Nutrient deficiencies under stress in legumes: An overview - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 60

optimum concentration of phosphorus is important,
even crucial, for better utilization of other nutrients like
nitrogen. Islam et al. (2013) further confirmed a 38%
increase in nitrogen fixation by phosphorus application;
however, this effect of phosphorus is indirect and is
probably due to enhanced growth and production of
dry matter. Both oversupply and undersupply of phos-
phorus can have a negative impact on plant growth,
and the phosphorus concentration in soil must be main-
tained at a certain level, depending upon the plant
species, to ensure proper functioning of the plant.

3.6.5 role of potassium
Potassium is the third major macroelement essential for
plant growth and is involved in photosynthesis, protein
synthesis and enzyme activation, increasing water
uptake and improving the quality of seeds and fruits
(Maathuis, 2009). Inadequate amounts of potassium
result in poor root development, small seeds and slow
growth. In alfalfa, Grewala and Williamsa (2002)
reported that potassium application has a direct and
vital impact on nodulation, yield and leaf-to-stem ratio.
Yamada and colleagues (2002) showed the effect of
potassium deficiency on two types of crops. In type 1
crops (wheat and soybean) potassium concentration
decreases when nitrogen concentration decreases,
whereas in type 2 crops (sunflower) potassium
concentration remains constant or is independent of
decreases in nitrogen concentration. They found that in
Type 2 crops potassium deficiency decreased the photo-
synthetic rate and increased the photorespiratory rate
and dark respiratory rate. On the other hand, in Type 1
crops the potassium deficiency had a less marked effect
on CO 2 and consequently a less marked effect on the
above-mentioned processes. Potassium therefore has
an important role to play in regulating the process of
nitrogen fixation and also in several other processes
that are vital to the proper functioning and growth
of plants.

3.6.4 role of sulphur
Sulphur, besides having important utility in many
biological processes, is also a constituent of amino acids
like cysteine and methionine, and of vitamins such as
biotin or thiamine. Sulphur deficiency affects plant
growth severely but when applied properly, it improves
plant growth and seed yield (Hafeez ur Rehman et al.,
2013). Sulphur is important for the production of amino
acids, seeds, enzymes and vitamins, and also promotes
nodule development in legumes. Deficiency of sulphur
can lead to reduced production of sulphur-containing
amino acids (methionine and cysteine) and hence
reduced nutritional value (Sexton et al., 1998). In
addition it forms a variety of organic compounds, mod-
ulation of which can provide tolerance against salinity.
Sulphur deficiency reduces N 2 fixation in legumes by
affecting nodule development and function (Scherer,
2008). Varin et al. (2009) demonstrated the effect of
sulphur on growth of legumes in white clover ( Trifolium
repens L.) and reported that application of sulphur
increased the whole-plant dry mass, nodule biomass
and volume, nodule protein content and root amino
acid concentrations. Plants supplied with little or no
sulphur showed poor nodule growth and low nitroge-
nase activity and leghaemoglobin content, which
ultimately reduced nitrogen content, confirming the
interactive role of these mineral elements. Proper sup-
plementation of sulphur enhances nitrogen fixation
and hence nitrogen content (Fazli et al., 2008). Islam et
al. (2013) also showed that sulphur has a direct
influence on nitrogen fixation, and that sulphur appli-
cation caused a 33% increase in nitrogen fixation over
controls. Therefore, sulphur has an important role to
play in nitrogen fixation in root nodules of legumes,
affecting the process of nitrogen fixation more or less
directly.

3.7 Storage proteins in legumes and
effect of nutritional deficiency

Legume seeds accumulate a large amount of protein
within membrane-bound organelles, protein bodies or
storage vacuoles. These are non-structural proteins and
mostly have no catalytic activity. The most abundant
class of proteins in legumes is globulins, which are clas-
sified as 7S and 11S according to their sedimentation
rates. These are salt-soluble proteins. Legume seeds
also contain some anti-nutritional compounds, which
are an evolutionary adaptation to help plants survive
and complete their life cycle under natural conditions
(Duranti, 2006). Storage proteins have many beneficial
effects on health, such as plasma cholesterol reduction,
anti-cancer and anti-inflammatory properties, weight
control, etc. But nutritional deficiency does have an
effect on these storage proteins. Tabe et al. (2002)
showed that the synthesis of proteins with moderate or