A peak at a wavelength of about nanometers is characteristic of the blue quinoidal base anion, the structure complexed with the aluminum. The second peak we found, at a lower wavelength, is characteristic of the flavylium cation. Evidently, each mechanism produces half of the final blue product. A second part of the complex, leading to enhanced bluing, is the stacking of a flavylium cation on this primary complex.
Thus, we have generated both a chemical mechanism and a model for the bluing. 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. 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.
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.
Consequently, a solution of citrate ions C 6 H 5 O 7 3- and citric acid forms around the roots at relative concentrations that are specific to the soil pH. Because sepals are simply modified leaves, such behavior might be expected. 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. The reverse spray to change blue sepals back to red has proven to be a more challenging chemical problem. The color occurs near the top surface of the sepals. Shades of lavender, magenta, and purple in the sepals have different proportions of these two end members of red and blue.
The illustration above provides a comprehensive model for the chemical incorporation of aluminum ions into the hydrangea, resulting in the bluing of sepals. 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 glucoside in the sepals. This content varies from zero for white cultivars to greater than micrograms of delphinidin 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.
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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. 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.
Incorporating new natural pigments could yield hydrangea blooms that fluoresce at night as well.
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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. 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.
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. 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 MoO 4 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. 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 MoO 4 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. 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 cyanidinglucoside, 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.
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.
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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. 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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Photographs courtesy of the author. Watch the chemistry behind hydrangea colors in action:. Facebook Twitter. Bibliography Dirr, Michael A. Hydrangea for American Gardens.
Portland, OR: Timber Press. Schreiber, H. Lariviere, and R. Developing hydrangea with yellow blooms by chemical manipulation. The Cut Flower Quarterly 24 4 — Jones, C. Lariviere, K. Mayhew, and J. And would you like to know the best thing about blackberry winter? Programs Tennessee History Day About.
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Locust Winter — Some connect this little winter to when leaves start to appear on locust trees in early April, and others to when the trees bloom in May. Redbud Winter — Mid-March to early April, when the redbud trees bloom. Dogwood Winter — Mid- to late April, when the dogwood trees bloom. Often a heavy frost falls in dogwood winter. Blackberry Winter — Early to mid-May, when blackberries are in full bloom.