Chlorine in plant life
Authors: Schwartau V. V. , Mykhalska L. M. , Makoveychuk T. I. , Tretiakov V. O. 
Published: February 2025

Chlorine is an essential nutrient, a deficiency of which reduces plant productivity. Chlorine-containing substances have been known and used for a long time. The most common chlorine compound, sodium chloride (table salt), has been in use since ancient times. It was used as early as 3000 BC and brine as early as 6000 BC. Cl substances are mentioned in ancient texts from different cultures. The discovery of chlorine was in 1774 by Carl Wilhelm Scheele. He obtained it by reacting pyrolusite (manganese dioxide, MnO 2 ) with hydrochloric acid (HCl, then known as muriatic acid). Scheele thought that the gas produced contained oxygen. It was Sir Humphry Davy's proposal and confirmation in 1810 that chlorine was an element, and he also named the element. Chlorine has been considered a biologically importa nt element almost since its discovery. Research into the effects of chloride fertilisers was carried out in the second half of the last century. In 1949, Warburg argued that chloride was an important trace element for plant growth and showed that it was necessary for the water distribution system at the site of photosystem II oxidation. In the 1954 Broyer et al. finally demonstrated the biological importance of chlorine for plants. Chloride is the most abundant inorganic anion in plant cells, an element available in most agrophytocenoses. The average Cl - content in plants ranges from 2.0 – 20.0 mg / g DM, but for Cl-sensitive and Cl-tolerant glycophyte species, the critical (often toxic) Cl-content in tissues can be around 4 – 7 and 15 – 35 mg / g DM, respectively. Chlorine deficiency in plants has characteristic symptoms: wilting, numerous spots, and reduced productivity. Chloride performs a wide range of functions in plants, primarily forming turgor and osmoregulation, respectively, affecting transport processes on membranes (plasmalemma, tonoplast, etc.), water & nitrogen use efficiency (WUE & NUE), and affects the functioning of photosystem II, and is therefore an important part of agricultural plant productivity. Chloride stimulates the structural and functional role of the plasma membrane, sugar transport, as well as nitrogen fixation and assimilation in the plant. Nitrogen assimilation , and photorespiration become more efficient when fed with chloride. Recent studies have discussed the role of chlorine in nitrogen assimilation and photorespiration. It has been shown that Cl plays an important role in the oxygen-evolving complex by adjusting the affinity of different amino acid residues for manganese (Mn). Chlorine acts as a counterion, balancing the positive charges of potassium (K⁺) and other cations in plant cells, which is essential for maintaining electrical neutrality and proper ionic balance in cells. Chlorine plays a significant role in soil salinity. Sources of chlorine in soil include mineral weathering, chlorine from marine species and anthropogenic pollution. Fertilisers such as potassium chloride help to increase the chloride content of the soil. Planting salt-tolerant crops can help maintain agricultural productivity on saline soils. The sensitivity of crops to chlorine varies according to the type of crop. Some crops can tolerate higher levels of chloride without adverse effects, while others are more sensitive and may show symptoms of toxicity or growth retardation when exposed to higher chloride concentrations. Understanding the response of specific crops to chloride is important for the development of nutrient systems and irrigation practices. Chloride increases plant resistance to diseases that require relatively large amounts of Cl - . These doses are much higher than those required for its use as a trace element, but much lower than those required to induce salinity control effects. Most of the research on chlorine nutrition has been devoted to studying the effect of the element on the incidence of physiological leaf spot (PLS) in cereals. PLS form on the leaves of cereal crops when there is a lack of chlorine in the nutritional systems. The necrosis that develops in Cl-deficient plants is thought to be associated with the accumulation of H 2 O 2 during the release of Cl from the Mn cluster of the oxygen-evolving complex. Physiological spotting in the form of completely/partially transparent dots/spots on the leaf was observed, which may indicate inhibition of chlorophyll synthesis rather than degradation. Given that chlorine at micromolar concentrations affects transport processes on membranes and that the element is easily leached through the soil profile, its deficiency occurs in the second half of the growing season, during the period of generative development, which may be the initial mechanism for the formation of PLS in the form of transparent/translucent leaf spots. The development of these spots in the generative period of development, during grain filling, can be significantly accelerated by high levels of actinic light and, accordingly, significantly limit the productivity of cereal crops and their quality. A possible component of chlorine deficiency and leaf damage in wheat and other cereals by PLS may be the application of phosphate fertilizers with high fluoride content, such as phosphate rock, etc. Therefore, in high productivity technologies, it is advisable to use phosphate fertilizers with a low fluoride content, such as potassium monophosphate. Therefore, the use of chlorine fertilisers, mainly potassium chloride in the basic application, ammonium chloride, calcium chloride, etc. in the foliar application, is important to provide plants with chlorine during the growing season to increase WUE & NUE, increase plant resi s tance to pathogens, control PLS, and increase productivity of cereals and other agricultural crops. Chlorine's role in increasing WUE & NUE is particularly important for the country's profitable crop production in the face of resource shortages.