In situ fixation of grape berries.

Usual immersion protocols in aldehyde solutions fail to fully preserve the fine structure of both exocarp and mesocarp cells of grape berries, especially for the veraison (onset of ripening) and post-veraison stages. In exocarp cells, fixative diffusion is hampered by the thick polysaccharide cell walls. In mesocarp cells, plasma membrane and tonoplast are disrupted before aldehyde cross-linking occurs, owing to the high osmotic pressure and cell wall texture. The fixative was therefore injected under pressure as small droplets in the outer and inner parts of the fruit, with limited changes in the steady-state organization of fruit tissues. Compared to a selective range of immersion protocols, a striking improvement in cell preservation was observed for all berry tissues, allowing new information on various compartments of grape berry cells. The preservation of organ integrity and local concentration of aldehyde molecules are the most critical parameters of improved fixation. This technique may be applicable to a large array of fleshy fruits containing mainly cells comprising a high volumetric proportion of vacuoles accumulating large amounts of organic acids and sugars and bounded by thick-walled exocarp cells.