It’s 2023, why haven’t seedless pomegranates been invented yet!

Nowadays, people have higher and higher requirements for fruits. In addition to being sweet, juicy, and sweet and sour, they also need to be easy to eat, and preferably without spitting out the seeds. As a result, various seedless fruits came into being, such as seedless watermelons and rice-core lychees. The seeds of many fruits have gradually disappeared.

However, some fruit seeds stick to their posts, such as pomegranates, which have a great taste and flavor, but those seeds that bite your teeth will greatly reduce the pleasure of eating pomegranates. Why can't horticulturists cultivate seedless pomegranates?

Seedless fruit, where to eat

Let’s first understand the edible parts of seedless fruits.

The edible part of watermelon is the placenta , which has a similar mission to the placenta of animals, providing a place for seeds to grow and provide nutrition.

Image source: pixabay

The edible part of the lychee is the aril , which is meant to be a bait to lure animals to eat and spread the seeds;

Image source: pixabay

The edible part of the banana is a mixture of the peel and the placenta ;

Image source: pixabay

As for jackfruit, what we eat are actually their fleshy tepals .

Image source: pixabay

Why can't you eat seedless pomegranates?

The simplest way to obtain seedless fruit is to cultivate or screen out individuals with poorly developed seeds . Seedless watermelon is obtained by hybridizing diploid watermelon with tetraploid watermelon to obtain triploid individuals that cannot effectively produce seeds, thus eliminating the trouble of spitting out watermelon seeds when eating watermelon; as for seedless bananas and lychees, they are obtained by planting suckers or grafting, "cloning" those individuals with poorly developed seeds, thus obtaining seedless fruits.

However, the above methods are not applicable to pomegranates. If we use these methods to cultivate pomegranates, we will eventually have to eat the bitter pomegranate skin. Seeing this, you may have guessed it. The reason why there is no seedless pomegranate is that we eat the seeds of the pomegranate.

Generally speaking, plant seeds are composed of embryo, cotyledon, endosperm and seed coat. In most angiosperm seeds, the seed coat is the least noticeable. This thin coat can only help the seeds lock in moisture and provide some defense against fungi (Note: In plant classification, seed plants are divided into gymnosperms and angiosperms.) However, this is not the case with pomegranate seeds. The peel is so plump, which is rare in the plant kingdom.

The seeds of a pomegranate are composed of a seed and a testa, but the testa is divided into two layers: the bony endocarp and the fleshy exocarp. This layer of exocarp is the "pulp" of the pomegranate that we like the most . Now the problem is clear. If we really cultivate seedless pomegranates, then we don't need to eat this pomegranate.

Image source: Wikipedia

The day when you can eat pomegranates without spitting out the seeds may not be far away!

The question is, is there a solution that has the best of both worlds?

Then we need to solve the problem by starting with the texture of the endocarp. The hardness of the endocarp is directly related to the lignin content in the cells, which is a bit like the process of bamboo shoots turning into bamboo. The increase in hardness of both is related to the accumulation of lignin. The goal of accumulating lignin is to protect itself and resist being eaten by animals.

The study found that during the growth of pomegranates, the lignin content in the seed coat showed a trend of first increasing and then decreasing, especially in Tunisian soft-seeded pomegranates and Mengzi soft-seeded pomegranates. In the future breeding process, genome analysis and molecular in situ hybridization technology can be used to screen out individuals with defects in seed coat lignin accumulation and expression, so that excellent soft-seeded pomegranates can be obtained.

However, the soft-seeded pomegranates we eat so far are still the result of long-term screening of soft-seeded budding materials and propagation through grafting technology. According to seed hardness, Chinese fresh-eating pomegranate varieties are divided into soft-seeded varieties (seed hardness <3.67 kg/cm2), semi-soft-seeded varieties (seed hardness 3.67-4.20 kg/cm2) and hard-seeded varieties (seed hardness >4.2 kg/cm2).

In addition to cultivating special varieties, improving planting techniques can also help us get pomegranates with better taste. In the experiment, it was found that light intensity affects the hardness of pomegranate seeds. On the same pomegranate tree, the hardness of pomegranate seeds on the west and south sides of the crown is significantly higher than that of pomegranate seeds on the north and inner sides of the crown. This suggests that by controlling the light intensity, it is possible to control the hardness of pomegranate seeds.

From this it seems that the day when we can eat pomegranates without spitting out the seeds is not far away.

References

[1] Lu Lijuan, Gong Xuemei, Zhu Liwu. Study on seed hardness characteristics of Chinese pomegranate cultivar resources. Journal of Anhui Agricultural University, 2006, 33(3): 356-359.

[2] Hu Qingxia et al. Study on the growth and development of Tunisian soft-seeded pomegranate fruit and its quality formation. Fruit Science, 2022, 39(3): 426-438.

Planning and production

Author: Shi Jun, PhD in Botany

Review丨Yang Laisheng, Researcher at Lanzhou Academy of Agricultural Sciences

Planning｜Xu Lai

Editor: Yang Yaping