freeze. Several new cultivars, most notably the popular “Endless Summer” cultivars, set their blooms on new wood, the current year’s growth, and will flower each year, unaffected by the weather of the winter or spring. However, such cultivars usually have more subdued colors than other hydrangea cultivars, for reasons yet unclear. One direction for breeding and genetic engineering has been to generate similar cultivars with brighter or more intense coloration.
Alternatively, we tried a chemical approach to enhance the color brightness by infusing magnesium ions (Mg 2+ ) into the plant, because other researchers have shown that such an approach brightened the color of grapes as well as other floral anthocyanin-based blooms, although the mechanism remains unknown. Surprisingly, instead of intensifying the coloration of the hydrangea sepals, we were able to produce sepals with red, white, and blue colors simultaneously (see figure below) on certain cultivars. It’s still an open research question as to why and how this happens.
Knowledge of hydrangea chemistry allows the creation of novel colors. A white, red, and blue sepal was generated by infusing magnesium ions into blue hydrangea blooms (top). A compound of molybdenum created yellow blossoms from white ones (middle). Intentional mixed colors of red and blue on a single bloom have been created in cut red hydrangea flowers by the controlled infusion of aluminum (bottom).
Photographs courtesy of the author.
But these results inspired us to see if it is possible to manipulate hydrangea chemistry to produce totally new colors. Hydrangeas lack the necessary pigment to generate yellow or orange sepals. Our initial strategy to produce these colors was to replace Al 3+ with some other metal, at first using solutions in the laboratory. However, the chemical mechanism we found for bluing indicated that such an approach might not work, because aluminum did not really contribute to the blue color, but only acted as an anchor for the blue form of the delphinidin core. Therefore, we were not surprised when our experiments with other metals that complexed with the delphinidin generated only subtle differences in blue hues.
But in another case of serendipity, during attempts to create a slightly different shade of blue with the infusion of the molybdate ion (MoO4 2- ) through the cut stems of red blooms, the sepals did not turn blue like the solutions, but showed yellowing. The yellowing occurred via a different mechanism than the bluing, observed not on the top surface but on the lighter bottom surface of the sepals. Accordingly, we changed strategy and started over with a white bloom of the hydrangea to generate, with success, the yellow color. We hypothesize that the normally colorless MoO4 2- bonds with the same phosphate groups that usually anchor Al 3+ , creating a yellow phospho- molybdate entity that gets stuck on the bottom surface of the sepals, for reasons we are still trying to determine.
Similar to some hydrangea cultivars being better “bluers” than others, it appears that a sepal’s yellowing ability is also a function of the cultivar. Such yellowing, as shown in the figure above, has only been successful thus far through cut stem infusions and sprayings. Introduction of the molybdate ion into the hydrangea shrub through the soil has proven toxic to the plant, but long-term experiments are in progress exposing hydrangeas to very low concentrations of MoO4 2- in the soil to create a viable plant with attractive, strongly yellow-colored sepals.
Another result of our research is the production of unique red-blue patterns on hydrangea sepals by controlled aluminum diffusion through cut stems and by direct sprayings onto blooms. Whereas the incorporation of Al 3+ through the roots always resulted in the relatively homogeneous distribution of blue color in the sepals, the rapid forcing of aluminum into the initially red sepals quickly generates unusual and novel patterns of red and blue.
There is still much to learn about natural changes in the color in hydrangea blooms. Near the end of the growing season, the sepals of the blooms of some plants flip over and change colors from blue to red, even though they retain the same aluminum content. The reason for this transformation is still unknown. A possible explanation is that the operational anthocyanin changes its composition, perhaps from the delphinidin- 3-glucoside to cyanidin-3-glucoside, which requires much more of an aluminum excess to stabilize the blue complex.
One last example of our current research on hydrangea coloration focuses on the blooms and leaves of the oak-leaf hydrangea, H. quercifolia. The green leaves of this hydrangea turn red in the autumn, much like its blooms age from white to red, with increasing intensity over time. Such blooms and leaves do not turn blue in the presence of Al 3+ as for H. macrophylla sepals, probably once again due to differences in anthocyanin types and levels. But we hope with more research, we may be able to generate intensely blue, “Smurf”-colored oak-leaf hydrangea leaves during the fall months. We envision beds of blue-leafed hydrangeas to complement the autumn spectrum of reds and golds of other shrubs and trees.
In the future, we plan to investigate the incorporation of entirely new natural pigments, not just the insertion of other metals, from various flowers to the hydrangea sepals. Perhaps we can attain this result via a spray to localize the desired changes—such that the changes are permanent for selective blooms without affecting neighboring plants and blooms. We imagine not only color changes but also controlling other properties, such as fluorescence, so that one can achieve hydrangea blooms that glow at night as well. For example, we are investigating the transfer of the fluorescent betaxanthin pigment from yellow flowers of the night-blooming four o’clocks (Mirabilis jalapa) and of portulacas to hydrangea blooms. Adding specific metal ions, such as those of rare earth elements, to the betaxanthin pigment seems to fine-tune the desired fluorescent hue.
It seems that when it comes to hydrangea colors, there are always more chemical mysteries awaiting a solution. And exploration of the coloring of hydrangea blooms illustrates that metal ions as well as pH play a key role in being able to create designer colors for flowers. Although such chemical manipulation of these colors has been underused in the past, this method may represent a fertile approach to generate novel floral colors for the future.
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Chromatography Plant Pigments
Chlorophyll often hides the other pigments present in leaves. In Autumn, chlorophyll breaks down, allowing xanthophyll and carotene, and newly made anthocyanin, to show their colors.
The mix of pigments in a leaf may be separated into bands of color by the technique of paper chromatography. Chromatography involves the separation of mixtures into individual components. Chromatography means “color writing.” With this technique the components of a mixture in a liquid medium are separated. The separation takes place by absorption and capillarity. The paper holds the substances by absorption capillarity pulls the substances up the paper at different rates. Pigments are separated on the paper and show up as colored streaks. The pattern of separated components on the paper is called a chromatogram.
Gather leaves from several different plants. CAUTION: Avoid poisonous plants. Autumn leaves from deciduous trees are especially interesting. Sort the leaves by kind (maple, etc.) and color. Review a diagram of a plant cell . Find the grana and the chloroplasts of the cell.
Chromatography solvent (92 parts Petroleum ether to 8 parts acetone)
Chromatography paper (or filter paper) about 1 cm x 15 cm
Test tube rack
Scissors and Ruler
Fresh leaves of plants
Glass stirring rod
Cork (to fit test tube)
Mortar and pestle
10-ml Graduated cylinder
Leaves should be grouped by kind (maple, etc.) and color. Work with a spinach leaf and with one or more other types. CAUTION: Chromatography solvents are flammable and toxic. Have no open flames maintain good ventilation avoid inhaling fumes.
1. Cut a strip of filter paper or chromatography paper so that it just fits inside a 15-cm (or larger) test tube. Cut a point at one end. Draw a faint pencil line as shown in figure 1. Bend a paper clip and attach it to a cork stopper. Attach the paper strip so that it hangs inside the tube, as shown. The sides of the strip should not touch the glass.
2. Tear a spinach leaf into pieces about the size of a postage stamp. Put them into a mortar along with a pinch or two of sand to help with grinding. Add about 5 ml ethyl alcohol to the leaf pieces. Crush leaves with the pestle, using a circular motion, until the mixture is finely ground. The liquid in which the leaf pigments are now for paper chromatography dissolved is called the pigment extract.
3. Use a glass rod to touch a drop of the pigment extract to the center of the pencil line on the paper strip. Let it dry. Repeat as many as 20 times, to build up the pigment spot. NOTE: You must let the dot dry after each drop is added. The drying keeps the pigment dot from spreading out too much.
4. Pour 5 ml chromatography solvent into the test tube. Fit the paper and cork assembly inside. Adjust it so that the paper point just touches the solvent (but not the sides of the tube). The pigment dot must be above the level of the solvent. Watch the solvent rise up the paper, carrying and separating the pigments as it goes. At the instant the solvent reaches the top, remove the paper and let it dry. Observe the bands of pigment. The order, from the top, should be carotenes (orange), xanthophylls (yellow), chlorophyll a (yellow-green), chlorophyll b (blue-green), and anthocyanin (red). Identify and label the pigment bands on the dry strip. Write the species of leaf on the strip as well.
Record the species, external color, and chromatogram pigments in the DATA TABLE of your report sheet.
5. Each pigment has an Rf value, the speed at which it moves over the paper compared with the speed of the solvent.
Rf = Distance moved by the pigment / Distance moved by the solvent
Measure the distance in cm from the starting point (pencil line) to the center of each pigment band. Then measure the entire distance traveled by the solvent. Remember, the starting point for the solvent is also the pencil line and the ending point for the solvent is the top edge of the paper. Do the required divisions and record your Rf values in the DATA TABLE of your report sheet.
6. Wash the mortar and pestle thoroughly, using a little alcohol to remove any remaining pigment.
Bluebell flowers are perennials that come in colors ranging from deep purple to pinks as well as white and blues. They are also known as Wood Hyacinths.
- Partial sun to partial shade (they grow better out of direct sunlight)
- Humus-rich, moist and well-drained soil
- They grow best when planted in shady areas like around trees or underneath shrubs
Devil is in the detail: Evolution of color in plants and animals
Researchers have looked at a species of fish to help unravel one of the biggest mysteries in evolutionary biology.
In many species of plants and animals, individuals from the same population often come in different color variants. But the mystery has remained as to why one color doesn't eventually replace the other through natural selection.
Research published in the Journal of Evolutionary Biology has looked at a species of Central American freshwater fish to look at how different colors are maintained in the species.
Lead researcher, Will Sowersby, a PhD student at Monash University, said the reasons why different color morphs (color variants) existed in a population -- when in theory they should be equally subjected to natural selection -- was still a major question for evolutionary biologists and remained unknown for many species.
"The importance of this work lies in the fundamental question: how and why do variants of the same animal exist in nature," he said.
"Color variants of the same species are a striking example of biological variation, yet the adaptive significance and what evolutionary processes maintain them, remains unknown."
Sowersby said the team looked at a species of fish called the red devil cichlid, which comes in two colors -- one is dark (grey through black with dark patterns) and the other is gold, (yellow through red).
The gold colored fish is genetically dominate but the darker colored fish is much more common.
"With this species, the darker individuals appear to be able to alter the shade of their body color and patterns to better match their environment," he said.
"We wanted to assess whether this had a part to play in how different color morphs (color variants) can exist in a population, and why the gold color fish is rarer."
The researchers filmed the red devil cichlids over both dark and light surfaces. Screenshots were then analysed to measure the amount of change to the shade of the fish's body color. After analysis they found that the darker fish could alter its brightness to match the surface it was on, while the gold colored fish could not.
"These results suggest that differences in the ability to match backgrounds could play a potentially important role in maintaining color frequencies in the wild," Sowersby said.
The research team, including Associate Professor Bob Wong, School of Biological Sciences, and Dr Topi Lehtonen, University of Turku, hope to do more work in this area.
"Given the complexities of color variants in species, more work is needed to understand how differences in coloration might influence the susceptibility of dark and gold individuals to different predators and under different environmental conditions," Sowersby said.
Chromoplasts are found in fruits, flowers, roots, and stressed and aging leaves, and are responsible for their distinctive colors. This is always associated with a massive increase in the accumulation of carotenoid pigments. The conversion of chloroplasts to chromoplasts in ripening is a classic example.
They are generally found in mature tissues and are derived from preexisting mature plastids. Fruits and flowers are the most common structures for the biosynthesis of carotenoids, although other reactions occur there as well including the synthesis of sugars, starches, lipids, aromatic compounds, vitamins, and hormones.  The DNA in chloroplasts and chromoplasts is identical.  One subtle difference in DNA was found after a liquid chromatography analysis of tomato chromoplasts was conducted, revealing increased cytosine methylation. 
Chromoplasts synthesize and store pigments such as orange carotene, yellow xanthophylls, and various other red pigments. As such, their color varies depending on what pigment they contain. The main evolutionary purpose of chromoplasts is probably to attract pollinators or eaters of colored fruits, which help disperse seeds. However, they are also found in roots such as carrots and sweet potatoes. They allow the accumulation of large quantities of water-insoluble compounds in otherwise watery parts of plants.
When leaves change color in the autumn, it is due to the loss of green chlorophyll, which unmasks preexisting carotenoids. In this case, relatively little new carotenoid is produced—the change in plastid pigments associated with leaf senescence is somewhat different from the active conversion to chromoplasts observed in fruit and flowers.
There are some species of flowering plants that contain little to no carotenoids. In such cases, there are plastids present within the petals that closely resemble chromoplasts and are sometimes visually indistinguishable. Anthocyanins and flavonoids located in the cell vacuoles are responsible for other colors of pigment. 
The term "chromoplast" is occasionally used to include any plastid that has pigment, mostly to emphasize the difference between them and the various types of leucoplasts, plastids that have no pigments. In this sense, chloroplasts are a specific type of chromoplast. Still, "chromoplast" is more often used to denote plastids with pigments other than chlorophyll.
Using a light microscope chromoplasts can be differentiated and are classified into four main types. The first type is composed of proteic stroma with granules. The second is composed of protein crystals and amorphous pigment granules. The third type is composed of protein and pigment crystals. The fourth type is a chromoplast which only contains crystals. An electron microscope reveals even more, allowing for the identification of substructures such as globules, crystals, membranes, fibrils and tubules. The substructures found in chromoplasts are not found in the mature plastid that it divided from. 
The presence, frequency and identification of substructures using an electron microscope has led to further classification, dividing chromoplasts into five main categories: Globular chromoplasts, crystalline chromoplasts, fibrillar chromoplasts, tubular chromoplasts and membranous chromoplasts.  It has also been found that different types of chromoplasts can coexist in the same organ.  Some examples of plants in the various categories include mangoes, which have globular chromoplasts, and carrots which have crystalline chromoplasts. 
Although some chromoplasts are easily categorized, others have characteristics from multiple categories that make them hard to place. Tomatoes accumulate carotenoids, mainly lycopene crystalloids in membrane-shaped structures, which could place them in either the crystalline or membranous category. 
Plastids lining which pollinators visit a flower, as specific colors attract specific pollinators. White flowers tend to attract beetles, bees are most often attracted to violet and blue flowers, and butterflies are often attracted to warmer colors like yellows and oranges. 
Chromoplasts are not widely studied and are rarely the main focus of scientific research. They often play a role in research on the tomato plant (Solanum lycopersicum). Lycopene is responsible for the red color of a ripe fruit in the cultivated tomato, while the yellow color of the flowers is due to xanthophylls violaxanthin and neoxanthin. 
Carotenoid biosynthesis occurs in both chromoplasts and chloroplasts. In the chromoplasts of tomato flowers, carotenoid synthesis is regulated by the genes Psyl, Pds, Lcy-b, and Cyc-b. These genes, in addition to others, are responsible for the formation of carotenoids in organs and structures. For example, the Lcy-e gene is highly expressed in leaves, which results in the production of the carotenoid lutein. 
White flowers are caused by a recessive allele in tomato plants. They are less desirable in cultivated crops because they have a lower pollination rate. In one study, it was found that chromoplasts are still present in white flowers. The lack of yellow pigment in their petals and anthers is due to a mutation in the CrtR-b2 gene which disrupts the carotenoid biosynthesis pathway. 
The entire process of chromoplast formation is not yet completely understood on the molecular level. However, electron microscopy has revealed part of the transformation from chloroplast to chromoplast. The transformation starts with remodeling of the internal membrane system with the lysis of the intergranal thylakoids and the grana. New membrane systems form in organized membrane complexes called thylakoid plexus. The new membranes are the site of the formation of carotenoid crystals. These newly synthesized membranes do not come from the thylakoids, but rather from vesicles generated from the inner membrane of the plastid. The most obvious biochemical change would be the downregulation of photosynthetic gene expression which results in the loss of chlorophyll and stops photosynthetic activity. 
In oranges, the synthesis of carotenoids and the disappearance of chlorophyll causes the color of the fruit to change from green to yellow. The orange color is often added artificially—light yellow-orange is the natural color created by the actual chromoplasts. 
Valencia oranges Citris sinensis L are a cultivated orange grown extensively in the state of Florida. In the winter, Valencia oranges reach their optimum orange-rind color while reverting to a green color in the spring and summer. While it was originally thought that chromoplasts were the final stage of plastid development, in 1966 it was proved that chromoplasts can revert to chloroplasts, which causes the oranges to turn back to green. 
Genetic Variation Examples
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Favorable genetic traits in a population are determined by the environment. Organisms that are better able to adapt to their environment survive to pass on their genes and favorable traits. Sexual selection is commonly seen in nature as animals tend to select mates that have traits that are favorable. As females mate more often with males considered to have more favorable traits, these genes occur more often in a population over time.
A person's skin color, hair color, dimples, freckles, and blood type are all examples of genetic variations that can occur in a human population. Examples of genetic variation in plants include the modified leaves of carnivorous plants and the development of flowers that resemble insects to lure plant pollinators. Gene variation in plants often occurs as the result of gene flow. Pollen is dispersed from one area to another by the wind or by pollinators over great distances.
Examples of genetic variation in animals include albinism, cheetahs with stripes, snakes that fly, animals that play dead, and animals that mimic leaves. These variations enable the animals to better adapt to conditions in their environments.