Shoot Length Distribution and Its Relation to Yield of Alternate-bearing Pistachio Trees

Pistachio ( Pistacia vera L.) is known to strongly exhibit alternate bearing. Over 19,500 individual shoots were measured on eight alternate bearing `Kerman' pistachio trees on P. atlantica Desf. rootstock. Average length of “on” year (5.4 cm) and “off” year new shoots (5.6 cm) were not significantly different. New shoot length distribution was skewed toward the shorter length categories, with a mode of 2 and 4 cm in “on” and “off” trees respectively. These results contrast with previous studies which have shown that “on” year new shoots of pistachio are much longer than “off” year new shoots. There were about twice as many “on” year new shoots in the 18 to 30 cm class as compared to “off” new shoots, and fruiting wood length was associated positively with fruit number. However, ≈80% of tree yield occurred on fruiting wood that was <10 cm long, with shoots longer than 15 cm contributing <5% to yield. We suggest that results from earlier studies regarding the bud abscission process in very long pistachio shoots should be confirmed on shorter shoots, which contribute significantly to yield. Shoots >30 cm in length may be important in establishing vegetative buds in a position above the main tree crown for canopy expansion during the following “off” year.