FAST FACTS
Macadamia nut orchards are under-pollinated due to a lack of diversity in cultivars.
Inbreeding occurs when there is a monoculture of varieties.
Substantial yield increases can be gained from better pollination.
Better pollination in macadamia nut orchards is key to improving yields and increasing kernel recovery. While bees play a crucial role in pollinating these crops, obtaining maximum pollination is an issue that most farmers grapple with.
Coupled with this is the underestimated role that cross-pollination between different macadamia nut varieties plays in getting maximum pollination and nut set. For this reason, it is not uncommon to find whole farms consisting of a single variety.
Looking into the exact role that monoculture plays in pollination, an ongoing study in Australia has revealed that not only are macadamias under-pollinated for the most part, but improving this one aspect could add 1t/ ha of nut-in-shell (NIS) to yield. At current nut prices, this equates to around R65 250.
Prof Helen Wallace from the Agricultural Ecology Department at Griffith University in Queensland, Australia, has been a macadamia researcher for more than 20 years and is part of a team investigating pollination in macadamias. Debunking myths on how pollination in orchards works, she clarifies that bees and other insects are primarily responsible for pollination.
“Macadamias are not adapted for wind pollination since they have nectar. Plants have nectar to attract insects or animals, who then need to spread it further. The idea that orchards are filled with pollen and all that is needed is for wind to disperse it, is not accurate.”
Most notably, Wallace explains that pollination that occurs between two flowers of the same cultivar constitutes inbreeding, with the same dire consequences as it would have elsewhere in nature.
“Cross-pollen is any pollen from a different variety, and self-pollen is any pollen from the same variety. This means that even if you get pollen from a 344 variety in Australia and pollen from a 344 in South Africa, it’s still self-pollen. It doesn’t matter where it comes from, it’s all about the genetics of that pollen.
“Macadamias, just like animals, don’t like inbreeding. This results in offspring that are likely to be deformed, weak, small and have all kinds of problems, so they may not survive. It’s exactly the same for plants.
“Macadamias have evolved to stop inbreeding from happening. Since the male and female parts are right next to each other, they’re going to accidentally self-pollinate all the time. To prevent these potentially inbred, weak nuts from developing, macadamia trees have a way to chemically recognise pollen that’s self-pollen and then stop it from growing. They don’t want to waste any of their effort making a nut that’s going to be small and weak and unlikely to survive. “If the self-pollen does make it, which it sometimes will, you’ve got an embryo that’s going to be small and weak and not as strong as those from cross-pollination. That’s why macadamias need cross-pollination. They’ve evolved both to prevent self-pollination and to ensure the offspring is as healthy as possible.”
‘WE USE PATERNITY TESTING OF CROPS TO DETERMINE THE EXTENT OF CROSS-POLLINATION IN AN ORCHARD’
AWAY WITH MONOCULTURE
Cross-pollination is important for nut set, yet the most common scene in macadamia orchards around the world are blocks of single varieties, sometimes covering the whole farm. Wallace says this has been a particular point of concern over the years, with research only recently being able to show the true effect of monocultures.
“We’ve started using paternity testing of crops to determine the extent of cross-pollination in an orchard. This allows us to see where the pollen came from that made the embryo, which eventually turned into a nut. The techniques involve massARRAY and genotype-by-sequencing, and we looked at what was happening in big blocks of 816 and Daddow varieties.
“We looked at long sequences of DNA from the nuts, which we then try to match to the parents. As we now have a library where we can see what a 344 or an 816 variety looks like, for example, we can go back and match those sequences to the nuts to find out who the likely father is.
“When we went into some blocks (there were about 48 rows of Daddow and 48 rows of 816), we were expecting a lot of cross-pollination on the edges of those blocks where the Daddow is next to the 816. We thought we’d find a lot of self-pollinated nuts right in the middle of those blocks.
“We paternity-tested all the nuts, and much to our surprise, found that 90% were crosspollinated, even in the middle of the blocks. So right in the middle of those great, big, pure blocks, there were hardly any self-pollinated nuts at all.”
However, Wallace cautions against making the conclusion that monoculture is as effective as a diversity of varieties.
“There is the possibility that pollination is adequate among big blocks of the same variety. But there is also another explanation that is really important for growers to think about, as it might impact on the yield.”
She explains that since self-pollinated nuts abort, or produce a small crop, the possibility exists that many more nuts have been produced by the tree, but since they were self-pollinated, did not reach maturity. Therefore all the nuts left on the tree had been cross-pollinated, which gives the impression of adequate pollination, but is in fact only a fraction of what could have been produced.
The study also highlighted the important role that bees play in cross-pollination, with evidence of pollen on trees 1km apart.
Wallace adds that another reason why there might be nut set in the pure blocks is that the block may not be just one cultivar.
“There are always some varieties that aren’t what they were supposed to be, or that don’t actually look right, and that might be because there’s been a rootstock that’s grown up. Or it might be because one variety somehow got mixed up with other varieties, or it might be trees that get replaced after storm damage. The result is that instead of having a pure block, you’ve got a mostly pure block with a few trees here and there that are not the same variety, which turns out to be a good thing.”
A third reason is currently being investigated and could hold further potential for increasing nut yields in the future. Wallace has discovered evidence of nut varieties that are self-fertile, but the question remains how big the impact on kernel recovery is in entirely self-pollinated nuts.
“The handful of self-pollinated nuts that we got from the 816 variety had a kernel recovery of about 30% compared with 44% for the crosses. This goes back to the consequences of inbreeding, where the result is a weak and small nut set.
“We’re in the middle of doing paternity testing of about 20 varieties to find out which ones are self-fertile. Just because they’re self-fertile doesn’t mean that the whole crop will come from self-pollinated nuts. It might be that they’re self-fertile, but only 30% of the crop comes from self-pollinated nuts.
“But we also want to know which pollen parents make big nuts and which pollen parents make small nuts. Think about a dog, for example. If you cross a big dog with a small one, we know that the puppies are going to be bigger.
“We think that the same thing is going to happen with the nuts. We know that different cross-pollen parents might be able to produce bigger nuts or smaller nuts but, most importantly for growers, we want to know how cross-pollen parents affect kernel recovery.”
YIELD POTENTIAL
Wallace conducted an experiment to deduce how much yield could be lost due to inadequate pollination. This entailed pollinating as many racemes as possible in big blocks of Daddow and 816 orchards. “We wanted to know what the maximum yield was that we could get from the trees if we threw all the pollen in the world at them. We had 10 trees cross-pollinated and 10 trees as controls in each row.
“We had one row of Daddow next to the 816 and one row of Daddow in the middle of the block. We did the same for 816, with one row of 816 next to the Daddow and one row of 816 in the middle of the big block.
“For the 816, we lifted NIS yield from 1,3t/ ha to 2,5t/ ha in the middle of the block, which is a massive increase. For the Daddow, in the middle of the block, we lifted it from 2,8t/ ha to 3,9t/ha, also a very big increase in yield.
“Even on the edge of the blocks, right next to the other variety where you would think pollination was really good to start with, we still managed to lift NIS yield by about 30%. For 816, yield went from 2,1t/ha to 2,7t/ha, and for Daddow from 3,1t/ha to 4t/ha. So, by throwing all the cross-pollen in the world that we could at these trees, we managed to lift the yield by about 1t/ha. We think that that’s how much the macadamias are under-pollinated.”
She admits she was surprised that trees in the middle of large blocks were under-pollinated by about 1,1t/ha to 1,2t/ha, and that even trees right next to another variety were still under-pollinated by about 0,7t/ha to 0,9t/ha. “This indicates that under-pollination might be much more widespread than we thought.” To gain a better understanding of the level of pollination in an orchard, Wallace advises farmers to check for clumps of pollen on macadamia flowers in the early afternoon, which would indicate that bees have not been there.
‘WE WANT TO KNOW HOW CROSS-POLLEN PARENTS AFFECT KERNEL RECOVERY’
“If farmers are seeing really low initial nut set at about three weeks after flowering, that might mean that they have a pollination problem.”
The research underscores the importance of inter-planting nut varieties in an orchard.
“It’s really important to make the job as easy as possible for the honeybees by putting the cultivars close together. Hopefully that will help to get the cross-pollen in. The other important thing is to bring the honeybees in and to spread them around the orchard as much as possible,” Wallace says.
Email Helen Wallace at [email protected]. *Research obtained through the Australian Macadamia Society with the help of industry development manager Leoni Kojetin.
Source: Scribd