Alternate bearing and the possible role of carbohydrates in bud abscission of pistachio (Pistacia vera L.)

Pistachio produces an abundant number of inflorescent buds every year (during spring and early summer) which abscise mainly on fruiting branches during seed development. Competition of the developing ovaries/seeds with inflorescent buds for nutrients was suggested as being primarily responsible for bud abscission. Although leaf N correlated with bud retention, foliar application of N did not reduce bud drop. Inflorescent buds on defruited branches accumulated twice as much C-photosynthate as those on fruiting ones. A depletion of carbohydrates in flower buds in "on" year trees was found by some researchers during seed development. Sucrose movement from leaves to flower buds was also reduced, resulting in a decrease of flower bud development on the "on" year trees which was followed by abscission. The possibility that depletion of carbohydrate in several organs of "on" trees during seed development triggers bud abscission resulting in alternate bearing is reviewed.