Endogenous carbohydrate status affects postharvest ethylene sensitivity in relation to leaf senescence and adventitious root formation in Pelargonium cuttings

This study investigated the role of ethylene action in postharvest leaf senescence of two cultivars of pelargonium (Pelargonium x hortorum L. H. Bailey 'Patriot Bright Red' and 'Patriot White') cuttings to determine whether endogenous carbohydrate status interacts with ethylene action in relation to leaf senescence and adventitious root formation. In the first experiment, ethylene or receptor-directed ethylene-action blocker, 1-MCP, was applied to cuttings harvested early in the morning, when carbohydrate status is relatively low. Cuttings were stored in sealed bags at 20±1°C for 3 days in darkness. Ethylene promoted chlorophyll breakdown and leaf senescence during propagation, whilst application of 1-MCP decreased those responses. In the second experiment, application of ethylene had no significant effect on the leaf senescence of cuttings that were harvested at the end of the day, when carbohydrate status was relatively high. In contrast, ethylene application to cuttings with low carbohydrate status (harvested from stock plants kept in darkness for 9h) resulted in substantially higher leaf senescence. Highly significant positive correlations were calculated between the total chlorophyll content of the oldest leaf after 4 days of initial propagation and sucrose, total sugars, and total non-structural carbohydrate concentrations in the whole cutting, irrespective of postharvest ethylene concentration during storage. Adventitious root formation was inhibited by 1-MCP, whereas it was promoted by exogenous ethylene only when applied to cuttings with a higher carbohydrate status. For those cuttings, ethylene concentration was positively correlated with final root number. These results indicate that ethylene action is involved in postharvest leaf senescence, and that ethylene sensitivity decreases with the increase in preharvest endogenous carbohydrate status of the cuttings. In addition, when the endogenous carbohydrate status is high, ethylene sensitivity promotes adventitious root formation without triggering rapid leaf senescence. These results collectively suggest that preharvest endogenous carbohydrate status interacts with postharvest ethylene action to regulate leaf senescence and adventitious root formation in pelargonium cuttings.