EasyBOven t1_j2caqfv wrote on December 31, 2022 at 4:36 AM

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

KainX t1_j2e8vga wrote on December 31, 2022 at 4:47 PM

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.

EasyBOven t1_j2eayw7 wrote on December 31, 2022 at 5:01 PM

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.

kct11 t1_j2ettth wrote on December 31, 2022 at 7:07 PM

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.