Physiological mechanisms underlying nitrogen remobilization in the vegetative organs of maize in response to sink strength

Abstract

While N remobilization is well-studied, the mechanisms by which grain sink strength feeds back on presilking-accumulated N remobilization, storage, and assimilation remain less well understood. This study aimed to determine how grain sink strength influences the presilking-accumulated N remobilization and the regulation of amino acid transport and nitrate assimilation, by examining two sink strength treatments (normal pollination, 3/4 grain removal), using two high-yield maize hybrids (ZD958 and XY335), over 3 years. Presilking-accumulated N remobilization dynamics in vegetative organs and grain N accumulation were assessed using ¹⁵N field-tracer labeling, along with measurements of N components and enzyme activities. The use of a ¹⁵N double-labeling assay revealed that reduced sink strength greatly decreased presilking-accumulated N remobilization in the leaves, stem, husk, cob and roots. Sink strength did not regulate protein decomposition in the leaves, stem, or leaf sheath but mainly regulated the import and export of amino acids (which come from the leaves) in the stem post-silking. When the sink strength is full, excess N in the form of amino acids is stored in the stem, and some amino acids synthesize proteins in the cob for cob growth. Nitrates absorbed post-silking might be assimilated, mainly in the stem and cob, regulated by the GS/GOGAT pathway. The negative feedback by reducing sink strength regulates the GS/GOGAT activity in the stem and cob, which weakens nitrate assimilation, resulting in nitrate accumulation. These findings suggest that sink strength does not directly regulate N decomposition in vegetative organs but only regulates the import and export balance of N in the buffer sinks (stem and cob). However, sink strength can directly regulate the GS/GOGAT-mediated nitrate assimilation process in stem and cob, which affects the total amino acid capacity of the buffer sink and might provide feedback on postsilking N uptake.