*2.8. Heat Tolerance Index*

Heat tolerance index (HTI), a concept that indicates the extent of yield reduction due to heat stress compared to the potential yield under control condition, was determined according to Fernandez [35] and used to separate cultivars yield response into heat sensitive and heat tolerant. The seed yield was obtained by small plot combine harvest of individual plots at maturity.

$$\text{HTI} = \frac{\left(\text{Yield}\_{\text{c}}\right)\left(\text{Yield}\_{\text{h}}\right)}{\left(\text{Yield}\_{\text{c.ave}}\right)^{2}}\tag{11}$$

where Yieldc is seed yield for each cultivar in a replication under non-heat stress (control conditions), Yieldh is seed yield for each cultivar in a replication under heat stress, Yieldc.ave is the grand mean seed yield from all control plots of all replications per environment under non-heat stress conditions. When HTI is close to zero or zero, crops do not yield under heat and are heat sensitive. When HTI is high (>1), then the cultivar would be deemed heat tolerant for yield compared to the grand mean yield under the control conditions.
