The mechanism and underlying physiology perpetuating alternate bearing in 'Pixie' mandarin (Citrus reticulata Blanco)

Alternate bearing citrus trees produce a heavy (on) crop followed by a lighter (off) crop, but the mechanism and underlying physiology by which fruit influence flowering the next spring was previously unresolved. Fruit removal from entire on-crop 'Pixie' mandarin (Citrus reticulata Blanco) trees in July demonstrated the significant contribution of summer and fall (SF) vegetative shoots to return bloom. Fruit removal in July also increased spring bud break and the number of floral shoots and flowers per parent shoot, independent of SF shoots (-SF), to a value greater than off-crop trees. December fruit removal from on-crop trees increased bud break and floral shoot and flower number in the return bloom to that of off-crop trees. Bud break on parent (+SF) shoots accounted for 71% of the variation in flower number in the return bloom. For all treatments spring vegetative shoot number was minimal and never inversely related to floral shoot number. Therefore, fruit inhibit bud break, reducing the number of summer-fall vegetative shoots and number of spring floral shoots that develop on parent (-SF) shoots, thereby, reducing the intensity of the return bloom, rather than inhibiting the transition of vegetative buds to floral buds. To determine the underlying physiology, the effect of crop load on endogenous carbohydrate and hormone concentrations of shoot and root apices was quantified. In August, on-crop trees had a greater shoot indole-3-acetic acid (IAA) concentration and lower isopentenyladenosine (IPA) concentration than off-crop trees. Fruit were confirmed to be the source of IAA and fruit removal in July decreased shoot IAA and increased shoot IPA concentrations by August to those of off-crop trees. SF shoot number per parent shoot was negatively correlated with August shoot IAA concentrations and positively correlated with August shoot IPA concentrations. Spring bud break per parent (-SF) shoot was significantly correlated with January shoot glucose and starch concentrations. This research is the first to provide evidence that fruit perpetuate alternate bearing, in part, by inhibiting the development of SF vegetative shoots that bear flowers in spring due to high IAA and low IPA concentrations in the shoot apices, a mechanism similar to apical dominance.