Metabolic Responses to Contrasting Levels of N Fertilization during Ear Leaf Transition from Assimilation to Remobilization at Grain Filling in Maize

Abstract

MAIZE grain yield is shaped by source-sink relationships during grain filling. Such relations are driven by complex metabolic changes, responsive to N availability and are not fully understood. Here, the impact of limited and sufficient N fertilization on the metabolic interconversions in the ear leaf of a new maize field-grown hybrid (Tzi8 × Mo17) during its critical transition from assimilation to remobilization were chronologically investigated at 0, 5, 10, 15, and 20 days after pollination. N-deficient plants produced 43% less grain yield and 13% less biomass however, they had 51% higher root dry weight and higher Root/Shoot ratio at anthesis. The low N-induced reduction in yield and biomass accumulation was associated with earlier chlorophyll degradation and overall decrease in leaf chlorophyll, total soluble proteins, carbon (C: sucrose & total soluble sugars), and N assimilates (leaf N, nitrate, ammonia & amino acids). In contrast, N-deficient plants accumulated 18% more starch and 24 % flavonoids than N-sufficient plants and such responses were driven by low N-induced sink limitation. N-deficient plants also had significantly higher activities of N remobilizing (asparaginase & protease) but lower activities of N assimilating (nitrate reductase & glutamine synthetase) enzymes compared to N-sufficient plants. Glutamate and aspartate followed by branched amino acids dominated the amino acids pool under both N conditions. Altogether, the accumulation of starch and flavonoids and the induction of N remobilizing enzymes represent low N-specific responses whereas the rest of responses depict the common metabolic interconversions between adequate and limited N-induced responses during maize grain filling.