Wednesday, July 31, 2013

#Chemsummer: The color of hydrangeas

This is written as a very late submission to C&EN's #chemsummer blog carnival.

Why are these hydrangeas blue? Seems to be an
aluminum complex of some sort.
Credit: Chemjobber's parents
On a recent warm summer Saturday, my sainted mother was walking around her yard with her grandchildren in tow. She pointed at her blue hydrangeas and said, "I heard that these change color with the pH of the soil. Do you know anything about that, Mr. Chemist?" I shrugged and said something about pigments changing their absorbance and changed the subject quickly by redirecting her attention to her lovely grandchildren.

Well, Mom, thanks for asking the question -- it's actually an interesting story. It appears to be a combination of factors; apparently, more acidic soils will generate blue flowers and more basic soils will generate pink flowers. (The ranges seem to be pH 5-6 for blue flowers, and ph 6 or higher for pink flowers. This pedantic chemist notes that both seem to be fairly acidic, but then again, I don't know much about soil pH.)

This Natural Products Report review indicates that it's actually an interesting complex between a specific member of the anthocyanins (the class of molecules that's involved in the classic cabbage pH experiment), another organic acid (5-O-acylquinic acid) and aluminum ions. The work that went into determining this is quite remarkable (single cell micro-spectrophotometry?!?), so I'll quote liberally:
In hydrangea sepals, the colored cells are located in the second layer; therefore, Yoshida et al. prepared protoplast mixtures, from which they collected and analyzed only colored cells.Vacuolar pH (pHv) measurements of colored cells illustrated the difference between blue and red cultivars. The pH of blue cells in the blue cultivar was approximately 4.1, significantly higher than that in red cells (pH = 3.3). Ito et al. analyzed the composition of anthocyanin (49) and three co-pigments (50–52) by collecting approximately 150 colored cells . The results indicated that the molar ratio of 5-O-acylquinic acids to 49 was much higher in the blue cells than that in the red cells. The amount of Al3+ was the same; in blue cells, the molar equivalent of Al3+ to 49 was greater than 1 eq., while the amount in red cells was lower than 0.1 eq. These results were significantly different from data obtained from whole sepal tissue. This discrepancy emphasizes the importance of isolating and analyzing only colored cells in flower color studies. To measure the composition in colored cells with greater sensitivity, Yoshida et al. developed a single-cell analysis method. Monitoring the cell color by micro-spectrophotometry, a single cell was collected, and then the organic or inorganic components were quantified. These results showed an obvious correlation between cell color blueing and increase in the levels of 5-O-acylquinic acid and Al3+.
The authors go on to propose a rather remarkable aluminum ion/quinic acid/anthocyanin complex. It is still a little unclear to me how plants change the color of their hydrangea according to the mechanism suggested by Yoshida et al. -- does the acidity of the soil determine the amount of the 5-O-acylquinic acid produced or sequestered in cells?

So, Mom, sorry, that Ph.D. in chemistry your tax dollars paid for still won't tell you exactly why your hydrangea could change color -- and yes, I'll be happy to water the lawn and pick the peas while you're on vacation. (Psst - it still has something to do pigments changing their absorbance.)

Love, your son CJ

5 comments:

  1. Wasn't that something to do with bioavailability of aluminium? Al2O3 being insoluble, but hydrolized species at low pH and Al(OH)3 at high pH being soluble, and thus bioavailable?

    ReplyDelete
    Replies
    1. This was my understanding too - I have various folklore about hydrangea flowers. The most entertaining being that a friend's grandmother used to have two hydrangeas either side of to the table where she sat out to drink tea. She would always tip the tea dregs/leaves on the soil next to the hydrangea, but always on the same side. One bush had pink flowers, the other blue, because of the organic acids she kept 'feeding' it with.

      On a more serious note, the Royal Horticultural Society suggests that bioavailability of aluminium is the key factor. You can buy 'hydrangea bluing agents', which are essentially aluminium phosphate, but these only work if your soil is acidic enough. Of course, you can make the soil more acidic by adding acid-rich organic matter like manure (or tea...) http://apps.rhs.org.uk/advicesearch/Profile.aspx?PID=122#section3

      White and green-flowered hydrangeas, however, will never turn blue, no matter how much aluminium you feed them.

      Delete
  2. The Aqueous LayerAugust 1, 2013 at 8:43 AM

    I transplanted one from my childhood home and the damn thing has NEVER flowered in 13 years.

    ReplyDelete
  3. Anthocyanines usually transition to purple in acidic environment and to blue in alkaline. (think of color of blueberries or red wine on your tongue when hit with a toothpaste containing baking soda). Are sure these flowers are more blue in more acidic soils? I would wager it is rather the other way around.

    ReplyDelete
  4. @Milkshake: yes, they are, and so of course it cannot depend on protonation/deprotonation of anthocyanines.

    @cj: my mother has both multicoloured hydrangeas - just discovered the English name of "ortensie"! - AND a son who should be a coordination chemist with a bioinorganic dissertation, a well-known expert of colour causes and analysis, and so on. She also is puzzling about the utility of spending on education (not sure this includes the reason for the State to pay me any wage).

    Anyway, this degenerate son knows that also iron is turning blue the flowers, and so it is commonly added to the soil. Speciation between complexes of +2, +3 and less usual oxidation states in the flower is by far out of my limited mind.

    Soil chemistry is really strange, and more strange ar "soil chemists", as it is usual to hear about "calcium - iron complexes" to justify acidity changes... decades ago, I heard the same wonderful idea (from supposed expert chemists) as the reason to use Ca(OH)2 or CaCO3 to deacidify archive papers written with iron-gall inks...!!

    ReplyDelete