Pollen limitation occurs when plants cannot get enough pollen to fertilise all their ovules, limiting the number of seeds set. It’s most common in bee-pollinated plants, self-incompatible plants and tropical plants. But it’s unclear if pollen limitation is a problem for mass-flowering trees, even though trees produce over 600 million tons of food each year. Stephen Trueman and colleagues decided to take a closer look at pollen limitation by studying Macadamia trees. Their findings indicate that changes in bee management could improve crop yield by a substantial amount.
If adding more pollen to a plant causes it to set more seeds, it is said to be ‘pollen limited’. Pollen limitation occurs when a plant either cannot get enough pollen or if the pollen it does get is of low quality, causing it to leave ovules unfertilised. The way botanists test if plants are pollen limited is to compare one group of plants that are left to fend for themselves with a second group that get supplemental pollen and see if there’s a difference in fruit set. It’s a simple experiment, but Trueman and colleagues point out it’s not been done for trees like macadamia.
Macadamia is untested because it’s a mass flowering tree, and that mass flowering means there is a lot of work to do. They point out that some tests have been done, pollinating a few flowers on a tree and comparing them with other flowers on the same tree. There are a couple of problems with this approach.
The first is that the flowers are not fully independent units. They draw the energy to develop fruits from the same central store. If some flowers are pollinated better, then the plant might direct more resources to them and dispose of the unpollinated or poorly pollinated flowers early. There’s good reason to believe that macadamia would do this because of the sheer number of flowers it produces.
Trueman and colleagues note that macadamia also produces many more female or hermaphrodite flowers than they can support. If all the flowers were pollinated, there simply wouldn’t be the energy to turn them all into fruits. Macadamia is happy to drop immature fruits thanks to pests or infections, so any problem with pollen limitation could be hidden among this big effect of the tree ditching any sub-par fruits.
For this reason, the botanists decided they needed to pollinate whole trees to test results between trees instead of between branches. But supplemental pollination for macadamia is not a simple task. They write that macadamia trees produce up to 3500 racemes, being a cluster of flowers attached by short stalks at equal distances along a central stem. Each raceme can hold between 100 and 300 flowers. That means each tree has hundreds of thousands of flowers. The team have developed a method to hand pollinate the flowers rapidly.
The team planted the trees in multiple blocks. They used two cultivars, ‘Daddow’ and ‘816’. In one section, there were two large blocks of each cultivar. In the other section, there were stripes of one cultivar then another.
In their article Trueman and colleagues write: “We collected pollen by rubbing the inside of a test tube (25-mm internal diameter) over at least four donor racemes that bore freshly opened flowers, until pollen was visible in streaks on the inside of the tube. Supplementary cross-pollination was performed by rubbing the inside of the test tube over receiver racemes of the adjacent cultivar that bore freshly opened flowers.
“Each test tube was used to pollinate a maximum of ten racemes, before it was washed with 70 % (v/v) aqueous ethanol, rinsed with water and dried in sunlight. We hand-pollinated each tree every second day during the flowering period, commencing on 8 and 9 September 2018 and concluding on most trees on 18 or 19 September 2018. The last hand-pollination of any tree was performed on 24 September 2018.
“Typically, two to four people pollinated each tree on each occasion, attempting to reach all racemes with open flowers up to about 2.4 m above ground level. We estimated that this represented approximately 70 % of the racemes on each tree.
“The ‘816’ and ‘Daddow’ trees each possessed, on average, 1940 and 2140 racemes, respectively, based on extrapolations from the percentage of racemes that we could reach and the average number of test tubes used to pollinate each tree. Racemes of these two cultivars possessed 199 ± 5 and 104 ± 3 flowers (n = 40), respectively, so that trees of ‘816 and ‘Daddow’ each produced approximately 386 060 and 222 560 flowers, respectively.”
The results showed that the trees were pollen limited and that the pollen quality mattered. In her commentary on the paper, Megan van Etten writes: “This study provides insights on several aspects of floral evolution and pollen limitation. First, this study shows that there may be a high cost of producing many flowers through a decrease in the quality of pollen received. This cost should be considered when examining the evolution of masting since it would affect economies of scale, which is thought to be the ultimate driver of the evolution of masting.
“Additionally, at least for this species, the strong pollen limitation during a masting year with added pollinators suggests that it is unlikely that masting evolved in response to economies of scale for increased pollination.
“Second, this study shows that even with abundant managed pollinators, pollen limitation in both quantity and quality may still be quite severe. This finding is even more concerning given the possibility of current and future declines in native pollinators due to increasing human disturbance and climate change.”
The importance of bee pollination is underlined in Trueman and colleagues’ conclusion. “The reliance of bee-pollinated macadamia flowers on cross-pollination for fruit set and the strong xenia effects on kernel mass and oil concentration demonstrate the high value that pollination services can provide to food production and food quality. Improved pollination led to increases in macadamia kernel yields of 0.31–0.59 tons ha–1, which equate to increases in farm-gate income of $US3720–$US7080 ha–1 at current prices of about $US12 000 per ton of kernel.”
READ THE RESEARCH
Trueman, S.J., Kämper, W., Nichols, J., Ogbourne, S.M., Hawkes, D., Peters, T., Hosseini Bai, S. and Wallace, H.M. (2021) “Pollen limitation and xenia effects in a cultivated mass-flowering tree, Macadamia integrifolia (Proteaceae),” Annals of Botany. https://doi.org/10.1093/aob/mcab112