Freelance Science Writer & Editor

The cool shade of trees

An occasional blog about plants, people and landscape.

Peak chlorophyll

Viki Cramer - Thursday, September 07, 2017

It’s the Nyoongar season of Djilba in the south-west just now, a time of transition between the cold rain and blustering wind of Makuru (June–July) and the wildflower season of Kambarang (September–October). Some early bloomers have already bolted: the candy-cane pink of Geraldton wax (Chamelaucium uncinatum) and sunlight yellow of acacias festoon the banked earth along the edges of the freeway, the sheer exuberance of their display enough to bewitch even the most surly of commuters.

In our garden, the hardenbergia (Hardenbergia comptoniana) has spent the winter silently draping and twining itself over the two-leaved hakea (Hakea trifurcata), but now declares itself in cascades of neat purple pea-flowers that tumble over the edges of the hakea’s spiny branches. The hakea’s blooms, soft and white, hang like puffs of cloud in between.

But the colour that most captures my imagination at this time of year is, simply, the green. In this brief space of time while the days are warming but the sun is yet to regain its desiccating strength, vegetation has a chance to relax. Old leaves unfurl and plumpen, and fresh crowns of new shoots are optimistically thrust into the lengthening daylight. It’s peak chlorophyll, and every species models its signature shade of green.

The marri (Corymbia calophylla) looks sleek in its gum green, deep and matte, bluish with a hint of yellow. The clear yellow-green of the peppermints (Agonis flexuosa) matches the intensity of the cloudless spring sky. The lilly-pilly (Syzygium sp.), not from around here but well-suited to the deep shade under the eaves, is all dark glossy green except for the pinky-brown-green of the soft and shiny new leaves. The coastal daisy (Olearia axillaris), on the other hand, stubbornly remains a wizened silver-grey-green, but softer than in the depths of the summer drought.

Leaves are green owing to chlorophyll a and chlorophyll b, the pigments primarily responsible for harvesting the sun’s energy for photosynthesis. Both chlorophylls absorb red and blue light but reflect green light in a characteristic way that is the same in every plant. Yet all leaves are not the same green. It’s the type and amount of other pigments within the leaves and the form of the protective structures on the outside of leaves that creates the spectrum of greens.

With four atoms of nitrogen each, chlorophyll is a big and expensive molecule for a plant to make, and it’s an investment worth protecting. Within the leaves, much of this protection comes from other types of pigments: yellow and orange carotenoids, and red and purple anthocyanins. These pigments protect chlorophyll a and b from damage in two ways: by absorbing excess light energy and scavenging atoms of reactive oxygen (free radicals). Think of them as a team of personal minders, heading off over-enthusiastic fans and intrusive paparazzi before they get too close to the hardworking, yet somewhat fragile, creative duo of chlorophyll a and b. The more stressful the environment becomes, the more likely we are to see these minders: as chlorophyll begins to break down in a leaf, the colours of the other pigments become more visible.

On the outside, the defence is structural. Leaves produce hairs and waxes to protect themselves from the bite of the sun’s radiation and to reduce the loss of water to a thirsty atmosphere. Waxes are embedded in the cuticle layer that forms a thin protective covering over each leaf. The structure and chemical properties of the cuticle varies not just between species, but even between the developmental stages of leaves on the same plant. Cuticles can be glossy, shiny or dull, highlighting or dampening the leaf pigments underneath. Leaf hairs, on the other hand, tend to have one chromatic effect. Masses of tiny fine hairs make for whitish or silvery-grey leaves; the more hairs, the lighter leaves appear.

Under normal lighting levels – those of a clear spring day, perhaps – the human eye is most sensitive to the green wavelengths of light, followed by yellow wavelengths. Scientists have suggested that the development of trichromatic (red-green-blue) colour vision in primates was so that they could more easily find their preferred orange and yellow fruits amongst foliage, or to better discriminate between edible and nutrient-poor foliageā , although this is not universally agreed.

While scientists are equivocal about the evolutionary importance of our ability to recognise subtle variations of green, experts of ’colour psychology’ are united about what green signifies to us as humans: renewal, growth and feelings of abundance. And it does, at least for now, until the leaves start to slightly yellow as their chlorophyll fades, and they hang on, in a state of suspended animation, until the rains come again.

What do you  think?

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    Science Writer and Editor Perth Australia
    Viki Cramer   PhD BSc
    Freelance science writer specialising in ecology and the environment.
    Ecologist - Science Communicator - Editor
    Perth, Australia
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