Quantification of thermos-inhibition response of seed germination in different oilseed rape cultivars

Introduction Some seeds will enter thermoinhibition when imbibed at warm temperature regimes. Seed thermoinhibition is the situation when germination of seeds that have been imbibed at warm temperature is prevented, but the seeds will subsequently germinate rapidly when the temperature is reduced. It is therefore a temporary inhibition of germination that can be alleviated simply by lowering the temperature. This process generally occurs at temperatures considerably lower than ceiling temperature. It may also occur over a small range of temperatures, such that germination can go from 100 to 0 % when the upper temperature limit is exceeded by only a few degrees. This germination behavior is often associated with a winter annual life cycle common in Mediterranean-type climates with wet winters and dry summers, as seeds that are shed in early summer and have subsequently after-ripened nonetheless may not germinate when hydrated at warm temperatures, waiting instead until the fall for cooler temperatures and more certain rainfall before germinating. Patterns of seed germination behavior of different populations in response to warm temperatures can be described using the models based on thermal-time units. In this study, thermal-germination model was developed based on four different probability distribution functions of Normal, Lognormal, Gumbel and Weibull. Seed thermoinhibition in six cultivars of spring oilseed rape (Brassica napus L.) were then quantified using these models. Materials and methods Experiment was conducted at the Seed Technology Laboratory, Ramin Agriculture and Natural Resources University, Khuzestan. In this study, germination responses of six spring oilseed rape cultivars (Sarigol, RGS003, Dalgan, Hyola-401, Jerry and Julius) were investigated at constant temperatures. Germination test were conducted in the dark at eleven constant temperature regimes of 8, 12, 16, 20, 24, 28, 32, 33, 34, 35 and 36 ºC with a range of ±0.2 ºC. For each cultivar, germination test was carried out in a completely randomized design with four replications and the entire experiment was repeated three times. The germinated seeds (criterion, radicle protrusion of > 2 mm) were counted and removed at specified intervals. Seeds of all cultivars germinated more rapidly at 32 ºC than at higher or lower temperature regimes. Therefore, the optimum temperature for germination was assumed to be 32 ºC. Above this, oilseed rape seeds showed thermoinhibition of germination. So that, ungerminated seeds of each cultivar at these warm temperature regimes were germinated rapidly at low temperature of 20 ºC. Cumulative-germination curves of oilseed rape cultivars in temperatures beyond 32 ºC were used to perform non-linear regression procedures to assess the relative accuracy of different thermal-germination models in predicting germination response under constant temperatures. The thermal-germination models were fitted to the germination data of each cultivar using the PROC NLMIXED procedure of SAS. Assessment of goodness-of-fit was performed by the Akaike information criterion (AIC). Results and discussion Assess the goodness of fit indicated that precision of the thermal-germination models was different in describing the germination behavior of early- and mid-maturing cultivars of oilseed rape. Thermal-germination model based on Normal distribution provide the best fit to data of germination progress over time for early-maturing cvs. Dalgan (AIC=-237.5) and Hyola-401 (AIC=-168.8). For mid-maturing oilseed rape cultivars (Sarigol, RGS003, Jerry and Julius), the smallest AIC values were obtained for the Gumbel thermal-germination model, while the model based on Normal distribution gave the worst fit to germination data. The germination responses of early- and mid-maturing oilseed rape cultivars to supra-optimal temperature regimes were different. Early-maturing cultivars showed 5, 50 and 95% thermoinhibition of germination at averaged temperatures of 33.52, 33.99 and 34.37 ºC, respectively. While, seed thermoinhibition in mid-maturing cultivars reached to 5, 50 and 95 % of maximum at averaged temperatures of 33.63, 34.34 and 35.59 ºC, respectively. Based on the model prediction, the germination completely inhibited at an average temperature of 34.54 ºC in early-maturing cultivars and an average temperature of 36.64 ºC in mid-maturing cultivars. Abscisic Acid (ABA) has been reported as the major regulator of seed thermoinhibition. Conclusions Early-maturing oilseed rape cultivars have lower thermal-thresholds compared to mid-maturing cultivars. A different thermal-germination model was necessary for predicting the probability of seed germination in response to warm temperature regimes depending on the type of germination behavior of early- and mid-maturing cultivars. Thermal-germination models proposed here well described the observed thermoinhibition response of seed germination in oilseed rape cultivars.