Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

206
K. Janardhan Reddy
Table 15. Threshold values of boron for deficiency,
sufficiency and toxicity in certain crops*
Crop Age of plant Boron content in leaves (ppm)
(days) Deficiency Sufficiency Toxicity
Wheat (Kalyansona) 68 11 29-60 140
Maize (Ganga-101) 46 10 20-60 200
Paddy (IR-8) 48 18 20-72 400
Barley(K-19) 68 13 14-126 476
*Adapted from Agarwala et al. (1969).
14. CHLORINE
Chlorine mainly found in soils as monovalent ion as chloride (Cl - ). Plants usually contain
chloride in the range of 2 to 20 mg/g dry matter although the chlorine requirement
for optimal growth is in the range of 0.20 to 0.40 mg/g dry weight. Chloride is available
to plants from several sources like soil reserves, rain, irrigation water, fertilizers and
through air pollution. Therefore there is much concern about chloride toxicity rather
than deficiency in plants.
Chloride is mainly involved in the photolysis of water by photosystem II.
Several experiments were conducted to show the evolution of oxygen by taking chloroplast
fragments from different plant species. Chloride may either act as a bridging ligand
for stabilization of the oxidized state of manganese (Critchley, 1985) or as a structural
moiety of the extrinsic protein (Coleman et al., 1987). Proton pumping ATPase located in
the plasma membrane is stimulated by monovalent cations particularly K + whereas
ATPase at the tonoplast is not influenced by K + or any other cation but specifically
stimulated by Cl - (Churchill and Sze, 1984).
Chlorine plays an important role in the stomatal movement. Stomatal opening
and closure is mediated by fluxes of K + and accompanying anions like malate and
chloride (Raschke et al., 1988). Chloride has important functions in osmoregulation and
plant water relations (Flowers, 1988). Due to its biochemical inertness chloride is able to
fulfill osmotic and cation neutralization roles which may have biophysical or biochemical
consequences of importance (Clarkson and Hanson, 1980).
Chlorine deficiency induces reduction in leaf area resulting in lower dry matter
production. The decrease in leaf area is mainly due to reduction in cell division (Terry,
1977). Wilting of leaves is a typical symptom of Cl - deficiency. In tomato, leaves
show chlorotic mottling, bronzing and tissue necrosis. Wilting and premature senescence
of leaves, frond fracture and stem cracking are general symptoms of Cl - deficiency
in coconut plants. When these plants were treated with KCl growth disorders
disappeared to a larger extent with the increase in Cl - content of leaves (Table 16).