What the relation between nitrogen (or phosphorus) and roots grow ?It is a simple adaptation. Plants have to make the decision on how they have to allocate their resources (primarily carbon assimilated via photosynthesis). In the wet environment, they would invest more to the leaves to get more photosynthates. Roots don't photosynthesize as well as leaves, so they are wasteful. On the other hand, if there isn't enough water, plants don't get enough water for photosynthesis (and other functions), so they have to shut down the photosynthesis by wilting. So they have to invest the resource to roots. Plants have mechanisms to fine-tune and optimize root:shoot ratio. Nitrogen (and other macro nutrients) is another factor, and air humidity influences the ratio, too.
Breabantia, in general, the behavior of the plants is to optimize their growth rate. So if there is sufficient N (or other mineral nutrients), they reduce allocation to roots. When one of the mineral nutrients, they invest on root production. There are fair amount of research about this, but here is an open-accessible one:What the relation between nitrogen (or phosphorus) and roots grow ?
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