It's probably not just the scarlet colour of Photinia spring foliage that warns off herbivores. It's probably that aggressive-looking glint too...
Botanical note: it's Photinia x fraseri that's under the spotlight. That's Fraser's Photinia, aka Photinia Red Robin in the UK, and, I believe, Photinia Red Tip in the States ,that being where the hybrid was first spotted in Alabama (see later).
More on the botany (and evolution) later, like the Far Eastern origin (China and Japan) of the two contributing species that came somehow to make a cross.
So what you may ask caused this science bod to get focused on this particular plant, given the gardens here and in Switzerland (from which I've just returned) are bursting with the colour of scores of Spring flowering shrubs and bedding plants?
There were still more gardens sporting Photinia on the approach to this village:
Here's another taken nearby, still lacking sunshine on those amazing leaves.
These pictures, under cloudy skies, hardly do justice to Photinia and its descriptive name (more later).
Here's one I took from a moving bus at our small village on the south-facing vine-terraces above Lake Geneva. (We ate at this excellent restaurant on our first night; the local rose wine is superb too - with for me the obligatory good 'bite'). Spot the botanical street furniture.
Here's an archive picture that does it justice: luscious sun-kissed Photinia.
Note below the clever two-tone effect that can be achieved with clipping, which relies on the fact that the red leaves are new Spring growth on the previous year's foliage (Photinia is an evergreen, contributing no doubt to its popularity)
|(someone else's picture from image archives)|
.At least I think it's done by selective clipping (but on second thoughts someone may have used a mix of two different shrubs). Either way it's Fun Photinia.
So then, what about the science?
Perhaps not surprisingly it's the red colour of the spring foliage that attracts all the attention. So let's insert a brief word about that to get it out the way, not because it's uninteresting, far from it, but because it distracts from the interesting etymology of Photinia (a word that gives no clue to colour).
The red colour is due to a class of plant pigments that is ubiquitous throughout the entire plant kingdom - from algae to autumn leaves - and which has been the subject of much puzzlement. I refer to the anthocyanins.
Anthocyananins are flavonoid compounds whose aromatic polycyclic molecules bristle with phenolic -OH groups. Why are they there, in so many different species and habitats, often with no obvious explanation?
For those interested in pursuing that question, I would recommend a short and concise review with a highly apt title:
by Kevin Gould
Here's the Abstract (but I recommend the rest too).
Anthocyanins, the pigments responsible for spectacular displays of vermilion in the leaves of deciduous trees, have long been considered an extravagant waste of a plant's resources. Contemporary research, in contrast, has begun to show that the pigments can significantly influence the way a leaf responds to environmental stress. Anthocyanins have been implicated in tolerance to stressors as diverse as drought, UV-B, and heavy metals, as well as resistance to herbivores and pathogens. By absorbing high-energy quanta, anthocyanic cell vacuoles both protect chloroplasts from the photoinhibitory and photooxidative effects of strong light, and prevent the catabolism of photolabile defence compounds. Anthocyanins also mitigate photooxidative injury in leaves by efficiently scavenging free radicals and reactive oxygen species. Far from being a useless by-product of the flavonoid pathway, these red pigments may in some instances be critical for plant survival.
I've bolded up three words, for reasons that will become clear later when one considers the presence in Photinia leaves of toxic chemical compounds (toxic to grazing animals that is).
In fact, of the reasons listed for the presence of red anthocyanins. none except arguably that warning to visual "eat-me-at-your peril" sign to herbivores is self-evident. While I have no doubt that the anthocyanins absorb harmful UV-B rays, they don't need to be coloured to do that, colour being a characteristic of absorption in the visible spectrum - red pigment abstracting green and yellow light. Lots of completely colorless substances are capable ot absorbing uv (like those in sunscreen lotions that come out the bottle white - fortunately not bright red, the aim being to avoid that coloration).
Why my preference for the idea that the red anthocyanins are a warning to herbivores?
The striking visibility of Photinia from afar is not just the red colour of those leaves, is it? It's their glossy reflective character. In fact, if one looks at photos of Photinia in full sun, one finds that a sizeable proportion of the leaves are not red, but "white". As hinted at earlier, the very name "Photinia" is not a reference to red colour, or indeed any colour, but to "light", Gk: photos.
The botanical genus name derives from the Greek word photeinos for shiny and refers to the often glossy leaves.
Photinia leaves have an unusual degree of shininess, even indoors under artificial light.
|Is it seen simply as red from a herbivore's eye perspective?|
Shininess - the property of being shiny. So what's that about? Why are some things, like metal surfaces, glass etc shiny, while others are matt and dull?
A more scientific term than "shiny" would be "reflective", inasmuch as it gives a clue as to mechanism. "Reflective" in turn brings to mind everyday mirrors, but that can be a step too far, given that mirrors are glass plus a reverse-side coating ("silvering"), whereas we know that plain glass is also capable of reflecting light (shop windows etc).
If one reads up on the subject, as I have just done (briefly!), all is explained. First, while reflection is most efficient off a metallic surface, like the silvering of a mirror, it is not essential to have metal (which Photinia leaves lack). All that's required is an interface between two layers that differ in their refractive index, explaining why air/glass can act as a mirror, and by extension, the interface between air and the waxes and cutins of leaves. That's the physics, basically, but there's chemistry too, or at any rate physical chemistry to be grasped, when one learns that reflection is not a mere passive process, but actively involves the atoms of the reflecting surface, more specifically the electron clouds, provided the latter are of the right kind. To cut a long story short, the incident light produces changes in the electron clouds (polarization etc) and the energised atoms each become point sources of light, re-radiating light as if a new light source, and doing so (obviously) in conformity with the laws of reflection (angle between normal and reflected ray = angle between normal and incident ray).
But it's not events at that level that are my chief interest where Photinia is concerned, which could be said to apply to any reflected light. We are not talking about any old reflected light. We are talking about the way that Photinia leaves glint in the sunshine in a manner that distinguishes the shrub from other ones, red ones included, such that a Photinia can be spotted at 100 metres or more. Glinting? What's that about? It's obviously to do with reflection, but special reflection.
Yes, there'e ordinary reflection and there's special reflection . The first is non-dazzling, the second is, wait for it, dazzling, if only slightly. Here's an example of the first, non-dazzling reflection, encountered on my recent travels:
|Vaud canton, Switzerland near Lake Geneva, April 2014. Shame about the absence of sunshine that day (one tanned instantly the preceding day).|
Here's a more striking example - Mirror Lake, Oregon - but still "ordinary reflection" for present purposes.
|Here's the geometry set out in a survivalist guide to attract attention of an aircraft. A signal mirror is being deployed (hole in centre to assist with sighting and alignment).|
The mirror in the above diagram needs to be correctly oriented in two dimensions. It has to be tilted upwards towards space and have the correct orientation in a horizontal plane, level with the horizon, for refelected light to reach the chosen target (a passing aircraft). In practice of course one does a lateral sweep such that the intended observer receives flashes of light (spotted more readily than a fixed beam). What's the relevance to those glossy Photinia leaves? The answer should be, er, glaringly obvious. Photinia has to be smart in the way it orients its stalks and leaves if it is to function as a deterrent to grazing animals that can have an angle of approach on a radius that sweeps 360 degrees of arc. The question is therefore: ARE Photinia plants designed to "glint" a 'hazard warning' at herbivores no matter what the angle of approach?
But Photinia is not trying to catch the attention of high flying aircraft, but ground-grazing herbivores. How can ot do that most effectively? is it by "ordinary" or "special reflection"? You can probably guess what's coming, and yes, it's to do with the sun acting not merely to illuminate everything on which it falls, but as an incandescent orb in the sky that can be projected as a virtual image from ground level mirrors - the latter including those glossy Photinia leaves - into the eyes of herbivores. Would the abiility of leaves to act as tilted mirrors be left to chance, or would there be evidence of specific adaptation for that purpose?
Here's a telling passage gleaned from a plantsman's catalogue (my bolding):
Others too have commented on "loose" growth in a variety of terms, which one suspects is a euphemism for "a bit untidy looking, not the correct choice if what you are seeking is a geometrically-precise box hedge".
Here's an archive picture that taken with the above helps build the case for what I'm about to suggest:note the sizeable proportion of 'mirror-like' leaves that reflect the sky and (probably) in some cases the sun itself (even if a discernible image of the sun is not visible, merely hinted at)?
OK. let's return to the mainstream first - with the attention focused on those bright red leaves. Theories abound. Some say they are a protection against damaging radiation which, as indicated, hardly squares with visible colour. Others suggest that herbivores that grazed on Photinia in its natural habitat were colour-blind to red, so would have ignored red leaves. Maybe, but where's the hard evidence?
There's another explanation - to do with the presence in Photinia leaves of cyanogenic glycosides. They are protective chemicals that aren't just feeder-unfriendly re taste. As the name implies, they release hydrogen cyanide (HCN) in the gut, especially of ruminants with multiple stomachs that nibble foliage on an industrial scale. HCN, the stuff of execution gas chambers, blocks respiration at the cellular level, producing the mother of all bad turns, the kind that is usuaally described in medical and no doubt veterinary jargon as death.
But maybe Photinia has adopted a belt-and-braces approach, reasoning (if plants under evolutionary pressure, to say nothing of personal survival when there is no fight or flight mechanism) can be said to reason, that red color in the leaves is not a sufficien t defence. After all, there are any number of plants with red leaves, notably in autumn, but in spring and summer too.
So maybe Photinia adopted an extra defence in the form of shiny leaves. How would that work in practice? i guess one could take a leaf from the "colour is bad news" school of thought, and suggest that anything unusual about appearance, whether colour or shininess, aids survival if there's a toxin present. Or it could be a lethal one that simply wipes out grazers with a genetic predisposion to prefer red over green, say. Or it could work more subtly, and simply give the grazer a bad headache, such that it forms an association ("red is bad" or "shiny is bad" so the initial bad experience is less likely to be repeated.
One is hesitatnt to dismiss any "evolutionary" kind of hypothesis that works slowly, maybe through countless generations, slowly modifying the gene pool to shift the odds in favour of survival. So if shininess is added to leaf colour as a deterrent to feeding it might well operate by that same mechanism of simply conferring a "difference" that in time is perceived as a threat to survival. One cannot help but wonder if there's not a more immediate warning that comes with shiny leaves that glint in the sun, especially when the wind blows. foliage becomes agitated.
What are the natural enemies of herbivores, excepting toxin-laden plants. Carnivores, obviously.How is the approach of carnivores detected? Hearing and small, obviously. What if the wind is blowing, and both hearing and small are compromised? what is the last line of defence, especially against a stalking predator? Answer - sight. And which part of the anatomy is the hardest for a predator tpo conceal if sneaking up, keeping low to the ground?
Answer - the eyes, the light-reflecting eyes. Eyes glint.
Might a Photinia shrub, with its glinting leaves, maybe shifting in the wind, given the "growth that is somewhat loose" make grazing animals nervous of approaching too closely? Is it too much of a distraction, masking the presence of 'real eyes'?
Natural habitat? Tricky, since given that the common-or-garden Photinia x fraseri reported to be a cross between two species. Viz:
Why is that?
Sorry about the scrappy ending - down to that poorly-sited Publish key that is so easy to hit accidentally.
The order of tabs should arguably be changed to SAVE, PREVIEW, PUBLISH, CLOSE. Certainly SAVE (in draft) should be well-separated from PUBLISH.
I'll tidy things up in a day or two (busy right now). The hypothesis being proposed here is that the somewhat open 'untidy' look about Photinia (the smaller specimens especially that are still getting established) is perhaps no accident - it's the result of leaves and stems being oriented generally upwards but through a wide range of angles to ensure that a sizeable proportion of leaves are always reflecting the sun towards an approaching herbivore, thus flashing a warning signal so to speak: this plant is a threat to your health and wellbeing.