When Is Luxury N Uptake Actually Luxurious? a 15N Approach in Maize Experiencing Post-Silking Water and/or N Stress.

It has been hypothesized that increasing the amount of nitrogen (N) stored in the vegetative biomass of maize can help buffer kernel abortion and kernel weight against water and/or N stresses experienced during grain-fill. This hypothesis rests upon the idea that the remobilization of N from vegetative tissues provides a stable source of N to meet grain demand during stressful periods. To test this hypothesis a greenhouse study was conducted, where we impose water stress, N stress, or both, on maize plants either before silking during vegetative growth, or after silking during reproductive growth. ¹⁵N, a stable isotope tracer, was delivered either before or after silking, which allowed for the accurate quantification of both i) N remobilization, and ii) the partitioning of post-silking N uptake between different plant organs. Maize was grown in pots filled with inert calcined clay, which allowed for both water and N availability to be carefully regulated, and for the availability of ¹⁵N for plant uptake to be tightly controlled through leaching. Green leaf area, senescence and photosynthesis measurements were also taken. Implications of this hypothesis are that it may not be possible to determine luxury or economically optimal N uptake during vegetative growth, because in years where stresses are experienced during grain-filling, it is desirable for maize to have accumulated higher N concentrations than if grain-fill occurred in nutrient replete conditions