Cell Wall Metabolism in Developing Strawberry Fruits

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Abstract
Cell wall metabolism was studied in strawberry receptacles (Fragaria ananassa, Duchesne) of known age in relation to petal fall (PF). Polysaccharide and protein composition, incorporation of [14C]glucose and [14C]proline by excised tissue, and the fate of 14CO2 fixed by young, attached fruits were followed in relation to cell division, cell expansion, fine structure, and ethylene synthesis. Cell division continued for about 7 d after PF although vacuolation of cells was already beginning at PF and the subsequent cell expansion was logarithmic. There was an associated logarithmic increase in sugar content per cell and a decreasing rate of ethylene production per unit fresh weight. During cell expansion radioactivity from [14C]glucose was incorporated into fractions identified as starch and soluble polyuronide and into glucose and galactose residues in the cell wall. Radioactivity from [14C]proline was also incorporated into the cell wall, but only 10 per cent of this activity was found in hydroxyproline. Correspondingly wall protein contained a low proportion of hydroxyproline residues. The proportion of radioactivity from 14CO2 fixed by fruitlets remained constant in most sugar residues in the cell wall. The proportion of radioactivity in galactose fell, indicating turnover of these residues. Between 21 and 28 d after PF receptacles became red and softened but there was no change in the rate of ethylene production. Cell expansion continued for at least 28 d. Tubular proliferation of the tonoplast and hydration of middle lamella and wall matrix material had begun 7–14 d after PF but became extreme during ripening. Associated with the hydration of the wall, over 70 per cent of the polyuronide in the wall became freely soluble, and arabinose and galactose residues lost from the wall appeared in soluble fractions. There was no increase in total polysaccharide during ripening and incorporation of [14C]glucose into polysaccharides ceased, although protein increased and incorporation of [14C]proline into wall protein continued.