Nuclear magnetic resonance (NMR) micro-imaging of ripening red raspberry fruits

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Red raspberry (Rubus idaeus) fruits were subjected to nuclear magnetic resonance (NMR) micro-imaging to study the changes in spatial distribution of mobile protons (1H) during a 3-day ripening period while the fruit was still attached to the receptacle. All imaging measurements were made using the protons from a single absorption peak at 4.6 ppm, which corresponds to water, and data were collected by pulsed gradient spin-echo and gradient-echo methods with voxel dimensions of 70 × 70 × 500 μm. Individual drupelets were clearly visible by this non-invasive histological method in distal transverse ‘slices’ of a newly harvested red ripe fruit viewed by spin-echo; highest mobile proton densities were found at the lines of contact between drupelets. The mesocarp tissues of each drupelet showed intermediate grey tones with some evidence of radial striations. In more proximal ‘slices’ the surface of the receptacle and its central vascular cylinder had the highest mobile proton densities. The seeds produced black images in all ‘slices’, indicative of low mobile proton densities or long relaxation times.

After 3 days the proximal ring of drupelets separated from the receptacle, the spaces between drupelets enlarged slightly and the boundaries between drupelets were less pronounced. Vascular bundles in the receptacle lost their whitish radiating pattern in proximal ‘slices’ almost completely, indicating that mobile protons were less abundant after abscission of drupelets.

The same ‘slices’ viewed by gradient echo were less discriminating for localization of Seeds and pronounced tangential wave patterns, often overlaying radial striations, presented more complex images than those seen by spin echo.

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