Georgofili World

Newsletter of the Georgofili Academy

The role of legumes in limiting agricultural consumption of fossil fuels and the greenhouse effect

The invasion by fungi and bacteria of vegetables does not always result in a diseased state. On the contrary, some invasion types, i.e., when micro-organisms break into radical cells, are beneficial. By supplying essential nutrients, micro-organisms help both the host plants and crop rotations if not even entire agricultural systems. In certain symbiotic interactions, the roots are infected with mycorrhizal fungi that help plants acquire more phosphorus from the soil. In other agricultural symbioses, the rhizobia in the roots of such legumes as peas, soybeans and alfalfa (PHOTO) produce the nitrogen form necessary for growth. Recently, some biochemists and agronomists have begun to develop a thorough knowledge of the genes that determine the onset of these mutual interrelations. It was found that both the plant’s and the microorganisms’ genes contribute to the symbiosis. The two partners are in fact involved in a complex molecular conversation.
The "fungus-plant" symbiosis was established well before the "rhizobium-legume" association. The presence of useful fungi in fossil plants indicates that the first associations date back to about 400 million years ago. Legumes instead date back to about 70 million years.
In establishing this dialogue, the plants first "talk", beginning a molecular conversation with the specific bacterium needed for their symbiotic interaction. The condition for which different legumes produce different biochemical messages (flavonoids) helps to create different specificity of a symbiotic action, with the rhizobium itself contributing to this specificity. Therefore, once the specifics have been met, the symbiosis can be activated and the "conversation" can continue. There are fluctuating messages that continue until the end of the mutual discussion.
This plant type has the specific ability to fix atmospheric nitrogen, which is a renewable resource, through the formation of nodules within which the relatively specific microsymbiont radicals are hosted (rhizobia). Biological nitrogen fixation is an efficient, large-scale process; the ammonium production via "Rhizobium-legume" symbiosis is higher in industrial production of synthetic nitrogen.
Using nitrogen fertilizers we consume fossil energy, with the equivalent (in energy) of two tons of petroleum required to produce and spread a ton of nitrogen fertilizer. Instead, the use of organic nitrogen, like the one fixed by the Leguminosae to produce vegetable protein, permits considerable consumption savings of non-renewable energy and therefore reduces agriculture's contribution to the greenhouse effect.  The cultivation of a legume leads to conserving about 0.2 tons of petroleum per hectare, equivalent to a CO2 production of 600 kg/ha. Legumes are therefore one of the pillars of sustainable development of agricultural systems. It has been estimated that 20-30% of the energy costs required to grow plants is absorbed by the nitrogens produced and distributed in the field.
Lastly, it should be remembered that legumes promote diversity and efficiency in crop rotation and contribute significantly to maintaining the fertility of agricultural land. Legumes also contribute to the wholesomeness of food in general and are an integral part of the so-called "Mediterranean diet". Although still under-represented in European agriculture today, legumes instead deserve to be developed further, in relation to their numerous advantages.