Potential Physiological Frameworks for Mid-Season Field Phenotyping of Final Plant Nitrogen Uptake, Nitrogen Use Efficiency, and Grain Yield in Maize

Improved phenotyping tools for simultaneously characterizing maize (Zea mays L.) genotypes with superior grain yield (GY) and N use efficiency (NUE) would be beneficial for breeding progress. Possible phenotypic predictors of the crowding intensity and N availability effects on maize plant N uptake, GY, and NUE were evaluated for different genotypes in two environments. Our objectives were to develop phenotyping framework tools to predict plant N uptake, GY, and NUE via (i) identification of important mid-season morpho-physiological traits (from a total of 80 parameters), (ii) assessment of correlations between predictive traits (principal component analyses identified 21 traits), and (iii) arrangement of key traits into sequential pathways of mechanistic functions (3 traits). Plant phenotyping measurements taken during vegetative stages were poor predictors of GY and NUE. Plant N status at silk emergence was strongly associated with grain components. At silking, the chlorophyll contents (Soil Plant Analysis Development [SPAD] readings) were highly correlated to leaf N concentration, and the latter with the N nutrition index (NNI). As expected, NNI fairly reflected plant N uptake at silking and correlated well to relative GY. Maize plant biomass and N uptake at maturity were predicted via stem volume estimation at silking. The latter predictive model accurately simulated both GY and NUE in other field experiments. Physiologically based frameworks for mid-season prediction of maize GY and NUE require further testing but hold promise.