# Functional Anatomy, Impact Behavior and Energy Dissipation of the Peel of Citrus × limon: A Comparison of Citrus × limon and Citrus maxima

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## Abstract

**:**

## 1. Introduction

## 2. Results

#### 2.1. Anatomy

^{3}compared to 417.72 ± 60.02 kg/m

^{3}, respectively. Thin cross sections were made to analyze the radial cell arrangement from the pulp to the epidermis. Figure 3 shows a comparison of the number of cells per millimeter, in relation to the distance from the pulp of Citrus × limon and Citrus maxima. The number of cells per millimeter for Citrus × limon increases strongly with increasing distance from the pulp, after a short (initial) decrease (Figure 3a). Comparing the number of cells per millimeter of Citrus × limon with those of Citrus maxima, the average number of cells of Citrus maxima is lower. The difference between the average cell numbers per millimeter in Citrus × limon and Citrus maxima is especially large in the areas close to the epidermis (flavedo); however, close to the pulp in the endocarp region (interpolated lines 1–3), only the number of cells per millimeter at the second interpolated line shows a significant difference between Citrus × limon and Citrus maxima (0.01 ≤ p < 0.05). In the flavedo region, which is represented in the median by the last four (IQR:0) interpolated lines (interpolated lines 17–20) in Citrus × limon and by the last (IQR:1) interpolated line in Citrus maxima, the cells are more densely packed than in the central peel region farther away from the epidermis, i.e., the albedo region, where more intercellular spaces are visible. The toluidine blue-stained thin section of the peel of Citrus × limon (Figure 3b) illustrates the gradual change in cell arrangement from the endocarp (left) to the epidermis (right), which from a qualitative perspective does not differ from that of Citrus maxima [2], except for absolute sample thickness and tissue density. The cross section shows that the peels of both fruits mainly consist of parenchyma cells (pc), vascular bundles (vb), oil glands (og) and intercellular spaces (is) (Figure 3b).

#### 2.2. Sample Thickness

#### 2.3. Drop-Weight Tests

#### 2.4. Different Drop Heights

## 3. Discussion

## 4. Materials and Methods

#### 4.1. Plant Material

#### 4.2. Anatomy

#### 4.3. Sample Preparation

#### 4.4. Mechanical Testing

_{1}) (intersection of green and red regression lines) and after (v

_{2}) the impact (intersection of red and blue regression lines). A linear regression was assumed since the observed period of time was very short [3].

_{2}) and the velocity immediately before the impact (v

_{1}) is the coefficient of restitution (COR).

_{kin}) is calculated as one-half the product of an object’s mass and the square of its velocity.

_{diss}) is the relative ratio of the kinetic energy before (E

_{kin}

_{1}) and after the impact (E

_{kin}

_{2}). The rel. energy dissipation is the energy of an impact that becomes dissipated by the impacted object.

#### 4.5. Statistics

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

- Thielen, M.; Speck, T.; Seidel, R. Viscoelasticity and compaction behaviour of the foam-like pomelo (Citrus maxima) peel. J. Mater. Sci.
**2013**, 48, 3469–3478. [Google Scholar] [CrossRef] - Thielen, M.; Schmitt, C.N.Z.; Eckert, S.; Speck, T.; Seidel, R. Structure-function relationship of the foam-like pomelo peel (Citrus maxima)—An inspiration for the development of biomimetic damping materials with high energy dissipation. Bioinspir. Biomim.
**2013**, 8, 25001. [Google Scholar] [CrossRef] - Thielen, M.; Speck, T.; Seidel, R. Impact behaviour of freeze-dried and fresh pomelo (Citrus maxima) peel: Influence of the hydration state. R. Soc. Open Sci.
**2015**, 2, 140322. [Google Scholar] [CrossRef] [PubMed][Green Version] - Gentile, A.; La Malfa, S.; Deng, Z. The Citrus Genome; Springer International Publishing: Cham, Switzerland, 2020; ISBN 978-3-030-10799-4. [Google Scholar]
- Moore, G.A. Oranges and lemons: Clues to the taxonomy of Citrus from molecular markers. Trend Genet.
**2001**, 17, 536–540. [Google Scholar] [CrossRef] - Klock, P.; Klock, M.; Klock, T.A. Das Große Ulmer-Buch der Zitruspflanzen; Ulmer: Stuttgart, Germany, 2007; ISBN 9783800146932. [Google Scholar]
- Ladaniya, M.S. Citrus Fruit: Biology, Technology and Evaluation; Elsevier Academic Press, ScienceDirect: Amsterdam, The Netherlands; Heidelberg, Germany, 2008; ISBN 9780123741301. [Google Scholar]
- Nicolosi, E.; Deng, Z.N.; Gentile, A.; La Malfa, S.; Continella, G.; Tribulato, E. Citrus phylogeny and genetic origin of important species as investigated by molecular markers. Theor. Appl. Genet.
**2000**, 100, 1155–1166. [Google Scholar] [CrossRef] - Fang, D.; Krueger, R.R.; Roose, M.L. Phylogenetic Relationships among Selected Citrus Germplasm Accessions Revealed by Inter-simple Sequence Repeat (ISSR) Markers. J. Am. Soc. Hortic. Sci.
**1998**, 123, 612–617. [Google Scholar] [CrossRef][Green Version] - Gross, J.; Timberg, R.; Graef, M. Pigment and Ultrastructural Changes in the Developing Pummelo Citrus grandis ‘Goliath’. Bot. Gaz.
**1983**, 144, 401–406. [Google Scholar] [CrossRef] - Morton, J.F. Fruits of Warm Climates; Julia F. Morton: Miami, FL, USA, 1987; Available online: https://hort.purdue.edu/newcrop/morton/index.html (accessed on 21 November 2021).
- Fischer, S.F.; Thielen, M.; Loprang, R.R.; Seidel, R.; Fleck, C.; Speck, T.; Bührig-Polaczek, A. Pummelos as Concept Generators for Biomimetically Inspired Low Weight Structures with Excellent Damping Properties. Adv. Eng. Mater.
**2010**, 12, B658–B663. [Google Scholar] [CrossRef] - Janzen, D.H. Why Fruits Rot, Seeds Mold, and Meat Spoils. Am. Nat.
**1977**, 111, 691–713. [Google Scholar] [CrossRef] - Bührig-Polaczek, A.; Fleck, C.; Speck, T.; Schüler, P.; Fischer, S.F.; Caliaro, M.; Thielen, M. Biomimetic cellular metals-using hierarchical structuring for energy absorption. Bioinspir. Biomim.
**2016**, 11, 45002. [Google Scholar] [CrossRef] - Sharma, B.D.; Hore, D.K.; Gupta, S.G. Genetic resources of Citrus of north-eastern India and their potential use. Genet. Resour. Crop Evol.
**2004**, 51, 411–418. [Google Scholar] [CrossRef] - Ford, E.S. Anatomy and Histology of the Eureka Lemon. Bot. Gaz.
**1942**, 104, 288–305. [Google Scholar] [CrossRef] - Scott, F.M.; Baker, K.C. Anatomy of Washington Navel Orange Rind in Relation to Water Spot. Bot. Gaz.
**1947**, 108, 459–475. [Google Scholar] [CrossRef] - Underhill, S.J.; McLauchlan, R.L.; Dahler, J.M.; Bertram, J. Flavedo and albedo changes in ‘eureka’ lemons caused by static compression and impact loading. J. Texture Stud.
**1998**, 29, 437–452. [Google Scholar] [CrossRef] - Yang, B.; Chen, W.; Xin, R.; Zhou, X.; Tan, D.; Ding, C.; Wu, Y.; Yin, L.; Chen, C.; Wang, S.; et al. Pomelo Peel-Inspired 3D-Printed Porous Structure for Efficient Absorption of Compressive Strain Energy. J. Bionic Eng.
**2022**, 19, 448–457. [Google Scholar] [CrossRef] - Wang, B.; Pan, B.; Lubineau, G. Morphological evolution and internal strain mapping of pomelo peel using X-ray computed tomography and digital volume correlation. Mater. Des.
**2018**, 137, 305–315. [Google Scholar] [CrossRef][Green Version] - Van Opdenbosch, D.; Thielen, M.; Seidel, R.; Fritz-Popovski, G.; Fey, T.; Paris, O.; Speck, T.; Zollfrank, C. The pomelo peel and derived nanoscale-precision gradient silica foams. Bioinspired Biomim. Nanobiomater.
**2012**, 1, 117–122. [Google Scholar] [CrossRef] - Li, T.-T.; Wang, H.; Huang, S.-Y.; Lou, C.-W.; Lin, J.-H. Bioinspired foam composites resembling pomelo peel: Structural design and compressive, bursting and cushioning properties. Compos. Part B Eng.
**2019**, 172, 290–298. [Google Scholar] [CrossRef] - Fischer, S.F.; Thielen, M.; Weiß, P.; Seidel, R.; Speck, T.; Bührig-Polaczek, A.; Bünck, M. Production and properties of a precision-cast bio-inspired composite. J. Mater. Sci.
**2014**, 49, 43–51. [Google Scholar] [CrossRef] - Speck, T.; Bold, G.; Masselter, T.; Poppinga, S.; Schmier, S. Biomechanics and functional morphology of plants—Inspiration for biomechanic materials and structures. In Plant Biomechanics: From Structure to Function at Multiple Scales; Geitmann, A., Gril, J., Eds.; Springer International Publishing: Cham, Switzerland, 2018; pp. 399–433. ISBN 978-3-319-79098-5. [Google Scholar]
- Gibson, L.J.; Ashby, M.F. Cellular Solids: Structure and Properties, 2nd ed.; Cambridge University Press: Cambridge, UK, 1997; ISBN 0521495601. [Google Scholar]
- Birman, V.; Byrd, L.W. Modeling and Analysis of Functionally Graded Materials and Structures. Appl. Mech. Rev.
**2007**, 60, 195–216. [Google Scholar] [CrossRef] - Tarlochan, F. Sandwich Structures for Energy Absorption Applications: A Review. Materials
**2021**, 14, 4731. [Google Scholar] [CrossRef] [PubMed] - Gibson, L.J.; Ashby, M.F. The mechanics of three-dimensional cellular materials. Proc. R. Soc. Lond. A
**1982**, 382, 43–59. [Google Scholar] [CrossRef] - Gibson, L.J. Biomechanics of cellular solids. J. Biomech.
**2005**, 38, 377–399. [Google Scholar] [CrossRef] [PubMed] - Apetre, N.A.; Sankar, B.V.; Ambur, D.R. Low-velocity impact response of sandwich beams with functionally graded core. Int. J. Solids Struct.
**2006**, 43, 2479–2496. [Google Scholar] [CrossRef][Green Version] - Ortiz, J.; Zhang, G.; McAdams, D.A. A Model for the Design of a Pomelo Peel Bioinspired Foam. J. Mech. Des.
**2018**, 140, 114501. [Google Scholar] [CrossRef][Green Version] - Liu, Z.; Meyers, M.A.; Zhang, Z.; Ritchie, R.O. Functional gradients and heterogeneities in biological materials: Design principles, functions, and bioinspired applications. Prog. Mater. Sci.
**2017**, 88, 467–498. [Google Scholar] [CrossRef] - Stover, E.; Castle, W.; Chao, C.-C.T. Trends in U.S. Sweet Orange, Grapefruit, and Mandarin-type Cultivars. Horttech
**2005**, 15, 501–506. [Google Scholar] [CrossRef] - Abouzari, A.; Mahdi Nezhad, N. The Investigation of Citrus Fruit Quality. Popular Characteristic and Breeding. Acta Univ. Agric. Silvic. Mendel. Brun.
**2016**, 64, 725–740. [Google Scholar] [CrossRef][Green Version] - Jenks, M.A.; Bebeli, P.J. (Eds.) Breeding for Fruit Quality; Wiley-Blackwell: Ames, IA, USA, 2011; ISBN 0470959355. [Google Scholar]
- Niklas, K.J. Plant Biomechanics: An Engineering Approach to Plant form and Function; University of Chicago Press: Chicago, IL, USA, 1992; ISBN 0226586308. [Google Scholar]
- Lewicki, P.P.; Witrowa-Rajchert, D.; Mariak, J. Changes of Structure During Rehydration of Dried Apples. J. Food Eng.
**1997**, 32, 347–350. [Google Scholar] [CrossRef] - Caliaro, M.; Schmich, F.; Speck, T.; Speck, O. Effect of drought stress on bending stiffness in petioles of Caladium bicolor (Araceae). Am. J. Bot.
**2013**, 100, 2141–2148. [Google Scholar] [CrossRef] - Westerman, B.; Stringfellow, P.M.; Eccleston, J.A. EVA mouthguards: How thick should they be? Dent. Traumatol.
**2002**, 18, 24–27. [Google Scholar] [CrossRef] [PubMed] - Kirugulige, M.; Kitey, R.; Tippur, H.V. Dynamic fracture behavior of model sandwich structures with functionally graded core: A feasibility study. Compos. Sci. Technol.
**2005**, 65, 1052–1068. [Google Scholar] [CrossRef] - Verissimo, C.; Costa, P.V.M.; Santos-Filho, P.C.F.; Fernandes-Neto, A.J.; Tantbirojn, D.; Versluis, A.; Soares, C.J. Evaluation of a dentoalveolar model for testing mouthguards: Stress and strain analyses. Dent. Traumatol.
**2016**, 32, 4–13. [Google Scholar] [CrossRef] [PubMed] - Kampowski, T.; Mylo, M.D.; Speck, T.; Poppinga, S. On the morphometry, anatomy and water stress behaviour of the anisocotyledonous Monophyllaea horsfieldii (Gesneriaceae) and their eco-evolutionary significance. Bot. J. Linn. Soc.
**2017**, 185, 425–442. [Google Scholar] [CrossRef] - R Core Team. R: A Language and Environment for Statistical Computing; R Foundation for Statistical Computing: Vienna, Austria, 2019. [Google Scholar]

**Figure 2.**Lemon (Citrus × limon) fruit divided into three parts (lower (L), middle (M) and upper part (U)), (

**a**) showing unripe lemon fruits in their natural orientation on the tree. (

**b**) Ripe Citrus × limon with samples cut out using a cork borer. (

**c**) Freshly cut-out sample of the peel of Citrus × limon.

**Figure 3.**(

**a**) Average cell number per millimeter counted at interpolated lines in thin sections of Citrus × limon and Citrus maxima. (

**b**) Cross thin section of the peel Citrus × limon stained with toluidine blue, showing the 20 interpolated lines along which the cell numbers have been counted. The cross thin section of Citrus × limon is characteristic for both species from a qualitative point of view. The peel of both species mainly consists of oil glands (og), parenchyma (pc), intercellular spaces (is) and vascular bundles (vb). The cell density shows a gradual decrease from the outside of the peel (right) to the inside. In the flavedo close to the epidermis (right) the cells are visibly more densely arranged than in the part of the albedo which is close to the endocarp (left).

**Figure 4.**Sample thickness according to the parts within the fruit. The sample thickness of each sample was measured three times with a digital caliper. The level of statistical significance is indicated in the figure as follows: n.s.: p ≥ 0.05; ***: p < 0.001.

**Figure 5.**Characteristic force-time diagrams of a drop-weight test for a fresh and a freeze-dried sample of Citrus × limon. The area under each graph characterizes the impulse of the impact. The impactor (mass = 0.061 kg) was dropped from a height of 0.81 m onto the samples.

**Figure 6.**Comparison of fresh and freeze-dried samples of Citrus × limon. Drop-weight tests with an impactor (mass 0.061 kg) dropped from a 0.81-m height. The level of statistical significance is indicated in the figure as follows: n.s.: p ≥ 0.05; ***: p < 0.001.

**Figure 7.**Force-time diagrams for fresh Citrus × limon samples at different drop heights (0.25 m (n = 7), 0.81 m (n = 10), 1.25 m (n = 6)). For clarity, only 10 out of 208 samples tested at a drop height of 0.81 m are plotted. The drop-weight tests were performed with a cylindrical impactor (mass = 0.61 kg).

**Figure 8.**(

**a**) Coefficient of restitution (COR) of the impact on fresh and freeze-dried samples of Citrus × limon for different drop heights, and (

**b**) relative energy dissipation of fresh and freeze-dried samples of Citrus × limon for different drop heights. All drop-weight tests were performed with a cylindrical impactor with a mass of 0.061 kg.

**Figure 9.**(

**a**,

**b**) Coefficient of restitution (COR) of fresh and freeze-dried samples, and relative dissipated energy of the impact (

**c**,

**d**) on Citrus × limon and Citrus maxima for different drop heights. All drop-weight tests were carried out with a cylindrical impactor having a mass of 0.061 kg. The data of Citrus maxima are taken from [3].

**Figure 10.**Average velocity of an impactor with mass = 0.061 kg dropped from a height of 0.81 m onto a fresh lemon sample. The impactor’s position is calculated for each point in time by averaging five tracking points on the impactor. The intersections of the regression lines show the velocities immediately before (v

_{1}) (intersection of the green and red regression lines) and after (v

_{2}) the impact (red and blue regression lines).

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**MDPI and ACS Style**

Jentzsch, M.; Becker, S.; Thielen, M.; Speck, T. Functional Anatomy, Impact Behavior and Energy Dissipation of the Peel of *Citrus* × *limon*: A Comparison of *Citrus* × *limon* and *Citrus maxima*. *Plants* **2022**, *11*, 991.
https://doi.org/10.3390/plants11070991

**AMA Style**

Jentzsch M, Becker S, Thielen M, Speck T. Functional Anatomy, Impact Behavior and Energy Dissipation of the Peel of *Citrus* × *limon*: A Comparison of *Citrus* × *limon* and *Citrus maxima*. *Plants*. 2022; 11(7):991.
https://doi.org/10.3390/plants11070991

**Chicago/Turabian Style**

Jentzsch, Maximilian, Sarah Becker, Marc Thielen, and Thomas Speck. 2022. "Functional Anatomy, Impact Behavior and Energy Dissipation of the Peel of *Citrus* × *limon*: A Comparison of *Citrus* × *limon* and *Citrus maxima*" *Plants* 11, no. 7: 991.
https://doi.org/10.3390/plants11070991