What makes the blue color on some conifers?
I am by no means an expert on plant physiology, but I believe it is the waxy coating (cuticle) on the needles that makes them look blue, as confirmed in this article about a variety of Colorado blue spruce by Edward Gilman and Dennis Watson on the website of University of Florida Extension:
“[….] the wax coating on the needles of Blue Spruce which give
the blue color can be washed off by some pesticides.”
Here is similar information from Montana State University Extension (page 14):
Excerpt re: Colorado blue spruce:
“The bluish color of the leaves of some of the trees results from a wax (cutin) accumulation, which is genetically controlled.
This doesn’t satisfactorily explain why blue conifer needles would be any different from those of green conifers (whose needles are also waxy), though.”
The following article by John Clark and Geoffrey Lister in Plant Physiology, vol. 55, 1975 has a complex technical explanation:
Excerpt:
“The observed differences in relative pigment complements can, therefore, partially account for the differences between the action spectrum for red alder and those of the conifers as a whole. In particular, the increasing carotenoid-Chl ratios determined for red alder (0.38) and the two green conifers, Douglas fir (0.54) and Sitka spruce (0.67) would seem to be the factor responsible for the differences between their action spectra. The same explanation, however, cannot alone account for the range of differences seen in the action spectra for the four conifers. No evidence was found to support differential degrees of screening by an extrachloroplast blue-absorbing pigment reportedly present in some conifer needles (2). One is therefore led to believe that an additional factor must be
responsible for the differences between the green and ‘blue’ spruces. Differences in apparent leaf coloration, arising from changes in relative spectral reflectance attributable to varying leaf cuticle structure, seems to be the most plausible explanation.”