Effect of heat stress on growth and plant characteristics of three commercial sugar beet (Beta vulgaris L.) cultivars

To evaluate the effect of heat stress on three commercial sugar beet cultivars (Aria, Rosaflor and Paya), two experiments were conducted in laboratory (20-44°C; at germination stage) and greenhouse (30°C; at seedling establishment stage) in factorial layout, based on completely randomized design in Sugar Beet Seed Institute, Karaj, Iran, in 2015 and 2016. Severe loss of germination of sugar beet cultivars was obtained at 32°C, and at 38°C, no any normal germination observed. The maximum germination percentage of Rosaflor and Aria cultivars obtained at 20°C, while that of Paya obtained at 26°C. Paya and Aria cultivars had the lowest (41%) and the highest (65%) decrease in germination at 32°C, respectively, in comparison to their maximum germinations. In greenhouse experiment, principal component analysis showed that Rosaflor performed better than other cultivars regarding to; photochemical efficiency of PSII (Fv/Fm), total dry weight, SPAD value and seedling emergence, while Paya cultivar was superior regarding to; leaf area, total fresh weight, stomatal conductance, leaf area ratio (LAR), specific leaf area (SLA) and leaf temperature depression. No significant difference was observed among sugar beet cultivars in leaf area and photochemical efficiency of PSII (Fv/Fm) traits. Paya cultivar had higher fresh weight, leaf area and stomatal conductance which led to more decline in leaf temperature through higher transpiration rate. The highest dry weight belonged to Rosaflor cultivar. According to results of laboratory and greenhouse experiments, it seems that Rosaflor and Paya cultivars were more tolerant to heat stress at early growth stage compared to Aria cultivar.