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.
