UvA biologists unravel the mystery of sour and sweet lemons

Why is some citrus sour and other citrus sweet? A fundamental, biologic mechanism is behind this, and scientists have tried to discover this to no avail for decades. However, biologists of the University of Amsterdam, under the leadership of Prof Dr Ronald Koes and Dr Francesca Quattrocchio, have now solved this mystery. They’ve published an article on this in scientific journal Nature Communications.

Oranges, lemons, grapefruit, tangerines, pomelos: all citrus belongs to the same family. Yet they all taste different. This is mostly caused by the amount of acid in the fruit. While oranges often contain little acid, the amount of acid in lemons can cause your mouth to contract. But sweet lemons also exist, and sour oranges. How is that possible?

Organic pumps
This question goes deeper than you might suspect. Organically, it’s quite odd that a fruit manages to collect so much acid. Looking at the flesh of citrus, you might notice it consists of a type of follicle filled with moisture. These are the plant’s cells. On the edge of a moisture-filled room within the cells – the so-called vacuole – organic pumps can be found, and these regulate where the moisture ends up. Some of these pumps send the acid inside.

But to get lemons this extremely sour, the pumps have to perform very well. Compare it to pumping up a ball. It’s not that difficult to pump a little air into it, but if the ball is already full, it’ll take more and more strength to get some more air into it. The air would prefer to come out. It’s the same with acid and the cells of the fruit. If there’s already too much acid in the vacuole of the cell, you need a powerful pump to add any more acid. This pump had not yet been found, despite decades of searching.

Thanks to petunias
Ronald Koes, Francesca Quattrocchio and their team have now managed to discover this pump. They found it thanks to their work with a completely different plant: petunias. With petunias, the flowers can have more or less acid in their leaves. This can easily be seen in their colours: more acid results in blue flowers The biologists knew which genes decide the amount of acid in petunias. They therefore decided to look for related genes in citrus, and to see if these genes are responsible for the amount of acid as well.

Their search was successful. The biologists researched a collection of citrus, sweet and sour, lemons, oranges and pomelos. They discovered the same thing over and over: in the sour fruits, two genes called CitPH1 and CitPH5 were very active, and these genes were inactive in sweet fruits. They had therefore found the genes that provide the code for the powerful acid pumps.

Valuable for growers
Besides an old biologic mystery being solved, the discovery of the Dutch biologists is also valuable for growers. Koes: “All varieties of citrus we know are the result of years of cross-breeding and selecting from trees with fruit containing certain characteristics. These are processes that take a very long time, and you’re never certain what you’ll end up with. It takes years before a seed results in a tree that bears fruit. Now that we know which genes are responsible for the amount of acid, and are thus largely responsible for the fruits’ flavour, growers can more easily select for that. Even in young plants that take years before they start producing fruit.”

Koes adds: “Another interesting fact is that these genes are also active in other fruits, including grapes and apples. It’s very probable that they also cause the amount of acid, and therefore influence flavour, in these fruits. Perhaps not just the growers of citrus can benefit from this discovery, but growers of other fruit might benefit as well.”

In conclusion a fun trivia fact: just as with petunias, you can sometimes tell by the appearance of the tree whether the citrus will be sweet or sour. Koes: “In petunias, too much acid causes the flowers to turn blue. There’s a citrus tree, the Faris variety, which has branches that give sweet and branches that give sour lemons. The leaves of branches producing sour lemons are purple when they’re young, and the seeds of the lemons have purple spots. However, this isn’t the case with the branches producing sweet lemons. This also appears to correspond nicely with how active the genes are that we have found.”

Source: UvA Amsterdam

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