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High temperature adversely affects the development and growth of wheat crop. It disturbs photosynthesis and transpiration due to canopy temperature and senescence of leaves that cause reduction in wheat grain yield. Genetic divergence among genotypes and suitable selection criteria is imperative for efficient breeding programs to develop tolerance against heat stress. Therefore, 158 wheat genotypes were evaluated under normal and delayed planting mediated heat stress. Current study revealed reduction in grain yield (42.41%), grains per spike (16.51%), photosynthetic rate (39.84% and 48.52%), transpiration rate (55.88% and 51.85%) at vegetative and reproductive stage respectively. Furthermore, cell membrane injury (11.4-52.0% and 10.4-32%), canopy temperature depression (15.5-10.8 degrees C and 11.3-6.1 degrees C) at vegetative and reproductive stage, leaf angle (6 degrees-42 degrees) and stay green (68.8-17.5) were observed from heat tolerant to susceptible genotypes. Principal component analysis indicated highest negative loading components viz., stay green followed by grain yield and canopy temperature depression at reproductive stage whereas positive loading component such as heat susceptibility index for grain yield that would be used as effective selection criteria against heat stress. Correlation analysis suggested that high values of stay green, thousand grain weight, grains per spike, photosynthetic rate, transpiration rate and proline content while low values of cell membrane injury, heat susceptibility index for grain yield and canopy temperature can improve grain yield. Cluster analysis characterized 158 wheat genotypes into three groups based on different physiomorphic traits. Cluster-III retained genotypes with high value for heat tolerance indices such as stay green, grain yield, canopy temperature depression at reproductive stage while least value of heat susceptibility index for grain yield those were selected in PCA. These results would enhance the efficiency and precision as selection criteria for improvement in wheat against heat stress.

Authors are gratefully thankful to Department of Plant Breeding and Genetics, PMAS Arid Agriculture University Rawalpindi, Pakistan and CIMMYT for providing seeds and funds for this research work.

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