Developing Hydrangea with Yellow Blooms by Chemical Manipulation
This project is funded in part by the ASCFG Research Foundation
Henry D. Schreiber, Corinne M. Lariviere, and Ruthann P. Hodges
The bountiful, striking, and large blooms (inflorescences) of hydrangea make them high-value specialty cut flowers. The diversity of hydrangea cultivars provides a selection of bloom colors ranging from white to green, pink to blue, and varying shades of lavender to purple. However, none of the hydrangea cultivars possesses the necessary pigments in their blooms to generate yellow and orange colors. Extending the palette of available colors would correspondingly expand the appeal and market for cut hydrangea blooms.
The Underlying Chemistry of Hydrangea Sepal Coloration
The color of the sepals comprising a hydrangea bloom relies on the chemistry of two primary components—aluminum and anthocyanin. The default color of the anthocyanin (delphinidin-3-glucoside) is pink to red in the cellular environment of the individual sepals. This anthocyanin can also react with aluminum to form a blue entity. However, in order to react with the anthocyanin, the aluminum must first assimilate into the hydrangea. Aluminum compounds in the soil are soluble, and thus available to the hydrangea, only under acidic conditions (low pH), but are not available under neutral to basic conditions (pH of 7 and above). Sepals of white blooms lack the anthocyanin pigment, and remain white regardless of the aluminum content.
As a result of understanding this underlying chemistry, hydrangea growers manipulate the color of blooms of certain cultivars from pink to blue, or vice versa. Thus, when hydrangeas are grown in neutral to basic soils, produced by adding lime to the soil, aluminum is not available to hydrangeas and their blooms remain pink to red. Upon addition of aluminum sulfate, the supply of aluminum increases and the soil pH is lowered. The aluminum then becomes mobile in the soil and incorporates into the hydrangeas. Consequently, the hydrangea blooms turn blue. Accordingly, the pink-or-blue color in hydrangea blooms is all about the aluminum content in the sepals (figure 1), which in turn is controlled by the availability of this aluminum to the hydrangea as a function of the soil pH.
Because aluminum is central to the bluing of hydrangea sepals, we first approached the development of unique colors by replacing the aluminum with other metals. We studied the reaction of the hydrangea pigment with common metals such as iron and tin as well as the uncommon like uranium and scandium under ideal “test tube” conditions. The result of the metal-anthocyanin reaction in these model systems was always bluing (figure 2), with molybdenum (as the molybdate ion) being almost as effective a bluing agent as aluminum with the anthocyanin. However, when we placed a cut stem of a hydrangea bloom into an aqueous molybdate solution, we surprisingly observed yellowing instead of the expected bluing.
Some of the properties of molybdenum within the hydrangea were similar to that of aluminum. Both metals need citric acid, or rather an aqueous solution of a citrate – citric acid buffer, to be assimilated into and transported throughout the hydrangea to get to the sepals in the bloom. In a sense, the citric acid detoxifies the metals, allowing the hydrangea to be one of the few plants to tolerate high concentrations of aluminum in the soil. In addition, the content of molybdenum in the sepals needed for yellowing is about the same amount of aluminum required for bluing. On the other hand, the red-blue coloration caused by the aluminum-anthocyanin reaction occurs in the upper surface of the hydrangea sepal, while more of the yellowing happens in the lower surface. We attribute this molybdenum-induced yellowing of the hydrangea sepals to be possibly due to the formation of yellow phosphomolybdates before the molybdenum even encounters the anthocyanin.
Producing a Hydrangea with Yellow Blooms
Because the molybdenum was not reacting with the anthocyanin in the sepals, we changed our approach to add molybdenum (as the molybdate ion) to hydrangea with white blooms. We have studied the delivery of the molybdenum to the white blooms through the soil and into the roots, by infusion in cut flower stems, and with spraying directly onto the blooms.
At too high a concentration, we determined that molybdenum directly mixed into soil or added as watering solutions became toxic to the hydrangea, and likely will be even more toxic to other plants. We are currently performing long-term studies of white-blooming hydrangea planted in soils with trace levels of molybdenum to see whether the hydrangea will adapt to this environment and produce yellow blooms. After all, aluminum sulfate or lime additions to the soil may take more than a growing season to produce the desired blue or red blooms in hydrangea (and too high a concentration of aluminum in the soil also becomes toxic to the hydrangea).
We have infused the molybdenum in a citric acid – citrate buffered solution through cut stems of white hydrangea blooms. Even very dilute solutions of molybdenum generate yellow blooms within a day of soaking the stem. This yellow color is permanent, as the molybdenum solution can be replaced by water after a day without any loss of color, and appears quite natural (figure 3). Higher concentrations of molybdenum in the solution and/or longer soak times yield a greater intensity of yellow, but unfortunately decrease the effective vase life and also brown the sepal edges.
We are still optimizing the molybdenum concentration and soak time; that is, we are developing a protocol for cut flower processing to achieve both acceptable yellow blooms and an adequate vase life. A further complication is that, similar to some cultivars of red hydrangea being better at bluing with aluminum than other cultivars, certain white hydrangea cultivars appear to be better at yellowing with molybdenum than other cultivars.
Finally, we have sprayed a dilute molybdenum-containing solution once a day directly onto the white blooms of hydrangea with successful yellowing of the bloom (figure 4). The water-based spray is buffered to a pH of 6 with citric acid and sodium citrate, and includes a surfactant. Advantages of the spray include the localization of the molybdenum concentration onto the bloom with no effect on neighboring plants or even adjacent blooms, the development of a permanent yellow without any loss of color from rain, and essentially no waste. A disadvantage is that it takes almost a week of spraying to generate a sufficiently intense yellow color of the bloom. Once again, we are developing a protocol to define the molybdenum concentration in the spray and length of spraying to optimize the yellowing, before harvesting the bloom as a cut flower.
With an understanding of the chemistry of hydrangea coloration, albeit aided by serendipity in stumbling upon the effect of molybdenum on hydrangea sepals, we have now developed yellow blooms on hydrangea by chemical manipulation of their environment. Throughout the next growing season, we plan to ascertain the best delivery mechanism to produce yellow blooms (via cut stems or direct spraying), the best cultivars to accept the yellowing, and the protocol for optimization of the yellow color.
Figure 1. Cut blooms of Hydrangea macrophylla ‘Endless Summer’. Regardless of bloom color (pink, lavender, purple, or blue), the concentration of anthocyanin in this cultivar is always about 0.01 wt% in a fresh sepal. However, the aluminum content controls the color as it systematically changes from 0 wt% in the pink to 0.004 wt% in the blue, with the lavenders to purples having intermediate concentrations.
Figure 2. Cut blooms, initially pink, of Hydrangea macrophylla ‘Tovelit’ infused with metal ion solutions (of pH 6 buffered by a citric acid – sodium citrate mixture) in water for two days. Stems were soaked in a molybdate solution (LEFT), control with just water (CENTER), and in an aluminum-containing solution (RIGHT).
Figure 3. Molybdate-induced yellowing of Hydrangea macrophylla ‘Regula’ (TOP) and Hydrangea paniculata ‘Incrediball’ (BOTTOM) blooms by infusion of a molybdate solution through cut stems (RIGHT) compared to a control in water (LEFT) for two days.
Figure 4. Blooms of Hydrangea paniculata ‘Incrediball’. Left bloom is control, while right bloom has been sprayed with a dilute solution of molybdate, citric acid-citrate, and cocowet® once a day for five days.
Henry Schreiber is a Professor of Chemistry
at the Virginia Military Institute and operates
BackCountry Research in Lexington, Virginia.
Contact him at [email protected]
Corinne Lariviere and Ruthann Hodges are students at VMI and Roanoke College, respectively.