The protocol is based on Krause (2010) with some modifications proposed by the Feeley Lab in 2016. This protocol is for finding heat tolerance or the critical thermal maximum temperature of plant leaves. Plants can undergo heat damage that can be repaired as long as the critical value is not exceed. This value is called critical thermal maximum (CTmax) or heat tolerance (T50) and is the highest temperature a leaf can take before and irreversible damage occurs, and when exceeded leads to necrosis. The way we measure this is using chlorophyll fluorescence, more specifically, the maximum efficiency of PSII value or Fv\/Fm, which optimal values are around 0.83 in non stressed leaves. Fv\/Fm can be obtained with a fluorometer or with an Irga, in both cases using dark adaptation clips. Plant collection is recommended to be done on the day of the procedure or, as long as leaves are kept well hydrated, the material can be even measured a day after the collection. The total time required for the measurement to be compleated is 2 days (including a 24h wait). Apart from the fluorometer, the equipment needed includes a water heating device (stable water temperature is imperative), such as a water bath or if possible a Sous Vide (recommended to us by Ken Feeley; works great!).