The more wrinkled the pepper is, the spicier it is? Here’s the truth!

In fact, just like people age and develop wrinkles on their faces, plants also develop wrinkles on their surfaces for various reasons. The wrinkles on the surface of peppers have attracted scientists to explore the mystery behind them.

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01

The reason why the pepper surface wrinkles

It is a common phenomenon in daily life that peppers develop wrinkles due to drying, but it is not a simple task to analyze in detail why such complex wrinkles are formed.

Recently, Chinese scientists have used relevant knowledge in mathematical physics to conduct evolution experiments on the complex surface morphology of ring-shaped core-shell structured materials and a series of theoretical calculations to explain the causes of wrinkles on the surface of peppers.

Ring-shaped core-shell pepper. Image source: Reference [1]

As they continue to grow, many peppers develop into a curved ring structure. In addition, the inside of the peppers is hollow, so the researchers classify the peppers as a ring-shaped core-shell structure material .

Researchers said that the wrinkles on the surface of peppers are affected by many internal and external factors, and the geometric structure is an important physical factor affecting the wrinkles on the surface of peppers.

In terms of geometric structure, the curvature of the ring-shaped core-shell structure is the most typical feature. The ring structure contains changing curvatures, including positive, negative and zero Gaussian curvatures. These changing curvatures will affect the changes in the shape of peppers during their growth, and then induce the formation of complex wrinkles on the surface of peppers.

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In order to study the causes of wrinkles, the researchers used the finite element method to peel off the pepper into surface shell elements and internal soft core elements.

As peppers dehydrate during their growth, they have different mechanical properties on the surface and inside, which results in bending deformation of the structure and strain mismatch between the surface and the base. In the annular structure of peppers, the curvature is constantly changing, which results in uneven stress distribution of the peppers as a whole, leading to symmetry destruction. In summary, the peppers are subjected to different forces inside and outside, and in the pulling of the internal and external structures, the peppers gradually change their shape.

We have solved the problem of why peppers deform as they dry, but different peppers have different wrinkles . To solve this problem, we need to analyze the structure of peppers.

Through the nonlinear ring shell mechanics model, it is found that slender structural objects are more likely to form wrinkles on the inner ring, while wrinkles are more likely to form on the outer ring in the ring structure with small holes. At the same time, the hardness of the object will also affect the morphology. In a softer ring shell, dimple-like concave morphology is more likely to be produced. In a harder ring shell, bidirectional stripes or spiral wrinkles are more likely to be produced.

This research result not only solves the problem of pepper wrinkles, but the theoretical model constructed by the researchers is also applicable to other things with slightly different shapes. It can help predict the changes in the morphology of objects under different conditions and help guide the development of product shape design based on wrinkle morphology regulation .

Simulation diagram of the morphological evolution of the ring-shaped core-shell structure. Image source: Reference [1]

02

The relationship between pepper wrinkles and spiciness

An important factor affecting the spiciness of peppers is the content of capsaicin in the peppers. The higher the capsaicin content, the spicier the peppers. The wrinkles of peppers are related to the lignin in the peppers. The higher the lignin content, the stiffer the peppers are and the fewer wrinkles they have. Otherwise, the more wrinkled they are.

So it seems that capsaicin determines the spiciness and lignin determines the wrinkles, as if the two have their own functions and do not affect each other? In fact, there is a mutual influence in the synthesis process of the two.

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Phenolic substances are needed in the synthesis of capsaicin. Studies have shown that the peroxidase in peppers can oxidize and decompose the phenolic substances that synthesize capsaicin to produce the synthesis of lignin. It can be seen that the spiciness and hardness of peppers are in a trade-off relationship . The lower the activity of peroxidase, the lower the content of lignin, the more wrinkles the pepper has, the more capsaicin it will contain, and it will taste spicier. It turns out that the peppers that look weathered have not reduced their lethality at all.

03

Other secrets hidden in the folds of plants

The phenomenon of peppers becoming dry and wrinkled due to dehydration and other factors can also be seen in other plants. Various plants have different appearances and structures, and the wrinkling process also brings different inspirations to researchers.

1. Passion fruit

As passion fruit matures, its skin gradually wrinkles. Researchers describe the wrinkling process of passion fruit as a chiral wrinkling pattern . Using a mechanical model, the researchers revealed the universal laws of the wrinkling process of passion fruit and the structural-elastic mechanism behind the wrinkle formation. Based on this mechanism, the researchers designed a robot that can grasp objects, promoting the development of adaptive grasping robots.

Passion fruit, copyrighted image, unauthorized reproduction

2. Lotus Leaf

The morphological evolution of lotus leaves during their growth process has aroused the research interest of researchers. The researchers have constructed a thin plate model that can explain the differential growth of suspended leaves and floating leaves in different directions to accurately predict the evolution of aquatic plant morphology .

The results show that lotus leaves growing on the water surface usually have curved cones and long-wave folds on the outer edges of the lotus leaves. For floating lotus leaves, the outer edges have short-wave folds. The research results can provide new ideas for the regulation of bionic structural morphology.

Lotus leaf, copyrighted image, unauthorized reproduction

It can be seen that many laws of biological development can be explained by science, and will provide novel ideas for the development of new things. What other interesting natural phenomena have you discovered?

Finally, the picture below shows some peppers from Shanghai Chenshan Botanical Garden. Everyone is welcome to leave a message and guess which of the six numbered peppers is the spiciest?

Photo taken by: Xu Lai

References:

[1] Wang, T., Dai, Z., Potier-Ferry, M. & Xu, F. Phys. Rev. Lett. 130, 048201 (2023).

[2] A chilli's wrinkles and a cherry's dimples explained. https://doi.org/10.1038/d41586-023-00132-y.

[3] Xu, F., Huang, Y., Zhao, S. et al. Chiral topographic instability in shrinking spheres. Nat Comput Sci 2, 632–640 (2022).

[4] Zhou Tao. Evaluation of pepper variety resources and preliminary study on factors affecting capsaicin content[D]. Hunan Agricultural University, 2005.

[5] Xu, F., Huang, Y., Zhao, S. et al. Chiral topographic instability in shrinking spheres. Nat Comput Sci 2, 632–640 (2022).

[6] Xu, F., Fu, C., Yang Y. et al. Water affects morphogenesis of growing aquatic plantleaves. Phys. Rev. Lett. 124, 038003 (2020).

Produced by: Science Popularization China

Author: Ishimiya

Producer: China Science Expo

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