Turgor pressure in vegetable tissues: comparison of the performance of incipient plasmolysis technique using mannitol and polyethylenglycol

To determine turgor pressure of melon and kiwifruit edible parts (Cucumis m elo, L. and Actinidia deliciosa, A. Chev., respectively), we measured volum e change of tissue cylinders immersed in different mannitol or polyethylene glycol 400 (PEG) solutions up to equilibrium and sought the osmotic potent ial that led to incipient plasmolysis. Compression tests were also performe d to compare the mechanical response of these tissues at different turgor c onditions. Cellular vitality was evaluated through fluorescein diacetate hy drolyisis by lipases located in the active cellular membrane. This enzyme-c atalyzed reaction was observed by fluorescence microscopy. Results indicate d that turgor pressure of kiwifruit could only be evaluated by using PEG as a solute. Immersion in PEG solutions did not affect the mechanical respons e of this tissue 'per se' based on a comparison of tissue firmness. On the one hand turgor pressure values determined in melon samples were different using both solutes and on the other the immersion affected melon tissue bec ause a significantly (P < 0.05) higher firmness was obtained for raw (unsoa ked) melon cylinders. Cellular membranes were not affected by mannitol or P EG shown by fluorescence microscopy of isotonic melon samples. Some solute- specific effect could change the water movement into the tissue affecting t he value of melon turgor pressure finally estimated. Thus, we emphasized th e importance of using the same solute to compare the effect of different tr eatments and/or ripeness grade on turgor pressure when incipient plasmolysi s technique is used. In this way, PEG showed to be a useful alternative sol ute due to its wide solubility range. (C) 2000 Canadian Institute of Food S cience and Technology. Published by Elsevier Science Ltd. All rights reserv ed.