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

190
K. Janardhan Reddy
World nitrogen consumption has increased from 22 to 80 million tons in the
last 20 years. Anhydrous ammonia is the main ingredient of most of the nitrogenous
fertilizers.
3. PHOSPHORUS
Phosphorus is the second important nutrient required by plants .It is an essential
component of nucleic acids, phosphorylated sugars, lipids and proteins which control
all life processes. Phosphorus forms high energy phosphate bonds with adenine, guanine
and uridine which act as carriers of energy for many biological reactions.
Phosphorus is present in the soil in inorganic and organic forms. Much of the
inorganic phosphorus is present mainly as H 2
PO 4-
and HPO 4
2-
. Availability of these
ions depend on soil pH. The lower pH favours H 2
PO 4-
and the higher pH HPO 4
2-
. The
main source of organic phosphorus is plant and animal debris residue and this is degraded
by microorganisms to release in organic phosphorus. Vance et al (2003) found that plant
growth is limited because of the inaccessible and unavailable form in the soil. Arbuscular
mycorrhizae (AM fungi) promote the plant growth by the improved supply of phosphorus
from the soil (Tinker et al., 1992). Since the phosphate availability is usually low in the
soils, the plants have developed special adaptations to acquire the same with the help
of multiple high affinity transporters (Raghothama, 1999).
The requirement of phosphorus for optimal growth is in the range of 0.3 to
0.5% of the plant dry matter. The toxicity may occur if the tissue concentration is more
than 1% in the dry matter. Phosphorus deficiency decreased photosynthetic rate in
soyabean leaves (Lauer et al., 1989). Root growth is less affected under phosphorus
deficiency than shoot growth leading to a typical decrease in shoot –root dry weight
ratio (Fredeen et al., 1989).Phosphorus deficiency not only retard shoot growth but also
affects the formation of reproductive organs (Barry and Miller, 1989). Toyota et al.,
(2003) observed that phosphorus deficiency induced the expression of
phosphoenolpyruvate carboxylase in Tobacco.
Phosphorus deficient plants show stunted growth, leaves develop
characteristic dark blue green colour and some times purplish appearance. Because of
the high mobility of phosphorus older leaves become chlorotic as compared to younger
leaves. Leaf shape may be distorted and also leads to reduction in the number of leaves
(Lynch et al., 1991). Phosphorus deficiency caused decrease in primary root elongation
and increased lateral root formation.(Lynch and Brown, 2001; Hodge, 2004). Phosphorus
toxicity induces iron and zinc deficiency. At higher levels of P the yield was decreased
in Abelmoschus esculentus L. due to the deficiency of zinc (Table 2).
Main source for phosphorus fertilizers is the rock phosphate. super phosphate
and Diammonium phosphate are important phosphorus fertilizers. In response to
phosphorus deficiency several genes are expresssed. Expression of these genes could
be used to develop the crop plants for the improved phosphorus use efficiency. (Wang
et al., 2002; Hammond et al., 2003; Franco-Zorrilla et al., 2004).