Grain Yield and Quality Traits of Rice (Oryza sativa L.) Cultivars under Intermittent Drought and Contrasting Temperatures : physiological, biochemical and molecular characterisation

Abstract: Rice (Oryza sativa) has evolved in a semi-aquatic tropical environment and therefore thrives better in a water intensive system and optimal temperature between 25 and 30°C. With the future climate change, frequent droughts and heat stress are predicted to happen simultaneously in field but little is known about their combined effects on rice productivity and quality. This thesis investigated the effects of high temperature combined with recurring drought at different stages of plant development and identified quantitative trait loci associated with drought tolerance. Drought sensitivity, growth characteristics, grain yield and its component traits, and grain quality traits were also evaluated in seven rice cultivars grown in Rwanda. The results showed that grain yield was strongly influenced by the number of panicles per plant and spikelet fertility. A combination of high temperature and repeated drought at seedling and tillering stages dramatically limited panicle development, whereas spikelet fertility was negatively affected by drought at the reproductive stage. Grain yield was reduced by 30% to 100% as a result of drought and temperature stress as compared with well-watered control conditions. Quantitative trait loci (QTL) for number of panicles per plant were found on chromosomes 1, 4, and 8 whereas QTL for spikelet fertility were located on chromosomes 1, 5 and 9. Four QTL each were found for grain yield after drought at seedling stage (located on chromosomes 1, 4, 7, 9) and for grain yield after drought at tillering stage (located on chromosomes 6, 8, 11, and 12). These QTL could be introgressed into elite cultivars for improvement of their adaptation to intermittent drought. The cultivars displayed genetic diversity in their tolerance to stresses and quality characteristics. ‘Intsindagirabigega’ and ‘Jyambere’ were adapted to high temperature site conditions. ‘Intsindagirabigega’ had the highest amylose content. ’Ndamirabahinzi’ and ‘Mpembuke’ were rich in total phenolic and total antioxidant capacity, and displayed a beneficial stress memory. ‘Jyambere’ had high total protein content and high content in mineral elements together with ‘Nemeyubutaka’ and ‘Ingwizabukungu’. The results in this thesis indicate potential for developing resilient, high-yielding and nutritionally rich rice cultivars that can be grown in stressful environments.