Evaluation of high and low temperature stresses tolerance and secondary dormancy as affected by drought stress in canola cultivars

Introduction oilseed rape has a wide range of adaptation and grows well in many regions under variable temperatures. However, the ability of the varieties varies in response to the favorable and unfavorable conditions. Temperature stresses (high or low temperatures) have harmful effects on crops. Generally, temperatures below 10 ° C can damage germination and emergence. Low temperatures can also have a negative effect on the post-emergence stages of crops. The high temperatures can also reduce the total dry matter produced, the number of pods under development, the number of seeds per pod, the weight of seeds, and, finally, the yield of the plant. Also, high temperatures affect the development and maturity of the seeds, resulting in a higher level of yield. Identifying high or low tolerant cultivars can help researchers improve the new cultivars and increase the flexibility of selecting the right cultivar for farmers. There are several methods for evaluating cultivars at low and high temperatures based on farm or laboratory surveys. Field surveys are difficult, unsustainable and seasonal. Because in field conditions, temperatures and humidity fluctuate a lot. Various studies have been done to evaluate the variation of germination in heat or cold stress in different plants such as lettuce, alfalfa, sorghum, cotton, sesame, etc. However, there is not much information on canola cultivars in Iran. The aims of this study was to determine the cardinal temperatures (2) evaluate the tolerance of different genotypes at high or low temperatures and (3) and to study the secondary dormancy potential in Iranian genotypes. Materials and methods In order to, germination tests were conducted in fixed temperatures incubators with 5, 10, 15, 20, 25 and 30 oC on 10 oilseed rape genotypes, in 4 replications with 50 seeds. Finally, cardinal temperatures were estimated using segmented (for germination rate) and beta (for germination percentage) functions. The tolerance of genotypes to high or low temperatures were determined using estimated cardinal temperatures. Also, an experiment was conducted to investigate the potential of different cultivars for induction of dormancy, in which seeds of 10 oilseed rape genotypes were subjected to drought stress conditions to determine the percentage of total induction due to drought for each genotype. The percentage of germinated seeds, the percentage of seeds with dormancy and dead seeds can be determined. Finally, the percentage of induction of the secondary dormancy was calculated for each genotype. Statistical analysis was performed using SAS software. Results and discussion Marshall and Squire (1996) showed that the minimum temperature (Tb) for oilseed rape germination was about 3 °C. Soltani et al. (2013) showed that the minimum temperature for oilseed rape germination was 2.7 °C and under dry conditions up to 6.7 °C at -0.8 MPa increased. Also, they showed that the average thermal time for germination (TT50) increased from 29.5 °C/day at 0 MPa to 57.9 °C/day at -0.8 MPa. Results indicated that mean of base (Tb), optimum (To) and ceiling (Tc) temperatures for germination percentage were 4.99, 18.23 and 34,20 oC and also mean of base (Tb), optimum (To) and ceiling (Tc) temperatures for germination rate were 6.18, 24.76 and 39.40 oC. The Heat and Cold tolerance indexes were also different in different cultivars. Okapi cultivar was the most tolerant in heat tolerance index (5.95) and in cold tolerance index (8.48). X-Power had the lowest tolerance to heat (4.78) and cold (6.50). The potential of secondary dormancy induction also was at the highest level in Okapi (about 40 %). It seems that there was connection between secondary dormancy induction and tolerance to temperature stresses which needs more studies.