No field trip this week. Travel…so an early post from the lab instead wild plants from the forest.
Gadgets reveal how nimble plants are in responding to environmental cues. Usually slowly but sometimes zippy…they wiggle, waggle, adjust, turn on and off metabolic processes, and reconfigure. The complex interplay of chemical and electrical signals in plants is coming into better focus scientifically, and is astounding with no need to jump onto the goofy “plant intelligence” bandwagon. Plant are far more dynamic than we tend to think traditionally. But intelligent?—get real.
In this blog there has already been a good bit of attention to leafy behavior:
Shade leaves and variegation: CLICK
Nyctinasty (folding at night): CLICK
So today, more of all that with the help of time lapse photography condensing stretches of hours to seconds.
That many plant collapse leaves at night is clear to anyone with a flashlight. Daytime movements are more surprising. The relationship between foliage and sunbeams and shadows is not merely a matter of “collect all the sun you can.” There are three strikes against such simplicity:
- Excess light hitting a leaf can diminish photosynthesis, even damage the leaf.
- Bernie Sanders would agree that if the elite leaves at the top of the plant grab all the light, those below get cheated. If Ronald Reagan had debated Bernie though, The Gipper might have contended that top leaves let light trickle down. Foliar positional adjustments through the day can optimize the light economy. Not too much for those greedy CEOs, and not too little for the struggling rank and file.
- A plant’s response to any one environmental condition is intertwined with other factors. For example, a leaf on a sunny day is at risk from too much light per se, and also from heat. Only about 1% of the light striking a leaf powers photosynthesis, most of the rest converts to heat. Leaf temperatures often read well above the surrounding air temperatures. Stresses come hand in hand.
Now the stage is set so we can look at daytime leaf adjustments to light conditions by reversible movements.
Some plants orient their leaf blades to track incoming light. Think of a solar energy facility keeping its panels aimed at the sun. Leaves able to orient to face the sun can gather angled light early and late in the day, or from a low-hanging winter sun. They maximize photosynthesis when the sun is indirect, and they may reorient away from it when overhead and too intense.
This Sand Mat was photographed in late afternoon, then again the next morning. At both times the leaf blades faced the sun directly, morning and night. The afternoon photo follows the sun in directly onto the faces of the blades. The morning photo along the same line defined by the afternoon sun shows the blades now tilted to face the morning sun.
Strongly sun-tracking leaves sometimes reveal their adjustments by what I call the “Egyptian dance” position…the blades on the side of the stem toward the sun with tips down, in contrast with those across the stem from the sun with tips up.
Leaves that orient more or less vertically can take in plenty of angled sun morning, evening, and winter, and may or may not face the mid day sun. Desert plants, such as agaves, and many plants in bright hot places have vertical leaves.
The vertical arrangement allows light to penetrate deep into the plant. In some species upper foliage may be vertical (and/or small), while lower leaves at risk for under-exposure are more horizontal (and often larger).
One way to adjust to the spotlight is by reorienting part of the leaf, by cupping, that is, tilting the edges inward all around. Interestingly, in this Desmodium the variegated region is the central portion of the blade unable to tip into a sloped configuration, the one zone under continuous heavy illumination. (Variegation may in part exist to protect leaf surfaces from excess light intensity.)
A made-up term today A walk in any hot sunny scrub quickly reveals a common phenomenon in many species: leaf blades twisted, held at varied angles, and undulate-crisped like fried bacon. Crazy leaves. Some meadow, marsh, or desert-ish plants do it too. Pathological conditions can cause curling but we’re not talking about that here. So can severe drying. But bacon leaves in sun-drenched habitats appear to be adaptive. Those wacky twists, turns, curves, bends, and curls prevent full frontal sun exposure yet at the same time assure a good measure of exposure at any moment, no matter where the sun is. As the sun moves across the sky the plant is always catching a tan somewhere but a blistering sunburn nowhere.
Maybe more importantly, foliar contortions prevent the full force of the wind from sweeping water away directly from the blade surface. The convolutions are partial windscreen.
The wavy leaves may offer a compromise between “normal” and “desert” structure. Sun-protection and wind protection, yet also broad surfaces for gas exchange, evaporative cooling, and sun-gathering.
The crinkled configurations could be hard-wired and evolutionary. Or a growth response. Or reversible short-term adjustments. Or a mix. (Or none of these things.) Designing experiments to find out would be easy.
Dancing in the Sunlight
Leaves tracking the sun. Before becoming interested in the topic I assumed that, well, the sun-trackers turn to follow the sun across the sky. Not untrue, but way too simple. Time lapse photography of several species reveals how complex the sun-influenced leaf movements can be. In most species I’ve photographed there’s a fancy dance between leaf blades and sun.
In the Erythrina time lapse below, the camera faces south as the sun crosses the sky all day from east to west, left to right. You might agree the overall average drift is subtly left to right, but that is obscured by constant adjustment with all the blades waving in sync to and fro in a solar current. (The time lapse was indoors with no wind. When the same plant was placed in constant light there is no movement.)
Is it possible to use light to reorient leaves? Yes. This green bean sat in the dark with light coming first from the left, then the light did a 180 to directly from the right. CLICK
I wonder if the leaves move on a night with a bright full moon.