Application of Thermal-time Concept to Modeling Oilseed Rape (Brassica napus L.) Seed Germination Response to Temperature

IntroductionIn seed plants, seed germination is one of the important life history events, because it determines the time when a new life cycle is initiated. Temperature (T) is one of the most important environmental determinants of capacity and rate of germination. Base, optimum and ceiling T (cardinal temperatures) characterize the limit of this environmental factor over which the germination of a particular species can occur. The thermal-time approach has been successful in describing germination time courses in response to T, and most models predicting crop phenological development use a thermal-time scale to normalize for T variation over time. A clear understanding of the seed germination patterns is helpful in screening for tolerance of crops and cultivars to either low or high temperatures and in identifying geographical areas where a species or genotype can germinate and establish successfully by using the critical lower and upper temperatures for germination. Information on cardinal temperatures is lacking for germination of spring oilseed rape (Brassica napus L.), as one of the world’s major oilseed crops. The aim of the present work was to evaluate the relative accuracy of different thermal-time approaches for the description of germination in three cultivars of spring oilseed rape.
Materials and MethodsGermination responses of three spring oilseed rape cultivars were investigated at different constant temperatures. The seeds were incubated in the dark using germinators with controlled environments at eleven constant T regimes of 8, 12, 16, 20, 24, 28, 32, 33, 34, 35 and 36 ºC with a range of ±0.2 ºC over a 21-day period. These T regimes cover both the sub- and supra-optimal T ranges. The trial was replicated three times with 4 Petri-dishes in each replication, for a total of 12 Petri-dishes for each cultivar at each T regime. The germinated seeds (criterion, radicle protrusion of > 2 mm) were counted and removed at frequent time intervals (every 4-8 h). Germination counts at each replicate of each T regime were pooled by cultivar across trials for data analysis. Cumulative germination percentage was calculated for every cultivar and T regime for every count-hour. The time taken for cumulative germination to reach subpopulation percentiles of 10, 50 and 90% of maximum in each T regime were calculated by interpolation from the progress of germination (%) versus time curve. Experimentally obtained cumulative-germination curves were used to perform a non-linear regression procedure to assess the relative accuracy of different thermal-germination models in predicting germination response under constant incubation temperatures. Assessment of goodness-of-fit was performed by the Akaike information criterion (AIC).
Results and DiscussionThe most accurate approach for simulating the thermal-germination response of all three spring oilseed rape cultivars achieved by assuming a normal distribution of both thermal-time required to complete the germination of each given seed fraction in sub-optimal T range (θT(g)) and maximum germination temperatures (Tm(g)), while base T (Tb) or supra-optimal thermal-time (θTm) were considered constant for the entire population. According to this model, the base T for different cultivars ranged from 5.66 (cv. Sarigol) to 7.13 ºC (cv. Dalgan). Estimated θTm varied between 31.62 to 34.55 ºC h for different spring oilseed rape cultivars. A θT(50) of 369.27 ºC h and a Tm(50) of 34.32 ºC were identified for seed population of cv. Sarigol. The θT(50) was estimated to be 378.76 ºC h for cv. Dalgan and 357.89 ºC h for cv. RGS003. The Tm(50) for germination of cv. Dalgan and cv. RGS003 was estimated to be 33.98 and 34.42 ºC, respectively. In all three cultivars, calculated values for optimum T (To) were not constant across subpopulations. The To(50) was estimated to be 31.85 ºC for cv. Sarigol, 31.78 ºC for cv. Dalgan and 32.06 ºC for cv. RGS003. Thermal-time analysis, although an empirical method, is considered by many researchers to have physiologically and ecologically relevant parameters and, in its standard form, provides several useful indices of seed germination behavior in response to T. Despite its popularity, the generality of its assumptions has not been examined systematically. If these assumptions do not hold, at least approximately, in a particular situation, misleading interpretations can easily arise.
ConclusionsThe thermal thresholds for seed germination identified in this study explain the differences in seed germination detected among populations of different spring oilseed rape cultivars. The thermal-time model described here gave an acceptable explanation of the observed seed germination patterns.