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 (R 2 ) 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), (R 2 = 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 and 7.38 degree-days, respectively.

The effects of water deficit and nitrogen fertilizer were studied on antioxidant enzymes activity and quantum yield of barley. An experiment carried out in greenhouse in factorial subject based on a completely randomized design with three replications. Irrigation schedules imposed at three levels of 85%, 60% and 35% field capacity (FC), and nitrogen were applied in quantities of 40, 80 and 120 kg N ha-1. We determined Catalase (CAT), Peroxidase (POX) Polyphenol oxidase (PPO) activities, proline, chlorophyll and carotenoid content, quantum yield and grain yield. The results showed that severe stress (35% FC) increased the activities of CAT, POX and PPO enzymes and proline content, whereas the carotenoids, chlorophyll a and chlorophyll b decreased. Water deficiency caused the reduction in the quantum yield and the grain yield by 34%. Application of 80 kg of N during stress treatments resulted in higher enzyme activity and proline content. High amount of nitrogen reduced carotenoids, chlorophyll a, chlorophyll b, and in contrast, enhanced quantum yield. Application of 120 kg N ha-1 increased the yield up to 37% under mild stress (35% FC). Correlation coefficient and path coefficient showed that, grain yield was affected directly by amount of carotenoids and quantum yield.

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Effects of seed priming and aging on some physiological characteristics of Milk thistle was studied in a factoral experiment based on Complete Randomized Design (CRD). Tratments were included hydro priming (using distilled water), halo priming (0, 1.5, 3, 4.5 and 6% KNO 3 ) and accelerated aging (0, 2, 4 and 6 days under 45°C and 95% humidity) in three replications. Determined parameters were germination charactristics including germination percentage, daily germination speed, mean time of germination, seed vigor index, hypocutile length and hypocutile dry weight. Activity of catalase, peroxidase and polyphenol oxidase were determined at 12 hours after imbibition and seedling stage. According to results of this experiment, germination percentage, seed vigor and seedling growth of seeds were increased under all priming treatments. Improving the catalase and peroxidase activity led to decrease the aging damages. Germination characteristics were improved under both priming treatments at the beginning of germination as well as seedling growth. Polyphenol oxidase activity was increased in the pre-treated seeds but decreased in seedling growth stage. Aging treatments led to reduce the germination percentage, daily germination speed, seed vigor and seedling growth while the germination time was increased. Accelerated aging caused to reduce the germination rate and seedling growth of milk thistle that is probably due to increasing the lipid peroxidation, free radical increment and decreasing the antioxidants activity. The greatest and lowest antioxidants activity, the germination percentage, germination speed and seed vigor were respectively observed under priming using 3% KNO 3 concentration and control seeds.

In order to investigate the effects of some plant growth regulators on photosynthetic pigments and growth of fennel plants, a greenhouse experiment was conducted based on the randomized complete block design with three replicates in 2017. Treatments were the application of methyl jasmonate (25, 50, 100 and 200 μM), putrescine (0.25, 0.5, 1 and 2 mM) and 24-Epibrassinolide at 0.001, 0.01, 0.1 and 1 μM and distilled water as a control. The results indicated that application of 0.5 Mm putrescine, exhibited significant effects on the chlorophyll a (62%), b (104%), total chlorophyll (72%), carotenoids (51%), flavonoids (51%), anthocyanin content (-14%), phenolic compounds (13%) and maximum quantum efficiency (17%) in dark condition and in light condition. Application of 24-Epibrassinolide resulted in a significant increase of chlorophyll a and total chlorophyll, carotenoids, phenol content, maximum quantum efficiency in the dark condition and photochemical quenching of fluorescence. The highest chlorophyll content and carotenoids were observed in treated plants with 0.1 µM 24-Epibrassinolide, while the maximum phenol content was obtained by application of 0.01 µM 24-Epibrassinolide. The application of methyl jasmonate significantly affected the major chlorophyll and accessory pigments (except phenol) of fennel. Plants treated with 50 µM methyl jasmonate exhibited higher concentrations of chlorophyll a (3.25 mg per g FW-1), total chlorophyll (4.35 mg per g FW-1), carotenoids (0.87 mg per g FW-1) and flavonoids (4.75 µg per g FW-1). A significant dry weight increased after the application of methyl jasmonate and it can be concluded that the most effective treatment in this regard for fennel plants was 50 µM methyl jasmonate.