Pollination and resource limitation as interacting constraints on almond fruit set

Pollination and resource availability are factors determining reproductive success of plants, and in agriculture these factors influence yield of fruit-bearing crops. Our understanding of the importance of crop pollination is fast improving, but less is known about how the interaction between pollination and resources constrains fruit production. We conducted an experiment with almond trees (Prunus dulcis) to examine how the number of flowers, light availability and competition for resources affected nut (fruit) production on individual spurs (fruit-bearing structures) exposed to open-pollination or hand-pollination. We found a positive relationship between flower number and nut number on spurs with up to four flowers, but no further benefit after four flowers, suggesting a resource threshold expressed by individual spurs. Spurs with few flowers increased the conversion rate of flowers to nuts when supplemented with hand-pollination, but spurs with more flowers were more likely to achieve the threshold number of nuts even under open-pollination. Our experiment included a further treatment involving spraying whole trees with pollen. This treatment reduced nut production by spurs with many flowers and high light availability, suggesting competition is experienced by well-resourced spurs when resources need to be shared among developing nuts across the whole tree. Our study supports the hypothesis that excess flower production in fruit trees increases the potential for fruit production when pollinator and resource availability is variable (bet-hedging). Spurs with more flowers typically produce more nuts (within a limited range), but only if both resources and pollen supply increase with flower number. For almond growers, a focus on maintaining high flower numbers, especially in high 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 tree architecture and orchard design. However, fruit set would be lifted above that achieved by current practice by an increase in the pollination rate of flowers.