No abstract

Summary• Here, nitrogen management within the plant was compared in an early-senescing maize hybrid and in a late-senescing maize hybrid, both grown under field conditions with a high fertilisation input involving large quantities of fertiliser.• We monitored, in representative leaf stages, the changes in metabolite content, enzyme activities and steady-state levels of transcripts for marker genes of N primary assimilation, N recycling and leaf senescence.• The hybrids differed in terms of persistence of leaf greenness, the expression of marker genes and the concentration of enzymes used to describe the transition from N assimilation to N recycling. The transcription of leaf-senescence marker genes did not differ. Agronomic studies confirmed the ability of the late-senescing hybrid to absorb and store more N in shoots.• Despite the differences in the mode of N management adopted by the two hybrids, we conclude that leaf senescence occurs independently of the source-to-sink transition at the high level of fertilisation used involving large quantities of fertiliser. The possibility of improving N metabolic efficiency in the latest maize hybrids is discussed.

To illustrate the development of the source‐to‐sink transition in maize leaves during the grain‐filling period, an integrated physiological‐agronomic approach is presented in this study. The evolution of physiological markers such as total leaf nitrogen (N), chlorophyll, soluble protein, amino acid and ammonium contents was monitored from silking to a period close to maturity in different leaf stages of three maize genotypes grown at high and low levels of N fertilization. In addition, the activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH), two enzymes known to play a direct or an indirect role during leaf N remobilization, were measured. In the three genotypes examined, we found that a general decrease of most metabolic and enzyme markers occurred during leaf ageing and that this decrease was enhanced when plants were N starved. In contrast, such variations were not observed between different sections of a single leaf even at an advanced stage of leaf senescence. We found that there is a strong correlation between total N, chlorophyll, soluble protein and GS activity, which is not dependent upon the N fertilization level, which indicates the N status of the plant, either in a single leaf or during ageing. In contrast, ammonium, amino acids and GDH activity were not subject to such variations, thus suggesting that they are indicators of the metabolic activity of the whole plant in response to the level of N fertilization. The use of these markers to predict the N status of maize as a function of both plant development and N availability is discussed.

No abstract

No abstract