RCA enhances nitrogen acquisition in low N soil
Suboptimal soil nitrogen (N) availability is a primary constraint for crop production in developing nations, while in rich nations intensive N fertilization carries significant environmental and economic costs. Understanding root traits enhancing nitrogen acquisition is therefore of considerable importance. Soil nitrogen is heterogeneous and dynamic. The availability of soil N to plants depends on the balance between the rates of mineralization, nitrification, and denitrification. These processes are determined by several factors including soil composition, microbial activity, soil temperature, and soil water status . The predominant form of soil nitrogen available to plants in most agricultural systems is nitrate, which is highly soluble in water and thus mobile in the soil [16,17]. Mineralization of organic matter and/or the application of nitrogen fertilizer at the beginning of the growing season followed by precipitation and irrigation cause a pulse of nitrate which may exceed the need of seedlings and leach away from the root zone . Therefore, increasing root production to explore deep soil strata could benefit nitrogen acquisition. However, the structural investments and metabolic expenditures of root systems are substantial and can exceed half of daily photosynthesis . Full consideration of the costs and benefits of root systems is therefore crucial for identifying root traits to improve crop production especially in water and nutrient deficient environments . Taking rhizoeconomics of root systems and spatiotemporal availability of nitrogen in the soil into account, Lynch (2013) proposed a root ideotype for enhanced N acquisition in maize called “steep, cheap, and deep”, in which ‘steep’ refers to architectural traits and ‘cheap’ refers to traits that reduce the metabolic cost of soil exploration. One element of this ideotype is abundant root cortical aerenchyma. The overall objective of this research was to assess the utility of RCA for nitrogen acquisition in maize under nitrogen limiting conditions. We employed maize recombinant inbred lines (RILs) with a common genetic background but contrasting in RCA formation under nitrogen stress and planted them in mesocosms and in the fields to test if RCA formation in maize lines grown on low N soils is associated with reduced root respiration, greater rooting depth and enhanced N acquisition.
Fig. 6. Rooting depth, defined as D95, which is the depth above which 95% of roots are located, of maize lines at 35 DAP in mesocosms and 63 DAP in the field in South Africa under low N conditions. Data shown are means of 4 replicates + SE of the mean. Different letters represent significant differences (p<0.05) within the experiment (Saengwilai 2013).
We found that N stress induced RCA formation. High RCA RILs had greater RCA than low RCA RILs by 360% in mesocosms, by 210 % in the field in South Africa and by 320 % in the field in Pennsylvania. High RCA RILs had greater rooting depth under low N consitions than low RCA RILs by 15% in mesocosms and by 30% in the field (Figure 6). High RCA RILs had greater shoot mass in low N soil than low RCA RILs by 66% in mesocosms and 52% in the field in South Africa and 31% in Pennsylvania (Figure 7). High RCA also associated with 70% increased in yield under low N conditions at the field in Pennsylvania (Figure 8). Our results are consistent with the hypothesis that RCA enhances N acquisition by reducing root metabolic costs, permitting greater rooting depth and enhanced N acquisition under suboptimal nitrogen conditions .
Fig. 7. Shoot biomass under high N and low N conditions at 35 days after planting (DAP) in soil mesocosms and at flowering (63 DAP) in the field at South Africa (SA) and Pennsylvania (PA). The data shown are means of 4 replicates ± SE of the mean. Different letters represent significant differences (p<0.05) compared within each location (Saengwilai 2013).
Fig. 8. Correlation of yield and percentage of root cortical aerenchyma (% of cortex) under low N conditions (R^2=0.16, p=0.05).