Impact of variable water limited environments on grain sorghum yield and lodging in Australia

Abstract

Water availability is the primary constraint to sorghum production worldwide. The crop is grown in water-limited environments of varying extent and timing with water stress during grain filling being common. Varying extent and timing of water availability generates substantial genotype × environment interaction (GEI) for sorghum production, which restricts genetic improvement of grain yield. The objective of this thesis is to better understand sorghum performance under drought conditions and identify potential strategies to improve the rate of genetic gain for grain yield. To better understand the impacts and to deconstruct GEI by removing the confounding effects of flowering time (DTF) and tillering (FTN) on grain yield, linear mixed models were used to evaluate the impacts of DTF and FTN on grain yield of 1741 unique test hybrids. Hybrids were derived from crosses between 1078 elite male parents and 3 female testers and grown in 21 yield testing trials at 15 locations across the major sorghum production regions in Australia in three seasons. DTF made a significant contribution to genetic variation in grain yield of male lines in 14 of the 48 tester/trial combinations, explaining 0.2% to 61.0% variation, and FTN in 12 tester/trial combinations, explaining 1.4% to 56.9% variation. The relationship of DTF and FTN with grain yield of hybrids in the whole dataset was frequently positive, but varied depending on the genetic background of testers and growing environments. However, between trial genetic correlations for grain yield were not significantly improved after accounting for the effects of DTF and FTN using linear models. These results suggests that other genetic factors affecting canopy development dynamics and grain yield might contribute more of the genetic variation in grain yield. It is possible that linear mixed models did not capture the non-linear effects of flowering time and fertile tiller number on yield. One of the consequences of water stress during grain filling is lodging, with major consequences on grain yield and quality. Hence, resistance to lodging has been a major target of selection in sorghum breeding programs in Australia. A retrospective analysis was conducted on 37 commercial hybrids grown in 83 yield testing trials in major sorghum production regions in Australia in 14 seasons to quantify the geographical and seasonal variations in lodging occurrence and severity. Lodging occurred more frequently in Central Highlands and less frequently in Liverpool Plains, in comparison to the overall average across regions. The severity of lodging also varied across regions, with the most severe lodging (>20%) occurring in the Central Highlands and Western Downs. Lockyer Valley was the only region free from lodging. In addition, seasonal patterns of lodging frequency and severity were also observed. Over the 14 growing seasons, the frequency of lodging varied from 0 to 100%, with the most severe lodging (>20%) observed in 2005, 2016, and 2017. The Southern Oscillation Index explained 29% of the seasonal variation in lodging frequency. Results also showed that the levels of lodging resistance in commercial hybrids were not improved. This is possibly because sorghum breeders trade of improvements in lodging resistance to increase grain yield due to the complex nature of lodging. Currently, sorghum breeders use strategies of direct selection against lodging susceptibility and direct selection for stay-green (delayed leaf senescence) rather than selection for lodging resistance to improved lodging resistance. A better understanding of the genetic architecture of lodging and its association with other traits would enable faster and greater genetic gain in lodging resistance and grain yield. Large scale genome-wide association studies on lodging were conducted using data from 17 Australian sorghum trials over three seasons involving 2308 unique test hybrids. The identification of 213 QTL for lodging clearly demonstrate the complexity of lodging. The majority (92%) of the lodging QTL showed a significant association with leaf senescence and plant height, suggesting lodging in grain sorghum is primarily driven by carbohydrate remobilisation and plant height. Additionally, the thesis found that one third of the previously reported QTL for stalk rots co-located with lodging QTL identified in this study, 11 of which were associated with leaf senescence and/or plant height. These results indicate that genes providing classical resistance to stalk rots are likely to be of limited effectiveness in breeding for enhanced lodging resistance. However, the observation of significant enrichment of genes in lignin biosynthesis pathways within lodging QTL identified in this thesis indicates that lodging resistance is likely to be improved by selecting for stem strength and composition traits such as lignin content.