Dissecting the QTLome for Osmotic Adjustment and Chlorophyll Fluorescence in Field Grown Durum Wheat

Among the proxy traits involved in the adaptive response of wheat to drought and other abiotic stresses, information on the QTLome governing osmotic adjustment (OA) and chlorophyll fluorescence imaging (CFI) is very limited, mainly due to the difficulty to measure these traits on a scale suitable for GWAS. In this study, OA and CFI were measured in plots of 248 elite accessions of durum wheat grown (two replicates) under a fully automated Lemnatec Field Scanalyzer (LFS) and exposed to a progressively severe drought treatment starting 5 days prior to flowering which lasted for two weeks. Leaf relative water content (RWC), chlorophyll content (SPAD) and leaf rolling (LR) were also measured. CFI was measured (i) in the dark in four time series using the LFS camera and (ii) during daytime using a light-induced fluorescence transient (LIFT) sensor mounted on a manually pushed cart. The high variability in OA and CFI among the durum accessions resulted in high to medium repeatability (h2 = 72.3 and 54.6%, respectively). At the end of the drought treatment (mean leaf RWC = 62.1%), OA and leaf RWC were positively correlated (r = 0.78). Association mapping using flowering time as covariate revealed 15 QTLs for OA (global R2 = 63.6%) as well as eight major QTL hotspots on chromosome arms 1BL, 2BL, 4AL, 5AL, 6AL, 6BL and 7BS where a higher OA capacity was always positively associated with leaf RWC, SPAD and negatively associated with LR, hence indicating a beneficial effect of OA on the water status of the plant. Additionally, the comparative analysis with previous field trials showed concurrent effects for five of these hotspots on normalized difference vegetation index (NDVI), thousand kernel weight (TKW) and/or grain yield (GY), hence supporting OA as a valuable proxy for marker-assisted selection aimed at enhancing drought resistance in wheat.