Effects of postharvest mechanical and climatic stress on carrot tissue water relations

Using a 'Scholander'-type pressure bomb, the effects of climatic or mechanical impacts on the water relations of carrots during postharvest handling were investigated to characterise the stresses during the different steps of the whole chain. Water potential (Psi) provided a valuable means to easily, rapidly and repeatedly measure a large number of roots during an experiment. Water potential of freshly harvested carrots ranged between -0.2 and -0.6 MPa. Independent of whether the leaves were detached from the roots, Psi declined by more than 200% (-0.8- -1.5 MPa) during two days of storage at 18 degrees C even if the air humidity was very high ( > 98% RH). While osmotic potential (Psi(pi)) was unaffected, pressure potential (Psi(P)) decreased from approximately 0.7 to approximately 0.3 MPa, as determined by pressure-volume (PV) analysis or cryoscopy. Storing bunched carrots at moderate air humidity (85% RH) resulted in their water potential falling beyond the turgor loss point (i.e. Psi(P) = 0 at Psi approximate to -2.1 MPa in this experiment) within 2 days. Repeated falls from a conveyor belt onto either a steel plate or a belt did not enhance the reduction of Psi in intact, leafless carrots relative to controls during subsequent storage. On the other hand, shaking roots on an oscillation table stress-simulator resulted in a significant decrease in Psi, reflecting the degree of stress (i.e. time on the stress-simulator). In contrast to Psi, changes in water content (from measuring dry mass) of randomly selected samples did not clearly indicate the effects of climatic and mechanical stress on root water relations. (C) 1999 Published by Elsevier Science B.V. All rights reserved.