Disentangling the Roles of Plant Water Status and Stem Carbohydrate Remobilization on Rice Harvest Index Under Drought.

BACKGROUND: Harvest index is an important component of grain yield and is typically reduced by reproductive stage drought stress in rice. Multiple drought response mechanisms can affect harvest index including plant water status and the degree of stem carbohydrate mobilization during grain filling. In this study, we aimed to dissect the contributions of plant water status and stem carbohydrate mobilization to harvest index. Pairs of genotypes selected for contrasting harvest index but similar biomass and days to flowering were characterized at ICAR-RCER, Patna, India and at IRRI, Philippines. RESULTS: Multiple traits were related with harvest index across experiments, including mobilization efficiency at both sites as indicated by groupings in principal component analysis, and plant water status as indicated by direct correlations. Biomass-related traits were positively correlated with harvest index at IRRI but biomass was negatively correlated with harvest index at ICER-RCER, Patna. We observed that some pairs of genotypes showed differences in harvest index across environments, whereas other showed differences in harvest index only under drought. Of all time points measured when all genotypes were considered together, the stem carbohydrate levels at maturity were most consistently (negatively) correlated with harvest index under drought, but not under well-watered conditions. However, in the pairs of genotypes grouped as those whose differences in harvest index were stable across environments, improved plant water status resulted in a greater ability to both accumulate and remobilize stored carbohydrate, i.e. starch. CONCLUSION: By distinguishing between genotypes whose harvest index was improved across conditions as opposed to specifically under drought, we can attribute the mechanisms behind the stable high-harvest index genotypes to be more related to stem carbohydrate remobilization than to plant water status. The stable high-harvest index lines in this study (Aus 257 and Wanni Dahanala) may confer mechanisms to improve harvest index that are independent of drought response and therefore may be useful for breeding improved rice varieties.

Can yield, soil C and aggregation be improved under long-term conservation agriculture in the eastern Indo-Gangetic plain of India?

Deteriorating soil health, diminishing soil organic carbon (SOC), development of subsurface hard compact layer and declining system productivity are barriers to achieving sustainable production in the traditional rice-wheat cropping system (TA) in the eastern Indo-Gangetic Plain of India. Conservation agriculture (CA), which favours minimum soil disturbance, crop residue retention and crop diversification could be a viable alternative to the TA to address most of those major problems. With that in mind, a long-term experiment is being implemented at ICAR-RCER, Patna, Bihar, India, with four treatments: (a) TA, (b) full CA (fCA) and (c and d) partial CA (pCA1 and pCA2), differing in crop establishment methods, cropping system and crop residue management in a randomized complete block design. Measurement of soil health parameters was carried out in the 11th year of the experiment. The results revealed a beneficial effect of CA and 46 and 40% increase in SOC concentration and stock, respectively, under fCA over TA in the 0-7.5-cm soil layer. The effect of partial CA (pCA1 and pCA2) was variable, but an increasing trend was always observed under pCA compared to TA. There was an enrichment in SOC content of aggregates under CA irrespective of size class; however, no relation was found between SOC content and aggregate diameter. The contribution of macroaggregates to SOC stock was larger (36-66%) under CA in the 0-7.5-cm soil layer. Adoption of CA improved the macroaggregate content, MWD and GMD of aggregates, and aggregation ratio. Soil macropore content was greater under fCA, whereas other parameters were similar among treatments. The impact of CA was mostly limited to 0-7.5 cm soil layer and a maximum up to 15 cm soil depth while evaluation until 60 cm soil depth was realized. The yield of rice in CA was comparable to or higher than in TA, whereas the system rice equivalent yield was always higher (38-53%) under CA than under the conventional practices. Therefore, a CA-based cropping system must be encouraged, to increase SOC status, improve aggregation stability and, consequently, sustain or increase system productivity, in order to achieve food and nutritional security in the eastern Indo-Gangetic Plain of India. HIGHLIGHTS: Effects of long-term conservation agriculture (CA) on soil C, aggregation and yield were evaluated.CA improved SOC concentration and stock by 46 and 40%, as well as macroaggregate SOC stock by 36-66%.Macro-aggregation and mean weight diameter improved in CA but was mostly limited to a shallow soil depth.CA can be promoted for sustainability of a rice-wheat system due to higher productivity (38-53%).

Enhanced antioxidant enzyme activities in developing anther contributes to heat stress alleviation and sustains grain yield in wheat.

Climatic variations along with a rise in temperature during the winter season impose severe heat stress during the anthesis stage of spring wheat, resulting in severe yield losses. The present study was conducted to evaluate the influence of heat stress on redox homeostasis in developing anthers and flag leaves of wheat. Five Indian bread wheat genotypes were studied under field conditions during the dry season, with two extreme sowing dates (timely and very late sown) to explore the effect of heat stress on anthesis stage. Results showed that elevated temperature during anthesis caused significant increase in reactive oxygen species (ROS) content and malondialdehyde (MDA) accumulation in developing anthers, triggering pollen mortality. Moreover, defective source (leaf) to the sink (anthers) mobilisation of starch also contributes in reducing pollen viability. However, ROS-induced oxidative damage of developing anthers under heat stress varied among the wheat genotypes depending upon differential antioxidant enzyme activities. Wheat genotype with enhanced antioxidant activities and reduced ROS built up in developing anthers sustained their grain yield, suggesting thermo-tolerance in wheat to be associated with antioxidant enzyme-mediated improved ROS-scavenging mechanism not only in leaves even in developing anther also. In the present study, heat stressed wheat genotype WH 730 exhibited effective source to sink mobilisation and sustainable grain yield with improved ROS scavenging, conferring greater potential for heat tolerance. We conclude that redox homeostasis and balanced source sink activity played a significant role for sustainable yield and heat tolerance in wheat.

Comprehensive physiological analyses and reactive oxygen species profiling in drought tolerant rice genotypes under salinity stress.

Rice being a staple cereal is extremely susceptible towards abiotic stresses. Drought and salinity are two vital factors limiting rice cultivation in Eastern Indo-Gangetic Plains (EIGP). Present study has intended to evaluate the consequences of salinity stress on selected drought tolerant rice genotypes at the most susceptible seedling stage with an aim to identify the potential multi-stress (drought and salt) tolerant rice genotype of this region. Genotypic variation was obvious in all traits related to drought and salt susceptibility. IR84895-B-127-CRA-5-1-1, one of the rice genotypes studied, exhibited exceptional drought and salinity tolerance. IR83373-B-B-25-3-B-B-25-3 also displayed enhanced drought and salt tolerance following IR84895-B-127-CRA-5-1-1. Variations were perceptible in different factors involving photosynthetic performance, proline content, lipid peroxidation, K+/Na+ ratio. Accumulation of reactive oxygen species (ROS) disintegrated cellular and sub-cellular membrane leading to decreased photosynthetic activities. Therefore, accumulation and detoxification of reactive oxygen species was also considered as a major determinant of salt tolerance. IR84895-B-127-CRA-5-1-1 showed improved ROS detoxification mediated by antioxidant enzymes. IR84895-B-127-CRA-5-1-1 seedlings also displayed significant recovery after removal of salt stress. The results established a direct association of ROS scavenging with improved physiological activities and salt tolerance. The study also recommended IR84895-B-127-CRA-5-1-1 for improved crop performance in both drought and saline environments of EIGP. These contrasting rice genotypes may assist in understanding the multiple stress associated factors in concurrent drought and salt tolerant rice genotypes.

Improvement of submergence tolerance in rice through efficient application of potassium under submergence-prone rainfed ecology of Indo-Gangetic Plain.

Potassium (K) is one of the limiting factors that negatively influenced rice growth and yield in submergence-prone soils. We conducted an experiment during the wet season of 2014-15 to achieve optimal doses of K and understand the effect of K application on submerged rice in terms of survival, chlorophyll content, non-structural carbohydrates (NSC), anti-oxidant activities and yield. Results revealed that chlorophyll and NSC content were significantly (P≤0.05) lower whereas the activity of anti-oxidants (catalase, superoxide dismutase and total peroxidase) were significantly (P≤0.05) higher after submergence compared with pre-submergence. Further, application of K at a higher basal dose (40kgha-1) was more beneficial to improve survival after de-submergence by maintaining NSC, chlorophyll content and higher activity of anti-oxidants with lower level of lipid peroxidation. Furthermore, results showed superiority of the treatments having application of higher doses with one foliar spray (T9-40kg K2O ha-1 (basal)+one foliar spray at 0.5% K at panicle initiation (PI) stage) for grain yield. We conclude that application of a higher dose of K with one foliar application at PI stage is more beneficial to enhance plant survival, better recovery and yield gain of rice during complete submergence.

Characterization of rice (Oryza sativa L.) genotypes on the basis of morpho-physiological and biochemical traits grown under aerobic situation in rainfed ecosystem .

The objective of the present study was to examine the effect of aerobic situation on yield, physiological and biochemical traits of advanced breeding lines of rice. Experiment was conducted with two set of rice genotypes under two water regimes (aerobic and irrigated), during three consecutive wet seasons 2010-2012. Significant decrease in yield was observed in rice genotypes grown under aerobic situation as compared to the irrigated ones. Promising rice genotypes having the ability to maintain high plant biomass, harvest index, early vegetative vigour, improved physiological and biochemical traits in terms of relative water content (RWC), leaf area index (LAI), total soluble sugar, starch, protien and proline content help to sustain higher grain yield under aerobic situation. The yield gap between aerobic and irrigated rice ranged between 24% to 68%. Grain yield showed positive correlation with harvest index (0.434), test weight (0.647), plant biomass (0.411) and effective tiller numbers (0.473), whereas spikelet sterility was negative associated (-0.380). The current study suggested that promising genotypes viz., IR77298-14-1-2-130-2, IR84899-B-182-3-1-1-2, IR84887-B-157-38-1-1-3 and IR 84899-B-179-1-1-1-2 for aerobic situation, showing yield advantage due to better performance of physiological and biochemical traits, might be adopted in large area of rainfed ecosystem as well as in irrigated areas where water scarcity was a major problem.