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Responses of Contrasting Rice Genotypes to Anaerobic Condition During Germination
Dissertation Abstract:
Most Asian rice farmers are shifting from labor, water, and energy intensive transplanted puddle rice (TPR) to direct-seeded rice (DSR) system due to scarcity of these resources. Although weed problem in DSR can be suppressed with flooding after dry seeding of rice, flooding can cause adverse effects on germination and subsequent growth of rice, resulting in poor crop establishment. However, crop establishment under flooding can be enhanced by developing anaerobic germination (AG)-tolerant genotypes for DSR irrigated and rainfed areas, where waterlogging during germination is expected because of unleveled fields. The aim of this study was to better understand the adaptive mechanisms of newly-identified AG tolerant genotypes— namely, Bota Bara, Vavilovi, Zi Gan Nan Gu, and Kon Suito, compared to the tolerant check Ma_Zhan Red and the sensitive check IR42. The researcher evaluated the morphological and physiological responses related to AG tolerance and studied the genetic variation in the major (quantitative trait loci) QTLs, AG1, and AG2 markers identified before.
Allelic variation in AG1 and AG2 markers in the new tolerant Bota Bara, Vavilovi, and Kon Suito confirmed the genetic variations of AG tolerance with potentially differences mechanism. Although all tolerant genotypes showed similar pattern of carbohydrate depletion in germinating seeds and efficient ROS-scavenging activities. This study highlights the involvement of several mechanisms associated with the tolerance of anaerobic condition during germination in rice.Six greenhouse and two growth chamber experiments were conducted with three replications in each experiment. AG tolerance in all of the genotypes is associated with better seedling emergence, longer coleoptile, longer shoot and root length, heavier biomass production, well-developed shoot aerenchyma, and faster water absorption rate during the first 48 hours after sowing and watering. Rapid starch depletion, higher-soluble sugar concentration, higher-amylases activities, and higher-trehalose-6-phophate phosphatase (TPP) activities in all tolerant genotypes was observed, explaining their improved survival rate and growth under flooding. Increased activities of alcohol dehydrogenase (ADH), pyruvate decarboxylase (PDC), and aldehyde dehydrogenase (ALDH) were observed in all tolerant genotypes under flooded (AG, anaerobic germination) condition compared with saturated (control) condition and also in comparison with the sensitive under flooded condition. Tolerance of anaerobic germination was associated with lower superoxide and hydrogen peroxide (H 0 ) concentration, lower malondialdehyde (MDA) concentration, higher DPPH (2,2-diphenyl-l-picrylhydrazyl) scavenging activity, total phenolic compounds concentration, increased concentration of reduced ascorbate, together with higher superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX) activities in all tolerant genotypes. These antioxidants probably reduced the cell damage caused by anoxia during germination.
