MSc

The level of CO2 in the atmosphere is rising at an unprecedented rate.
According to NOAA (National Oceanographic and Atmospheric Administration)
2014, global concentration of CO2 has reached 400 ppm for the first time in
recorded history. This rise, along with other trace gases in the atmosphere is
widely thought to be a primary factor driving global climate change. Moreover the
report of IPCC, 2012 has reconfirmed the increasingly strong evidence of global
climate change and projected that the globally averaged temperature of the air
would rise by 1.8–6.4°C by the end of the century.
Pulses are the main sources of protein and is commonly called poor man’s
meat. They are also used as fodder and concentrate for cattle. Pulses are
responsible for improving soil fertility by increasing the amount of N2 in the soil.
Drought is a recurring problem limiting pulse production in rainfed areas.
High frequency of crop failure and yield instability due to biotic and abiotic
stresses also contribute to low productivity in pulses. The pulse production
scenario is also getting affected by the changing climate and the resulting rise in
temperature and decline in rainfall. Under such changing climatic scenario, soil
microbes play an important role in the maintenance
of physicochemical
properties of soil and also in making the soil nutrients available to the plants.
In this context, the current programme “Carbondioxide enrichment
mediated plant- microbe interaction in cowpea (Vigna unguiculata L.) under water
stress ” attempts to study the water stress tolerance character and N2 fixation
efficiency of cow pea as influenced by microbial inoculants under elevated CO2
condition. This investigation will help to design improved production
technologies with suitable varieties for a changing climatic scenario.
Two pot culture experiments were conducted at different carbon dioxide
concentrations with three different levels of soil moisture regimes i.e Field
capacity, 75% field capacity and 50% field capacity. The cowpea seeds were
sown in pots inside OTCs and in open field, one set with microbial seed
inoculation and the second set without inoculation. The technology used for
subjecting the plants to elevated CO2 environments is the Open Top Chambers
(OTC) system. In both set of experiment entire crop period was completed in
OTCs. Respective moisture stress levels were imposed during the second month.
Experimental plants were maintained for a period of three months. Observations
on growth parameters and microbial population studies were done at the end of
CO2 exposure period and all the other parameters were taken
at monthly
intervals. The experiments were laid out in CRD with 18 treatments and three
replications.
The observations on growth parameters during first month of CO2 exposure
on cowpea inoculated with Rhizobium revealed a reduction in specific leaf area
by 21.39% under elevated CO2 condition compared to absolute control. Among
the physiological and biochemical parameters studied, highest relative water
content was recorded under elevated CO2 (4.69%). Carbon dioxide enrichment
significantly lowered the stomatal frequency by 28.40 % and transpiration rates by
89.27%. Significant increase in total chlorophyll contents by 50 % was registered
under elevated CO2 conditions. Per cent leakage was found significantly lower
(38.74%) under CO2 enriched treatment compared to control. Among
physiological parameters, a marked rise in phenol content was noticed by 56.68%
under elevated CO2. Significant increase in reducing sugars, free amino acid, and
ascorbic acid contents by 5.97%, 23.92% and 63.79% was recorded in elevated
CO2. Protein content was found decreasing under elevated CO2 by 29.02%.
The observations on growth parameters during water stress period in
cowpea inoculated with Rhizobium revealed a reduction in specific leaf area by
17.24% under elevated CO2 condition compared to absolute control. Root and
shoot dry weights were also found to be higher by 56.08% and 140.77% under
elevated CO2 resulting an increase in root shoot ratio by 36.51%. Dry matter
production was recorded 116% higher under elevated CO2. Parameters related to
nitrogen fixation was recorded lower leaf nitrogen status (17.15% reduction),
nitrogen use efficiency (36.46% reduction), and soil nitrogen status (8.79%
reduction) under elevated CO2. But the Rhizobial inoculated plants was found to
have a positive influence on soil nitrogen status (8.46% increase). Under elevated
CO2 tremendous increase in root nodule number plant and nodule dry weight, but
Rhizobium doesn't have any significant influence on this parameter. Among the
physiological and biochemical parameters studied, highest relative water content
was recorded under elevated CO2 (18.57%). Carbon dioxide enrichment
significantly lowered the stomatal frequency by 15.53% and transpiration rates by
88.12%. Significant increase in total chlorophyll contents by 13.26 % was
registered under elevated CO2 conditions. Per cent leakage was found significantly
lower (51.45%) under CO2 enriched treatment compared to control. Among
biochemical parameters, significant increase in reducing sugars, free amino acid,
phenol, SOD and ascorbic acid contents by 22.63%, 30.65%, 67.56%, 42.12% and
34.06% was recorded in elevated CO2. Protein content was found decreasing
under elevated CO2 by 44.92%.
Nitrogen fixation efficiency was found to be decreased in terms of reduced
leaf nitrogen status under elevated CO2. But nodule number per plant and nodule
dry weight were increased. Rhizobial inoculated cowpea plants were observed to
have better growth and improved stress tolerance in terms of better leaf water
status and membrane integrity under elevated CO2.
In the case of cowpea inoculated with P.indica during first month of CO2
exposure, number of leaves (33.30%)was found to be highest under elevated CO2.
Lower stomatal frequency (35.78%), transpiration rate (86.89%) and per cent
leakage (63.25%) were observed prominently under elevated CO2 compared to
open control. Significant increment in reducing sugars by 21.78%, phenol by
77.71%, free amino acid content by 27.42% was recorded under elevated CO2.
SOD and ascorbic acid content was found increased by 49.39 % and 8.05 % under
elevated CO2 treatment compared with control. Root weight, shoot weight and
total dry matter production were found enhanced by 42.39%, 27.27% and 35.31%
under elevated CO2 in comparison with control.
In the case of water stress also, elevated CO2 was found to have positive
influence on growth like number of leaves (57.27%). Significant increment of
relative water content (13.933%), total chlorophyll (124.9%), reducing sugar
(23.43%), phenol(135.5%) was recorded under elevated CO2 in comparison with
control.
P. indica inoculated plants under CO2 was found to have better stress
tolerance. This was achieved through maintenance membrane integrity and
stomatal modifications. There was reduction in stomatal characters like stomatal
frequency and stomatal conductance resulting in reduced transpiration and better
tissue water status. There were enhanced accumulation and increased activity of
antioxidants like ascorbic acid and SOD. This also would have helped the
experimental plants in achieving better stress tolerance.
The present investigation was carried out with the objective to study the
water stress tolerance character and N2 fixation efficiency of cowpea as
influenced by microbial inoculants under elevated CO2 condition. Considering all
the physiological, biochemical studies conducted, it can be concluded that carbon
dioxide enrichment has a positive role on water stress tolerance character of
cowpea variety Bhagyalakshmy. There was further enhancement of stress
tolerance by both microbial inoculants, Rhizobium sp and P.indica. The
underlying tolerance mechanisms were found to be stomatal modifications
resulting in reduced transpiration and better tissue water status, activation of
antioxidants like ascorbic acid and better maintenance of membrane integrity.
Nitrogen fixation efficiency was improved tremendously by elevated CO2 in terms
of nodule number per plant and nodule dry weight but leaf nitrogen content and
nitrogen use efficiency were reduced by CO2 enrichment.
The outcome of the programme reveals the possibility of improving yield
potential and stress tolerance under elevated CO2 by integrating photosynthesis
and nitrogen use efficiencies with the application of microbial inoculants like
P.indica.
References