Ramani Kumar Sarkar

Submergence effects on rice genotypes during seedling stage: Probing of submergence driven changes of photosystem 2 by chlorophyll a fluorescence induction O-J-I-P transients

The effects of submergence on chlorophyll (Chl) a fluorescence were compared in seven Oryza sativa (L.) cultivars, namely FR 13A, Khoda, Khadara, Kalaputia (tolerant), Sabita, and Hatipanjari (avoiding type), and IR 42 (susceptible). Seedlings were submerged for 4 d under complete darkness. Oxygen concentration of flood water decreased with the period of submergence with concomitant increase in concentration of carbon dioxide. Submergence caused diminution in the amount of total Chl. Genotypic differences were observed for Chl content and survival percentage. Quantification of the Chl a fluorescence transients (JIP-test) revealed large cultivar differences in the response of photosystem 2 (PS2) to submergence. The kinetics of Chl a fluorescence rise showed complex changes in the magnitudes and rise of O-J, J-I, and I-P phases caused by submergence. The selective suppression of the J-I phase of fluorescence especially after 2 d of submergence provided evidence for weakened electron donation from the oxygen evolving complex whereas under severe submergence stress (4 d) both O-J and J-I steps were suppressed greatly with highly suppressed P-step, which resulted in lowering of variable fluorescence. Grouping probability or energetic connectivity between PS2 obtained through JIP-test from the data after 2 d of submergence showed a direct relation with survival percentage, i.e. fluorescence measurements contained the information of the survival chance of a plant under submerged conditions. The information could be used in identifying the submergence tolerant cultivars when the damage is not very severe.

Rice Genotypes with SUB1 QTL Differ in Submergence Tolerance, Elongation Ability during Submergence and Re-generation Growth at Re-emergence

Submergence tolerance is an important trait where short term flash flooding damages rice. Tolerant landraces that withstand submergence for 1–2 weeks were identified. Due to the heterogeneity in flood-prone ecosystem many different types of traditional rice cultivars are being grown by the farmers. The local landraces adapted to extremes in water availability could be the sources of genetic variation are to be used to improve the adaptability of rice to excess water stress. Greater genotypic variability was observed for plant height, elongation and survival %, absolute growth rate, non-structural carbohydrate retention capacity, chlorophyll content, different chlorophyll fluorescence parameters (FPs) characteristics, and re-generation growth at re-emergence. Twenty days submergence caused greater damage even in Submergence 1 (SUB1) introgressed cultivars compared to the 14 days of submergence. The FPs, carbohydrate content and dry weight at the end of submergence showed positive and highly significant association with re-generation growth. The presence of SUB1 associated primers, either SC3 or ART5, was noticed even in greater elongating types of rice genotypes. These genotypes possess one or more of the adaptive traits required for the flood-prone ecosystem, which range from temporary submergence of 1–2 weeks to long period of stagnant water tolerance.

Distinction and characterisation of submergence tolerant and sensitive rice cultivars, probed by the fluorescence OJIP rise kinetics

Rice (Oryza sativa L.) plants experience multiple abiotic stresses when they are submerged. In addition to the effects of submergence on gas exchange, water also creates shading of submerged plants. It is believed that responses to submergence are actually responses to low light stress, although during complete submergence in addition to low light other environmental factors like reduce movement of gases affect the plant growth, and therefore, the consequences of submergence are not always alike to shade. We monitored the extent to which shade and submergence change the plant height, chlorophyll a fluorescence characteristics and CO2 photosynthetic rate in three Indica rice cultivars, namely Sarala, Kalaputia and Khoda, which differed in submergence tolerance. There were both similarities and dissimilarities between the consequence of shade and submergence on rice plants. Under shade conditions, elongation growth was greater in submergence tolerant cultivars than the sensitive cultivar, whereas elongation growth was greater under submergence in sensitive cultivar. The reduction in chlorophyll content, damage to PSII, and decrease in CO2 photosynthetic rate was more notable under submergence than the shade conditions. Our results show that several JIP-test parameters clearly distinguish between submergence tolerant and sensitive cultivars, and responses to submergence among different rice cultivars differ depending on their sensitivity to submergence. There were different interactions between cultivar and shade (~low light) and cultivar and submergence.

INFLUENCE OF PHOSPHORUS APPLICATION TO FLOODWATER ON OXYGEN CONCENTRATIONS AND SURVIVAL OF RICE DURING COMPLETE SUBMERGENCE

SUMMARY Low concentrations of phosphorous were added to the floodwater of submerged rice in an attempt to regulate algal growth, so as to increase floodwater O2 concentration and plant survival during submergence for up to 12 days. Increasing the phosphorus concentration of the floodwater by 1 ppm increased algal growth by at least 4-fold based on chlorophyll concentrations, and increased floodwater O2 concentrations to over twice air saturation. However, additions of phosphorus reduced rice plant survival during submergence by up to 35%. The adverse eAects of a high phosphorus concentration in the floodwater were mainly attributed to competition between the algae and the submerged rice for CO2 and, perhaps, light. The importance of photosynthesis during the submergence of rice was supported by the results of experiments in which floodwater CO2 concentration was manipulated by altering pH. The survival of an intolerant rice cultivar during submergence increased from 0 to 17 and 62% at pH 8, 7 and 5 respectively, while floodwater CO2 concentrations at these pHs would have increased from 0.02 to 0.3 and 1.0 mol m 73 respectively. The results were used to question the importance of floodwater O2 concentrations above anoxia for submergence tolerance of rice.

Distinction and characterisation of salinity tolerant and sensitive rice cultivars as probed by the chlorophyll fluorescence characteristics and growth parameters

Soil salinity is a major abiotic stress that limits rice productivity worldwide. The problem is intense - particularly in areas with extremely dry and hot climatic conditions. Designing an effective phenotyping strategy requires thorough understanding of plant survival under stress. The investigation was conducted using 12 rice cultivars differing in salinity tolerance. Among these cultivars, seedling survival on day 10 of salt treatment (12dSm-1) was above 85% during wet season and 75% during dry season in FL478, AC39416, Pokkali and Kamini. Highly salt-tolerant cultivars maintained greater proportion of green leaf and chlorophyll content under salt stress. Unlike sensitive cultivars, tolerant cultivars taken up less Na+ and more K+, resulting in lower Na+:K+ ratio in leaf and sheath. Normalised chlorophyll a fluorescence data revealed that the Fv/Fm and PIABS values decreased on days 3 and 7, respectively, of salt stress in susceptible rice cultivar. Salinity factor index (SFI) calculated by giving different weights to relative PIABS values after variable days of salinity stress clearly distinguished the level of tolerance among rice cultivars. The SFI can be used for grouping of moderately to highly salt-tolerant cultivars based on their tolerance level. We conclude that maintenance of greater proportion of green leaf, and restricted transport of Na+ to sheath and leaf helps the plant to counteract adverse effects of salinity on rice growth.

Leaf Photosynthetic Activity and Antioxidant Defense Associated with Sub1 QTL in Rice Subjected to Submergence and Subsequent Re-aeration

The influence of submergence on photosynthesis and antioxidant capacities in rice varieties Swarna and Swarna-Sub1 with or without Sub1 QTL were evaluated under control, simulated complete submergence and subsequent re-aeration. The leaf photosynthetic rate and stomatal conductance decreased in the both varieties during the progression of submergence as compared to the control plants, but significant varietal differences were observed after 1 d of submergence. Submergence also altered the PSII activity, as reflected in a decrease in the values of Fo, Fm and Fv/Fm and degradation of chlorophyll, more in Swarna than in Swarna-Sub1. During early submergence period, the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), glutathione reductase (GR) against reactive oxygen species were increased in the both varieties. However, with the progress of submergence period (after 7 d), the activities of SOD, catalase (CAT), APX, guaiacol peroxidase (GPX), GR and DHAR declined, more in Swarna than in Swarna-Sub1. During re-aeration, Swarna-Sub1 showed significant increase of above antioxidant enzymes but not in Swarna. Swarna-Sub1 improves photosynthetic activity, showing higher photosynthetic rate compared to Swarna under submergence and subsequent re-aeration because of less degradation of chlorophyll, higher stomatal conductance, and efficient PSII activity along with better antioxidant protection from oxidative damage.

Mechanism associated with nonstructural carbohydrate accumulation in submergence tolerant rice (Oryza sativa L.) cultivars

Nonstructural carbohydrate (NSC) accumulation in submergence tolerant rice cultivars (cv) was studied in six Indica rice [Oryza sativa (L.)] cv under control and simulated submerged conditions. Tolerant cultivars accumulated greater contents of NSC compared to the susceptible cultivars. Starch and total NSC content showed significant positive association with survival percentage. On the other hand, elongation due to submergence was significantly a negative association with survival. The CO2 photosynthetic rate, chlorophyll content, maximum photochemical efficiency of PS II (Fv/Fm), and activities of Rubisco were not significantly different between tolerant and susceptible cv under control condition. The ADP glucose pyrophosphorylase (AGPPase) activity was significantly higher in the tolerant cv and was a positive association with starch/NSC, whereas Fructose 1,6-diphosphatase (FDPase) activity was significantly higher in susceptible cv compared to tolerant cv and was a negative association with starch/NSC. Greater activities of AGPPase along with lower activities of FDPase might facilitate greater accumulation of NSC in tolerant rice cultivars.

Submergence-tolerant rice withstands complete submergence even in saline water: Probing through chlorophyll a fluorescence induction O-J-I-P transients

Plants experience multiple abiotic stresses during the same growing season. The implications of submergence with and without saline water on growth and survival were investigated using four contrasting rice cultivars, FR13A (submergence-tolerant, salinity-susceptible), IR42 (susceptible to salinity and submergence), and Rashpanjor and AC39416 (salinity-tolerant, submergence-susceptible). Though both FR13A and IR42 showed sensitivity to salinity, FR13A exhibited higher initial biomass as well as maintained greater dry mass under saline condition. Greater reduction of chlorophyll (Chl) contents due to salinity was observed in the susceptible cultivars, including FR13A, compared to the salinity-tolerant cultivars. Exposure of plants to salinity before submergence decreased the survival chance under submergence. Yet, survival percentage under submergence was greater in FR13A compared to other cultivars. Generally, the reduction in the Chl content and damage to PSII were higher under the submergence compared to salinity conditions. The submergence-tolerant cultivar, FR13A, maintained greater quantities of Chl during submergence compared to other cultivars. Quantification of the Chl a fluorescence transients (JIP-test) revealed large cultivar differences in the response of PSII to submergence in saline and nonsaline water. The submergence-tolerant cultivar maintained greater chloroplast structural integrity and functional ability irrespective of the quality of flooding water.

ROLE OF NON-STRUCTURAL CARBOHYDRATE AND ITS CATABOLISM ASSOCIATED WITH SUB 1 QTL IN RICE SUBJECTED TO COMPLETE SUBMERGENCE

The present study is to characterise the non-structural carbohydrate (NSC) status and its catabolism along with elongation growth in rice cultivars either possessing or not possessing the Sub 1 quantitative trait locus (QTL), i.e. Swarna and Swarna Sub1 exposed to seven days of complete submergence. During submergence, Swarna accelerated the rate of stem and leaf elongation and rapidly consumed NSC. In contrast, Swarna Sub1 consumed energy resources more slowly and maintained similar growth rate to that of non-submerged plants. Swarna Sub1 showed better utilisation of carbohydrate than that of Swarna by progressive induction of alcohol dehydrogenase, starch phosphorylase and total and α-amylase enzyme activity during submergence. Overall, submergence tolerance conferred by the Swarna Sub1 QTL is correlated with better maintenance and utilisation of NSC than that of Swarna.

Stability analysis of backcross population for salinity tolerance at reproductive stage in rice

Significant yield reduction was recorded at flowering stage in rice varieties under salinity stress. But due to high genotype x environment interaction (GEI) the adaptability of the salt tolerant elite lines is generally poor. The identified source of tolerance, AC41585, was used in developing a backcross population. One hundred eighty BC3F4 lines were evaluated under stress (EC 8dSm−1) and non-stress conditions in net-house during the years 2012 and 2013. Under salinity stress plant yield was observed to be associated positively with the number of panicles/plant, panicle length, harvest index and negatively with the percentage of spikelet sterility and degeneration. Genotypes such as, L-41, L-45, L-112, L-171, L-192 with low IPCA and higher mean were identified with general adaptability through AMMI analysis. In addition, ‘which-won-where’ pattern of GGE Biplot detected L-192 and L-41 as highest performing genotypes in saline and non-saline environments, respectively. Both the analyses identified stable introgression line L-171 with high yield stability index having phenotypic similarity with recurrent parent, IR 64. The elite lines selected through the present study could be used in rice breeding and also to investigate the molecular basis of salt tolerance at reproductive stage.