Yield Alternation: Horticulture, Physiology, Molecular Biology, and Evolution

Yearly yield fluctuations are normal in fruit trees. Yield alternation might be induced by environmental stresses, but in many fruit tree species yield alternation is an inherent characteristic, resulting in perpetual biennial cycling. Endogenous cues are thought to be the major players in determining the phenomenon, although the genetic basis, if it exists, is still enigmatic. In most cases, alternate bearing is due to inhibition of floral induction, following a heavy fruit load ON year. Understanding of flowering control in model plants has made great progress during recent decades. Due to conservation of many molecular components of the flowering-control pathways among higher plants, there has also been considerable progress in the identification of these components and in the understanding of their role in fruit trees. In this review, flowering control by exogenous and endogenous cues in Arabidopsis along with updated findings in fruit trees are summarized. The effect of fruit load on flowering-control genes is also examined. Along with these fundamental traits, more practical aspects, namely mitigation strategies of alternate bearing, are also eviewed. Recent developments in mitigation practices of six representative fruit trees are also summarized. Finally, some basic questions, including the genetic and epigenetic background of alternate bearing, an evolutionary perspective, and possible common mechanisms among various fruit trees, are thoroughly discussed.