1. What green peas are: pea seeds eaten before they ripen
Green peas are pea seeds eaten while they are still green. If you let the peas ripen and dry, they become red peas or green peas, used in different dishes [1] [2]. If you split a pea pod with your fingers, you find round, swollen beans inside. Peas are also called "real peas" because people eat these beans [3]. Snow peas, which are eaten whole with the pod, and pea shoots, which are eaten as young sprouts, are also the same plant [2]. So, how you eat a pea depends on when you harvest it. But where does that single seed start growing? Let’s look at the seasons first.
2. How pea seeds sown in autumn survive the winter as small seedlings
Peas are vegetables that can handle cold weather. In Japan, farmers usually sow seeds in autumn, let the plants survive the winter, and harvest them around April of the next year. However, in northern Tohoku and Hokkaido, seeds are sown in spring, while in warmer areas, they may be sown at the end of summer for a winter harvest [1]. It is said that sowing too early in autumn is not good. If the plants grow too big before winter, they are easily damaged by the cold. The best time to sow changes depending on the region. The plants stay as small seedlings, enduring the winter cold quietly. Then, they grow rapidly in spring. But when the sprout comes out of the ground, where is the bean itself, which holds the nutrients?
3. Why the bean stays underground when a pea sprout appears
Pea germination is a type called "hypogeal," where the cotyledons (seed leaves) do not come above ground. The nutrient-filled cotyledons stay in the soil, while the stem above them (the epicotyl) grows up, and the sprout appears above the surface [5]. Not all beans do this. Kidney beans and soybeans push their cotyledons up to the surface. But for peas, the lump of nutrients stays underground while the stem emerges [5]. So, a small stem appears above ground. Now, how does this thin stem manage to stand tall?
4. How pea tendrils grab supports so the plant can stand up
Pea stems are vine-like and can reach heights of 1.5 to 2 meters (about 5 to 6.5 feet), though some varieties stay low at around 50 centimeters (about 20 inches). The tips of their compound leaves turn into tendrils that wrap around supports like stakes, lifting the stem up [1]. There are experimental reports that these tendrils move according to the support. A research group in Italy reported that pea tendrils move faster when they get close to a support and slower if the support is thick, changing their movement based on the object [6]. However, scientists do not yet know how the tendrils "sense" the support. The research paper lists several hypotheses, but it is not proven that the plant "sees" or "thinks" about it [6]. As the stem stands up and leaves grow thick, the plant needs lots of nutrients. Yet, peas are known to grow well even in poor soil. Where do these nutrients come from?
5. How bacteria in pea root nodules turn nitrogen from the air into plant food
If you pull up a pea plant, you might see small bumps on the roots. These are root nodules. Legume plants make these nodules and house root nodule bacteria (nitrogen-fixing bacteria) inside them. These bacteria change nitrogen from the air into a form the plant can use [7]. Dictionary explanations state that root nodule bacteria fix nitrogen and pass amino acids to the plant. In return, the plant gives the bacteria carbohydrates made through photosynthesis [8]. Plants need nitrogen to make amino acids, so this partnership helps them grow in soil with little nitrogen. This cooperation has a cost for the plant. Giving away sugars means the plant has less sugar for its own use. Research shows that when there is enough nitrogen in the soil, legume plants may reduce their partnership with nodules. However, this study was done on a legume called Miyakogusa, not on peas [7]. With help from root nodules, the plant flowers and makes fruit. So, how long does it take for the pods and beans to reach harvest size after the flowers bloom?
6. How the part of the pea you eat changes with harvest time
Approximate days from flowering to harvest are about 15 days for snow peas and about 25 days for snap peas. Snow peas are eaten when the pods are small and young. Snap peas are eaten whole with the pods after the beans inside have grown larger. Green peas (shelled peas) are harvested about one month after flowering [4]. These days are rough estimates and change depending on the variety and temperature. Besides differences in variety and use, the harvest time determines whether you eat the pod, the whole pod, or just the beans. If you let them ripen and dry, they become dried beans like red or green peas [2]. The green peas in your rice are immature pea seeds. They grew from autumn soil, survived winter, borrowed help from root nodules, and flowered in spring. Next, let’s see how you can check this process in your own home.
7. How to look inside a green pea and observe its shape yourself
At home, take raw green peas out of their pods and observe the shape of the beans with a family member. The bean is made of two cotyledons joined together. These cotyledons are the part that stays underground during germination to keep nutrients [5]. At a store, if you can find them, line up snow peas, snap peas, and green pea pods to compare them. The difference in pod size and how developed the beans are inside is thought to be related to the number of days since flowering [4]. To learn more, the English abstract of the tendril experiment report (Scientific Reports) is readable by anyone [6]. The JST announcement is in Japanese and explains the details of root nodule symbiosis [7].