OJIP curves throughout chlorophyll a fluorescence normally is very sensitive to heat and cold stress.

Bernardo,

You can estimate heat tolerance by seed (https://journals.uair.arizona.edu/index.php/jrm/article/viewFile/4560/4171) and pollen germination reaction at high temperatures. Pollination by heat-treated pollen is a good method for selection of superior plant genotypes (http://www.liliumbreeding.nl/HS-CHI.pdf).

Yield

There are different options: you either can test what happens when you expose the plants to really extreme temperatures or you can cultivate them at temperatures that are a bit above the plant's optimum. In the first case you address the question until what temperature do the plants survive and which genotypes survive higher temperatures than others. In the second case you check out which genotypes are sutiable for cultivation in regions with higher average temperatures than those where the plants normally grow. You then analyse which genotypes still grow well although temperatures are above optimum. As traits you should select various, that cover different aspects of the plant's life, e.g. germination, growth, photosynthesis, biomass, yield; also with regard to your institute's technical equipment: if you detect an interesting hint you might look for collaboration with technical specialists having dedicated equipment for certain questions.

When you heat the environment you will also be chaning the vapour pressure deficit unless you aree adding water to the air to compensate. If you are not compensating then you will be changing atmospheric waer stress so you will be causing water stress at different levels, whether it be by reducing stomatal aperature directly or by reducing soil moisture and causing feed back effects. In this case you should be aslo be measuring plant water relations traits.