Centre for Planetary Health and Food Security, School of Environment and Science, Griffith University, Nathan, QLD 4111, Australia.
Department of Animal Ecology, Evolution and Biodiversity, Ruhr-University Bochum, 44780 Bochum, Germany.
Ann Bot. 2022 Jan 28;129(2):135-146. doi: 10.1093/aob/mcab112.
Pollen limitation is most prevalent among bee-pollinated plants, self-incompatible plants and tropical plants. However, we have very little understanding of the extent to which pollen limitation affects fruit set in mass-flowering trees despite tree crops accounting for at least 600 million tons of the 9200 million tons of annual global food production.
We determined the extent of pollen limitation in a bee-pollinated, partially self-incompatible, subtropical tree by hand cross-pollinating the majority of flowers on mass-flowering macadamia (Macadamia integrifolia) trees that produce about 200 000-400 000 flowers. We measured tree yield and kernel quality and estimated final fruit set. We genotyped individual kernels by MassARRAY to determine levels of outcrossing in orchards and assess paternity effects on nut quality.
Macadamia trees were pollen-limited. Supplementary cross-pollination increased nut-in-shell yield, kernel yield and fruit set by as much as 97, 109 and 92 %, respectively. The extent of pollen limitation depended upon the proximity of experimental trees to trees of another cultivar because macadamia trees were highly outcrossing. Between 84 and 100 % of fruit arose from cross-pollination, even at 200 m (25 rows) from orchard blocks of another cultivar. Large variations in nut-in-shell mass, kernel mass, kernel recovery and kernel oil concentration were related to differences in fruit paternity, including between self-pollinated and cross-pollinated fruit, thus demonstrating pollen-parent effects on fruit quality (i.e. xenia).
This study is the first to demonstrate pollen limitation in a mass-flowering tree. Improved pollination led to increased kernel yield of 0.31-0.59 tons ha-1, which equates currently to higher farm-gate income of approximately $US3720-$US7080 ha-1. The heavy reliance of macadamia flowers on cross-pollination and the strong xenia effects on kernel mass demonstrate the high value that pollination services can provide to food production.
传粉限制在传粉媒介为蜜蜂的植物、自交不亲和植物和热带植物中最为常见。然而，尽管树木作物至少占全球每年 9.2 亿吨粮食产量的 6 亿吨，但我们对于传粉限制在大规模开花树木中影响结实率的程度了解甚少。
我们通过人工异交授粉对大规模开花的澳洲坚果（Macadamia integrifolia）树的大部分花朵进行授粉，从而确定了传粉媒介为蜜蜂的、部分自交不亲和的亚热带树木的传粉限制程度。澳洲坚果每棵树可产生 20 万至 40 万朵花。我们测量了树木的产量和果仁质量，并估算了最终的结实率。我们通过 MassARRAY 对单个果仁进行基因分型，以确定果园中的异交水平，并评估父本对坚果质量的影响。
澳洲坚果树存在传粉限制。补充异交授粉可使带壳坚果产量、果仁产量和结实率分别提高 97%、109%和 92%。传粉限制的程度取决于实验树木与另一品种树木的距离，因为澳洲坚果树的异交程度很高。即使距离另一个品种的果园块 200 米（25 行），也有 84%至 100%的果实来自异交。带壳坚果质量、果仁质量、果仁回收率和果仁油浓度的较大差异与果实亲代的差异有关，包括自交和异交果实之间的差异，因此证明了花粉亲代对果实质量（即异交效应）的影响。
本研究首次证明了大规模开花树木中的传粉限制。授粉的改善导致果仁产量增加 0.31-0.59 吨/公顷，相当于目前每公顷农场收入增加约 3720-7080 美元。澳洲坚果树花朵对异交的高度依赖以及对果仁质量的强烈异交效应表明，授粉服务对粮食生产具有很高的价值。
