Determining the effect of drouth stress on the values of critical temperature thresholds for seed germination of volunteer rapeseed (Brassica napus L.) using hydrothermal time model

Seed germination is largely regulated by two factors, temperature (T) and the amount of seedbed water potential (ψ). Hence, hydrothermal time models have been widely used to describe seed germination patterns in response to these two environmental factors. In this study, a new hydrothermal time model based on the Gumble distribution is presented that 1) by considering the thermoinhibition of germination at high temperatures, it simply explains the germination dynamics of seeds in response to drought stress in both sub- and supra-optimal range; 2) includes mathematical solutions for estimating critical T thresholds; and 3) explains well any systematic change in critical T thresholds due to water availability for different germination fractions (g). The developed model was fitted to the germination data of volunteer rapeseed in response to two factors T and ψ, based on which the effect of moisture stress on the values of critical germination T thresholds of this plant was modeled.

The experiment was conducted at the Seed Technology Laboratory of Agricultural Sciences and Natural Resources University of Khuzestan in 2020. Germination test was performed at temperatures of 10, 15, 20, 25, 30, and 35 °C. In each of these T regimes, the germination response of seeds to different levels of drought stress, i.e., osmotic solutions with concentrations of 0, -0.3, -0.6, and -0.9 MPa was evaluated. The germination test was performed with four replications (each Petri as one replicate). In each replicate, 40 seeds were placed on a layer of Whatman No 1 filter paper in an 8-cm glass Petri, and then moistened with 5 ml distilled water or other osmotic solutions. The number of germinated seeds was counted twice daily until the end of the test or cessation of germination in each T regime. The following model was fitted to the cumulative germination data of volunteer rapeseed to explain the germination behavior of this species in response to different levels of ψ and T.(1)The optimum (To) and maximum (Tm) temperatures for germination were also determined using the followiequations:   (2) (3)Fitting of models to germination data was performed using SAS (version 9.4) and PROC NLMIXED procedure in this program.

At each of the T regimes, the Gumble hydrotime model provided a good fit to the cumulative germination data of volunteer rapeseed in response to different water potentials. The two parameters θH (hydrotime constant) and μ (location) (and thus ψb(50) (median base water potential)) showed a defined trend with increasing T, but the coefficient σ (scale) was not affected. The value of ψb(50) linearly increased toward more positive values in response to an increase in T, whereas θH decreased curvilinearly with T. Whereas the increase in ψb(50) implies that seeds need more water availability to proceed germination at higher temperatures, the decline in θH indicates a promoting effect of increased temperatures on germination speed. The Gumbel hydrothermal time model was able to describe the germination pattern of volunteer rapeseed reasonably well, as there was a close match between observed and fitted values. The model estimated the coefficients θHT (hydrothermal time constant), Tb (base T), ψbase(50) (median base water potential at T=Tb) and KT (slope of changes in ψb(g) with T) as 305.50 MPa °C h, 5.17 °C, -1.375 MPa and 0.044 MPa °C-1, respectively. For this species, To and Tm were warmer for low percentiles, but they gradually became cooler with increasing g. Both these critical T thresholds also decreased proportionally with increasing drought stress intensity. The hydrothermal time model developed herein was able to reveal some adaptive characteristics in the germination response of volunteer rapeseed to T and ψ environments.

Based on the results obtained here, ψb(g) showed an increasing trend in response to an increase in T in the range between Tb to Tm(g), and its changes were limited to temperatures beyond To. Both To and Tm become cooler for higher germination percentiles and more severe drought stress levels. This means that volunteer rapeseed seeds can germinate only in a narrower T range under drought stress, which itself is a conservative strategy.