Factors Influencing the Efficacy of Glucose Pulsing as a Technique Commercially Employed to Control Leaf Blackening in Protea ‘Sylvia’

Leaf blackening is a postharvest disorder affecting almost all commercially important Protea cut flower cultivars. The current commercially recommended treatment to delay and/or control leaf blackening is to pulse harvested stems with a 6-10% glucose solution. Existing protocols mostly prescribe a certain length of time as the standard guideline for exposure to the pulsing solution. However, biophysical factors such as vapour pressure differentials, water potential of stems, as well as the osmotic potential of the pulsing solution may seriously impact on the rate and volume of pulse uptake within a certain timeframe. In this study the pulse solution uptake rate was evaluated in flowering stems of Protea ‘Sylvia’ by either dehydrating stems up to 15% of their original fresh weight before pulsing with a glucose concentration solution or by pulsing stems with a glucose concentration range of between 0 to 17.7% for four hours. Stem and pulse-solution weight was used to calculate the solution uptake, water retained and glucose accumulated by the stem as well as to estimate transpiration. Pulse solution uptake volumes increased with a decrease in stem hydration, illustrating the possible water potential and osmotic potential deficits between stems and the pulse solution, as would differ in stems harvested from irrigated Protea orchard or between multiple harvests at different times during the day. Glucose concentration of the pulse solution was found to similarly affect the solution uptake, due to osmotic potential differences. Correct pulsing protocols are key for Protea ‘Sylvia’, as these different factors play an important role in the uptake rate of glucose and thus also in the success rate to alleviate leaf blackening. Pulsing for leaf blackening, if not controlled, could result in toxic or inadequate levels of glucose, therefore reducing the efficacy of this treatment.