Quantifying of germination rate response to temperature of three sunflower varieties using nonlinear regression models

Germination and seedling establishment are two critical and important stages during plants growing season. From seed technology viewpoint, germination refers to emergence and development from seed embryo, so that could produce normal seedling. Many studies have been done to determine germination cardinal temperatures and required thermal time for emergence occurrence, but there's not a lot of information about the sunflower cultivars. This study was aimed to investigate the germination response of three sunflower varieties (Farrokh, Hisan and Progress) and assessment of nonlinear regression models to determine cardinal temperatures and required biological days for germination of three aforementioned varieties. For this, Farrokh, Hisan and Progress varieties seeds were exposed to five constant temperatures (15, 25, 30, 35 and 40 0C) in Seed Research Lab., Gorgan Univ. Agric. Sci. and Natur. Res. (GUASNR) in 2012. Germination duration and percentage were measured, germination rate was calculated and biological time and thermal time for germination were estimated. In order to quantify cardinal temperatures, three nonlinear regression models (segmented, dent-like and beta) were used. Results indicated that temperature and variety simple and interaction effects had significant effect on maximum germination percentage (MGP), germination rate (reciprocal time to 50% germination), and time to 10, 50 and 90% germination. Assessment of three nonlinear regression models including segmented, dent-like and beta models revealed that segmented and beta had the same quality and selected as appropriate models for Farrokh, while dent-like model was selected as the superior model for Hisan and Progress varieties. For Farrokh, base, optimum and ceiling temperatures were estimated as 8.51, 33.26 (lower and upper optimum were 30.44, 37.05) and 48.47 0C, respectively. the base, lower optimum, upper optimum and ceiling temperatures were estimated as (8.9, 30.41, 37.77 and 45 0C) and (9.85, 28, 38 and 47 0C) for Hisan and Progress, respectively. Biological time for 50% germination for Farrokh, Hisan and Progress also were estimated as 18.14 (20.47), 18.52 and 17.05 hours. Results revealed that different sunflower varieties have different cardinal temperatures. Therefore, if we are planning to use these coefficients to quantify responses of sunflower to temperature and prediction of germination occurrence time, variety-specific coefficients should be determined and used.