Blueberry is one of the most important fruit trees all over the world and also the major economic tree species cultivated in China.Blueberry was suggested to be planted only in Northern China, because the warmer climate in Southern China could not meet its chilling duration.Several blueberry cultivars have recently been introduced to subtropical China and these cultivars can blossom and bear fruits.However, the temperature in subtropical China often approach 40 ℃ or even higher in summer, and thus high temperature has become the most significant abiotic stress limiting the growth and production of blueberry in this area.Meanwhile, previous studies also reported that the heat endurance ability varied with different blueberry cultivars.In this study, we examined the effects of high temperatures on the leaf stomatal traits and gas exchange of blueberry cultivars with four growth chambers controlling different high temperatures including control (25 ℃), mild high temperature(30 ℃), moderate high temperature(35 ℃), and severe high temperature (40℃).Two-year-old seedlings of three highbush blueberry cultivars including ‘O’Neal’, ‘Gulfcoast’, and ‘Blue Ridge’ were selected from field plots and transplanted into pots (10 cm diameter × 25 cm long) filled with fritted clay (one plant per pot) and grown in a greenhouse with an average temperature of 25/20 ℃(day/night) and about 1 000 μmol m-2·s-1 photosynthetic active radiation (PAR) in natural sun light, and 60%~75% relative humidity for 30 d(March-April 2014) to establish canopy.During the establishment period, plants were irrigated to water-holding capacity daily and fertilized once per week with half-strength Hoagland's solution.We selected 20 healthy and uniform growth plants for each cultivar and then randomly planted the plants into each of four walk-in growth chambers (5 plants for each cultivar), where the temperature was determined as 25, 30, 35, or 40 ℃, respectively.Other environmental factors maintained throughout all the four chambers include humidity (60%~75%), light intensity (1 000 μmol m-2·s-1 PAR), photoperiod (light on at 8 am, and off at 8 pm), soil type (fritted clay, same brand and package for all), water amount (200 ml, watered once per week), and nutrition type (plain tap water).Plants were fertilized once weekly with half-strength Hoagland's solution throughout the growth period.In order to minimize confounding effects of environmental variation between different chambers, we randomly changed the temperature of each growth chamber every week, and then we relocated the high temperature treated plants to the growth chambers with corresponding temperature.The large volume of the pot with frequent watering and fertilization ensured enough space for root growth and ample nutrient supply to avoid “bonsai effect”.Our results showed that comparing with the control, high temperatures increased the stomatal density of Gulfcoast and Blue Ridge, but had little effect on the stomatal density of O′Neal(P>0.05).Moderate high temperature increased the stomatal aperture width, stomatal aperture length, and stomatal area of O′Neal and Blue Ridge, whereas the stomatal aperture length of Gulfcoast was significantly reduced by 23.5% under moderate high temperature(P<0.05) compared with the mild high temperature(30 ℃).High temperatures resulted in a more regular stomatal distribution pattern on the leaves of O′Neal, while had little effect on the stomatal distribution pattern of Gulfcoast and Blue Ridge.Mild and moderate high temperatures significantly increased the net photosynthetic rates (Pn), stomatal conductance (Gs) and transpiration rates(Tr) of the three blueberry cultivars, whereas the severe high temperature caused them sharply decrease, although their maximum values varied across the three cultivars.In Conclusion, high temperatures increased the efficiency of leaf gas exchange by adjusting the structural characteristics and optimizing the spatial distribution pattern of stomata on the leaves of the south highbush blueberry.However, the ability of optimization for stomatal structure and function in the leaves of the south highbush blueberry was cultivar dependent, and thus resulted in the different responses to high temperatures in leaf gas exchange across cultivars, especially for heat stress resistance under extremely high temperature.Our results may not only be helpful for further understanding the potential mechanisms of high temperatures on leaf gas exchange of south highbush blueberry from the changes in leaf stomatal traits, but also provide theory for the selection and introduction of heat tolerance cultivars.