Ultrastructure des parois de cerises Bigarreau Burlat de textures différentes au cours de la maturation

The changes in texture of fruits during ripening are linked to cell wall degradation involving synthesis and degradation of polymers. An increase in pectin solubility leads to cell sliding and an elastic aspect of tissues. The biochemical cell wall process differs between soft and crisp fruits originating from a same cultivar but cultivated under different agroclimatic conditions. Although the proportions of cell wall material are similar, the composition and structure of the two cell walls are very different at maturity. A solubilization of the middle lamella and a restructuration of the primary cell walls arising from the cells separation is observed in crisp fruits. In contrast, the middle lamella of the soft fruits is better preserved and the primary cell walls are thin and show degradation bags delimited by residual membrane formations. In addition, the macroendocytosis process by endosome individualization is more important in soft fruits. In conclusion, the fruit texture depends on the extent of the links between cell wall polymers. Keywords: cherry, cell wall, texture, ultrastructural study.

Control of ripening

Ripening of fleshy fruits involves major changes in physiology and biochemistry that alter their colour, flavour, texture, aroma and nutritional value. These changes affect all cell compartments and require the expression of new genes encoding enzymes that catalyse reactions essential for the development of quality attributes. In climacteric fruits, such as tomato, ethylene functions as a hormone to stimulate changes in gene expression required for ripening. Molecular cloning experiments have led to the isolation of cDNAs encoding many ripening proteins. This has enabled the identification and manipulation of novel plant genes encoding enzymes involved in cell wall texture change, carotenoid biosynthesis, ethylene synthesis and the identification of gene control regions involved in fruit-specific, ripening-specific, and ethylene-regulated gene expression. Antisense and partial sense gene techniques have been developed to generate genetically modified plant lines in which specific genes have been permanently inactivated. These fundamental studies have led to production and evaluation of genetically modified tomato lines with improved colour, texture, storage life, and processing characteristics. Zeneca Seeds has established a new business division, the aim of which is to utilize these techniques for the development of improved fruit and vegetable varieties. In collaboration with Petoseed, Zeneca Seeds is in the process of transferring the genes leading to quality im provement of tomatoes to Petoseed’s elite tomato germplasm. The primary focus is on the development of improved processing hybrids. These are being evaluated in collaboration with Hunt Wesson, a large and diversified tomato processing company. It is planned that products based on this research will be introduced in the USA in 1995.

Cell wall texture in root hairs of the genus Equisetum

Based on cell wall texture of root hairs, two groups can be distinguished within the 10 species of Equisetum listed in Flora Europaea. This distinction coincides with the division of the genus Equisetum into two subgenera: Equisetum (horsetails) and Hippochaete (scouring rushes). All species of the subgenus Equisetum have a helicoidal cell wall texture in young growing root hairs as well as in full-grown hairs. All species of the subgenus Hippochaete deposit an additional inner cell wall layer against this helicoidal layer when elongation has stopped. The microfibrils in this additional layer do not form a helicoidal texture, but are helically arranged, forming a Z-helix. The presence of a helical layer in full-grown hairs is not a prerequisite for growth in soil, but an exclusively helicoidal root hair wall texture might be favourable for life in water. The wall texture is not influenced by the consistency of the substratum.