Narrow root cone angle promotes deeper rooting, cooler canopy temperatures and higher grain yield in a rice (Oryza sativa L.) recombinant inbred line population grown under different water availabilities in aerobic production systems

Context: Rice crops are one of the world's major consumers of irrigation water. With water becoming more limited, water-saving aerobic production (AP) systems may provide an alternative to traditional flooded culture. Genotypes with narrow root cone angle (RCA) and subsequently deeper rooting systems are hypothesised as key for AP adaptation ensuring water uptake at depth. Objective and Methods: Using a recombinant inbred line (RIL) population and differential genotypes (narrow and wide RCA groups) derived from a Sherpa/IRAT109 cross, a glasshouse experiment and four field experiments with different water inputs in well-watered (WW, in two seasons), mild water deficit (MWD), and intermittent water deficit (IWD) conditions were carried out to characterise the relationship of RCA with grain yield (GY) and key physiological traits. Results: Significant genotypic variation and group differences were found in root traits, canopy temperature and GY. Expression of genotypes with narrow RCA were found to be consistent across experiments and water availability conditions. Genotypes in the narrow RCA group produced a greater proportion of roots below 20 cm (7.4-17.7%), longer total root length (69.7%), cooler canopies (0.3-1.0 degrees C) and higher GY (14-62%) compared with wide RCA group. Two promising RILs were identified which expressed high GY in WW (12.0-13.4 t/ha) conditions while maintaining GY in IWD (up to 9.4 t/ha). Higher GY was obtained in genotypes with cooler canopies (r =-0.72) and narrow RCA (r =-0.39) in IWD. Narrower RCA was associated with higher total root length (r =-0.70) and higher stomatal conductance (r =-0.71) in the glasshouse and proportion of deep roots (r =-0.65) and cooler canopies (r = 0.66) in the field. Conclusions: To our knowledge, this was one of the foremost studies exploring and demonstrating such re-lationships in rice AP systems. Root traits determined in the field were congruent with the other root-related traits driving differences between RCA groups measured in the glasshouse providing confidence with the field phenotyping methods. This work demonstrated the advantage conferred by the development of narrow RCA in aerobic conditions, presented improved physiological understanding of RCA and with genomics assisted breeding, is considered valuable for the development of AP-adapted, sustainably produced rice.

Automated summary

Genotypes with narrow root cone angle (RCA) are advantageous in water-saving aerobic production (AP) systems, as they allow for deeper water uptake and higher grain yield. Results from glasshouse and field experiments demonstrated the stability and advantages of genotypes with narrow RCA in different water availability conditions, such as longer total root length, cooler canopies, and higher grain yield.