Nutrient deficiencies under stress in legumes: An overview - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 59

3.6 roles of macronutrients
in growth of legumes

nitrogen fixation. The cost of symbiotic nitrogen fixa-
tion varies from 2 to 3 mg of carbon per mg of fixed N 2
(Valentine et al., 2011).

Every nutrient is of biochemical and physiological
importance and has a minimum as well a maximum
concentration for plant homeostatic function. The mac-
ronutrients are also called 'primary nutrients', except
for sulphur, which is called a 'secondary nutrient'
because it is needed in lower concentrations than the
others. It has been observed by Sanchez (2002) that
annual nutrient depletion rates across 37 African coun-
tries are 22 kg N/ha, 2.5 kg P/ha and 15 kg K/ha. These
nutrients are essential for plants and their deficiency, or
complete absence may result in retarded growth and
hence reduced yield.
The presence of appropriate concentrations of mac-
ronutrients is important in various cycles, such as the
nitrogen, phosphorus and potassium cycles. Nitrogen,
phosphorus and potassium (NPK) are the principal
macronutrients needed for plant growth; nevertheless
others also make significant contributions to growth
(Ahanger et al., 2014). Mineral nutrients may also
influence N 2 fixation in legumes and non-legumes at
various stages of the symbiotic process, including infec-
tion, nodule development, nodule function and host
plant growth (Weria et al., 2013). These macronutri-
ents also regulate the amount of storage proteins in
legumes.

3.6.2 role of nitrogen
Nitrogen is one of the prime macronutrients, impacting
nutrient uptake and N 2 fixation. It is involved in amino
acid production, is a component of vitamins and affects
energy reactions in plants. Nitrogen is present in very
large quantities in the atmosphere as dinitrogen gas
(N 2 ) However, this atmospheric nitrogen cannot be
used directly by plants, so they have evolved several
mechanisms to overcome this. The most important of
these is symbiotic nitrogen fixation. Nodule formation
and nitrogenase activity are both inhibited by the
presence of mineral nitrogen (Vieira et al., 1998), but
the inhibition of symbiotic nitrogen fixation by mineral
nitrogen in the soil depends upon the time at which the
nodulation starts along with the concentration of
nitrogen (Anne-Sophie et al., 2002). Sarah et al. (2013)
also supported the hypothesis that N 2 fixation is con-
trolled by the availability of nitrogen. N 2 fixation
therefore has a direct relationship with the presence or
absence of mineral nitrogen in the soil and with its
concentration.

3.6.3 role of phosphorus
Phosphorus is also an important nutrient and nodules
are great sinks of it (Hart, 1989). It is important in var-
ious biochemical and molecular processes (Epstein &
Bloom, 2005). Many developmental processes depend
upon phosphorus, including root growth, as well as ATP
production. It is actively involved in photosynthesis and
respiration, and is vital for seed formation. Phosphorus
availability has an influence on N 2 fixation and nodule
formation (Saxena & Rewari, 1991). A 35% reduction
in the number of legumes in soybean plants was seen by
Tsvetkova and Georgiev (2003) by over supplying phos-
phorus as compared to controls, while its deficiency
caused a reduction in the size of nodules rather than
number. Hence, phosphorus concentration is critical
with respect to nitrogen fixation in legumes, and both
higher and lower concentrations can reduce both
nitrogen fixation (Weria et al., 2013; Sarah et al., 2013)
and the biomass of the symbiotic partners (Sarah et al.,
2013). When P is deficient, N 2 fixation decreases as a
result of adaptation to low N demand regulated by
feedback mechanisms (Almeida et al., 2000). So an

3.6.1 role of carbon
Carbon is an essential element in plant growth. Dry
matter of a typical green plant contains a large (35-50%)
and relatively constant proportion of carbon. Approxi-
mately 40 million tons of nitrogen is injected
symbiotically into the soil each year by rhizobia in
legumes mainly due to exchange of nitrogen from
bacteria for reduced carbon from the plants. Nitrogen
fixation is a finely regulated process that is crucial in
nutrient cycling and involves significant carbon alloca-
tions (Olivera et al., 2004; Valentine et al., 2011). Forests
with nitrogen-fixing trees accumulate more carbon
compared to non-nitrogen fixing trees (Sigrid et al.,
2002). Furthermore, the ability of legumes to exchange
carbon for nitrogen with their symbiotic partner gives
them a competitive edge over non-legumes. Therefore,
raised concentrations of CO 2 will provide more photo-
synthetic carbon fixation, which can then be exchanged
with nitrogen resulting in an increase in the rate of