The chloride ion (Cl-) has been found to be essential for plant growth (Broyer et al, 1954; Johnson et al., 1957; Ozanne et al., 1957; Ozanne, 1958), but seems to be involved in few biological reactions and only very small quantities are really necessary. Rains (1976) listed chlorine as a micronutrient. Chloride is required for the watersplitting protein complex of Photosystem II, and it can function in osmoregulation in particular in stomatal guard cells. The chloride ion is freely transported and many plants can tolerate the presence of high concentrations without showing toxicity. The chief role of chloride seems to be in the maintenance of turgor and in balancing rapid changes in the level of free cations such as K+, Mg2+ and Na+. Plants deprived of Cl- are liable to wilting (Johnson et al., 1957).

In isolated chloroplasts, chloride (together with Mn2+) ions are required for oxygen evolution in photosystem II of photosynthesis (Bov et al., 1963;

Mengel and Kirkby, 1982; Shkolnik, 1984), although there has been some doubt whether this requirement exists in vivo (Terry, 1977). Chloride ions are best taken into plants at slightly acid pH (Jacobson et al., 1971).

The most common concentration of chloride in culture media is 3 mM, the average 6 mM. MS medium contains 6 mM Cl-; Quoirin and Lepoivre (1977) medium, only 0.123 ^M. Some species are sensitive to chloride ions. McCown and Sellmer (1987) reported that too high a concentration, seemed to cause woody species to have yellow leaves and weak stems: sometimes tissues collapsed and died. An excess of Cl- has been thought to be one cause of the induction of hyperhydricity, and omission of the ion does seem to prevent the development of these symptoms in Prunus (Volume 2). Pevalek-Kozlina and Jelaska (1987) deliberately omitted chloride ions from WPM medium for the shoot culture of Prunus avium and obtained infrequent hyperhydricity in only one genotype. The presence of 7 mM Cl- can be toxic to pine suspension cultures (Teasdale, 1987).

As chlorine has only a relatively small nutritional significance, steps are sometimes taken to reduce the concentration of chloride ion in culture media, but in order to adjust the concentration of other ions, it is then often necessary to make a marked increase in SO42-. For example, using ammonium sulphate instead of ammonium chloride to supply NH4+ in Eeuwens (1976) Y3 medium, would increase the sulphate level from 2 to 12 meq/l (from 1 to 6 mM).

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Growing Soilless

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