Effects of Abscisic Acid and Abscisic Acid Analogs on Bud Break

Spring freeze damage is a significant risk for tree fruit and grape growers. Warm spring weather promotes bud break in woody perennial fruit crops and subsequently reduces bud cold hardiness.  A freeze event following the bud de-hardening often leads to damage and even death.  The plant hormone abscisic acid (ABA) plays a critical role in dormancy induction and maintenance.  Researchers at Valent BioSciences Corporation and university cooperators have recently tested the effects of exogenous application of ABA and ABA analogs on delaying bud break of grapevines and fruit trees.  Spray or soil drench application of ABA in the spring prior to bud break delayed bud break, but the effect was inconsistent and the delay was not as long as desired.   Efficacy of applied ABA for bud break delay is likely limited in part because of poor ABA uptake.  For a leafless eco-dormant plant, spray application uptake is limited to penetration of the water repellent bud scales and drench application uptake is limited by poor vascular movement from roots to the dormant buds.  In a study on potted Concord grapevines, a 1 L soil drench application of 1000 ppm ABA or 100 ppm ABA analog (PBI-429) was made in the fall prior to the onset of leaf abscission and dormancy.  The potted vines were held outside over the winter with the pots buried in bark mulch to protect the roots from freezing.  In the spring the vines were pruned and bud break was monitored.  The ABA treatment failed to delay bud break, however the ABA analog treatment did significantly delay bud break.  In a study on mature Cabernet Franc vines in the field, a post-harvest, pre-leaf abscission, foliar application of 300 ppm ABA analogs (PBI-429, PBI-524) significantly delayed bud break in the following spring.  In the spring following treatment the grapevines on a vertical shoot positioning trellis were pruned and bud development was monitored and rated using the BBCH system.  Bud break of the ABA analog treated vines was delayed by about 21 days.  Research is continuing to determine whether ABA or ABA analog can be used to consistently improve bud cold hardiness and/or delay bud break.