Marine estuaries act as better sink for greenhouse gases during winter in undisturbed mangrove than degraded ones in Sundarban, India

The estuaries provide the key pathway for travelling carbon across the land-ocean interfaces and behave as both source and sink of greenhouse gases (GHGs) in water-atmosphere systems. The sink-source characteristics of estuaries for GHGs vary spatially. The primary driving factors are adjacent ecologies (agriculture, aquaculture, etc.) and proximities to the sea. To study the sink-source characteristics of estuaries for GHGs (methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2)), the water samples were collected from 53 different locations in the estuaries for estimation of dissolved GHGs concentration and air-water GHGs exchanges. The locations represent five zones (Zone I, II, III, IV and V) based on the type and degradation status of mangroves (degraded and undisturbed), anthropogenic activities, and distance from the sea. Zone I, III, V represents to the degraded mangroves far from sea, whereas, Zone II, IV surrounded by undisturbed mangroves and nearer to sea. The average dissolved CH4 concentrations were higher in the estuaries which were adjacent to degraded mangroves (154.4 nmol L-1) than undisturbed mangroves (81.7 nmol L-1). Further, the average dissolved N2O concentrations were 48% higher in the estuaries nearer to degraded mangroves than that of undisturbed ones. Among the degraded mangrove sites, the dissolved CO2 concentrations were higher at Zone I (30.1 μmol L-1) followed by Zone III and IV, whereas in undisturbed sites, it was higher in Zone IV (22.3 μmol L-1) than Zone II (17.6 μmol L-1). Among the 53 locations, 36, 51 and 33 locations acted as a sink (negative value of exchanges) for CH4, N2O and CO2, respectively. The higher sink potential for CH4 was recorded to those estuaries adjacent to undisturbed mangroves (-791.69 μmol m-2 d-1) than the degraded ones (-23.18 μmol m-2 d-1). Similarly, the average air-water N2O and CO2 exchanges were more negative in the estuaries which were nearer to undisturbed mangroves indicating higher sink potential. The pH, and salinity of the estuary water were negatively correlated with air-water CH4 and N2O exchanges, whereas those were positively correlated with CO2 exchanges. Significantly lower dissolved GHGs and air-water GHGs exchange was observed in the estuaries adjacent to the undisturbed mangrove as compared to the degraded mangrove. The reason behind higher sink behaviours of estuaries nearer to undisturbed mangroves are higher intrusion of seawater, less nutrient availability, higher salinity, low carbon contents and alkaline pH compared to estuaries adjacent to degraded mangroves and far from sea.

Relative contribution of different members of OsDREB gene family to osmotic stress tolerance in indica and japonica ecotypes of rice

Drought/osmotic stress is the single largest production constraint in rain-fed rice cultivation. Different members of the DREB gene family are known to contribute to osmotic stress tolerance. In this study, an attempt was made to understand their relative contribution towards osmotic stress tolerance in indica and japonica ecotypes of rice. Two genotypes (one tolerant and one susceptible) from each ecotype were grown hydroponically, and 21-day-old seedlings were subjected to polyethylene glycol-induced osmotic stress (15% PEG-6000, equivalent to -3.0 bars osmotic potential). The tolerant genotypes CR143 and Moroberekan were found to have superior root traits (total root length, surface area and volume), better plant water status and increased total dry biomass as compared to their susceptible counterparts after 10 days of osmotic stress. Different members of the DREB gene family were differentially induced in response to osmotic shock (1 h after stress) and osmotic stress (24 h after stress), which also differed between the two rice ecotypes. From the gene expression profiles of 10 DREB genes (both DREB1 and DREB2 families), in indica two DREB genes, DREB1B and DREB1G, were significantly correlated with stress tolerance indices, whereas in japonica significant correlations with five DREB genes (DREB1A, DREB1B, DREB1D, DREB1E and DREB2B) were observed. We found that only one member, i.e. DREB1B, showed a significant correlation with drought tolerance indices in both indica and japonica ecotypes. This study provides an overview of the relative contribution of different members of the DREB gene family and their association with drought/osmotic stress tolerance in rice.

Elucidation of dominant energy metabolic pathways of methane, sulphur and nitrogen in respect to mangrove-degradation for climate change mitigation

Mangroves play a key role in ecosystem balancing and climate change mitigation. It acts as a source and sink of methane (CH₄), a major greenhouse gas responsible for climate change. Energy metabolic pathways of methane production (methanogenesis) and oxidation (methanotrophy) are directly driven by sulphur (S) and nitrogen (N) metabolism and salinity in coastal wetlands. To investigate, how mangrove-degradations, affect the source-sink behaviour of CH₄; the pathways of CH₄, S and N were studied through whole-genome metagenomic approach. Soil samples were collected from degraded and undisturbed mangrove systems in Sundarban, India. Structural and functional microbial diversities (KEGG pathways) of CH₄, S and N metabolism were analysed and correlated with labile carbon pools and physico-chemical properties of soil. Overall, the acetoclastic pathway of methanogenesis was dominant. However, the relative proportion of conversion of CO₂ to CH₄ was more in degraded mangroves. Methane oxidation was higher in undisturbed mangroves and the serine pathway was dominant. After serine, the ribulose monophosphate pathway of CH₄ oxidation was dominant in degraded mangrove, while the xylulose monophosphate pathway was dominant in undisturbed site as it is more tolerant to salinity and higher pH. The assimilatory pathway (AMP) of S-metabolism was dominant in both systems. But in AMP pathway, adenosine triphosphate sulfurylase enzyme reads were higher in degraded mangrove, while NADPH-sulfite reductase abundance was higher in undisturbed mangrove due to higher salinity, and pH. In N-metabolism, the denitrification pathway was predominant in degraded sites, whereas the dissimilatory nitrate reduction pathway was dominant in undisturbed mangroves. The relative ratios of sulphur reducing bacteria (SRB): methanogens were higher in degraded mangrove; however, methanotrophs:methanogens was higher in undisturbed mangrove indicated lower source and greater sink capacity of CH₄ in the system. Microbial manipulation in mangrove-rhizosphere for regulating major energy metabolic pathways of methane could open-up a new window of climate change mitigation in coastal wetlands.

A chloroplast Glycolate catabolic pathway bypassing the endogenous photorespiratory cycle enhances photosynthesis, biomass and yield in rice (Oryza sativa L.)

Photorespiration accounts for 20-50 % reduction in grain yield in C₃ crops. The process is essential to remove 2-phosphoglycolate produced due to the oxygenation activity of the ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) enzyme. Attempts were made to improve photosynthesis through enriched CO₂ concentration by installing numerous photorespiratory bypass modules in the chloroplast of several crops. In this study, we have introduced Escherichia coli glycolate catabolic pathway (ECGC) into rice chloroplast to bypass photorespiration partially (PB) or completely (FB). Five genes encoding glyoxylate carboligase (GCL), tartronic semialdehyde reductase (TSR), and three subunits of glycolate dehydrogenase (GDH) were introduced to get FB plants, whereas only the three subunits of GDH were introduced to get PB plants. Southern analysis confirmed stable integration of the transgenes and their expression was confirmed by RT-qPCR analysis in the T3 progenies. Both FB and PB transformed lines exhibited increased photosynthetic efficiency, biomass, and grain yield than wild type (WT) with empty vector control. The introduction of ECGC pathway favoured the carboxylase activity of RuBisCO while decreasing its oxygenase activity fostering the functioning of Calvin-Benson cycle and resulting in an increased carbon-assimilation that was manifested in their superior architecture and harvest index. These findings will support rice and related cereal crop breeding programs to increase yield under elevated temperature and arid conditions.

Inter-relationship between intercepted radiation and rice yield influenced by transplanting time, method, and variety

Photosynthetically active radiation (PAR) is one of the most important environmental factors that determine the productivity and grain quality of the crops. Continuous rainy days or cloudy weather throughout crop growth especially at critical stages often resulted in great loss of grain quality and yield in rice. Low light stress has rigorously constrained the rice production in various rice-growing regions, especially in Southeast Asia. Method and time of planting are the major management factors contributing to the higher yield potential of rice by influencing light harvesting and use efficiency. Present study was executed consecutively for 5 years (kharif seasons of 2012-2016) to determine whether planting time improves the radiation absorption and use efficiency in different duration rice cultivars. We evaluated the difference in plant growth and development leading to yield formation under different planting time which related to radiation incidence and interception. The results of the study revealed that PAR interception depends on morphological characters of cultivars and also with agronomic management such as transplanting time and method. Long duration cultivar intercepted more PAR but interception decreased due to late planting (3rd week of July), whereas short duration cultivars (Naveen) when planted earlier (1st week of June) could not effectively utilize intercepted PAR constraining the biomass accumulation and yield formation. Effect of planting density and crop architecture on PAR absorption was apparent among establishment methods as light interception at crop canopy was highest in the system of rice intensification and lowest in that of wet direct seeding. In general, Pooja as a long duration cultivar intercepted more PAR per day but when compared on same date of planting, the comparative absorption of radiation was 30.6% higher in Naveen. The lower yields in the wet season are attributed mostly to reduction in grain number per panicle or per unit land area, which is a consequence of high spikelet sterility. Grain yield of rice planted in July third week was reduced by 3.8, 12.3, and 6.9% over June first and third week and July first week, respectively, mainly due to spikelet sterility (26%) and lower grains per panicle (18%). Our results indicated that agronomic management like optimum time of sowing, cultivar duration, and establishment methods should be followed for yield improvement in tropical lowlands where light intensity is limiting due to prevailing weather situations.

Mechanism of plant mediated methane emission in tropical lowland rice

Methane (CH₄) is predominantly produced in lowland rice soil, but its emission from soil to atmosphere primarily depends on passage/conduit or capillary pore spaces present in rice plants. The gas transport mechanism through aerenchyma pore spaces of rice cultivars was studied to explore the plant mediated CH₄ emission. Seven rice cultivars, based on the life cycle duration (LCD), were tested in tropical eastern India. Three LCD groups were, (a) Kalinga 1 and CR Dhan 204 (LCD: 110-120 days); (b) Lalat, Pooja and CR 1014 (LCD: 130-150 days); and (c) Durga and Varshadhan (LCD: 160-170 days). Rate of CH₄emission, root exudates, root oxidase activities and shoot aerenchyma pore spaces were analyzed to study the mechanism of plant mediated emission from rice. Aerenchyma pore space was quantified in the hypothesis that it regulates the CH₄ transportation from soil to atmosphere. The ratio of pore space area to total space was lowest in Kalinga 1 cultivar (0.29) and highest was in Varshadhan (0.43). Significant variations in the methane emission were observed among the cultivars with an average emission rate ranged from 0.86 mg m^-2 h^-1 to 4.96 mg m^-2 h^-1. The CH4 emission rates were lowest in short duration cultivars followed by medium and long duration ones. The greenhouse gas intensity considering average CH₄ emission rate per unit grain yield was also lowest (0.35) in Kalinga 1 and relatively less in short and medium duration cultivars. Root exudation was higher at panicle initiation (PI) than maximum tillering (MT) stage. Lowest exudation was noticed in (197.2 mg C plant^-1 day^-1) Kalinga 1 and highest in Varsadhan (231.7 mg C plant^-1 day^-1). So we can say, the rate of CH₄ emission was controlled by aerenchyma orientation, root exudation and biomass production rate which are the key specific traits of a cultivar. Identified traits were closely associated with duration and adaptability to cultivars grown in specific ecology. Therefore, there is possibility to breed rice cultivars depending on ecology, duration and having less CH₄ emission potential, which could be effectively used in greenhouse gas mitigation strategies.

Effect of fly ash deposition on photosynthesis, growth and yield of rice

An experiment was conducted to assess the effect of fly ash deposition without and with (0.25, 0.50, 1.0 and 1.5 g m(-2 )day(-1)) foliar dusting on the photosynthesis, stomatal conductance, transpiration, leaf temperature, albedo and productivity of rice. Dusting of 0.5 g m(-2 )day(-1) fly ash and above significantly reduced the photosynthesis, stomatal conductance, transpiration and albedo. Panicle initiation and flowering stages were more influenced by the fly ash deposition as compared to active tillering stage. At higher rates of fly ash deposition, all growth and yield parameters were significantly influenced due to increased heat load and reduced intercellular CO2 concentration. A significant reduction of 12.3, 15.7 and 20.2 % in grain yield was recorded over control when fly ash was dusted at 0.5, 1.0 and 1.5 g m(-2 )day(-1), respectively.

Impact of elevated CO2 and temperature on soil C and N dynamics in relation to CH4 and N2O emissions from tropical flooded rice (Oryza sativa L.)

A field experiment was carried out to investigate the impact of elevated carbon dioxide (CO2) (CEC, 550 μmol mol(-1)) and elevated CO2+elevated air temperature (CECT, 550 μmol mol(-1) and 2°C more than control chamber (CC)) on soil labile carbon (C) and nitrogen (N) pools, microbial populations and enzymatic activities in relation to emissions of methane (CH4) and nitrous oxide (N2O) in a flooded alluvial soil planted with rice cv. Naveen in open top chambers (OTCs). The labile soil C pools, namely microbial biomass C, readily mineralizable C, water soluble carbohydrate C and potassium permanganate oxidizable C were increased by 27, 23, 38 and 37% respectively under CEC than CC (ambient CO2, 394 μmol mol(-1)). The total organic carbon (TOC) in root exudates was 28.9% higher under CEC than CC. The labile N fractions were also increased significantly (29%) in CEC than CC. Methanogens and denitrifier populations in rhizosphere were higher under CEC and CECT. As a result, CH4 and N2O-N emissions were enhanced by 26 and 24.6% respectively, under CEC in comparison to open field (UC, ambient CO2, 394 μmol mol(-1)) on seasonal basis. The global warming potential (GWP) was increased by 25% under CEC than CC. However, emissions per unit of grain yield under elevated CO2 and temperature were similar to those observed at ambient CO2. The stimulatory effect on CH4 and N2O emissions under CEC was linked with the increased amount of soil labile C, C rich root exudates, lowered Eh, higher Fe(+2) concentration and increased activities of methanogens and extracellular enzymes.

Growth, yield and photosynthesis of Panicum maximum and Stylosanthes hamata under elevated CO2

Plant height, biomass production, assimilatory functions and chlorophyll accumulation of Panicum maximum and Stylosanthes hamata in intercropping systems was influenced significantly under elevated CO2 (600 +/- 50 ppm) in open top chambers (OTCs). The plant height increased by 32.0 and 49.0% over the control in P. maximum and S. hamata respectively in intercropping system under elevated CO2 over open field grown crops (Ca). P. maximum and S. hamata produced 67 and 85% higher fresh and dry biomass respectively under elevated CO2. Rates of photosynthesis and stomatal conductance increased in both the crop species in intercropping systems under elevated CO2. The canopy photosynthesis (photosynthesis x leaf area index) of these crop species increased significantly under elevated CO2 over the open grown crops. The chlorophyll a and b accumulation were also higher in the leaves of both the crop species as grown in OTC with elevated CO2. The increased chlorophyll content, leaf area index and canopy photosynthesis led to higher growth and biomass production in these crop species under elevated CO2. The total carbon sequestration in crop biomass and soils during the three years was 21.53 Mg C/ha under elevated CO2. The data revealed that P. maximum and S. hamata intercropping system is the potential as a sink for the increasing level of CO2 in the atmosphere in the semi-arid tropics.

Drought stress induced changes in lipid peroxidation and antioxidant system in genus Avena

Seven species of genus Avena viz., Avena sativa, Avena strigosa, Avena brevis, Avena vaviloviana, Avena abyssinica, Avena marocana and Avena sterilis were used to study the impact of drought stress on lipid peroxidation and other antioxidant enzymes. Maximum increase in the catalase activity was recorded in A. vaviloviana (129.97%) followed by A. sativa (122.82%) and A. brevis (83.38%) at vegetative stage; however at flowering stage the maximum increase was reported in A. sativa (25.62%) followed by A. sterilis (20.46%) and A. brevis (18.53%). At vegetative stage drought, maximum increase in peroxidase activity was recorded in A. sativa (122.82%) followed by A. brevis (83.38%) and A. sterilis (49.78%). Flowering stage drought, showed maximum increase in A. Sativa (27.09%) followed by A. marocana (23.50%) and A. sterilis (20.46%). A. sativa and A. sterilis showed stress tolerance at both the stages by accumulating higher percentage of peroxidase followed by A. brevis at vegetative and A. marocana at flowering stage. Level of lipid peroxidation in terms of Malondialdehyde (MDA) content was increased in the leaves when plants were subjected to moisture stress. The rate of increase in lipid peroxidation occurs irrespective of stage however; maximum increase was recorded in A. strigosa at both the stages. Avena species which showed high level of MDA content, indicates more lipid peroxidation and more membrane permeability and are comparatively more susceptible for water stress than those which produce less Malondialdehyde (MDA) content at higher magnitude of water stress such species have better capability for moisture stress tolerance.