Question of the Month Submitted by Doris Bower, Arcadia, Calif., and answered by Dr. Elliot Meyerowitz, Professor of Biology, Caltech.
Only some vitamins are found in plants—others we must obtain from other sources such as bacteria and yeast, or from animal products. Vitamin B12 is an example of one vitamin that is not found in higher plants. Some plants are good sources of certain vitamins, however, or to be precise, they are good sources either of vitamins (such as vitamin B1 in rice husks or vitamin C in citrus fruits) or of provitamins, which are converted to vitamins in our bodies. An example is beta-carotene, also known as provitamin A, which is converted in our livers to vitamin A. Beta-carotene is abundant in carrots.
There are two answers to your question. The first is that each species of plant has its own methods of regulating the biosynthetic pathways by which they make provitamins, vitamins, and other substances. The plants themselves don't get vitamins from the soil, but they do get the raw materials they need to manufacture these vitamins for their own needs. These raw materials are phosphorus, potassium, nitrates, and about a dozen other elements in lesser quantities, such as iron and magnesium.
Also, the plants take in carbon dioxide from the air, and energy from sunlight. Each species of plant synthesizes different amounts of vitamins and provitamins from the available nutrients, both because of species differences and because plants regulate their biosynthetic pathways in response to the environment. Thus, even in the same environment, different species or varieties of plants will make different amounts of vitamins, and the same variety of plant will make different amounts of vitamins in different environments.
The second answer is that we eat different parts of different plants, and different parts will have different vitamin concentrations. Depending on the vegetable in question, we may eat the leaves, roots, or fruits. So you may get a good dose of vitamin A if you eat the root of the carrot plant. But if you develop a taste for fruits such as squash, you probably won't get nearly as much.
In the case of carrots and provitamin A, there is more to the story—plants synthesize beta-carotene as an aid to photosynthesis, and also as a pigment. As an aid to photosynthesis it is required mainly in leaves, where it is usually found in high concentrations. But beta-carotene is also a pigment; it is the substance that gives most of the orange color to carrots. Since consumers prefer bright orange carrots, plant breeders have deliberately bred carrots that contain high levels of beta-carotene.