Larger apples have lower natural defences against pests

Botanists find a clear trade-off between fruit size and phytochemical production, helping to explain why our modern varieties are highly susceptible to diseases and pests.

Most crops have been dramatically altered from their wild ancestors with the primary goal of increasing harvestable yield. A long-held hypothesis is that increased allocation to yield has reduced plant investment in defence and resulted in crops that are highly susceptible to pests. However, clear demonstrations of these trade-offs have been elusive due to the many selective pressures that occur concurrently during crop domestication.

apples
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To provide a robust test of whether increased allocation to yield can alter plant investment in defence, Whitehead and Poveda examined fruit chemical defence traits and herbivore resistance across 52 wild and 56 domesticated genotypes of apples that vary >26-fold in fruit size. Ninety-six phenolic metabolites were quantified in apple skin, pulp and seeds, and resistance to the codling moth was assessed with a series of bioassays.

The negative phenotypic relationship between fruit size and phenolic content, observed across a large number of wild and domesticated genotypes, supports the hypothesis of yield–defence trade-offs in crops. However, the limited effects of phenolics on codling moth highlight the complexity of consequences that domestication has for plant–herbivore interactions. Continued studies of crop domestication can further our understanding of the multiple trade-offs involved in plant defence, while simultaneously leading to novel discoveries that can improve the sustainability of crop production.

The authors conclude: “Our detailed data set provides valuable information on the potential chemical mechanisms of insect resistance in one of the most valuable speciality fruit crops worldwide.”

Further reading

Whitehead, S. R., & Poveda, K. (2019). Resource allocation trade-offs and the loss of chemical defences during apple domestication. Annals of Botany, 123(6), 1029–1041. https://doi.org/10.1093/aob/mcz010