Autumn Leaves

What’s blooming in the area: Áñil del muerto, purple asters.

What’s blooming in my garden: Snapdragon, coral bell bud, petunia behind hollyhock leaves, sweet alyssum from seed, blanket flower, chrysanthemum.

Inside: Aptenia, geranium.

Animal sightings: Quail, ants.

Weather: Some very cold mornings; last rain, 29 September.

Weekly update: We’re having a genuine autumn this year, after several years with no serious frost until late November. After the cold a week ago Monday, trees that had stayed green suddenly were brown. Those that had some intimation of the coming desolation were more intensely yellow. A few had brown, yellow, and green patches.

Beneath the surface, abscisic acid is sealing the joints between branches and leaves, so that no open wounds remain when leaves finally drop. Meanwhile, the rate of photosynthesis is slowing as perennials enter winter dormancy, and the quantity of chlorophyll in leaves is decreasing. As the green fades, other leaf pigments become visible, including the yellow xanthophyll.

Xanthophyll had been known for years when Frederick Addicott’s team reported abscisic acid as a long sought growth inhibiting hormone in 1963. It begins as xanthoxin with 15 carbon, 22 hydrogen, and 3 oxygen atoms which have split from the 40 carbon, 56 hydrogen, and 4 oxygen atoms in a molecule of violaxanthin.

Xanthoxin is inherently unstably, and loses two hydrogen atoms to become abscisic acid aldehyde. When exposed to oxygen, that molecule converts into abscisic acid with a half life of about eight hours and four oxygen atoms. That in turn becomes phaseic acid with another oxygen atom, then dihydrophaseic acid with two more hydrogen atoms.

Xanthophyll sounded much simpler when Jöns Jakob Berzelius described the pigments in fall leaves in 1837. Scientists have since applied the term to any oxygen compound with 40 carbon and 56 hydrogen atoms.

The most important xanthophyll in leaves at dawn is violaxanthin. Instead of shucking off atoms and moving through the plant as abscisic acid, it stays in the leaf and loses an oxygen atom to become antheraxanthin. Later in the day, it loses another oxygen atom to become zeaxanthin, before reverting to its original state in the night.

Scientists want to know more than that zeaxanthin is created to protect the photosynthesis centers in leaves from increased solar energy levels in the day or that abscisic acid is a response to threats from low temperatures and drought. They want to know the nature of the bonds that connect those carbon atoms, the type of oxygen, the enzymes that transform each of the compounds, and the system of communication that triggers the creation of xanthoxin.

I don’t think about molecules, when I look through my windshield at the changing patterns of color formed by groupings of trees against the open fields, the sky, and each other. Still, it’s reassuring to know laboratory scientists remain who use the simple methods we were taught in high school chemistry to submit any scientific postulation, no matter how complex, to the old tests of dissolution, heating, sniffing, and spectrum analysis to reveal the beauty beneath senescent skin.
Notes:
Compounds in the abscisic acid chain
C40 H56 O4 - violaxanthin
C15 H22 O3 - xanthoxin
C15 H20 O3 - abscisic acid aldehyde
C15 H20 O4 - abscisic acid
C15 H20 O5 - phaseic acid
C15 H22 O5 - dihydrophaseic acid

Compounds in the violaxanthin chain
C40 H56 O4 - violaxanthin
C40 H56 O3 - antheraxanthin
C40 H56 O2 - zeaxanthin

Photograph: Yellow and green leaves on young cottonwood, 3 November 2007.