High temperature index—for field evaluation of heat tolerance in wheat varieties

Abstract

High temperature (> 30 °C) at the time of grain filling is one of the major constraints in increasing productivity of wheat in tropical countries like India. Hence, wheat genotypes from national and international sources are regularly evaluated for tolerance to high temperature stress. Generally, genotypes are tested across space and time under field conditions by manipulation of date of sowing or choosing sites, which are featured by high temperature at grain filling (“hot spots”). Under such conditions, magnitude of heat stress determines accuracy of evaluation and selection of genotypes. Mean ambient temperatures during the day, month or crop season are often used to quantify the magnitude of heat stress. These temperature parameters, however, fail to account for changes in diurnal amplitude. As a consequence, actual duration of high temperature stress to which the crop is exposed to remains elusive. An attempt is made here to derive a thermal index to quantify heat stress under field conditions, taking into consideration the diurnal thermal amplitude. A mathematical equation is derived to quantify high temperature stress by integrating both the temperature and the duration. High temperature index (HTI) developed on the basis of this equation is compared with other thermal indices such as heat degree-days and mean temperatures. The index was found to be highly efficient in differentiating two crop environments. Further, HTI was used to quantify the magnitude of heat stress to which 12 advanced wheat ( Triticum aestivum) genotypes were exposed during two crop seasons. This index along with grain yield loss during hot season is used to identify genotypes performing better by tolerating the high temperature or by escaping from it.