In addition to these key functions, roots play additional key roles in plants: they provide anchorage to plants! They create porosity and soil structure. They are also important organs for storage of sugars, especially in perennial plants. They are a major source of carbon, hence soil organic matter that is stimulating soil microbial and faunal communities. These functions shall not be ignored. Nowadays, with increasing concerns about carbon sequestration in soils, roots are key features for sequestration at depth. With increasing concerns about biodiversity, roots and their rhizosphere microbiome are of considerable interest.
Let us come back to coping with drought and nutrient limitations, e.g., nitrogen deficiency, which are major issues in wheat production, especially in Mediterranean agriculture. What could be desirable belowground traits? Obviously, rooting depth is a major one, as soils dry in the topsoil in the first place, when water is still available at depth. For nitrogen, as nitrate is susceptible to leaching, it also makes sense to develop deep roots to capture nitrate at depth and avoid nitrogen losses and contamination of belowground water.
Efficient capture of resources also requires branched root systems and fine roots, possibly symbiotic associations with mycorrhizal fungi. This becomes even more crucial for the least mobile nutrients such as potassium, phosphate and micronutrients. We shall not overlook physiological traits involved in plant nutrition and all the complex interactions at play in the rhizosphere.
So why are we not considering belowground traits? Because they are not visible, they require tedious investigations to be assessed, because they are difficult to measure!
And on top of that, they usually exhibit considerable plasticity. That is why they need to be further taken into consideration actually: For the sake of designing more resilient crops, that are more efficient at capturing belowground resources.