Plant perception
Plant perception
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What plants see
What plants see
Plants see in many ways in colors that we can only imagine.
E.g. plants can see UV light that give sunburns and infrared light that heats us up, but we can’t.
They know if there’s little light (a candle or if sun's just set in horizon).
They know what direction light comes from, if another plant has grown over them, blocking light, how long the lights have been on.
“Light” refers to a portion of electromagnetic spectrum that we can only see a small part called visible light.
These range from 380-400 (violet; shortest/highest light waves) to 700-750 (red; longest/lowest light waves) nm.
we can't see slow smaller waves (ultraviolet light) and x-rays, which go through us and from gamma rays coming from the universe.
We can’t see:
Longer light waves/longer electromagnetic waves, which are infrared heat waves.
Microwaves, radio waves that can reach a kilometer in length.
We only see a tiny part of electromagnetic spectrum.
E.g. If light comes off a baseball, it bounces off it, goes in through eyes and hits the retina, where it sends the signal to the optic nerve.
Retinas have 2 types of photoreceptor cells attuned for visible light: cone and rod cells.
attuned: adjust, adapt
Visible light is the range of electromagnetic wavelengths our eyes can see.
Cone cells help see in high light/daylight (photopic) vision.
- mmn: Cones cells are higher so they are high lights and have more diverse color/happy.
- Have 3 types of cones, each sensitive to a different color:
some see red
some see green
some see blue
- located in retina's central part
- Give high visual acuity and help to see fine details and colors.
2. Rod cells help see black and white and different shades of grey and help to see in low light/night (scotopic) vision.
- mmn: Rod cells are only rods used for fishing, so they are low lights and only have black and white/depressed.
- Sensitive to light and don't detect color.
E.g. red light that's seen is absorbed only by red cone cells and send signals to the brain that we translate as red color.
E.g. If purple light is seen, it’s absorbed both by red and blue cone cells and our brain translates as purple.
Darwin’s experiments
Darwin’s experiments
How do plants see?
Beans seedle: each 12 hours, light is changed from one side to another, the seed bend.
We see plants seeing light and respond by growing and bending towards the light.
Charles Darwin was among the first to study how plants react to light and bend.
Though he's famous in his work on evolution, he spent his last 20-30 years mostly with plant biology.
With his most crucial works published in 1880 with his son, Francis Darwin, called "The Power of Movement in Plants".
E.g. he noticed that if you put a type of grass called canary grass in a room with only a dim candle on one side of the room.
He wrote that it’s so dim that the clock on the wall can’t be seen.
The plant, here, the grass, bent towards the candle.
Tropism Experiment by Cameron Wright
He did this experiment on tens of plant types and concluded (in his own words): "Heliotropism (bending to light), prevails so extensively among higher plants, that there are very few of which some part, either stem, flower, peduncle, petiole, or leaf, does not bend towards a lateral light”.
Heliotropism etm: (Greek) "Helios" (ἥλιος) means sun + "tropos" (τροπή) means turn/direction.
Darwin was also an expert experimentalist, asking:
“Where's the eye of the plant?
Which part of it is senses the light to to bend towards it?”
He thinks the answer was the the tip of the plant.
Thus, his observation: if he puts lateral light from the side, the plant bends to it.
- He then chopped off the tip, the plant couldn't bend towards the light.
- He then covered its top with a black light impermeable hat - it didn't bend to the light.
It may be that the cap was too heavy to inhibits it from bending.
inhibit: to prohibit from doing
A 4th time: he covered the plant's tip with a glass cap, to let light go through it: the plant bends.
1880 - Darwin succeed to prove that it's the plant’s tip sensing light signal and it transfers light energy/signal, down to its bottom - where bending starts.
In his words: “We must conclude that if seedlings are exposed to lateral light, some influence is transmitted from the upper to the lower part, causing it to bend”.
In other words, he concludes that there’s some way that the tip senses light, transduces a signal to the lower part to bend.
It's not so different from how we see.
If you catch a baseball, your eyes sense light, and sent signal to your hand.
In phototropism, a plant’s ability to bend to the light, the tip senses signal and transfers it down to where it bends.
a picture actually taken from his book, in his own drawing The Power of Movement in Plants
Darwin showed in his experiment that plants sense light and respond by bending to it.
Next is a 19th century experiment done by Darwin's colleague, Jules von Sachs in Germany.
He asked “Can plants differentiate between different colors?” and showed, the same seedlings Darwin studied will bend to blue light, but not to red light.
Can plants be colorblind?
Lights and bloom
Lights and bloom
Early 20th century, in Maryland - Tobacco farmers found new strains called Maryland Mammoth, but instead of flowering in late summer and give seeds, it made more leaves.
strains: specific variety of a plant species
The farmers weren’t happy as it never flowered and died in winter.
It couldn't get seeds to propagate this plant, a big issue.
What caused it to stop flowering? is a question taken by US Department of Agriculture (USDA) scientists.
They found that if they put Maryland Mammoth plants in a shed in afternoon, they can be induced to start flowering, which is how they found photoperiodism - plant's response to changing day length.
induce: convince, cause, persuade
photoperiodism: plants' reaction to changing day length
etm: "photo-": (Greek) "Phōs" (φῶς)means light + "period-": (περίοδος "periodos") means a cycle/a repeated series + "ism", English suffix that indicates a condition/process
Short-day plants: plants that only flower if day length is shorter: e.g. chrysanthemums, soybeans, or the Maryland Mammoth.
Long day plants only flower as days get longer: e.g. irises or wheat - often plants that flower in spring/early summer.
Day neutral plants are a 3rd class that don't really show photoperiodism, which flower if plant gets to a certain size: e.g. tomatoes or dandelions.
Photoperiodism tells plants day length and when to flower in a season.
Now plants adapted photoperiodism according to where they grew wildly.
E.g. Middle of summer in Canada, days are longer than in Florida. But Florida's winter days are longer and warmer than in Canada.
E.g. With Maryland Mammoth dying in winter in Maryland, if snow on ground and put it in Florida, there short days induce flowering. And there wasn't the cold for it to die.
E.g. Each plants called cocklebuer grows in northern Canada and US. It’ll only flower under a long day.
If it grew in the southern US or southern Europe it’ll never flower.
Is it the length of the day? Or is it the length of the night?
Experiment done by USDA scientists: turn on lights in middle of night.
E.g if you put a short day plant, like chrysanthemum in conditions where it should flower in a short-day, but turn on lights in middle of night, you inhibit the flowering, enough to turn on lights for few minutes.
If you keep a long-day plant in conditions of short-day, these are conditions that will inhibit flowering.
But you turn on the lights in the middle of the night, again, just for a few minutes. You can induce a small day plant to flower under conditions of short-day.
Overall, we learnt is plants aren’t sensing how long day is, but how long night is.
In an agricultural viewpoint, this is crucial, as it means that e.g. flower growers can inducers plants to flower if they want irregardless of the conditions outside by changing lights of greenhouses.
We can get chrysanthemums flowering all year by manipulate light conditions.
20th century - An experiment in the same lab of USDA: they turned on different light colors and found that only 1 color can either inhibit or induce flowering.
A red light flash in middle of the night inhibit flowering in short day plant, which induce flowering in long day plant.
Blue light and any other color can’t work - only red light.
here we see like weak in differentiate between blue and red light, plants also differentiate between blue and red light,
After giving flash of red light if it's followed by a flash called far red light (about 730 nnm; light we barely see as sun goes down, it’s long wavelengths, we're almost blind to. That inhibit flash of the red light.
After giving flash of red light if it's followed by a flash called far red light (about 730 nnm; light we barely see as sun goes down, it’s long wavelengths, we're almost blind to. That inhibit flash of the red light.
Far red light inhibits red light’s action. So if give a red light, you inhibit the flowering of the short day plant.
If give red light and then far red light, it's like you never turn on lights to and will flower. Or if we look at a long day plant, you give red light it’ll induce the flowering.
But if you give red and then far red, you inhibit it.
We can continue: after red, we give far red, then again give red, you have either an Inhibited flowering in the short-day or induced it in the long day plant. Give again far red, you inhibit or you've canceled out the effect of the red light, in other words, they have a type of memory.
The plant remembers what last color is seen.
If give red light, for a long day plant, you activate flowering.
If give far red light, you inhibite the flower.
Give it a red light again, activating.
References
References
Quizzes
Quizzes
[Q1] Understanding plants
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