Durum wheat (Triticum durum Desf.) is the 10th most important crop in the world and cultivated in vast agroenvironments. In tropical countries, high temperature stress is the major abiotic stress that affects the photochemical efficiencies of the plants/crop which in turn affect the productivity and yield of crop. Two high yielding and low yielding genotypes of durum wheat were selected for detailed analysis of their photo-chemical efficiencies. In low yielding genotypes (Malvi local and Sawer local), the whole primary photo-chemical reactions are affected before and after heat stress. The results show that low yielding genotypes show less efficiency in the usage of the available excitation energy. This is a case study to establish use of chlorophyll a fluorescence measurement as an effective tool to screen plants for their stress tolerance. The study is important for stress physiology and may be useful for assessment of stress tolerant plants.