Quantifying cardinal temperatures and thermal time required for germination of Silybum marianum seed

The response of seed germination to environmental factors can be estimated by nonlinear regression. The present study was performed to compare four nonlinear regression models(segmented, beta, beta modified, and dent-like) to describe the germination rate–temperature relationships of milk thistle(Silybum marianum L.) at six constant temperatures, with the aim of identifying the cardinal temperatures and thermal times required to reach different germination percentiles. Models and statistical indices were calibrated using an iterative optimization method and their performance was compared by root mean square error(RMSE), coefficient of determination(R2) and Akaike information criterion correction(AICc). The beta model was found to be the best model for predicting the required time to reach 50% germination(D50),(R2= 0.99;RMSE = 0.004; AICc =-276.97). Based on the model outputs, the base, optimum, and maximum temperatures of seed germination were 5.19 ± 0.79, 24.01 ± 0.11, and 34.32 ± 0.36 °C,respectively. The thermal times required for 50% and 90% germination were 4.99 and7.38 degree-days, respectively.

Performance of Iranian Okra Ecotypes Under Various Accelerated Aging Conditions

Knowledge of seed quality and preservation of okra seeds is very limited, and the main purpose of this study was to simulate the condition of seed quality loss during storage and the physiological implications, which has led to seed vigor loss. A factorial experiment was conducted with the aim of studying the mechanism of longevity loss of the three endemic okra ecotypes of Iran(Ahwaz, Isfahan, Mashhad) in response to different accelerated aging durations(24, 48, 96, and 192 h). The accelerated aging condition was incubation of seeds to the temperature of 40 °C with 99% humidity. Germination of okra seeds was improved at 24 and 48 h compared to non-aged seeds, while at 192 h of accelerated aging, seed germination(49%) reduced to 50% of its value in 48 h(96.4%). Our results revealed that Iranian okra ecotypes were able to survive accelerated aging conditions up to 48 h. Antioxidant enzyme activity decreased with the increase in aging durations. Among investigated cultivars, seedling vigor of Mashhad ecotypes was the most sensitive to aging treatments. Our results revealed that the seed reservoir use efficiency value was 66%lower when exposed to 192 h of aging compared to the control. The cultivar Isfahan exhibited the minimum reduction of antioxidant activity,and, interestingly, this ecotype was the most aging tolerant.