Alternate bearing in pistachio (Pistacia veraL.): a review

Key message In this review, the main internal and environmental hypotheses regarding alternate bearing and abscission of inflorescence buds and also, the methods to mitigate this physiological phenomenon have been investigated. Alternate bearing is annually fluctuating heavy ('on') and light ('off') crops, which occurs in most fruit trees. There are multiple visible causes, decreased shoot growth and flower buds or imperfect flowers, but all occur in association with a heavy crop, suggesting a carbohydrate resource limitation to shoot growth, or flower initiation, induction or differentiation. Pistachios bear on 1-year-old shoots from axillary buds produced the previous year. The visible mechanism is bud abscission on current season's growth during 'on' years and bud retention in low crop load ('off') years. Though bud abscission is visible, the underlying cause is unknown. This review classifies the multiple hypotheses as internal, genetic, carbohydrate resources and hormones or environmental, abiotic and biotic stresses. Our examination of potential hypotheses and recent research suggests alternate bearing in pistachio is a result of the carbohydrate resource limitation generated by the heavy 'on' year crop maturation. However, the specific signal producing bud abscission is unknown. Alternate bearing in pistachio can be mitigated by a vigorous rootstock, particularly if combined with proper pruning. We hypothesize the vigorous rootstock may produce a better carbohydrate resource status through a larger tree with more leaf surface area. The pruning does not address the bud abscission but alters the ratio of non-bearing to bearing shoots, resulting in a lower alternate bearing index. Furthermore, we conclude alternate bearing in pistachio, while a within shoot phenomenon, can be triggered by severe environmental events that damage a current crop.