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Nitrogen use efficiency is relatively low in oilseed rape (Brassica napus) due to weak nitrogen remobilization during leaf senescence. Monitoring the kinetics of water distribution associated with the reorganization of cell structures, therefore, would be valuable to improve the characterization of nutrient recycling in leaf tissues and the associated senescence processes. In this study, nuclear magnetic resonance (NMR) relaxometry was used to describe water distribution and status at the cellular level in different leaf ranks of well-watered plants. It was shown to be able to detect slight variations in the evolution of senescence. The NMR results were linked to physiological characterization of the leaves and to light and electron micrographs. A relationship between cell hydration and leaf senescence was revealed and associated with changes in the NMR signal. The relative intensities and the transverse relaxation times of the NMR signal components associated with vacuole water were positively correlated with senescence, describing water uptake and vacuole and cell enlargement. Moreover, the relative intensity of the NMR signal that we assigned to the chloroplast water decreased during the senescence process, in agreement with the decrease in relative chloroplast volume estimated from micrographs. The results are discussed on the basis of water flux occurring at the cellular level during senescence. One of the main applications of this study would be for plant phenotyping, especially for plants under environmental stress such as nitrogen starvation.The main physiological outcome of leaf senescence is the recycling of organic resources and the provision of nutrients to sink organs such as storage and growing tissues (Buchanan-Wollaston, 1997;Hikosaka, 2005;Krupinska and Humbeck, 2008). In crop plants, senescence progresses from the lower older leaves to the younger top leaves. Macromolecular degradation and the mechanism of reallocation of breakdown products are mediated by the up-regulation of senescence-related genes (Lee et al., 2001) in close relationship with both developmental and environmental conditions (Gombert et al., 2006). This leads to remobilization of carbon and nitrogen (N) compounds mostly from plastidial compartments (Martínez et al., 2008;Guiboileau et al., 2012), involving proteolytic activity in plasts, vacuole, and cytosol (Adam and Clarke, 2002;Otegui et al., 2005), chlorophyll breakdown (Hoertensteiner, 2006), galactolipid recycling (Kaup et al., 2002) in the plastoglobules (Brehelin et al., 2007), and loading of Suc and amino acids into the phloem through appropriate transporters (Wingler et al., 2004;Masclaux-Daubresse et al., 2008). In terms of leaf senescence at the cell level, where chloroplasts are degraded sequentially, relative organelle volume does not seem to be greatly modified, the vacuole remains intact, and in darkness-induced senescence the number of chloroplasts per cell decreases only slightly (Keech et al., 2007). However, major changes in metabolic fluxes and cell water...

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The changes in water proton transverse relaxation behavior induced by aggregation of whey proteins are explained in terms of the simple molecular processes of diffusion and chemical exchange. The water self-diffusion coefficient was measured in whey protein solutions and gels by the pulsed field gradient NMR method. As expected, water self-diffusion was reduced with increased protein concentrations. Whatever the concentration, the water molecules were free to diffuse over distances varying from 15 to 47 mum. Water diffusion was constant over these distances, demonstrating that no restrictions were found to explain the water hindrance. The modification in protein structure by gelation induced a decrease in water diffusion. The effects of protein concentration on water diffusion are discussed and modeled. Two approaches were compared, the obstruction effect induced by a spherical particle and the cell model, which considered two water compartments with specific self-diffusion coefficients.