1. Why Stripes? Experts Have No Definite Answer
A question about watermelon stripes appeared in the "Plant Q&A" section of the Japanese Society of Plant Physiologists. In a response dated September 12, 2007, it was stated that the difference in stripe color is due to the amount of chlorophyll, and other pigments are not involved. [2]
The expert considered various factors, such as the presence of vascular bundles (channels for water and nutrients) inside the dark stripes, and the fact that some seeded watermelons lack stripes. Despite this analysis, the response concluded that a definitive answer to why stripes exist could not be obtained. [2]
2. Dark and Light Stripes Differ in Green Intensity
Materials from the Plant Physiology Laboratory at Waseda University compare the dark and light parts of watermelon skin. Under a microscope, the dark parts appear to have more chloroplasts. [1]
When measured, the total chlorophyll per area was 1.06 g/m² in the dark parts and 0.56 g/m² in the light parts. Since 1.06 divided by 0.56 is about 1.9, the dark parts have nearly twice as much chlorophyll. [1]
However, a summary of the same materials notes that there was no significant difference in photosynthesis efficiency between the dark and light parts based on color. This means that the darker color did not result in noticeably higher photosynthetic efficiency. [1]
3. Watermelon Stripes Are Inherited, From a 1963 Study to Recent Genes
A 1963 paper in the Journal of Breeding Science counted the offspring (F2 generation) of a cross between striped and non-striped watermelons. The skin patterns split into 49 striped and 24 non-striped. The report stated that stripes are dominant over non-stripes, following a single-factor Mendelian inheritance pattern. [3]
The paper claims a 3:1 separation ratio, but the actual numbers in the abstract (49:24) look closer to 2:1. The reason for this discrepancy is not clear from the abstract. Also, these results are from specific lines studied in the paper and may not apply to all varieties. [3]
Recent research has identified specific genes. In 2025, a study reported that the protein ClSP is abundant in dark green stripes and activates the gene ClAPRR2, creating the contrast in stripe darkness. [4] Another study in the journal Plant Science reported that an incomplete structure of the ClAPRR2 gene causes light stripes. [5] These explanations are based on summaries of the papers. [4][5]
4. One Theory: Stripes Help Birds Find the Fruit
The Ministry of Agriculture, Forestry and Fisheries introduces a theory in its children's Q&A that stripes evolved to help birds find the fruit. The idea is that the watermelon, which originated in desert areas, uses the visual contrast of two colors to attract birds' attention so they will carry the seeds. [6]
This is presented as a hypothesis, not a proven fact. It is one possible explanation, but it remains a theory. It is also worth remembering that the 2007 society Q&A could not find a definitive answer to why stripes exist. [2]
5. Did Watermelons in the Edo Period Have Stripes?
According to the Agricultural and Food Products Information Center (ALIC), watermelons arrived in Japan around the 17th century. There are various theories, such as arrival in Nagasaki around 1630 or brought by Ingen Zenji around 1650. [7] However, another source, "Kotoba no Mado," suggests an earlier presence, showing that views differ depending on the material. [8]
ALIC materials state that Edo period watermelons were generally "plain black" without stripes. [7] On the other hand, "Kotoba no Mado" notes that paintings from the mid-Edo period depict watermelons with vertical stripes on a dark background, suggesting they were darker than modern ones but had stripes. [8] Because the sources disagree, we cannot decide here when stripes became widespread.
ALIC states that the foundation for striped watermelons is the "Yamato Suika" from Nara Prefecture and the "Miyako Suika" from Chiba Prefecture. Most current varieties are descendants of these two major types. Improvement progressed through crossbreeding with overseas seeds from the late Taisho to early Showa periods. [7] Some non-striped varieties may also be sold. [9]
6. How to Investigate Watermelon Stripes Yourself
If you see a watermelon, try counting the number and width of the stripe bands. Compare how the stripes run from the stem side to the opposite side with another watermelon; you might notice differences. When cutting the watermelon, always ask an adult for help.
If you look at watermelons drawn in encyclopedias or picture books, you will see that the way stripes are drawn differs by artist. If you are interested in Edo period paintings, you might also read the article in "Kotoba no Mado." [8]