|Why are these hydrangeas blue? Seems to be an|
aluminum complex of some sort.
Credit: Chemjobber's parents
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