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

Nutrient Stress
205
number and their viability (Agarwala et al., 1981). Boron is required for membrane integrity
and function. Tanada (1978) had shown that boron is essential for the formation and
maintenance of membrane potentials.
Boron deficiency induces callose formation and may block the sieve plate
pores and results in the impairment of sugar transport. Boron deficient plants contained
low levels of DNA and the synthesis of DNA was also decreased. Inhibition of DNA
synthesis can be attributed to either a primary (Krueger et al., 1987) or secondary (Ali
and Jarvis, 1988) effect of boron deficiency. RNA content was also decreased in borondeficient
tomato plants. Dave and Kannan (1980) had shown that boron deficiency
enhanced RNase activity in plants. So the decrease in RNA content can be correlated to
higher activity of RNase.
Boron deficiency induces accumulation of certain phenolic compounds like
caffeic acid and p-coumaric acid. Under boron deficiency, the carbon skeleton is shifted
to pentose phosphate cycle and enhances the phenol production. Boron deficiency
induces the accumulation of auxins. Necrosis in the growing points of boron-deficient
plants is mainly due to the accumulation of auxins. Synthesis of cytokinins was depressed
in sunflower roots under boron-deficient conditions (Wagner and Michael,
1971).
Boron deficiency reduced the phosphate uptake in field beans and it was
restored when supplied with 10µM of H 3
BO 3
for one hour (Robertson and Loughman,
1974). Boron deficiency is mainly seen in soils with high pH and under drought conditions.
Boron deficiency symptoms first appear at the apical growing points or in young
leaves. Young leaves become wrinkled and show darkish-blue green colour. Interveinal
chlorosis may occur in mature leaves. The common boron deficiency symptoms are
“top sickness” in tobacco, “heart rot” in sugar beet, “stem crack” in celary and “hollow
stem” in cauliflower.
Boron toxicity is usually occurs in arid and semi arid regions. The toxicity may
arise when boron fertilizers are used excessively to get more yields. The toxicity symptoms
usually appear in young leaves which develop chlorosis between the veins. The
leaf margins and the chlorotic areas become reddish brown. Later on the leaves become
dry and papery. Flowering is also affected by boron toxicity.
There exists a narrow range between boron deficiency and toxicity, so care
should be taken in applying the boron fertilizers to crop plants for increased yields. In
general the application of boron varies from 0.3 to 3.0 kg/hectare depending on the
requirement and sensitivity of crops to boron toxicity. Awarwala et al., (1969) had shown
the critical level of boron deficiency, sufficiency and toxicity for certain crops (Table
15).