<p>The photosynthesis and transpiration rates in 4-, 7-, and 11-day-old Hordeum vulgare L. seedlings exposed to high temperature were studied using IR spectroscopy. The state of the stomata and the real temperature of the primary leaf of these seedlings were assessed to analyze the mechanisms underlying their regulation. It was found that heat treatment (40°C, 3 h, illumination 120 μmol quanta·m^–2·s^–1) did not affect net photosynthesis and stomatal conductivity in rapidly growing 4-day-old H. vulgare seedlings with effective cooling of the leaf due to activation of transpiration. Heat treatment did not change the stomatal conductivity or the transpiration rate in the primary leaf fully formed by the age of 7 days. As a result, the leaf cooled less than in 4-day-old seedlings and net photosynthesis was suppressed. The stomatal conductivity for both water vapor and CO₂ decreased in the aging 11-day-old leaf under heat stress, which caused the suppression of transpiration and photosynthetic activity and significant heating of the leaf. We concluded that the different responses of the stomata and transpiration to heat in H. vulgare seedlings of different ages were aimed at maintaining the most effective balance between cooling the leaf and maintaining the water potential of the plants.</p>