Abstract
Many tree crops experience sub-optimal yields and low fruit quality due to inadequate pollination, low fruit set, and poor crop nutrition. Boron (B) is a critical crop nutrient for fruit set because B levels affect pollen germination and pollen tube growth. However, the relationship between floral B concentration and fruit set is not well understood. The aim of this study was to determine the effect of B applications on the initial fruit set, yield, quality, and paternity of macadamia (Macadamia integrifolia). Cultivar ‘816’ trees received one of three treatments: (a) 0 g, (b) 15 g, or (c) 30 g B per tree prior to flowering. Boron application increased the B concentration of macadamia flowers. Application of 15 g B increased fruit set at 3 weeks after peak anthesis, but this higher initial fruit set was not translated into higher fruit set at 6 or 10 weeks after peak anthesis or higher yield. Boron application increased B concentrations in kernels but did not affect nut-in-shell (NIS) mass, kernel mass, kernel recovery, kernel oil concentration or incidence of whole kernels. Cultivar ‘816’ was highly outcrossing, with 97–98% cross-paternity among kernels from all treatments. Our results indicate that higher B concentration in macadamia flowers can be associated with an increased initial fruit set. However, high B levels did not affect yield, nut quality, or the proportion of self-pollinated fruit at maturity. The heavy dependence on outcrossing highlights the importance of inter-planting different cultivars and managing bee hives to sustain the productivity of macadamia orchards.
Highlights
Global crop yields are currently trending below anticipated food demand [1,2]
Our results show that applying B to macadamia trees at the recommended rate of Our results show that applying B to macadamia trees at the recommended rate of 15 g per tree elevated floral B concentrations and increased initial fruit set at 3 weeks after g per tree elevated floral B concentrations and increased initial fruit set at 3 weeks after peak anthesis
This effect did not translate into higher fruit set at later stages peak anthesis
Summary
Global crop yields are currently trending below anticipated food demand [1,2]. Tree crops contribute over 600 million tons of the 10,600 million tons of annual global food production [3–5], and fruit number and fruit size are key components of tree crop yield [6–8].Tree yields can be constrained by the percentage of flowers that are pollinated and fertilised, the percentage of fruit that develop to maturity, and the mass of the fruit components that contribute to yield [9–12]. Global crop yields are currently trending below anticipated food demand [1,2]. Tree crops contribute over 600 million tons of the 10,600 million tons of annual global food production [3–5], and fruit number and fruit size are key components of tree crop yield [6–8]. Tree yields can be constrained by the percentage of flowers that are pollinated and fertilised, the percentage of fruit that develop to maturity, and the mass of the fruit components that contribute to yield [9–12]. Mass-flowering trees, which include many of the world’s tropical and subtropical tree crops, typically produce many more flowers than mature fruit [9,13–15]. Many of the flowers do not set fruit and many of the fruit abscise during the early fruit development period [9,14,16–18]. The final size of the remaining fruit that reach maturity can be affected by environmental conditions [19,20], crop nutrition [21–23]
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