Nodal root growth angle influences nitrogen acquisition and competition in maize (Zea mays)

Background/Question/Methods

Maize ecosystems cover 1% of the Earth’s land area and have major impact on the carbon and nitrogen cycles. Upwards of 60% of applied nitrogen may be lost to volatilization and leaching causing unprecedented pollution. Root system architecture (RSA) is the spatial deployment of roots. Shoot-borne nodal roots form the majority of the maize root system’s backbone. Nodal root growth angle (NRGA) plays a large role in determining the distribution of maize roots in soil. It was hypothesized shallow angled roots would capture more nitrogen in the shallow layers of soil, while steeper angles would allow leaching nitrogen to be captured. Mixtures of maize genotypes having phenotypes contrasting in NRGA were hypothesized to decrease competition and increase stand level performance, as in other experiments on effects of functional diversity. Field and simulation experiments were conducted across a range of soil types and nitrogen levels to test these hypotheses. Six genotypes exhibiting shallow or steep NRGAs were planted as monocultures and mixtures in a clay soil in Pennsylvania and a sandy soil in South Africa in a split plot design with low and high nitrogen. SimRoot modeled similar scenarios across more soil types, nitrogen levels, and root system phenotypes.

Results/Conclusions

In the field, nodal root growth angles were found to vary across genotypes from 40 to 70 degrees from horizontal. The angle for each genotype had small variation, which suggests genetic control. At the same time, angles were found to be shallower in low nitrogen, suggesting some amount of plasticity in this trait. Plants with shallow nodal root growth angles were found to have 18% larger shoot mass at 60 days after planting in high nitrogen with split application, while all plants performed equally poorly in low nitrogen. Mixtures of maize genotypes with contrasting nodal root growth angles produced greater shoot biomass than expected based on monoculture mass, which is a similar result to many agricultural polyculture experiments and natural experiments involving mixtures of species. However, mixtures did not beat the biomass production of the best performing monoculture. Structural-functional simulation results further demonstrated the utility of NRGA and its interaction with soil types and N regimes. Maize mixtures were shown to exhibit complementarity.  In conclusion, optimal root system architectures are context dependent and nodal root growth angle is an important governor of maize nitrogen acquisition efficiency. Genetic monocultures may lack functional diversity needed to drive productivity and efficiency.