Linking QTLs that Regulate the Distinct Epicuticular Layers in the Spike Glume, and Its Variable Composition to Improve Reproductive Stage Heat Tolerance in Wheat

Abstract

Global climate experiments project an average increase of ambient temperatures of 0.2°C per decade. Such prediction emphasizes the importance of crop varieties that have high heat tolerance. Wheat is significantly affected by high temperature. Optimizing heat and drought tolerance in wheat is one way to improve breeding efficiency. Previous studies on wheat leaf epicuticular wax (EW) have shown a strong association between wax load and high temperature stress tolerance. This study aimed to investigate the relationship between EW on wheat glume and high temperature tolerance. This study also compared the effect of glume EW to the effect of leaf EW on the plant agronomic productivity. A recombinant inbred line (RIL) population derived from the heat tolerant Australian cultivar ‘Halberd’ which we have previously identified as having a unique genetic loci regulating spike cooling, was used in this experiment. The RIL mapping panel contains 180 lines derived from Halberd and a heat susceptible cultivar, Len. The population was grown at multiple field locations at Collage Station, Texas, Uvalde, Texas, and Obregon, Mexico for the growing seasons of 2013 and 2014. The EW of leaves and glumes were extracted using published methods (Richardson et al, 2007). An alpha lattice design with 180 recombinants and 2 replications was used in four different environments over 2 years. The EW samples were collected at 10DAP and leaf/spike temperatures were recorded at the same time. Spectral canopy reflectance was measured between 350–1100 nm range. Yield components were estimated after harvest. Spike temperature depression was measured. The 180 RIL and their parents were mapped using 90K SNPs markers to identify linkage groups or QTL for EW.

A strong correlation was found between mean wax load for leaf and that for glume as function of mean high temperature recorded for 10DAP across the environments with R^2 = 0.6719 and R^2 = 0.8483 respectively . The maximum mean of leaf EW at OBR14 being 5.37 and that of CS13 was 2.51 mg/dm^2 , while the glume EW mean at OBR14 was 5.97 mg/dm^2 and CS13 was 2.38 mg/dm^2 . The EW mean was higher in glumes as compared to that of leaf for all locations except for CS13, which was considered a more optimum climate for wheat. A strong correlation between the two wax loads for UVL13 was observed with R^2 = 0.8285 and r=0.9195 significant at p≤ 0.001. A significant correlation also was observed for the two wax loads for OBR14 with R^2 = 0.0304 and r=0.1744 significant at p≤ 0.05. All yield and yield components data showed significant variation between the different growing locations. Correlation between WI was significant at p≤ 0.05 for most water status indices were associated with the glume wax with R^2 ranging from 0.274 to 0.2198, whereas there was no correlation between WI and leaf wax. The thermal index had negative correlation and was only significant with glume wax content with r = -0.5943 and significance level of P≤0.001. Spike temperature had a positive correlation with both leaf and glume wax content with an R^2 values of 0.078 and 0.1952 respectively. Two significant QTL for EW were detected on chromosome 5B. Leaf EW, QLWax.tam-5B, was on position 104.584 and explained 6.8% of the variation. Glume EW, QGWax.tam-5B, was located on position 102.098 and explained 6.6% of the variation.