Effects of high temperatures on leaf stomatal traits and gas exchanges of highbush blueberries

Abstract: 【Objective】Blueberry(Vaccinum corymbosum L.) is an important fruit worldwide, and the growth and distribution of blueberry cultivars are usually limited by temperature, especially in China.North highbush and south highbush blueberry cultivars are the two major highbush blueberry groups,which are widely planted in northeastern China. Highbush blueberries have recently been introduced into southeastern China, a typical subtropical area with high temperatures in summer. Understanding of the thermal tolerance in different genotypes of highbush blueberry is important for selecting appropriate cultivars, because blueberry cultivars may differ in tolerance to heat stress. In order to compare the tolerance to heat stress between the south highbush and north-highbush blueberry cultivars, we examined the effects of high temperatures on stomatal traits and gas exchange of six highbush blueberry cultivars(South highbush cultivars included‘O' Neal',‘Gulfcoast'and‘Blue Ridge'; and north highbush cultivars were‘Duke',‘Brigitta'and‘Bluecrop').【Methods】Two-year-old seedlings of six highbush blueberry cultivars including‘Bluecrop',‘Duke',‘Brigitta',‘Gulfcoast',‘O'neal'and‘Blue Ridge'were selected from field plots in the research farm at Dalian University in northeast China. The collected plants were 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-1photosynthetic active radiation(PAR) in natural sun light, and 65% relative humidity for 30 d to establish canopy. During the establishment period, plants were irrigated to water-holding capacity daily and fertilized twice per week with half-strength Hoagland's solution. Four growth chambers were set at different temperatures including control(25 ℃), mild heat stress(30 ℃), moderate heat stress(35 ℃), and severe heat stress(40 ℃). The stomatal traits were observed and analyzed with imprinting samples, light microscopy,scanning electron microscopy, and spatial analyzing techniques. Meanwhile, gas exchange parameters were determined with a Li-6400 portable photosynthesis system(LI-COR Inc. Lincoln, Nebraska, USA).【Results】Our results showed that high temperatures barely affected the stomatal density in the six blueberry cultivars, and no difference was found between the south highbush and north highbush groups. The temperature of 35 ℃ increased the stomatal density of‘Golfcoast'and‘Brigitta'by 30% and 70% compared with 25 ℃, respectively, while the stomatal density in‘Golfcoast'and‘Brigitta'grown at 40 ℃was about 40% and 30% lower than in the corresponding cultivars grown at 35 ℃. We also found no differences in response to high temperatures of stomatal structure and spatial distribution pattern between the south highbush blueberries and the north highbush blueberries. However, the response of stomatal traits to high temperature was cultivar dependent. We found that high temperatures changed stomatal traits(including length, width, area, perimeter, and shape index) in‘Gulfcoast',‘Blue Ridge',‘Brigitta', and‘Bluecrop', but had little effect on stomatal traits in‘O' Neal'and‘Duke'. The spatial distribution pattern of stomata was scale dependent, being regular at small scales and random at larger scales. High temperatures also resulted in a more regulated spatial distribution pattern of stomata in‘O' Neal'and‘Brigitta'with smaller L(t) values than that in the two cultivars grown under 25 ℃, with smaller L(t) values(Ripley's K-function, L(t) is an expectation of zero for any value of t). In addition to stomatal traits, parameters of gas exchange in the six highbush blueberries also showed different responses to high temperatures.The net photosynthetic rates(Pn) of the six blueberry cultivars followed a bell shape curve with temperature increase, but the maximum value differed in different cultivars. For example, the maximum Pnin‘Duke'occurred at 25 ℃, while that in the other five cultivars occurred at 35 ℃, indicating that the gas exchange in‘Duke'may be more sensitive to high temperatures than in the other cultivars. However, our results also showed that the maximum values in Pnof the south highbush cultivars were lower than those of the other two north high-bush cultivars(‘Brigitta'and‘Bluecrop'), suggesting north highbush cultivars may have stronger photosynthetic ability than south highbush cultivars under the same temperatures.Moreover, stomatal conductance(Gs) and transpiration rates(Tr) of the highbush blueberry cultivars were substantially increased as temperature increased from 25 ℃ to 35 ℃, whereas 40 ℃ caused sharp decreas-es, although their maximum values varied across the six cultivars. Response of Gsto high temperatures in the six cultivars was similar, with the maximum values of Gsall occurred at 30 ℃, but the maximum Pnof most cultivars took place at 35 ℃. These results suggested that the CO2 conductance may be more sensitive to high temperature than carbon assimilation. By comparing the values of Trand Pn, we found they had very similar trends in response to high temperatures. The maximum values of Trand Pnwere found at35 ℃, indicating that the highest Pnof blueberry cultivars at 35 ℃ may be attributed to the highest Trunder high temperatures, as higher transpiration helps to cool the leaves and thus protects the photosynthetic devices and enzymes【.Conclusion】These results suggested that high temperatures increased the efficiency of leaf gas exchange by adjusting the stomatal morphological characteristics and optimizing the spatial distribution pattern of stomata on the leaves of the highbush blueberries. However, the ability to optimize stomatal structure and function in highbush blueberries was cultivar dependant, which resulted in different responses to high temperatures of leaf gas exchange across cultivars and thus heat stress resistance to extremely high temperature. Our results may not only be helpful for further understanding the potential mechanisms of warming effects on leaf gas exchange in highbush blueberries, but also provide data for selection of heat tolerant cultivars.