In fact, just as people age and wrinkles on their faces, plants can also wrinkle on the surface for a variety of reasons. The folds on the surface of the pepper have attracted scientists to explore the mystery.
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01
The cause of the formation of folds on the surface of the pepper
Peppers wrinkling due to drying is a common phenomenon in daily life, but if you want to carefully analyze why such complex folds occur, it is not a simple matter.
Recently, mainland scientists have used mathematical physics and other related knowledge to carry out the evolution experiment of the complex surface morphology of the toroidal shell structure and a series of theoretical calculations to explain the cause of the surface folds of pepper.
Annular core-shell structure cayenne pepper. Image source: References[1]
As they continue to grow, many peppers grow into curved ring-shaped structures, and coupled with the hollow inside of peppers, researchers classify peppers as annular core-shell structural substances.
Researchers say that the folds on the surface of peppers are affected by a combination of internal and external factors. Geometry is an important physical factor affecting the surface folds of pepper.
Among the geometric structures, the curvature of the annular core-shell structure is the most typical feature. The ring structure contains varying curvatures, including positive, negative and zero Gaussian curvatures, which affect the morphological changes of peppers during growth, which in turn induces complex folds on the surface of peppers.
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In order to study the cause of folds, the researchers used the finite element method to peel off the shell unit on the surface and the soft core unit inside.
Due to the dehydration and other behaviors that occur during the growth of peppers, the mechanical properties of the surface and interior of peppers are different, resulting in bending deformation of the structure and strain mismatch between the surface layer and the substrate. In the toric structure of the pepper, the curvature is constantly changing, which leads to uneven stress distribution of the whole pepper, which in turn leads to the symmetry of the pepper. To sum up, the internal and external forces of the pepper are different, and in the pull of the internal and external structure, the pepper gradually changes its shape.
We have solved the problem of why peppers deform with drying, but different peppers have different folds. To solve this problem, we must then analyze the structure of peppers.
Through the nonlinear ring-shell mechanical model, it is found that elongated structural objects are more likely to form folds in the inner ring, while folds are easier to form in the outer ring in the annular structure of small holes. At the same time, the hardness of the object also affects the topography. In softer ring shells, dimple-like depression morphology tends to occur. In harder ring shells, bidirectional striped or spiral folded topography is more likely to occur.
This research result not only solves the problem of pepper folds, but also applies the theoretical model constructed by the researchers to other things with slightly different shapes, which can help predict the change of the morphology of objects under different conditions, and help guide the development of product shape design based on fold morphology control.
Simulation diagram of morphological evolution of annular core-shell structure. Image source: References[1]
02
The relationship between pepper folds and spiciness
An important factor affecting the spiciness of chili peppers is the amount of capsaicin in chili peppers. The higher the capsaicin content, the spicier the chili pepper. The folds of peppers are related to the lignin in peppers, and the higher the lignin, the harder and stiffer the pepper, the fewer wrinkles, and vice versa.
In this way, capsaicin determines the spiciness and lignin determines the folds, as if the two perform their respective duties and do not affect each other? In fact, in the synthesis process of the two, there is still a mutual influence relationship.
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Phenolic substances are required during the synthesis of capsaicin. Studies have shown that peroxidase in chili peppers can oxidize and decompose the phenolic substances that synthesize capsaicin to produce lignin synthesis. It can be seen that the spiciness and hardness of peppers are the relationship between one and the other. The lower the activity of peroxidase, the lower the content of lignin, the more pepper folds, the more capsaicin content will be, and the spicier it will taste. It turns out that the peppers, which seem to have experienced many vicissitudes, have not reduced their lethality at all.
03
The secret contained in the folds of other plants
For example, peppers become dry and wrinkled due to factors such as dehydration, and it is also reflected in other plants. A variety of plants have different shapes and structures, and the process of wrinkling also brings different enlightenment to researchers.
1. Passion fruit
During the ripening of passion fruit, the skin will gradually wrinkle. The researchers described the wrinkling process of passion fruit as a chiral wrinkling pattern. The mechanical model was used to reveal the universal law of passion fruit wrinkling process, and showed the structure-elastic mechanism behind the formation of folds. Based on this mechanism, the researchers designed a robot that can grasp objects, which promotes the development of adaptive grasping robots.
Passion fruit, gallery copyright image, not authorized to reprint
2. Lotus leaves
The morphological evolution of lotus leaves during the growth process has stimulated the research interest of researchers. The researchers 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 appear curved and conical, and long wavy folds will grow at the outer edges of the lotus leaves. For floating lotus leaves, there will be short-wave folds on the outer edges. The research results can provide new ideas for the regulation of biomimetic structure morphology.
Lotus leaf, gallery copyright image, not authorized to reprint
It can be seen that many biological development laws 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 is some peppers from Shanghai Chenshan Botanical Garden, welcome to leave a message to guess, which of the six peppers labeled is the hottest? The answer will be revealed in the comments section tomorrow morning!
Photo by Xu Lai
Bibliography:
[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.http://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 a preliminary study on the factors affecting the content of capsaicin[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).
Author: Shi Wuyao
(Popular Science China)