Modeling the Germination Timing of Bishop's Weed (Ammi majus L.) in Response to Temperature

After moisture, temperature is the main driving force affecting seed germination. Population-based models are useful tools for describing and predicting germination in relation to time and temperature. These models estimate the thermal thresholds for seed germination, taking into account both the speed and final percentage of germination in different temperature regimes. The population-based approach has been widely used to model the thermal response of various plant processes such as germination and dormancy of seeds as well as seedling emergence in the field. However, the assumptions of this approach on the germination of seeds of some plant species, especially weeds, are not always correct. Therefore, in this research, the accuracy of prediction of different thermal-time approaches for modeling the germination time courses of bishop's weed (Ammi majus L.) in response to constant temperature regimes was evaluated. Bishop's weed was selected because there are no reports regarding the germination response thresholds of this species to temperature in the scientific resources.

Experiment was conducted at the Seed Technology Laboratory of Agricultural and Natural Resources University of Khuzestan in November 2017. In this study, germination response of bishop's weed was evaluated at different constant temperatures. The seeds of bishop's weed were collected from the margins of several wheat fields at the time of their natural dispersal in June 2014. The seeds of bishop's weed were incubated in the dark using incubators with controlled environments at eight constant temperatures of 8, 12, 16, 20, 24, 28, 32 and 36 ºC with a range of ±0.2 ºC. These temperature regimes cover both the sub- and supra-optimal temperature ranges. The trial was performed in a completely randomized design with four replications. The germinated seeds (criterion, radicle protrusion of > 2 mm) were counted and removed at frequent time intervals. The event-time approach (package drc in R environment software) was applied to determine the time taken for cumulative germination to reach subpopulation percentiles of 20, 50 and 80% of maximum in each temperature regimes. 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).

The values of RMSE and AIC showed that the model had better and more accurate fit to bishop's weed germination data when the Tb (base temperature) and θTm (thermal-time required to complete germination at temperatures greater than optimal (To)) were assumed to be constant for the whole seed population, and Normal distribution was used to describe the variation in θT(g) (thermal-time required to complete the germination of each given seed fraction at a temperatures between the Tb and To) and Tm(g) (maximum temperature (Tm) for seed thermo-inhibition of given fraction g at temperatures between the To and Tm). Based on this approach, the Tb and θTm for this plant were estimated to be 1.06 ºC and 1155.41 ºC h, respectively. The values of θT and Tm for seed fraction of 50% (θT(50) and Tm(50)) were determined as 2708.62 ºC h and 34.55 ºC, respectively. The value of To was not constant for different germination fractions (To(g)), and for fraction 50% (To(50)) was determined to be 24.51 ºC. Thermal-time analysis 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 temperature. 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. The thermal-germination model presented here explained some of the adaptive characteristics of the germination response of bishop's weed to ambient temperature. Seed thermo-inhibition in this weed species occurred at temperatures beyond 25.43 ºC. In other words, the seeds of bishop's weed do not germinate when temperature will exceed this limit and thus remaining capable of germinating until the environmental conditions change.

In summary, in this study, the thermal thresholds for seed germination of bishop's weed were identified. Our results showed that the Tb was constant for the whole seed population. The thermal-germination model described here gave an acceptable explanation of the observed seed germination patterns. Almost all the concepts and mathematical models described in this study can be applied to modeling seedling emergence in the field.