Agriculture Reference
In-Depth Information
tolerance of sorghum seedlings to elevated temperatures by inducing physiological and biochemical
changes in the plant. Tolerance of plants to abiotic stresses depends on the type of plant and environ-
mental factors. A number of rhizobacteria, including
Pseudomonas
(Hong et al., 1991), are known
to promote plant growth. Timmusk and Wagner (1999) reported that inoculation with
Paenibacillus
polymyxa
protected
Arabidopsis thaliana
from drought stress by increasing the expression of stress-
induced gene
Erd15.
Ali et al. (2009) reported that inoculation did not affect plant growth and biomass at ambient
temperatures, whereas at elevated temperatures, significant differences in plant growth and survival
were observed due to inoculation, probably because
Pseudomonas
inoculation triggers some stress
responsive mechanisms that enable the plants to tolerate high temperature. The SDS-PAGE analysis
of leaf proteins revealed the presence of three high-molecular-weight polypeptides in the inoculated
plants exposed to elevated temperatures (Ali et al., 2009). Induction of heat shock proteins has been
reported in several plants (Howarth, 1991). McLellan et al. (2007) observed that the rhizosphere
fungus
Paraphaeosphaeria quadriseptata
enhanced the thermotolerance of
Arabidopsis thaliana
through the induction of HSP101 and HSP70 proteins. Similarly, Ali et al. (2009) suggested that the
inoculation with the
Pseudomonas
sp. strain AKM-P6 enhanced the tolerance of sorghum seedlings
to high temperature due to the synthesis of high-molecular-weight proteins and also to the improve-
ment of cellular metabolite levels.
Tropical plant species are greatly affected in growth and development by low temperature (Allen
and Ort, 2001). Photosynthesis is often the first physiological process to be inhibited by chilling.
Because the Calvin-Benson cycle enzymes are more sensitive to low temperature than photochemi-
cal reactions, inhibition of the enzyme-catalyzed reactions of the Calvin-Benson cycle by chilling
reduces the utilization of absorbed light energy for CO
2
assimilation and results in an increased
photosynthetic electron flux to O
2
(Allen and Ort, 2001; Alam and Jacob, 2002; Lu et al., 2013). All
these changes ultimately lead to lower N use efficiency by crop plants. Optimum soil temperature
for the growth and development of important field crops is given in Table 8.1.
8.2.1.2 Supply of Adequate Soil Moisture
Supply of adequate moisture during the crop growth cycle is essential to achieve maximum eco-
nomic yield and consequently higher nitrogen use efficiency. Excess as well as low soil moisture
adversely affects plant growth and consequently lower nitrogen use efficiency. However, there may
be some exceptions, such as flooded rice culture. Fernandez and Laird (1959) showed that, under
optimum soil moisture, wheat yielded 24 kg grain kg
−1
applied N but only 11 kg grain kg
−1
when
water was liming. Hence, the agronomic efficiency of wheat was less than half when the soil mois-
ture was limiting.
The recovered N in the grain decreased from around 20% to 4% when the water supply was
restricted in the experiment of Spratt and Gasser (1970), and from around 30% with water kept
at field capacity in the experiments of Thompson et al. (1975). In such experiments, water supply
would affect the growth rate as well as the availability of soil nitrogen (Novoa and Loomis, 1981).
Spratt and Gasser (1970) found that the recovery of N was greater from nitrate than from ammo-
nium under adequate water supply, but with limited water, the difference was smaller, probably
reflecting the much greater mobility of nitrate in soil. The use of irrigation is the best practice to
improve soil moisture content for crop production wherever possible. In addition, the use of conser-
vation tillage and mulching may also help in improving the soil moisture content.
8.2.1.3 Improving Bulk Density
Bulk density is a physical property of the soil that can be used as a simple index to the general
structural condition of the soil. Although it cannot be interpreted in a specific manner as with the
degree of aggregation, aggregate stability, or pore size distribution, bulk density does provide a gen-
eral index to air-water relations and impedance to root growth. Bulk density is defined as the mass
of dry soil per unit bulk volume. The value is expressed as megagram per cubic meter (Mg m
−3
) or
