Constraints to obtaining consistent annual yields in perennial tree crops. I: Heavy fruit load dominates over vegetative growth

Farmers lack effective methods to achieve and maintain stable production from year to year in many commercial fruit crops. Annual fruit yield within a region often alternates between high and low fruit load and is termed alternate bearing. The underlying cause of alternate bearing is the negative impact of high fruit load on vegetative growth and next year's flowering. In this review, we emphasize common responses of diverse perennials to heavy crop load. We present botanical, ecological and horticultural perspectives on irregular bearing. The later part of this review focuses on understanding how high fruit load dominates over vegetative growth. We discuss sink strengths and putative mobile signals (hormones), perhaps seed-derived. We highlight gaps in current understanding of alternate bearing, and discuss new approaches to better understand fruit load dominance. Assuming the effect of high fruit load may be related to other mechanisms of sink partitioning, other forms of dominance are presented such as apical, first fruit and king fruit dominance. Dominance seems to be enforced, in independent cases through the establishment of a polar auxin transport system from the stronger sink. Once established this somehow perturbs the transport of auxin out of weaker sinks. Possibly, fruit derived auxin may alter the polar auxin transport system of the shoot to inhibit shoot growth.

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► We present different perspectives on alternate bearing of trees.
► A primary effect of high fruit load is the inhibition of shoot growth.
► Fruit load inhibition of shoot growth may be related to apical dominance.
► We review current knowledge on polar auxin transport in model plants.
► We suggest that polar auxin transport from the developing fruit may define its dominance.