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EasyBOven t1_j2caqfv wrote

Mostly, the reason is the part of the plant. The seed needs to have a lot of protein because when it germinates, it needs to create a lot of structures very quickly to get things moving. So any edible seed is a good source of protein. Legumes are easier to process than most other seeds because they're softer, so it's easier to get to the protein, but pumpkin or sunflower seeds, nuts, and wheat are also high in protein

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KainX t1_j2e8vga wrote

This is not the reason. Legumes are nitrogen-fixing-plants, nitrogen is needed to make proteins. Legumes, and other N-Fixers have a symbiotic relationship with bacteria in the soil that extract nitrogen from the atmosphere and turn it into a stable form of nitrogen for the plants.

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EasyBOven t1_j2eayw7 wrote

Other plants use nitrogen in useable forms in the soil, so while nitrogen fixation is important to be able to thrive in poorer soil, I don't see how it would categorically lead to more nitrogen used.

The nutrition facts I found have pistachios at 11.6g of protein per half cup, and black beans at 7.3g for the same volume. Measured by calories, black beans do a lot better, since pistachios have a lot of fat, but the pistachios have a lot of protein in them. You can see the same thing in other human-edible seeds.

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kct11 t1_j2ettth wrote

Yes, all plants put lots of protein in their seeds to give their offspring plenty of resources to get off to a good start. This does not mean that there are not major differences in protein content between the seeds of different types of plants. OP asked about legumes vs non legumes, not seeds vs other parts of plants.

Comparing protein content on a per cup of the food as it is eaten is going to give you a misleading answer to the question. We add water to black beans while cooking before we eat them, which increases the volume of the beans, so fewer beans and less of the protein they contain, fit in a cup after cooking than before. Eating a half cup of pistachios will leave you much more full than eating a cup of black beans. It is more meaningful to compare foods on a dry weight basis, so how much protein per 100g of dry peas, wheat, rice or nuts.

Peas and beans have 20-25g of protein (there are lots of different species, so some may be higher or lower), wheat is 8-15g (some may get higher, different types of wheat have different protein contents), rice is 10-12g. Tree nuts are in the 20g range, so close to legumes, but remember they are trees with huge root systems. It is not surprising their seeds are different from non legume annuals like rice and wheat.

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KainX t1_j2earhh wrote

Other answers so far are incorrect.

Legumes, and few other types of plants are categorized as Nitrogen Fixers.

Nitrogen is one of the elements required to make protein, and not necessary in fats or carbs

A nitrogen fixing plant is one that has a symbiotic relationship with one of a few specific types of bacteria. These bacteria make colonies attached the roots of these plants (shown in the link). The bacteria take nitrogen from the atmosphere (gas) and turn it into a form of nitrogen fertilizer that plants can use (liquid).

Legumes, and other N-Fixers essentially get free fertilizer, where the rest of the plant kingdom does not. These N-fixers have a surplus of nitrogen compared to other plants, so they can use it their seeds, assumingly as a form of energy storage, instead of relying on carbs or fats to store energy in the seed.

In fact, Legumes and other nitrogen fixers are a component to regenerative, sustainable agriculture, because they can produce nitrogen naturally, instead of us making N fertilizer out of fossil fuels.

tl;dr legumes and their bacteria buddies get free nitrogen, which is needed to make protein, while almost all other plants have to struggle to get nitrogen so they use oils and carbs to store energy in the seed instead.

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kct11 t1_j2eonsv wrote

This is a good answer, but I have one minor change:

Legumes don't get the nitrogen for free. They trade for it by giving the bacteria sugars and a place to live. When legumes grow in soil with plenty of nitrogen, they stop trading with the bacteria (if you dig these plants up, you won't find as many nodules where the bacterialive on the roots). Still, legumes have much easier access to nitrogen than other plants.

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White_Lord t1_j2cewqh wrote

>why do legumes have so much more protein than other plants?

They don't, if you're comparing the whole plants. Plants have different parts and we don't always eat the same thing. Sometimes we eat the plant itself (like the leafs) which is usually rich in fibers, sometimes we eat the fruit which is mainly carbs, sometimes we eat the seeds which are generally rich in proteins.

Lentils, beans and so on technically are seeds.

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kct11 t1_j2epb35 wrote

Grains such as rice, wheat and corn are also seeds and they have less protein than legumes on a dry weight basis.

Edit to address whole plant differences: If you look at forage crops, legumes such as alfalfa have much higher protein content in the whole plant than grasses.

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