Maximum Efficiency of Photosystem II as a Freezing Stress Index in Perennial Ecotypes of Rye (Secale Montanum)

IntroductionChlorophyll fluorescence measuring is a quick and undestructive method, which is used as an important index to identify stress tolerant varieties for environmental stresses such as freezing. Rye planting is less prevalent comparing to other cool season cereals, but more investigations are needed because of suitable potentials in this crop for growing in cold area of Iran. In addition, low temperatures decrease physiological functions of plants and results in irreversible damages and disorders in physiological process of plants.
Material and MethodsIn order to study the possibility of using the chlorophyll fluorescence parameters for evaluation of freezing tolerance in perennial rye ecotypes, an experiment was performed using a factorial experiment based on completely randomized design with three replications at Agricultural Faculty of Ferdowsi University of Mashhad. Ten rye ecotypes (264, 941, 8425, 15771, 1587, 14947, 591, 1275, 3857 and 12460) were exposed to nine freezing temperatures (0 (control), -3, -6, -9, -12, -15, -18, -21 and -24◦C) and maximum efficiency of photosystem II (ME of PII) were measured four times (12, 24, 48 and 96 hours) after freezing. Correlation between ME of PII with Electrolyte Leakage percentage (EL %) and survival percentage (SU %) were tested.
Results and DiscussionThe results indicated that there was a significant difference among rye ecotypes for ME of PII, while ecotype 12640 had the highest ME of PII and the lowest efficiency was observed in ecotype 264. There was no difference in ME of PII among rye ecotypes until to -18 oC, but ME of PII decreased in -21oC and -24oC after 12 to 24 hours recovery period. This efficiency was zero in -24°C during 48 and 96 hours after recovery, while ME of PII did not get to zero in this temperature during 12 and 24 hours after stress. ME of PII impairment by freezing temperatures was similar in 48 and 96 hours and it seems that no changes happened in the efficiency after 48 hours. There was a rapid reduction in slope of efficiency from -15oC to -24oC in 264 and 941 ecotypes than the other ecotypes, while ecotype 12640 had the highest ME of PII than the other ecotypes in mentioned temperature range. In the four measuring times, ME of PII was not reduced until -18°C, but it was severely decreased by temperature reductions to -21oC and -24°C, as ME of PII decreased to the lowest value after 48 hours. Decreasing the ME of PII in rye ecotypes was different due to the ectypes in the times after freezing stress, the most reduction was observed in 264 and 941ecotypes and ecotype 12640 had less decrease in the slope of ME of PII. There were differences among rye ecotypes in reduction temperature for 50% of ME of PII; while ecotype 12640 get reduction temperature for 50% of ME of PSII in -24.8oC 12 hours after freezing stress; and ecotypes 264 and 941 had the highest reduction temperature of 50% ME of PII in -20.2 oC and -20 oC, respectively. Reduction temperature for 50% of ME of PII decreases in 24 hours after freezing stress; at this time, 12640 and 3857 ecotypes showed the lowest reduction temperature for 50% of ME of PII by -22.6°C and -22.2°C, respectively, and 264 ecotype had the highest reduction temperature for 50% of ME of PII by -19.2°C. There were significant correlations between ME of PII, EL% and SU%. Since EL test was conducted 24 hours after freezing stress, it seems that measuring ME of PII in 12 hours after freezing stress increases quickness in test and determining the stress effects rapidly. Higher correlations between plants survival percentage with ME of PII 12 hours after freezing stress, indicate that ME of PII is a non-destructive factor for estimating long term effects of freezing stress on rye plants. In conclusion, the mentioned factors can be used as a quick procedure to identify cold tolerant plants.