The effect of heat stress on photosystem II (PS II) efficiency and post-stress recovery was studied in four
wheat cultivars using chlorophyll fluorescence. The main aim was to examine the cultivar differences in
relation to inhibition and recovery of PSII functionality after heat stress at different growth stages. The
secondary aim was to investigate whether a pre-acclimation of plants to elevated temperature during the
growth period induces a better tolerance to heat stress than for plants grown in ambient temperature or
not. The plants were grown in two growth temperature conditions (15 ◦C and 25 ◦C) and subjected to heat
stress (40 ◦C) for two days at early tillering and three days at anthesis and early grain development stages.
The plants were returned to their original growth conditions after heat stress and recovery was observed
for three days.