There are 13 compounds currently identified as vitamins. With the exception of vitamin B12, plants are capable of synthesizing vitamins in small amounts when they are required. Plants also receive vitamins from fungi in the symbiotic mycorrhizal associations in their roots. Commercially-bred crops can use more vitamins than are supplied by nature, so supplementation can be extremely beneficial.
The roles of vitamins in humans have been studied extensively, but the mechanisms of their synthesis and function in plants have only recently begun to be uncovered in the scientific community.
The word vitamin is short for vital amine, which means nitrogen-containing compound required for life. While some vitamins have only one or two specific roles in plant metabolism, others are involved in numerous reactions. Most vitamins act as coenzymes and cofactors to help enzymes speed up chemical reactions. Production is triggered by developmental-stage cues and environmental stresses such as high light conditions and increased salinity.
Vitamin A (Retinol and Beta-carotene)
Vitamin A encompasses a group of compounds including beta-carotene and the other carotenoids: retinol and retinal. Beta-carotene is easily converted to retinol, which is then oxidized to retinal. Carotenoids are found in the plant’s photosynthetic tissues (chlorophyll) and are the most abundant antioxidant in chloroplasts, along with vitamin E.
Vitamin B1 (Thiamine)
Contrary to popular belief, vitamin B1 is not a remedy for transplant shock. Early experiments that demonstrated this had also supplied the test plants with auxins, which turned out to be what was responsible for the turnaround. Also known as thiamine, vitamin B1 activates systemic acquired resistance (SAR) in which a plant fights off invading pathogens such as fungi, bacteria or viruses. It also helps repair damaged DNA.
Vitamin B2 (Riboflavin)
Vitamin B2 protects plants from infections. Riboflavin forms flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), which act as cofactors of enzymes involved in plant metabolism reactions. These reactions include photosynthesis, fatty acid metabolism and the citric acid cycle.
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Vitamin B3 (Niacin)
Niacin is a precursor to the molecules nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), which are also important for plant metabolism. They carry and transfer electronic charges in the energy reactions such as photosynthesis and cellular respiration. Plants synthesize niacin from L-aspartate, an amino acid.
Vitamin B5 (Pantothenic Acid)
Vitamin B6 (Pyridoxine)
Vitamin B6 is a major antioxidant that absorbs harmful radiation from the sun. It helps plants tolerate stress from oxidation and high-salt conditions. An additional role as a coenzyme in amino acid metabolism has been identified as well. The vitamin is also involved in the biosynthesis of antibiotics to fight pathogens.
Vitamin B7 (Biotin)
Also called vitamin H, biotin is a cofactor for a few select enzymes and has a role in fat and carbohydrate metabolism.
Vitamin B9 (Folic Acid)
Folic acid is a precursor of amino acids and other biomolecules. It also stimulates the growth of beneficial microbes. When exposed to seedlings and cuttings, folic acid improves germination and growth rates during developmental stages. It also contributes to DNA biosynthesis.
Vitamin B12 (Cyanocobalamin)
Unique among vitamins, vitamin B12 cannot be synthesized by plants. It can only be produced by micro-organisms. Although it is found in kelp, the seaweed obtains it from symbiotic bacteria. Plant-based sources of B12 include seaweed, kimchi and wild mushrooms. Chemically speaking, vitamin B12 is a co-enzyme needed to synthesize methionine, an amino acid.
Vitamin C (Ascorbic Acid)
Plants can synthesize their own vitamin C, which is involved in many plant cell chemical reactions. Most animals can also make it, except for primates and bats. Vitamin C has two main roles in plants. As a cofactor, it carries and transfers electric charges in chemical reactions such as phytohormone synthesis. As an antioxidant, it acts as a scavenger of free radicals and toxic chemicals such as ozone and hydrogen peroxide, which in effect protects enzymes.
Vitamin D (Calciferols)
Calciferols are natural plant steroids that are precursors to cholesterol, which most plants contain a small amount of. Derivatives of calciferols are compounds that defend the plant when it is wounded. They also have a regulatory function and have been shown to stimulate root growth. Humans can synthesize calciferols from sunlight, but still need to obtain more from their diet.
Vitamin E (Tocopherol)
Along with carotenoids, vitamin E is the most abundant antioxidant in chloroplasts, the site of photosynthesis. Its presence increases stress tolerance in plants. It also contributes to signaling between plant cells. Vitamin E can only be produced by organisms capable of photosynthesizing (plants, algae and some bacteria).
Vitamin K (Phylloquinone)
Vitamin K is found in the dark green leaf tissues of plants, so it makes sense it has a role in photosynthesis as a redox cofactor. It also assists in the formation of sulfur-containing bonds in the amino acid cysteine. It is also found in cyanobacteria, and a close relative is found in red algae and diatoms.
Cold-pressed kelp contains all 13 of these vitamins, but in small amounts. Foliar spraying the kelp is a quick way to give your plants a multivitamin. Ask your local hydro shop which specialty products it carries that contain sources of these vitamins to obtain the many benefits of their application.