The 2012 ASHS Annual Conference

Postharvest needle abscission is a consequence of complex interactions among physiological, biophysical and hormonal factors. Efforts are being made to determine significant biophysical and physiological events related to postharvest needle abscission. It is known that initial postharvest average water consumption is 0.2 mL·g^-1 per day, but gradually decreases by over 75%. It is hypothesized that some degree of postharvest water stress exists, which is manifested in changes in several biophysical parameters. Further, the changes in key endogenous hormones postharvest are yet unknown . Branches were collected from a clonal orchard and placed in water. Parameters such as needle loss, water use, relative water content, capacitance, membrane injury, and xylem pressure potential were recorded once a week until complete needle shed occurred. In addition, needles were sampled at the beginning of the experiment and during peak needle abscission and then subjected to hormonal analysis. It was found that needle abscission occurred within 24 days. During this time water use decreased by 50%, relative water content decreased by 23%, capacitance decreased by 64%, membrane injury increased by 100%, and xylem pressure potential decreased 4-fold. Abscisic acid increased by 32-fold and trans-zeatin riboside increased by 4-fold during peak abscission. Other cytokinins such as cis-zeatin riboside, isopentenyl adenosine, trans-zeatin-O-glucoside, and dihydrozeatin riboside all doubled during abscission. Finally, there was a 95% decrease in indole-3-acetic acid. Observed changes in all biophysical parameters, as well as abscisic acid, could be indicative of a possible postharvest water stress or dehydration. It is possible that dehydration-induced changes in biophysical and hormonal factors trigger and/or modulate postharvest needle abscission.