Chlorine Deficiency

Chlorine is an important micronutrient and despite the fact that plant tissues usually contain substantial amounts of Cl^-, often in the range from 2 to 20 mg/g d.w., the demand for Cl^- for optimum growth is for most species considerably lower (deficiency symptoms usually occur in the range 70-700 μg/g d.w.). Chlorine is taken up by plants as Cl^- and it is highly mobile, so after absorption it can be easily transported inside plants. The negative charge of Cl in soil makes it prone to leaching in regions with high rainfalls. In contrast to that, in regions with high evapotranspiration (arid and semi-arid regions) Cl^- may be highly accumulated in surface soil horizons. Apart from the climatic conditions determining the accumulation or leaching of Cl^- in soils, the distance from the sea is another important factor influencing Cl concentrations in soils; so, Cl in soils exhibits a clear trend of decreasing concentration with increasing distance from the sea.

Chlorine is implicated in several physiological functions, such as in osmotic and stomatal regulation, in oxygen evolution in photosynthesis, in disease resistance and tolerance, as well as in fruit quality and crop yields. In recent publications it is referred that the critical Cl deficiency concentration is 2 g/kg d.w. (i.e., 2000 mg/kg d.w.). Below that concentration Cl deficiency symptoms, such as chlorotic leaves, leaf spots, brown edges, restricted and highly branched root system, as well as wilting of leaves at margins and leaf mottling, may occur. In order to alleviate Cl starvation symptoms, some Cl-containing fertilizers that may be supplied to plants are those of KCl (47% Cl), MgCl₂ and CaCl₂ (64% Cl). Other (anthropogenic) sources of Cl supply to plants are the irrigation water, the use of de-icing salt to frozen roadways during winter months and the atmospheric pollution.

Keywords: Cl availability, Cl deficiency, Cl leaching, Cl uptake, osmoregulation, photosynthesis, stomatal regulation.