Have you ever cut open a pepper or carved a pumpkin and wondered what the gas is inside the fruit? (Yes, technically they are both fruits and not vegetables.) You know the open space isn’t a vacuum because air doesn’t get sucked into an opening when you first make a cut. You can guess the gas isn’t flammable, like hydrogen or methane, because roasting a pepper doesn’t make it burst into flame. So, what is the gas inside a pepper or pumpkin? Here is the answer to the question and an explanation of why the gas is there at all.
Gas Inside a Pepper or Pumpkin
The gas inside a pepper or pumpkin has mostly the same composition as air, which is nitrogen, oxygen, argon, carbon dioxide, water vapor, and other trace gases. Depending on the ripeness of the produce, the plant hormone ethylene is present. However, the relative amounts of the gases in air change within a developing fruit. For example, researchers found the gas inside a cotton fruit was 46% nitrogen, 29% oxygen, 4% argon, and 20% carbon dioxide. In contrast, the air surrounding the plant was 73% nitrogen, 25% oxygen, 2% argon, and 0.3% carbon dioxide. So, the fruit contained enriched levels of of oxygen and carbon dioxide. The same study found that a diseased plant’s fruit contained much lower oxygen and much higher carbon dioxide.
Why Is the Composition Different From Air?
Plants use carbon dioxide for photosynthesis and oxygen for respiration, but these two gases play other important roles, too. If oxygen levels fall inside the fruit, it yields lower-weight seeds. Below a certain amount (15% oxygen, for peppers), embryo develop stops entirely. So, the gas within hollow fruits is enriched with oxygen to aid seed production.
In studies involving peppers, removing carbon dioxide did not affect seed weight, but it did accelerate fruit ripening, while reducing the amount of sucrose and starch in the pepper. A separate study, this time on wheat seeds, found that enriching a low-oxygen atmosphere with carbon dioxide offset the negative effect on seed development. Another study, involving rapeseed and soybean, found that plants need a higher concentration of carbon dioxide to maximize oil synthesis in seeds. Pumpkin seeds are rich in oil, so it makes sense the gas inside a ripe pumpkin contains higher percentages of both oxygen (for seed development) and carbon dioxide (for oil production in the seeds).
The composition of the gas within a pepper or pumpkin is not constant over time. It changes as the fruit develops and in response to factors that affect the plant’s health. The balance of these gases also plays a role in ethylene production, which ripens the fruit.
How Does Gas Get Inside the Fruit?
Young fruit, just like leaves and young stems, has a single-layer coating of cells called the epidermis. Just like in leaves, the fruit epidermis has tiny openings called stomata. Guard cells on either side of a stomatal pore control whether it is open or closed. When the pore is open, the fruit exchanges gases with the outside air. Green fruit performs photosynthesis, which involves carbon dioxide, oxygen, and water.
As the fruit ripens, tissue called a periderm replaces the epidermis. The periderm also allows gas exchange, this time through areas of loosely-connected cells called lenticels. Lenticels are easily observed on an apple or pear, but also occur on peppers, pumpkins, and other hollow produce.
Both the stomata and the lenticels are small openings. So, if you submerge a pumpkin or pepper in water, all of the gases inside don’t bubble out.
- Blasiak, J.; Kuang, A.; Farhangi, C.S.; Musgrave, M.E. (2006). “Roles of Intra-fruit Oxygen and Carbon Dioxide in Controlling Pepper (Capsicum annuum L.) Seed Development and Storage Reserve Deposition.” J. Amer. Soc. Hort. Sci. 131(1): 164-173.
- Goffmann, F.D.; Ruckle, M.; Ohlrogge, J.; Sachar-Hill, Y. (2004). “Carbon dioxide concentrations are very high in developing oilseeds.: Plant Physiol. Biochem. 42(9): 703-708. doi:10.1016/j.plaphy.2004.07.003
- Jacks, T.J.; Hensarling, T.P.; Legendre, M.G.; Buco, S.M. (1993). “Permanent Gases Inside Healthy and Microbially Infected Cotton Fruit During Development.” Biochem. Biophys. Res. Com. 191(3): 1284-1287. doi:10.1006/bbrc.1993.1356
- Ramonell, K.M.; McClure, G.;. Musgrave, M.E. (2002). “Oxygen control of ethylene biosynthesis during seed development in Arabidopsis thaliana (L.) Heynh.” Plant Cell Environ. 25:793–801.
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