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

192
K. Janardhan Reddy
nitrogen deficiency and may interfere with the uptake of divalent cations like Ca 2+ and
Mg 2+ .
Potassium fertilization is necessary to improve the yield of almost all crops. In
general K + application is about 50 to 250 kg /ha -1 year -1 . Potassium is mainly supplied as
chemical fertilizer to crops in the form of “muriate of potash”(KCl) and other sources
are K 2
SO 4
and KNO 3
. Potassium may not form structural organic compounds and yet is
required in high quantities to perform various roles such as osmoregulation, enzyme
activation, neutralization, transport processes (Table 3).
Table 3:
Effect of foliar application of potassium on yield
components of two pigeonpea cultivars*
Yield component C V PDM 1
C V LRG30
Control K + Control K +
Leaf area (cm 2 ) 776 2504 757.5 1450
Number of pods/ plant 107 221 119 174
Number of seeds/Plant 428 884 486 647
Grain yield (kg/acre) 333 637 334 539
*Adapted from Ravindranath et al. (1985).
5. SULFUR
The most important source of sulfur is SO 4
2-
taken up by roots. Reduction of SO 4
2-
is
necessary for its incorporation into various biomolecules particularly cysteine (Leustek
and Saito, 1999; Saito, 2000). Sulfur is a constituent of cysteine and methionine and
thus acts as an important component of proteins. These Sulfur containing amino acids
are the precursors for co-enzymes, intermediary metabolites and redox controllers. Sulfur
is the structural constituent of several co-enzymes and prosthetic groups like ferredoxin,
biotin and thiamine pyrophosphate. The SH groups act as functional groups in the
enzyme reactions of urease, sulfotransferase and co-enzymes. Glutathione, a tripeptide
(γ Glu-Cys-Gly) is the dominant nonprotein thiol in plants and is an important
antioxidant in plants. Sulfur also promotes nitrogen fixation in legumes.
Genes encoding for sulfate transporters were isolated from barley (Vidmar et
al., 1999) and Arabidopsis thaliana (Takahashi et al., 2000). Sulfur is required at a
concentration of 0.1 to 0.5% of dry weight of plants for optimal growth. Sulfur deficiency
decreased hydraulic conductivity of roots and net photosynthesis (Karmoker et al.,
1991). Chlorophyll and protein content were also decreased in tomato under low levels
of sulfur (Table 4). The growth of the shoot is more affected than root growth under
sulfur deficiency. Thus shoot/root dry weight ratio decreased from 4.4 in sulfur sufficient