**4.1 Screening criteria**

Breeding for high temperature tolerance requires an essential knowledge on plant adaptation response to heat shocks. In general, the genotypes exhibiting less detrimental effect on photosynthesis and reproductive development tend to survive well under heat prone areas [27]. Involvement of these two components in selection criteria would be beneficial in evolving thermo tolerant cultivars. Tolerant genotypes evolve several morphological, physiological and biochemical alterations in response to heat shocks. Knowledge on sensitivity of several phenological stages to high temperature will pave way for trait specific improvement. High temperature is often correlated with other environmental factors which poses a major limitation for selection under field conditions. At present, varied selection criteria has been developed by scientists, which favors delineation of superior variety at prevailing environment [28]. Heat tolerant index has been evolved for sorghum which depicts the proportion of growth recovery after exposure to high temperature stress. It is the ratio of increase in coleoptile growth in a heat stress environment [50°C] to the enhancement in coleoptile length under normal environment (non-stress) [29]. It proves cost effective and rapid method to screen a large population size within shorter period. A proper validation of such technique would facilitate the development of tolerant lines in other crop species. Pollen viability and fruit set was considered as major selection criteria to predict yield under high temperature stress in tomato [30]. Physiological based trait selection such as harvest index, photosynthetic efficiency, respiration rate, delayed senescence and canopy architecture will also contribute towards increased tolerance to heat stress [31, 32].
