Have you ever wondered why your plants don't grow in winter? Or why their leaves suddenly turn yellow, even though you water them well? You just want to make your plantbabies happy – we get that! And we are here to explain the role light plays in all this, and why it is absolutely vital for your plants to get the right amount of sunlight.
Here’s what’s ahead:
The Sun. Literally no life would exist on earth without it.
We feel this especially in the wintertime, as the days get shorter and the temperatures drop. But when spring comes and the sun finally wakes up from its seasonal nap, we feel a different energy. Our mood changes and we try to soak up as much of the sun's rays as possible.
Plants are no different. You have probably noticed how they will often turn their flowers and leaves towards the sun. They aren’t shy about how much they love it. But unlike humans, plants do not need the warm rays of light to produce endorphins. Plants need sunlight to produce energy on the most fundamental level; without it, all biological processes would come to a standstill, and the plants would die.
So here is how you can tell if your plants are suffering from too much or too little sunlight:
Too little sunlight: no growth or little growth
Too little sunlight: plant loses leaves
Too much sunlight: yellow leaves, brown spots on leaves (think of this as a “plant sunburn”)
But how exactly can we help our plants to live their best lives? Let's get out the magnifying glass and have a closer look.
There is an expression that goes, “some people eat to live, others live to eat”. But technically, we all eat to live. Animals and humans need certain nutrients in order to stay alive, and the primary way we obtain these vital nutrients is through food.
Plants, on the other hand, are autotrophs, which means they produce their own "food". And whereas your day may start with a luxurious cup of coffee and some avocado toast, your plants’ needs are much more fundamental, available in the naturally-occurring elements oxygen, carbon and hydrogen. More specifically, these nutrients come in the form of carbon dioxide (CO2) and water (H2O), and with sunlight as the catalyst, plants can produce what they need to survive: energy in the form of adenosine triphosphate (ATP).
The production of ATP takes place in the leaves of plants, specifically in the chloroplasts and mitochondria. The former are responsible for photosynthesis, the latter for cellular respiration.
Photosynthesis only takes place when the plant is supplied with light. This first part of the process is extremely important, because in this process glucose (simple sugar, dextrose or C6H12O6) is obtained.
Light-dependent reaction: The chloroplasts are activated by the sun's energy. They use the energy boost to break down water (H2O) molecules that the roots absorb from the soil. Oxygen (O2) is released into the air as a waste product, while hydrogen (H2) is retained.
Light-independent reaction: This part of photosynthesis is also called the “dark reaction”, since it does not require direct light energy. The plant absorbs carbon dioxide (CO2) from the air through gaps on the underside of its leaves. At this point, hydrogen (H2) and carbon dioxide are converted into glucose (C6H12O6) in the chloroplasts using energy (ATP).
Side note: Although this process is independent of direct sunlight, it requires heat. Glucose production increases up to 35 °C, but stops at 40 °C.
Plants produce simple sugars (glucose, C6H12O6) and oxygen (O2) from sunlight, carbon dioxide (CO2) and water (H2O). While keeping the sugar for themselves, they release the oxygen into the air.
In brief: CO2 + H2O + Light = C6H12O6 + O2
So your plant has now made glucose: Hurray! And glucose is nothing but pure energy. But only if you split the chemical compound. This splitting (= burning) now takes place in the mitochondria and is called "cell respiration". This produces ATP, i.e. energy that can be used by the plants.
All our pyromaniac friends out there know that you need air to burn things, and cell respiration is no different. Burning glucose requires oxygen (O2). Plants use the crevices on the underside of their leaves to get this important component. Put simply, oxygen starts the process that breaks down the glucose - releasing heat, energy (ATP), CO2 and H2O.
In brief: C6H12O6 + O2 = CO2 + H2O + Heat + ATP
The energy gained as ATP is used for all other processes: for the further use of the elements nitrogen, phosphorus, etc., for the production of starch, growth, the formation of seeds, and more. It literally allows the plant to carry out basic functions.
Unlike photosynthesis, cell respiration does not only take place during the day, but nonstop. That is why it is so important that the plants get enough sunlight to be able to carry out photosynthesis: It is the only way they can produce enough glucose for cell respiration to take place and vital energy to be generated.
By the way: You are more like your plant friends than you think! We humans – and all living things, in fact – also generate energy by processing glucose! But of course we get the oxygen we need for this with our noses (and sometimes with our mouths). And the glucose? It comes from…you guessed it: Plants! (or animals – but they also ate plants, soooo…)
You don't have to know all the individual chemical steps of photosynthesis and cell respiration by heart in order for your plants to survive. But it's good to understand the connections, because that's how many garden rules suddenly start to make sense. For example:
“Tomatoes need a place in the sun”: Tomatoes, peppers, aubergines and all other light-hungry vegetables can only be grown if there is a lot of sunlight; in the shade, they starve because they can't carry out photosynthesis to the extent required to generate the necessary ATP.
"Sowing should take place in March/April/May etc.": Sowing summer vegetables in autumn or winter is not very promising, because here too, there is a lack of light that the plants need to produce their own food.
"You should wait until the weather is good enough before sowing": A cold spring with little sunshine leads to slow plant growth.
"Don't put small plants in the shadow of large plants": If you plant large-leaved plants such as pumpkins or courgettes in front of small, light-dependent herbs, you can be sure that lavender and its herbalicious buddies will die. The large green leaf monsters of the pumpkin family rob them of the sunlight essential for survival.
"Seedlings should be in as bright a place as possible": Seedlings that are supposed to grow on a north-facing window will die. Few plants manage to get by with so little light.
"Don't fertilise in winter": There is no point in adding more fertiliser to the plants when they are "hibernating" and have too little glucose to process the extra portion of food.
"Water regularly/water when dry": A sufficient water supply is one of the prerequisites for a functioning photosynthesis.
Of course, light is not the only thing that plays a role in the success of your garden or the health of your plants. But it is one of the primary factors that determine whether or not your harvest is successful.
You see how, when we start paying closer attention to the biochemical reactions that sustain plant life, we can actually take care of our plants better and become even more passionate plant-lovers!
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