Researchers at the Noble Research Institute have discovered how differences in root systems can result in deeper rooting, greater nitrogen uptake, and increased plant growth. Roots are central to the ability of crops to acquire and use nutrients from the soil (see Why Roots Matter article). Understanding how root systems operate is imperative to maintaining yields while decreasing fertilizer use, which could also save producers money.
Fertilizers are typically used on agricultural fields in order to boost soil fertility and crop production. However, too much fertilizer can cause pollution by running off or leaching into waterways or by releasing greenhouse gases.
Haichao Guo, Ph.D., postdoctoral fellow, and Larry York, Ph.D., assistant professor, conducted research in the root phenomics laboratory at Noble to investigate earlier observations that reducing the number of nodal roots in maize (corn) could have beneficial effects on plant growth in less fertile soil.
Maize, like other cereals such as wheat, produces roots first from the seed, but then roots also emerge from rings, or nodes, around the base of the shoot over time. By the time maize flowers, these nodal roots are responsible for the majority of the root system biomass. Earlier simulation, field and greenhouse work had suggested that in low nitrogen soils, plants that produced fewer nodal roots might perform better.
In order to validate this observation and learn more about the underlying mechanism, Guo and York grew a popular maize line in 5-foot-tall pipes in the greenhouse with both high and low nitrogen, sand-based media, similar to soil. As the plants grew, the media was brushed away from the base of the shoot. For some plants, no nodal roots were cut away. For others, a scalpel was used to cut away up to two-thirds (67%) of the nodal roots as they emerged. After 41 days, a heavy version of nitrogen (15N) was injected in the bottom of the pipes in order to measure deep root activity. The next day, shoots were harvested and the root systems were washed out of the pipes. The root systems were divided into the different types of roots, including the seedling root system (including the tap or primary root), and each ring of nodal roots separately.
Actual scans of the tap root from 4 - 5 ft (120 - 150 cm) deep for a plant with no nodal roots cut away (left) and a plant with 67% of the nodal roots cut away (right). Increased lateral rooting is clearly visible.
In the low nitrogen media, the plants with two-thirds of the nodal roots removed actually produced 450% greater root length in the bottom of the pipe and had substantially more lateral (or branch) roots throughout the pipe compared to plants with roots that were not cut. These results may seem surprising but were actually expected based on previous experiments and simulations. The overall root system mass did not change substantially, so these results indicate that when maize produces fewer nodal roots, other roots can instead grow faster and deeper while also producing more lateral roots.
In fact, this study showed for the first time that reduced nodal rooting greatly increased the depth and branching of the tap root, which may play an important role in capturing deep soil resources despite being less massive. Remarkably, these plants with reduced nodal root number had 52% greater shoot mass and took up 232% more heavy nitrogen, indicating the modifications to the root system enhanced nitrogen uptake and allowed the plants to better tolerate a low nitrogen environment.
“The point of our work isn’t that farmers should cut away roots to increase yields,” York said, “but that it may be possible to breed new varieties that invest in cheaper roots that require less energy from the plant to grow and that more effectively explore the soil to gather water and nutrients.”
For the Future
The root phenomics laboratory is pursuing the idea of cheaper roots that allow the plant to invest carbon more efficiently. One important cheap root trait is smaller root diameter, which is exhibited by lateral roots. Reduced nodal rooting allowed more of these cheap lateral roots, and generally this experiment supports the idea that these root traits could be used to increase fertilizer and water use efficiency by crops. However, maize and other plants may produce thicker roots for other reasons, like plant stability to prevent lodging or to have extra roots in case of root disease or herbivory. Future work will be needed to address these possible tradeoffs.