This Halacha is an excerpt from our Sefer
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What grains or flours make a food Chametz:
Only the five grains; which are 1) wheat, 2) barley; 3) rye, 4) spelt, 5) oats, can become leaven and Chametz. All other flours, such as flour made from rice [and corn] and all other legumes and starches, cannot ever leaven and become Chametz. Although these flours also rise when kneaded with water, this rising is not a leavening but rather a spoiling. For example, if one were to take rice flour, and the like, and knead it with boiling water, and then cover it with a cloth until it rises like dough that has leavened, in truth this act of rising is not leavening but rather a spoiling of the dough. The dough thus remains permitted to be eaten on Pesach. [Nonetheless, certain flours are considered Kitniyos, and are accustomed by Ashkenazi Jewry not to be eaten on Pesach, as will be explained.]
Does there exist various species of the five grains?
Yes. There are various species of wheat and other grains of the five grains. All these species are considered under the category of the five grains that can become Chametz through contact with water.
Is “bulgur” from one of the five grains?
Yes. Bulgur is a cereal food made from the groats of several different wheat species, most often from durum wheat. Bulgur is a kind of dried cracked wheat and is most common in European, Middle Eastern, and Indian cuisine.
Is buckwheat a grain or a legume?
Despite its misleading name, buckwheat is not a grain. It is a seed of a plant which looks like grain, hence explaining its name. For all Halachic purposes, buckwheat cannot become Chametz. It does however fall under the category of Kitniyos.
 Admur 453:1; Michaber 453:1; Pesachim 35a
Other opinions: The Talmud [Pesachim 35a] records the opinions of Rav Yochanan Ben Nury who holds that rice and millet are considered a grain, and its leavening is actual Chametz for which one is liable for Kareis, just like the five grains. The Mishneh in Pesachim 2:5 rules that only the five grains can become Chametz, and the Gemara ibid establishes that this Mishneh does not follow the opinion of Rebbe Yochanan Ben Nury, and so is the final ruling of the Talmud, and all Rishonim and Achronim.
 See P”M 453 A”A 1 that Turkish wheat is not a grain and cannot become Chametz. Some translate this to refer to corn, which mainly cultivated in Turkey.