Chemical and biochemical changes of pumpkin (Cucumis moschata, Duch) tissue in relation to osmotic stress

MARINA F. DE ESCALADA PLA; NORA M PONCE; MARÍA E WIDER; CARLOS A. STORTZ; ANA M ROJAS; LIA N. GERSCHENSON

JOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE

WILEY InterScience

Año: 2005 vol. 85 p. 1852 - 1860

0022-5142

Abstract: Chemical and biochemical changes of pumpkin (Cucumis moschata, Duch) tissue in relation to osmotic stress were studied and related to tissue textural behavior. Turgor pressure of raw tissue was adjusted by immersion in hypotonic (0.0 molm−3), isotonic (250 molm−3) and plasmolyzing (1050 molm−3)buffered (20molm−3 potassium phosphate, pH 6.8) solutions of poly(ethylene glycol) 400 (PEG). It was found that an substantial proportion of polysaccharides located in the cell wall (CW)?middle lamellae (ML) of studied tissue was water extractable. This proportion decreased when CW-stretch increased. Ionically bound peroxidase (POX) presented an increased activity in swollen and isotonic tissue, probably associated to oxidative cross-linking of extensin located in the CW of this tissue, as a response to stress promoted by cut, immersion and turgor pressure change. The resistance of CW to elongation when equilibrated in hypotonic medium, might be ascribed to reinforcement of the CW by the oxidative crosslinking of extensin, with simultaneous formation of pectin-in-extensin entanglements. Plasmolyzed tissue where CWis relaxed, showed pectin?rhamnogalacturonan degradation. The water-soluble fraction (WSF) was shown to be a negative function of CW stretching and infinite force of relaxation.