UC Davis

Nitrogen fertilization of agricultural fields sustains global food security, but less than half of the fertilizer applied reaches the table. Under elevated CO2 levels anticipated in the near future, food production suffers from crop protein decline and declining or stagnant yields, threatening human nutrition worldwide. The natural variation in the ability of crops to assimilate different soil inorganic nitrogen forms, nitrate (NO3–) and ammonium (NH4+), into protein remains unexplored and may provide solutions to this grand challenge.

This doctoral dissertation explores 3 major questions pertaining to wheat and rice, staple food crops that provide 32% of the protein in the human diet: (i) how have past genetic modifications and breeding improved grain protein concentration and yield? (Chapter 1), (ii) what is the extent of natural variation in the ability to utilize different nitrogen forms for growth and development? (Chapter 2 and 3), (iii) how does adaptation to specific nitrogen sources influence crop responses to changing climates? (Chapter 3 and 4).

Chapter 1 summarized successful breeding attempts to modify nitrogen metabolism through genes that coordinate nitrogen and carbon metabolism. Gaps in our current understanding include (1) species differences among candidate genes in nitrogen metabolism pathways, (2) the extent to which relative abundance of these nitrogen forms across natural soil environments shape crop responses, and (3) natural variation in and genetic architecture of nitrogen-mediated yield improvement. The following chapters addressed these knowledge gaps.

Chapter 2 and Chapter 3 examined the growth responses to different nitrogen forms of rice and wheat populations that represent the genetic diversity within the global germplasm. Generally, most of the populations effectively use either ammonium or nitrate at moderate levels to support vegetative growth. Such plasticity may allow plants to be more resilient to fluctuations in soil nitrogen. Genome-wide analyses identified genetic markers associated with growth under different nitrogen sources that may be employed in future breeding programs.

Chapter 3 and Chapter 4 further evaluated the extent to which soil nitrogen sources alter wheat responses to atmospheric CO2 fluctuations using genotypes that demonstrated a preference for ammonium or nitrate, and contrasting degrees of ammonium tolerance. Nitrogen-form preference, but not ammonium tolerance, correlated with CO2 responses. Notably, ammonium-preferring genotypes maintained higher biomass and sustained grain nitrogen concentrations, thus avoiding CO2 acclimation, the decline in biomass stimulation after prolonged exposure to CO2 enrichment.

Overall, this dissertation provided strategies to guide matching crop genetic adaptations with fertilizer management to improve nitrogen-use efficiency and to sustain food security under the atmospheric conditions anticipated in the future. To continue improving grain yield and quality under changing climates, breeding strategies need to focus on both carbon and nitrogen assimilation. We recommend breeding for ammonium-adapted genotypes, which may not only improve climate resilience, but also potentially accelerate development and increase yield without any penalty on grain quality.