There are some traits that can be used in relative to yield of that crop. According to some studies, some secondary traits can be used in relative to primary trait to select. if you are conversant with path coefficient analysis, you can see that there are some traits that contribute to yield, such traits can be used since heat exposure to plant will have effect on the yield,its advisable to use these secondary traits in conjunction with yield.

Grain yield and harvest index can be measured. 

Hi

You can check out Membrane stability Index/Lipid peroxidation test, Relative water content and to a lesser extent  Proline and chlorophyll contents for determining  heat tolerance in different crop genotypes though large scale field level estimation can be quite tedious . Check this site. www.plantstress.com for more information.

Cheers

Characterize first what is heat ? Temp, humidity i.e environmental factors. Conduct experiment under controlled and heat environments. Calculate % reduction in trait which will reflect heat effect. Minimum reduction would indicate heat tolerance. Grain weight per plant per spike, grain number, grain fill duration are appropriate phenotypic parameters besides cell membrane stability, photosynthesis rate etc the physiological. Can see reference available in plenty.

Important trait is that check reduction in yield due to heat exposure and compare to normal conditions, we can also check pollen fertility traits under high temperature conditions,  Membrane stability is also a good measure to be related with heat tolerance. Early maturity can also be used but that will be escape mechanism. Mainly it will be grain yield produced under adverse conditions which may be due to many yield contributing trais   

Hi,

although it depend on the environment considered, a rapid and efficient way to screen for heat tolerance a large number of plant is to score the canopy temperature. In wheat, several papers found really high negative correlation between yield under heat stress and canopy temperature (a cooler canopy, i.e., low canopy temperature, is related to genotype with higher yield). Indeed, these plants are able to cool their canopy by an increase evapotranspiration leading to an increased protection.

See paper of Pinto et al (2010). Theor. Appl. Genet.

Cheers

Relative leaf water content and Membrane permeability index are important physiological traits, while proline and glycine are important biochemical traits for evaluating genotypes for heat tolerance. Harvest index and yield per se can also be recorded under heat stress to identify heat tolerant genotypes.

Hello,

i want to add some other plant traits in wheat for heat tolerance including days to heading and days to maturity under normal and heat stress conditions along with other plant traits, if temp. is raised then plant matures earlier and we can screen out heat tolerant genotypes

You can go for membrane thermostability study. Some secondary traits can be used. You can use some traits like pollen viability and stigma receptivity to ensure more seed set under heat stress. 

By the way, which crop you are working on. Secondary traits may differ crop to crop. I am working on Maize (heat stress) and secondary traits are different to some others.

And obviously yield is the ultimate responsive traits for any screening experiment.  

All biometric traits that you can measure for plants sampled before the harvest (useful for evaluation of growth dinamics),  total yield and plant weight at harvest time.