Functions in Plants

Chlorine is readily taken up by plants in the electrically charged form as chloride ion (Cl^-). Although chlorine occurs in plants as chlorinated organic compounds (11), chloride is the major form within plants, where it is bound only loosely to exchange sites or is a highly mobile free anion in the plant water. As an essential element, chlorine has several biochemical and physiological functions within plants.

Chloride appears to be required for optimal enzyme activity of asparagine synthethase (12), amylase (13), and ATPase (14). In photosynthesis, chloride is an essential cofactor for the activation of the oxygen-evolving enzyme associated with photosystem II (15,16). Chloride may bind (17) to the polypeptides associated with the water-splitting complex of photosystem II, and it may stabilize the oxidized state of manganese by acting as a bridging ligand (18-20). Chloride concentrations required for biochemical functions are relatively low in comparison to concentrations required for osmoregulation.

In rapidly expanding tissues such as elongating cells of roots and shoots, chloride accumulates in the tonoplast, to function as an osmotically active solute (21,22). This transport of chloride into the tonoplast occurs in association with the proton-pumping ATPase activity at the tonoplast, being specifically stimulated by chloride (14). This osmoregulatory function in specific tissues requires concentrations of chloride that are not typical of a micronutrient (23,24). The accumulation of chloride in plant cells increases tissue hydration (25) and turgor pressure (26). This osmotic function of chloride works closely with potassium to facilitate cell elongation and growth. The importance of this osmoregulatory role of chloride in plants depends on growing conditions and the presence of alternative anions, such as nitrate, which might function as substitutes for chloride.

Chloride along with potassium participates in stomatal opening by moving from epidermal cells to guard cells to act as an osmotic solute that results in water uptake into and a bowing apart of the guard cell pair (27). In many plant species, depending on the external supply of chloride, malate synthesis may occur in the guard cells and replace the need for chloride influx (28,29). Chloride, however, is essential for stomatal functioning in some plant species (30). In onion (Allium cepa L.), for example, where the guard cells are unable to synthesize malate, there is a requirement for an influx of chloride that is equivalent to potassium for stomatal opening to occur.

Relative differences in the uptake of cations (NH₄⁺, Ca²⁺, Mg²⁺, K⁺, Na⁺) and anions (NO₃^-, Cl^-, SO₄^2-, H2PO₄^-) by plants require the maintenance of electroneutrality in plant cells as well as in the external soil solution (31). As an anion, chloride serves to balance charges from cations. In plants well supplied with chloride, this inorganic anion may serve as an alternative to the formation of malate in its charge-balancing role (32). This role of chloride may be of greater importance when cation uptake exceeds anion uptake, as often occurs with plants provided with ammonium nutrition.

The functions of most of the over 130 chlorinated organic compounds (11) that have been identified in higher plants have not been determined. Some legume species contain chlorinated indole-3-acetic acid (IAA) in their seeds. The chlorinated form of IAA is more resistant to degradation, and this resistance may be responsible for increasing the rate of hypocotyl elongation over the rate of IAA production itself (4,33).