Ph.D

Rice cultivation in the Kole tracts, a unique wetland ecosystem of Kerala, is being drastically affected by the rising temperatures driven by climate change. The higher temperature during the second cropping season coincides with the reproductive stage of the crop leading to significant yield reduction, thereby affecting the overall productivity of the crop. To sustain rice production and harness the full potential of this fertile ecosystem in double cropping scenario, it is essential to understand the physiological and molecular mechanisms underlying the heat induced productivity decline which will facilitate the development of high yielding rice varieties having high temperature resilience. In this context, the current study employs physiological and molecular breeding approaches to improve resilience of rice to heat stress. The study consisted of three experiments using five rice varieties namely Nagina 22 (90-95 days), KAU Manu Ratna (95-100 days), MO 16 (Uma) (115-120 days), KAU Manu Varna (128-138 days) and Ptb 39 (Jyothi) (115-120 days). Variety Nagina 22 served as the heat tolerant check while the other four are popular high yielding rice varieties cultivated in Kerala. In the first experiment, the acquired thermotolerance of the above five rice varieties were evaluated through Temperature Induction Response Technique (TIRT). The experiment was laid out in a completely randomized design (CRD) with three replications in the laboratory. Two-day old seedlings were exposed to a gradual temperature rise (induction range: 32°C to 42°C for 5 h by increasing each degree from 32°C to 42°C every 30 minutes), followed by exposure to a challenging temperature (lethal temperature: 49°C for 3 h) and a subsequent recovery period of three days at ambient conditions. Nagina 22 registered a higher survival per cent (91.67), recovery growth (47.83 mm) and heat-stable protein levels (0.059 µg g -1). Jyothi recorded higher survival per cent (100%) and heat stable protein (0.040 µg g -1) indicating strong innate tolerance. KAU Manu Ratna, KAU Manu Varna, and Uma exhibited moderate to poor tolerance, particularly with respect to recovery growth and heat-stable protein levels, indicating their susceptibility to heat stress during seedling stage. The second experiment was a pot culture study using the five rice genotypes to evaluate the comparative potential of ameliorant sprays viz. salicylic acid (400ppm),
Hoagland solution (1/4x) and water spray over unsprayed control in high temperature stress mitigation and overall growth promotion. The varieties were evaluated simultaneously both under ambient and high temperature conditions between January to April, 2023, each following a completely randomized design (CRD) with three replications. The high temperature chamber builds up temperature 5-8oC higher than ambient condition, which was regularly monitored using data loggers. The rice genotypes grown under both the conditions, were subjected to four different foliar sprays viz. T1-salicylic acid (400 ppm), T2-Hoagland solution (1/4x), along with water sprayed (T3) and unsprayed (T4) controls, administered at maximum tillering stage and booting stage. The observations were taken one week after the second spray corresponding to panicle initiation in each variety. The day/night temperature regime in high temperature chamber (44oC/32oC) and ambient condition (38oC/25oC) was also monitored at the time of taking observations. Pollen viability was higher in salicylic acid (400 ppm) spray (T1) under high temperature, especially in Nagina 22 (98.07%) and KAU Manu Ratna (97.13%) suggesting better pollen function, successful fertilization and partitioning. In KAU Manu Ratna, salicylic acid (400 ppm) spray (T1) displayed significantly higher increase in spikelet fertility (8%) under ambient conditions while in the high temperature chamber, water spray (T3) displayed significantly higher increase in spikelet fertility (70%), compared to the unsprayed control in the respective conditions. Physiological parameters like membrane stability index, photosynthetic rate, transpiration rate, stomatal conductance, etc. were measured in the flag leaf at panicle initiation stage. Salicylic acid (400 ppm) spray improved photosynthetic rate and contributed to the grain yield significantly irrespective of variety in the ambient condition. Among varieties, the photosynthetic rate was significantly higher for KAU Manu Ratna (9.09 µmol CO₂ m⁻² s⁻¹). Under ambient condition, KAU Manu Varna (16.2 g/plant) effectively used salicylic acid (400 ppm) spray to improve yield, next to Nagina 22 (19.08 g/plant). Similarly, KAU Manu Varna displayed highest yield when sprayed with water (1.67g/plant) followed by salicylic acid (400 ppm) (1.64 g/plant) among all treatments across genotypes. Visible varietal difference in response to different ameliorants was noted owing to their contributory significance. Thousand grain weight data indicated that the grains from salicylic acid (400 ppm) treated plants were heavier (by 16 % in KAU Manu Ratna, 35% in Uma ,25% in KAU Manu Varna, and 21% in Jyothi) as compared to the respective unsprayed controls in ambient condition confirming better grain filling. Salicylic acid (400 ppm) emerged as the best treatment to enhance heat tolerance in Manu Ratna and Manu Varna while Hoagland (1/4x) worked better in Nagina 22. It can be inferred from the study that the ameliorative advantage of each foliar supplement used in the study is variety dependent and their dose needs to be standardized for each variety to improve heat tolerance without penalizing primary growth. The third experiment involved marker assisted breeding for improvement of heat tolerance of high yielding rice variety KAU Manu Ratna by hybridization with heat tolerant donor parent Nagina 22 employing single sequence repeats (SSRs) marker based genotyping. Parental polymorphism was ascertained by genotyping the parents using previously identified 44 rice markers associated to heat tolerance (Waghmare et al., 2018). Out of these 43 SSR markers, 21 exhibited parental polymorphism thereby showing a 48.83 per cent polymorphism rate between the two parents. The size of polymorphic markers identified ranged from 97 bp (RM473a) to 292 bp (RM10346). F1 plants were selfed to derive F2 population. Phenotyping of F2 population was done to identify heat tolerant progenies in a high temperature chamber with an average of 5oC higher than ambient temperature. Spikelet fertility was used to identify tolerant and susceptible bulks and was confirmed using the already identified polymorphic markers that differentiated the two bulks. Markers RM13 and RM495 were able to differentiate both the bulks, clearly showing co-segregation. Therefore, these markers, RM13 and RM495 may be putatively linked with spikelet fertility in the F2 population generated by crossing N22 and KAU Manu Ratna. Genotyping and phenotyping of the F2 population provided insights about the presence or absence of the heat tolerant marker signatures in the F2 progeny and their potential to maintain yield characters. The current study provides insights into assessing as well as enhancing high temperature tolerance in rice genotypes by integrating physiological screening through TIRT, exogenous amelioration and molecular breeding techniques. Further research could focus on refining the dosages of ameliorants and their possible combinations which may improve the overall performance of rice, irrespective of varieties. Unravelling the defective mechanisms of source-sink partitioning in susceptible varieties need to be focussed in the path ahead. The mapping population derived from the F2 could be effectively used for variety development and to study closely linked markers associated to heat tolerant QTLs. The F2 population could be effectively used to develop multiple mapping population targeting different heat tolerance mechanisms. Thus, the results of the current study offer a strong foundation for development of climate resilient rice involving mitigative amelioration and marker assisted breeding so as to address the global food security challenges consequential of climate change.