CHANGES IN THE BUFFERING CAPACITY OF CELL SAP IN SENESCING ROSE PETALS

An increase in the pH of cell sap (pH(s)) was observed during the ageing of rose petals (Rosa hybrida L. ) cultivar 'Sweet Promise'. Changes in the buffering capacity of cell sap (beta(s)) were determined during petal life time, in order to correlate the increase in pH(s) and the variation of the beta(s). The beta(s) was minimal in a region slightly more alkaline than that of the pH(s). The minimum value of beta(s) increased with age from 2 to 10 mumol H+ pH unit-1 ml-1 cell sap. In the acidic region, the beta(s), which was relatively low in young petals, increased sharply later. In the alkaline region the beta(s) of young petals (1 day) was very low (less than 4 mumol H+ pH unit-1 ml-1 at pH 7.5). In senescing petals, on the other hand, a very high buffering capacity could be observed. The increase in beta(s) in the low range of pH values was correlated with a marked increase in the malate content in senescing petals. It has been generally assumed that the pH(s) of cell sap extracted by a freeze-thaw method is representative of the vacuolar pH (pH(v)). The changes with time of pH(v), determined by a labelled chemical probe, were compared with those of pH(s). In senescing petals, pH(v), unlike pH(s), showed a slow but clear decrease. The variations observed in beta(s) and the increase of the difference (pH(s)-pH(v)) in ageing petals are discussed. They are consistent with increasing contamination of vacuolar sap by cytoplasm during cell sap extraction.