Pollination and resource limitation as interacting constraints on almond fruit set

Abstract

Pollination and resource availability are factors determining reproductive success ofplants, and in agriculture these factors influence yield of fruit-bearing crops. Ourunderstanding of the importance of crop pollination is fast improving, but less isknown about how the interaction between pollination and resources constrains fruitproduction.•We conducted an experiment with almond trees (Prunus dulcis) to examine how thenumber of flowers, light availability and competition for resources affected nut (fruit)production on individual spurs (fruit-bearing structures) exposed to open-pollinationor hand-pollination.•We found a positive relationship between flower number and nut number on spurswith up to four flowers, but no further benefit after four flowers, suggesting a resourcethreshold expressed by individual spurs. Spurs with few flowers increased the conver-sion rate of flowers to nuts when supplemented with hand-pollination, but spurs withmore flowers were more likely to achieve the threshold number of nuts even underopen-pollination. Our experiment included a further treatment involving sprayingwhole trees with pollen. This treatment reduced nut production by spurs with manyflowers and high light availability, suggesting competition is experienced by well-re-sourced spurs when resources need to be shared among developing nuts across thewhole tree.•Our study supports the hypothesis that excess flower production in fruit treesincreases the potential for fruit production when pollinator and resource availability isvariable (bet-hedging). Spurs with more flowers typically produce more nuts (within alimited range), but only if both resources and pollen supply increase with flower num-ber. For almond growers, a focus on maintaining high flower numbers, especially inhigh light regions of the canopy, is the foundation for high levels of production.Strategies to lift flower number and light are complicated by trade-offs inherent in treearchitecture and orchard design. However, fruit set would be lifted above thatachieved by current practice by an increase in the pollination rate of flowers