The chlorophyll fluorescence parameter Fv/Fm 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 Fv/Fm 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 Fv/Fm value) in
terms of growth and photosynthetic traits undermoderate 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 stresswas
positively correlated to Fv/Fm. The high Fv/Fm group maintained significantly
higher total chlorophyll and net photosynthetic rate (PN) than the low group,
accompanied by higher stomatal conductance (gs), transpiration rate (E) and
evaporative cooling of the leaf (ΔT). The difference in PN between the groups
was not caused by differences in PSII capacity or gs as the variation in Fv/Fm
and intracellular CO2 (Ci) was non-significant under the given heat stress. This
study validated that our three-tiered approach of phenotyping by Fv/Fm 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.