Effect of freezing damage at seedling stage in different sugar beet cultivars

This study was conducted to determine freezing damages on sugar beet seedling (at cotyledon, two and four leaves stages) in seven sugar beet cultivars (Shirin, Rasol, Gadook, 276, 428, 7233 and BR1) in Karaj in 2002 cropping season In first experiment, sugar beet seedlings at the four mentioned stages were transferred to the growth chamber and exposed to zero, -2 and –4°C for two periods of 4 and 8 hours. Second experiment, was carried out in the field to compare the results of greenhouse and field. Leaf samples were taken at three growth stages of sugar beet seedlings, transferred to growth chambers, and exposed to freezing temperatures. Assessment of freezing damages was performed using visual (survived or dead) and leaf chlorophyll florescence measurements. The results showed that freezing damage was 0.5, 8 and 85%, for low temperatures of zero, -2, and -4°C, respectively. Freezing damage on the detached leaves from field and the seedlings grown in growth chamber showed similar results. Chlorophyll fluorescence measurements as index for freezing damage showed that an increase of freezing stress caused a decrease in the amount of Fm (maximum chlorophyll fluorescence) and Fv/Fm (photochemical efficiency of photosystem II). Sugar beet cultivars were more susceptible at two-leaves stage as compared to four- leaves stage. The threshold of freezing damage had no significant differences among sugar beet cultivars, however, BR1 and Gadook cultivars showed the lowest and highest susceptibility to freezing, respectively. Results also showed that threshold of freezing damage on sugar beet seedling in concerned cultivars was -2°C and the lower the temperature the more damages occur in the field