What is the "belly button" of a navel orange? Are the fruits inside the belly button sweeter?

No way, no way, no one is going to start wearing belly-baring clothes this year, right? I've been wearing them for almost 200 years✌️

By A beautiful and confident little navel orange

In 1820, an ordinary sweet orange tree in a monastery in Brazil mutated and grew a batch of heaven-chosen oranges with fruits within fruits, no seeds, and extra sweetness. The old monk who discovered this batch of heaven-chosen oranges exclaimed "Oh my God, no need to spit out the seeds!" and grafted them to preserve them (let's say, thank you old monk!).

Over the past 200 years, this mutant sweet orange has traveled across the oceans, been grafted and bred, and has become the popular navel orange we eat today. Yes, most of the navel oranges we eat now come from this mutant Brazilian orange tree . The Latin name of the navel orange (Citrus sinensis Osb. var. brasiliensis Tanaka) also shows the evolution of the variety: Citrus sinensis is the original sweet orange, which was spread from China to Portugal in the 16th century and spread to all parts of the world; and the variant name brasiliensis indicates that this sweet orange variant originated in Brazil.

The sweet orange itself is a cross between a mandarin orange and a grapefruit, which is a common practice in the citrus family. | Ellen Levy Finch / Wikimedia Commons

I bought two fruits with one price. Did I make a profit?

The biggest feature that distinguishes navel oranges from other oranges, and also the origin of its name, is the beautiful and elegant belly button on the top of the fruit. This belly button is actually the secondary fruit of the navel orange, also known as the accessory fruit.

The secondary fruit of navel orange is not the same as the accessory fruit (false fruit developed from floral organs other than the ovary) of strawberry.

Generally speaking, under natural conditions, seeds produced after pollination and fertilization can stimulate the production of fruits. For example, in the flowers of normal citrus plants, the ovary develops into fruits after pollination, and the ovules develop into seeds after fertilization.

But the ancestor of the navel orange is the seedless sweet orange, which is sterile and can only reproduce asexually. How can it still bear fruit? Not even one fruit is counted, and the big fruit is wrapped in a small fruit. Is this some kind of buy-one-get-one-free pregnancy Virgin Mary orange?

Citrus flowers usually look like this｜Gema Ancillo & Alejandro Medina, 2015

In fact, navel orange is a typical parthenocarpic plant: pollen and ovules are aborted, and the ovary can develop into a seedless fruit without pollination. The abortion is due to a mutation in the AG gene involved in the development and expression of pistils and stamens .

The formation of secondary fruits in navel oranges is also closely related to this type of AG gene.

Plant growth is usually completed by the continuous differentiation of stem cells in meristems, and the same is true for flowers as reproductive organs. This growth usually has an end point, which is when the growth of the pistil carpel is completed.

The weird thing about navel oranges is that due to the mutation of the AG gene, the "termination" function fails : after the primary carpel is formed, the floral meristem does not disappear like the sweet orange ancestor, but remains active and further differentiates into secondary carpels. In the end, the incompletely developed secondary carpels are pitifully squeezed on the top of the axle placenta, turning into a small navel-shaped secondary fruit.

Drawing: Garlic; Navel Orange Image Source: dendyh7

Navel oranges are divided into male and female? Fake! Fake!

The confident, beautiful and generous little navel orange has a little worry. For many years, it has been troubled by a rumor full of gender bias: those who wear navel-revealing clothes are female oranges, and those who do not are male oranges. Navel orange: Please, please, I am an infertile little fruit that cannot even give birth to cubs. How can you tell me whether I am male or female? It’s the 21st century! Please stop applying your old-fashioned binary gender views to our little fruits!

No matter how big our belly buttons are, there is no gender distinction, and it has nothing to do with sweetness! | Islam M. Khattab

In fact, the size of the navel is only related to the shape and position of the secondary fruit, and the shape of the secondary fruit on the same fruit tree can also vary greatly : if it grows outward, the navel will be larger, and if it grows inward, the navel will be smaller. Secondary fruits are just a little willful, so what.

As for which one tastes better, there is currently no data to show that the shape and size of secondary fruits are related to the overall sweetness of the fruit .

Due to the distribution characteristics of the nutrient transport process, there are indeed differences in sweetness in different parts of a single fruit. Total soluble solids (TSS) are an important indicator for determining the maturity and intrinsic quality of the fruit. The TSS near the fruit stalk is usually lower than the top of the fruit; the secondary fruit at the top of the navel orange usually has a higher TSS content than the primary fruit . But whether it tastes good or not is a matter of personal opinion. After all, the secondary fruit is always wrapped in a thick white fruit vein (vascular bundle), and the moisture is not as abundant as the primary fruit, and the taste is relatively dry.

The sweetness of the secondary fruit of navel orange is usually higher than that of the main fruit｜Islam M. Khattab

More than just navel oranges

Secondary fruit is not unique to navel oranges, but the trait of navel oranges has been stably preserved through asexual reproduction. Thanks to the unstable genetic characteristics of citrus, secondary fruit is not uncommon in grapefruits, a member of the citrus family .

Hainan seedless honey pomelo (A) and its "navel pomelo" (B)｜Wang Qi

In addition, secondary fruits can also be produced in Solanaceae plants such as bell peppers and tomatoes, as well as papayas in the Carica family.

Bell pepper: Big pepper inside small pepper 😋｜Lavtian B; indefenseofplants.com/blog/2021/12/6/theres-a-pepper-inside-my-pepper

Papaya nesting dolls | Bergman, HF; imgur.com/a/qboj5.

References

[1] Ancillo G, Medina A. Citrus[M]. Universitat de València, Jardí Botànic, 2015.

[2] Bowman JL, Drews GN, Meyerowitz E M. Expression of the Arabidopsis floral homeotic gene AGAMOUS is restricted to specific cell types late in flower development[J]. The Plant Cell, 1991, 3(8): 749-758.

[3] Jiang Cailun, Peng Liangzhi, Lei Ting, et al. Differences in soluble solids content between individual citrus fruits and in different fruit parts [J]. Chinese Southern Fruit Trees, 2006, 35(2):3.

[4] Wan Xiaoyun. A preliminary study on the formation mechanism of secondary fruit in navel orange[D]. Huazhong Agricultural University, 2010.

Author: Garlic

Editor: luna

Cover image source: Tuchong Creative

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