Pollen limitation and xenia effects in a cultivated mass-flowering tree, Macadamia integrifolia (Proteaceae)

Abstract

Background and Aims 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 9,200 million tons of annual global food production. Methods 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. Key Results 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. Conclusions 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 per hectare, which equates currently to higher farm-gate income of approximately US3,720–US7,080 per hectare. 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.

Pollination services in a macadamia cultivar depend on across‐orchard transport of cross pollen

“…Currently, orchards are established with single‐cultivar blocks (Trueman, 2013; Vithanage et al., 2002) that could be, for example five rows wide (as in the narrow‐block orchard) or 48 rows wide (as in the wide‐block orchard). However, we have shown that the number of harvested nuts of cultivar 816 was reduced by three rows into the wide block, where the distance to another cultivar, Daddow, was 24 m. A recent study found that supplementary cross‐pollination increased yields of two cultivars, 816 and Daddow, and that the yield increase was greater when the experimental trees were far from another cultivar, suggesting that the flow of cross‐pollen across the orchard was suboptimal (Trueman et al., accepted). Yields of cultivar A16 trees have also been shown to decline by 14 rows into a wide block, where the distance to another cultivar, A4, was 98 m (Vithanage et al., 2002).…”

Hotter chili peppers by hybridisation: heterosis and reciprocal effects

“…This apparently anomalous phenomenon was first described by Darwin as “the direct action of the male element on the female form” and subsequently called ‘xenia’. Xenia is widespread in cultivated species, including cereals, vegetables and fruit trees (Trueman et al, 2021). To the best of our knowledge, however, no reports exist of xenia in Capsicum (Liu, 2018).…”

Boron Effects on Fruit Set, Yield, Quality and Paternity of Macadamia

“…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][10][11][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][14][15]. Many of the flowers do not set fruit and many of the fruit abscise during the early fruit development period [9,14,[16][17][18].…”

“…Many of the flowers do not set fruit and many of the fruit abscise during the early fruit development period [9,14,[16][17][18]. The final size of the remaining fruit that reach maturity can be affected by environmental conditions [19,20], crop nutrition [21][22][23] and fruit paternity [15,[24][25][26][27].…”