Reduction in plant biomass, wilting or curling of leaves, localized area of tissue necrosis, and abnormal root growth (extensive branching, stubby or thicken appearance) are common symptoms observed in Cl deficient plants grown in water culture. The question arises as to what role Cl plays in plant nutrition such that in its absence growth is reduced. Currently, not much is known about the physiologic role of Cl in plant nutrition. Several roles in plant metabolism have been proposed, but not all are universally accepted. In addition, many of these functions may not explain the observed symptoms of Cl deficiency in plants grown in water culture, pot experiments, or in the field.
Photosynthesis
A direct role for Cl in photosynthesis has been the subject of several investigations since Warburg and Luttegens (1944) first demonstrated that the water splitting system of photosystem II (PSII) required Cl. A number of studies have been conducted on isolated chloroplasts from spinach and sugarbeet plants. Chloride has been postulated to act as a bridging ligand between Mn atoms during the transfer of electrons from water to PSII (Critchley, 1985), or as a structural component of the associated (extrinsic) polypeptides (Coleman et al., 1987). The importance of Cl in PSII in vivo has been criticized by Terry (1977). He found sugarbeet growth was reduced when Cl concentrations in the leaf blades fell below 700 ppm. However, PSII was not affected even when the leaf Cl concentration fell to 35 ppm. A concentration at which severe deficiency symptoms would be expected in any species.
Tonoplast proton-pumping ATPase
Chloride has been implicated as being important in the function of several enzymes. The proton-pumping ATPase at the tonoplast has been show to be stimulated by Cl (Churchill and Sze, 1984). This pump is important in pH regulation of the cytoplasm and ion uptake. This topic has been review by Marschner (1995).
Osmotic regulation
Chloride has long been known to be important it in osmotic adjustment (Arnold, 1955). The ability of Cl to move rapidly across cell membranes and its biochemical inertness are two importance properties which make it well suited to serve as a key osmotic solute in plants (Maas, 1986). Chloride serves in this capacity at relatively low energetic cost to the plant (Sanders, 1984). If Cl is in short supply, plants may use more energy-costly organic salts for turgor control. The process of osmotic adjustment occurs when solutes such as Cl accumulate within a cell (or cell component), causing the water potential to decreases below the external potential. The resulting water potential gradient causes water to enter the cell and the plasmalemma to expand against the rigid cell water. This results in an increase in cell turgidity.
The importance of Cl osmoregulatory function on plant growth depends on its concentration in the plants. Marschner (1995) noted that at plant Cl concentrations of 0.2% (dry wt. basis), a concentration in close proximity to reported critical levels in several crops, there was insufficient Cl in the plant for it to be of great importance in the osmoregulatory function of the bulk plant tissue, unless it was partitioned preferentially into certain tissues (e.g. extension zones) or cells (e.g. guard cells). Not until plant concentrations are in the range of 1.7 to 5.3% (dry wt. basis) or 50 to 150 mM (fresh wt. basis) does Cl represent the dominant inorganic ion in the vacuoles of the entire plant.
Stomatal Regulation
In selected plant species Cl plays an essential role in stomatal regulation. Opening and closing of the stomata is controlled by fluxes of K and accompanying anions such as malate and Cl. If chloroplasts in the guard cells lack the capability of producing malate, such as they do in Allium cepa, Cl becomes essential for stomatal functioning (Schnabl, 1980). In coconut a close correlation exists between K and Cl fluxes during stomatal opening. In Cl deficient plants stomatal opening is delayed by about 3 hr. Impairment of stomatal regulation is considered to an important factor in growth reduction of Cl deficient coconut plants (von Uexjull, 1985; Braconnier and d'Auzac, 1990).