Nitrogen deficiency impacts on cell and tissue structure changes associated to leaf senescence in Brassica napus

During leaf senescence, improved remobilization of nutrients from vegetative to growing organs may represent an adaptive response of crop plants to nitrogen (N) stress. Recently, it has been demonstrated through NMR relaxometry and light microscopy that leaf senescence was associated to a process of cell and tissue structural modifications leading to cell enlargement and rehydration [1-2]. The aim of the present work was to investigate by NMR the impact of N deficiency on these changes and its link with yield reduction. Two studies were performed and two genotypes of winter oil seed rape (Aviso and Express) with different tolerance to N deficiency were assessed. In the first experiment, vernalized plants were grown under controlled conditions in 4L pots containing soil substrate (pot study) while in the second design, plants were grown under field conditions and studied after the winter period (field study). In well fed plants, mineral N was supply at the onset of stem elongation while in stress plants, N input was lowered in order to aim a 30% (pot study) or a 15% (field study) reduction of seed yield. Potted plants were transported into the lab prior to NMR measurement (pot study) while a mobile NMR lab especially set up for this purpose was brought into the field in order to perform NMR measurements without plant uprooting (field study). For NMR measurements and light microscopy, leaf disc sampling was performed on leaves of all leaf ranks throughout the canopy and performed at stem elongation for different genotypes and treatments. In addition, well fed plants of Aviso genotype (field study) were measured for NMR at stem elongation, flowering and seed filling stages. The genotypic response to N depletion was evaluated through dry mass production and seed yield. For each condition, measurements were performed in four replicates. Under controlled environment, lower tolerance to N depletion was associated with higher impact on senescence associated structural modification pattern. Under field conditions, at stem elongation development stage, NMR measurements were reproducible to those obtained under controlled conditions (Aviso genotype). NMR relaxation spectra changes throughout canopy appeared different under monocarpic senescence occurring during the reproductive period compared to changes observed during vegetative sequential senescence occurring at regrowth and stem elongation stages. These results demonstrated that despite a great variability of the environmental factors, the NMR method can provide robust indicators of structural changes associated with leaf development and that it could be used under field conditions in order to evaluate the effects of N deficiency on leaf development.