The chlorophyll fluorescence parameter F v/F m reflects the maximum quantum efficiency of photosystem II (PSII) photochemistry and has been widely used for early stress detection in plants. Previously, we have used a three-tiered approach of phenotyping by F v/F m to identify naturally existing genetic variation for tolerance to severe heat stress (3 days at 40°C in controlled conditions) in wheat ( Triticum aestivum L.). Here we investigated the performance of the previously selected cultivars (high and low group based on F v/F m value) in terms of growth and photosynthetic traits under moderate heat stress (1 week at 36/30°C day/night temperature in greenhouse) closer to natural heat waves in North-Western Europe. Dry matter accumulation after 7 days of heat stress was positively correlated to F v/F m. The high F v/F m group maintained significantly higher total chlorophyll and net photosynthetic rate (P N) than the low group, accompanied by higher stomatal conductance (g s), transpiration rate (E) and evaporative cooling of the leaf (DeltaT). The difference in P N between the groups was not caused by differences in PSII capacity or g s as the variation in F v/F m and intracellular CO 2 (C i) was non-significant under the given heat stress. This study validated that our three-tiered approach of phenotyping by F v/F m performed under increasing severity of heat was successful in identifying wheat cultivars differing in photosynthesis under moderate and agronomically more relevant heat stress. The identified cultivars may serve as a valuable resource for further studies to understand the physiological mechanisms underlying the genetic variability in heat sensitivity of photosynthesis.
