Chlorophyll Fluorescence as an Index for Freezing Injury Evaluation in Sugar beet (Beta vulgaris)

IntroductionA sustainable cropping system can be designed by choosing crops that can use the solar radiation and water more efficiently. Sugar beet (Beta vulgaris L.) is a highly productive crop which is widely under cultivation in different regions in Iran. Sugar beet production in Iran, scarcely limited by the solar radiation, but heavily relies on limited available water in such arid and semi-arid regions which makes the spring cultivation of it very expensive. Autumn cultivation of sugar beet is considered as an alternative to avoid water stress in Khorasan province, which is the main producing land of this crop in Iran. Variation of chlorophyll florescence parameter is an important criterion as well as suitable and nondestructive method which has been employed to determine the differences between plant species on resistance to environmental stresses such as freezing stress, and has been widely used as an important index to quantify the response of cold resistant varieties of corn and rice and heat resistance of sunflower cultivars.
Materials and MethodsIn order to study the possibility of using the chlorophyll fluorescence parameters for evaluation freezing tolerance of sugar beet varieties, an experiment was performed by using a factorial based on randomized complete block design with three replications at agricultural faculty of Ferdowsi University of Mashhad. Seven sugar beet varieties (Jolge, Giada,Monatunno,SBSI1, Palma, Suprema,andPP8), exposed to ten freezing temperature levels (Zero, -2, -4, -6, -8, -10, -12, -14, -16 and -18), and their yield of quantum efficiency (ΔF/Fms) were measured in four levels of recovery periods (2, 12, 24 and 72 hours). Chlorophyll fluorescence was measured on control plants along with plants which were under 2, 12, 24, 72 hr. recovery after freezing. Measurements were carried out by using a portable fluorometer in the marginal region of the youngest fully expanded leaves.
Results and DiscussionPhotochemical efficiency of photosystem II had strong regression with plant survival percentage (R2=0.90**). Monatunno variety had the highest and SBSI1 had the lowest photochemical efficiency of photosystem II. There were no differences between sugar beet varieties on photochemical efficiency of photosystem II until -14oC, but lower temperatures severely declined that parameter. There was a reduction on efficiency of photosystem II in the first 24 hr. duration of recovery, but after 72 hr. increased up to the before freezing conditions. Monatunno cultivar had the suitable recovery where exposed to the -16oC, but the efficiency of photosystem II on SBSI1 cultivar was decreased dramatically at the same temperature. Fluctuation trends of ΔF/Fmsbetween freezing treatments and recovery periods showed sensible decrease trend until -14oC, but negative trend started at -16oC and reached to lowest values at -18oC in recovery period. Hasselt (1996) showed significant decrease in (ΔF/Fms) between wheat varieties started at temperatures lower than -8oC. Fluctuation trend of ΔF/Fms parameter of Monatunna and Sbsi1 varieties as resistant and sensitive varieties with respectively highest and lowest survival percentage under severe freezing treatments (-14, -16 and -18oC) indicated clear difference between Monatunna and Sbsi1 at -16oC.
ConclusionsYield of quantum efficiency of sugar beet varieties showed highest differences in recovery period, this parameter quantity in Monatunna as a resistant genotype recovered near to before stress conditions during recovery period (except -18oC freezing treatment) but Sbsi1 as a sensitive variety did not show recovery especially in severe freezing levels. Based on this study, we can determine Monatunna variety as high resistant to freezing stress with quick recovery period compared to other study varieties in early growth period which is almost the most important part of growth cycle of sugar beet varieties.