Legumes have root nodules that are essential for what process?

Legumes have root nodules that play a crucial role in nitrogen fixation, a process whereby atmospheric nitrogen is converted into a form that plants can utilize for growth. These root nodules house symbiotic bacteria, specifically rhizobia, that help convert nitrogen gas (N₂) from the atmosphere into ammonia (NH₃), which the plant can then use to synthesize essential amino acids and proteins. This relationship is mutually beneficial; the plant provides carbohydrates and a suitable environment for the bacteria, while the bacteria provide the necessary nitrogen compounds that legumes need.

The other processes mentioned, such as water absorption, oxygen production, and carbon capture, do not involve the root nodules or the symbiotic relationship with rhizobia. Water absorption primarily occurs through the plant's roots, while oxygen production and carbon capture are related to the plant's photosynthetic processes, which take place in the leaves and do not directly involve root nodules.