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[Q] What plant is the most efficient?

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If there is only a set amount of Co2 (let’s say 1000), is there any one plant that would get the most energy from those units then any other? Or is chlorophyll kind of like a constant for all plants?

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If efficiency is measured in terms of carbohydrate formed per CO2 molecule then I believe that all plants are equally as efficient.
Photosynthesis efficiency is usually discussed in terms of CO2 to carbohydrate conversion per amount of light, which varies greatly between plants and also depends on temperature, water availability, light intensity & and concentration of CO2. So one plant type might be more efficient under low light whereas another is more efficient under intense light.

C4 photosynthesis is more light efficient than C3 and works better under extremes of high light and low CO2. But it's not just the efficiency that you will find in science literature as this relates to what happens when a photon hits a leaf. You also need to consider that if a photon is not used by a leaf it may then strike another leaf and be used. Thus photosynthesis is often said to be 3 or 4% efficient but the whole plant may achieve an efficiency of double or triple that, in act nearly rivalling photovoltaic panels in terms of their efficiency in converting photons to energy, with the added benefit that PV panels don't store energy whereas plants convert it into stored chemical energy ie carbohydrate. Solar panels to save the world? No thanks I'd rather have trees.
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Plants don't get there energy from CO2. They get it from sunlight. They do however store extra sunlight energy as sugar (CO2 combined with water). The plants can then modify the sugars (glucose) to make cellulose. to grow taller. The sugars(stored sunlight energy) can also be broken down for energy (say at night time). Similar to what animals and fungi do with sugars/carbohydrates.  

Under perfect conditions, I think that all plants have just about the same level of conversion efficiency.  

But if it was blackish water, certain plant would be stunted or even die outright while others would be fine.
You might find that in soils with low nitrogen levels legumes do much better, than other plants.
Or with heavy deer pressure or maybe insect xyz, everything is stunted except for plant ABC.

But overall the answer would be legumes family(low mineral) and grass family (low water).
I do however find that grasses and legumes, die back to the "root" very quickly and so very little carbon is actually stored in their body.
All the carbon that they capture is eaten by animals, humans, insects, fungi and quickly turned back into CO2 when we exhale and methane when we fart.

Trees are better because they regularly grow to 40ft sometimes 300ft+ and they live for 40yrs sometimes 2000yrs+
So the carbon that they capture from the air is stored for sometimes until it rots (eaten by fungi) and is turned back to CO2.

My preference would be to see it turned into biochar. Due to the fact that after 50% of the carbon in the "wood" is turned into heat+CO2. the remaining half is not food for animals/fungi/plants. SO that carbon is actually taken out of the atmosphere for millions of years vs for just a season or 50yrs.

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