Dissecting physiological traits for drought and heat tolerance in wheat

Intergovernmental panel on climate change has predicted a 1.5 degrees C increase in temperature and a 4-27% decrease in precipitation in the next decade. Drought and heat cause 60% and 40% wheat yield losses by altering the physiology of the crop. Understanding the impact of these stresses on wheat physiological traits can help in developing tolerant genotypes. Forty-two genotypes were evaluated under non-stress (TSIR-NS), drought (TSRF-DR), and heat stress (LSIR-HT) for two crop seasons. The experiments were laid in rectangular lattice (6x7) design with two replications. Data for grain yield and various physiological traits were recorded at GS70 and GS75. Grain yield (GY) was reduced by 29.0%, 16.4% under heat, and by 48.7% and 30.2% under drought stress during the two seasons. Heat susceptibility index (HSI) and drought susceptibility index (DSI) of these genotypes ranged from 0.3 to 1.8 and 0.4 to 1.4. Among the top five high-yielding lines under TSIR-NS, only one (G39) was drought tolerant, while under LSIR-HT, four high-yielding lines (G40, G41, G04, and G35) of the top five genotypes were heat tolerant. Conversely, the top five high-yielding lines under TSRF-DR were drought tolerant. Overall, 21 lines were tolerant to heat stress and 22 to drought and 12 genotypes were tolerant to both stresses. Chlorophyll was higher under LSIR-HT and TSRF-DR. Normalized difference vegetation index at GS70 (NDVIA) and normalized difference vegetation index at GS75 (NDVI15DAA) suffered a drastic reduction under TSRF-DR. The stomatal conductance (gs) and transpiration (E) were greatly reduced under drought and increased under LSIR-HT. Assimilation (A) and photosynthetic water use efficiency (WUE) were reduced under heat stress. The correlation of physiological traits with GY was also calculated. The traits chlorophyll fluorescence at GS70 (CFLA), NDVI 15DAA, and E contributed toward GY under TSIR-NS, CFLA, NDVI15DAA, WUE, Canopy temperature at GS75 (CT15DAA), and gs under TSRF-DR, and A, NDVIA, CFLA, and chlorophyll fluorescence at GS75 (CFL15DAA) under LSIR-HT. CFLA contributed toward GY under all environments. Finally, traits such as A, E, WUE, and CFL15DAA were impacted by both stresses, while NDVIA and NDVI15DAA and gs were only affected by drought and CFLA and CTA were only influenced by heat stress.