eli5: Why do seeds need indirect light while propagation?

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Not just to seeds but indoor plants in general as well. But what I want to know is why seeds need an indirect light source? wouldn’t an direct source give them more energy? Or will that dry them out?

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4 Answers

Anonymous 0 Comments

These are pretty broad questions, as different plants need different light and have different ways of handling it, but I’ll try!

Most species’ seeds don’t actually need light to sprout – there’s a common technique of putting them in a wet paper towel, for example, or just think of all the seeds that people plant by covering in dirt. They sprout just because the temperature is warm enough and they have enough water, and don’t need light at that stage because there is energy stored in the seed to get them started. Once they’re seedlings with leaves they do need light of course, but it actually depends on the type of plant, some need direct and some indirect.

Common indoor plants were chosen as houseplants because they can survive the indirect light of the house, but some of them will still grow faster in direct light, they just don’t die in indirect. Others have adapted to the indirect light of their native habitat (like a shady forest floor) by trying to maximize the amount of light they take in, for example by having really wide thin leaves so there’s a really big area to catch the light. But for the leaf to do the chemical reaction of photosynthesis, it has to let in air through openings that also let water escape from the plant. So those plants do get dried out from direct light, like you said, because their strategy to let in more light also lets out more water, and thin leaves aren’t as protected from damage.

Anonymous 0 Comments

These are pretty broad questions, as different plants need different light and have different ways of handling it, but I’ll try!

Most species’ seeds don’t actually need light to sprout – there’s a common technique of putting them in a wet paper towel, for example, or just think of all the seeds that people plant by covering in dirt. They sprout just because the temperature is warm enough and they have enough water, and don’t need light at that stage because there is energy stored in the seed to get them started. Once they’re seedlings with leaves they do need light of course, but it actually depends on the type of plant, some need direct and some indirect.

Common indoor plants were chosen as houseplants because they can survive the indirect light of the house, but some of them will still grow faster in direct light, they just don’t die in indirect. Others have adapted to the indirect light of their native habitat (like a shady forest floor) by trying to maximize the amount of light they take in, for example by having really wide thin leaves so there’s a really big area to catch the light. But for the leaf to do the chemical reaction of photosynthesis, it has to let in air through openings that also let water escape from the plant. So those plants do get dried out from direct light, like you said, because their strategy to let in more light also lets out more water, and thin leaves aren’t as protected from damage.

Anonymous 0 Comments

The answer is that it varies dramatically depending on species

Some species’ seeds are more likely to sprout after light exposure, some are indifferent

You can trick some species into sprouting by turning up soil such as tilling, exposing to light, raising chance of sprouting, and intentionally sprouting them too early or exposing them and then burying them.

There are blue light sensors on some seeds that make sprouting more likely

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6928806/

Exposure to light also helps avoid etiolation that can weaken stems and waste energy

https://en.m.wikipedia.org/wiki/Etiolation

It’s not so much that the young plant needs the energy, it’s more about managing the growth process with respect to light sensitive growth decisions the growing plant makes. The plant will do weird maladaptive stuff if it cannot tell where its light is coming from, even if it doesn’t need the energy yet and is still running on seed energy.

Also direct sunlight is difficult to manage and can kill a young plant. Same with a human, I’m sure a newborn gets sunburnt more easily than an adult

Direct strong sunlight causes all sorts of problems inside leaf cells and plants need to spend a lot of energy managing the excess light energy. Too much light can lead to reactive oxygen species forming that damage stuff and plants need to expend a lot of energy managing it. It starts overloading the chloroplast ETC stuff. And yes, also drying up and losing water. They waste more water and are more prone to drought when sunlight is very intense. It’s just hard for young plants to manage the excess light. Keep in mind that the young plants ideally start sprouting early in the season before the light intensity is very high so they ideally should be large enough to handle lots of light by the time summer rolls around. So when outdoors this problem is often solved simply by sprouting in spring. And if a plant sprouts in spring before sunlight is super intense, there’s no reason to evolve super hardy seedlings that can manage intense sunlight. I’m sure there are exceptions and some seedlings are great at it though.

Anonymous 0 Comments

The answer is that it varies dramatically depending on species

Some species’ seeds are more likely to sprout after light exposure, some are indifferent

You can trick some species into sprouting by turning up soil such as tilling, exposing to light, raising chance of sprouting, and intentionally sprouting them too early or exposing them and then burying them.

There are blue light sensors on some seeds that make sprouting more likely

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6928806/

Exposure to light also helps avoid etiolation that can weaken stems and waste energy

https://en.m.wikipedia.org/wiki/Etiolation

It’s not so much that the young plant needs the energy, it’s more about managing the growth process with respect to light sensitive growth decisions the growing plant makes. The plant will do weird maladaptive stuff if it cannot tell where its light is coming from, even if it doesn’t need the energy yet and is still running on seed energy.

Also direct sunlight is difficult to manage and can kill a young plant. Same with a human, I’m sure a newborn gets sunburnt more easily than an adult

Direct strong sunlight causes all sorts of problems inside leaf cells and plants need to spend a lot of energy managing the excess light energy. Too much light can lead to reactive oxygen species forming that damage stuff and plants need to expend a lot of energy managing it. It starts overloading the chloroplast ETC stuff. And yes, also drying up and losing water. They waste more water and are more prone to drought when sunlight is very intense. It’s just hard for young plants to manage the excess light. Keep in mind that the young plants ideally start sprouting early in the season before the light intensity is very high so they ideally should be large enough to handle lots of light by the time summer rolls around. So when outdoors this problem is often solved simply by sprouting in spring. And if a plant sprouts in spring before sunlight is super intense, there’s no reason to evolve super hardy seedlings that can manage intense sunlight. I’m sure there are exceptions and some seedlings are great at it though.