-Experiments were conducted in 2010 to determine the influence of plant density and seed position on the mother plant on seed physiological characteristics of cocklebur (Xanthium strumarium). Cocklebur burs were collected in fall of 2010 from Research Farm of University of Agricultural Sciences and Natural Resources of Gorgan, Iran. The experiment was established as factorial arrangement using a completely randomized design with three replications. The factors included different densities of cocklebur (0, 2, 4, 6 and 8 plant m -2 ) and the top and bottom parts of the canopy. Non dormant seeds were used for determining cardinal temperatures and tolerance to salinity and drought stresses. Base, optimum and ceiling germination temperatures were estimated between 7.09 to 12.33, 32 to 35 and 44 to 45 respectively in different treatments. Salinity stress up to 300 Mm and osmotic potential 8 bar inhibited the germination completely. Comparison of base temperatures and sigmoid equation coefficients showed that seeds produced in the top had higher germination than those that produced at the bottom of the mother plant. It seems plant densities through seed position on the mother plant affect seed quality. Likewise changes of light quality and quantity in shade environment increased seed dormancy in matured seeds. Shade environment affect seed germination on mother plant that increased dormancy of seeds maturing under shade be an adaptive response that reduces the probability of germination of offspring under unfavorable (shade, competitive) conditions. Keyword: shade environment, cardinal temperature, competition, dormancy. (0, 2, 4, 6 Palavras-chave: ambiente de sombra, temperatura cardeal, competição, dormência. FOROUGHI, A., et al. RESUMO -Em 2010, foram conduzidos experimentos para determinar a influência da densidade de plantas e posição das sementes na planta-mãe sobre as características fisiológicas de carrapicho (Xanthium strumarium). Espinhos de carrapicho foram coletados no outono de 2010 na Fazenda de Pesquisa da Universidade de Ciências da Agricultura e Recursos Naturais de Gorgan, Irã. O experimento foi estabelecido como um arranjo fatorial usando um delineamento inteiramente casualizado, com três repetições. Os fatores incluíram diferentes densidades de carrapicho

BACKGROUND: Understanding the relationship between physiological traits with yield and yield components is an essential step towards developing high-yielding and high-quality canola (Brassica napus L.) cultivars. This study aimed to explore further the relationship between some physiological features, including radiation use efficiency (RUE), and seed yield in canola.RESULTS: Significant differences were found among cultivars regarding maximum leaf area index (LAI max ) and required days to achieve maximum LAI (DLAI max ). All cultivars obtained the minimum LAI required to intercept 90% of the incident radiation, but at different times. Some cultivars like SW102 and Shirali had the same fraction of intercepted photosynthetically active radiation (IPAR) when LAI was maximal, but SW102 had higher IPAR. This indicated that SW102 was more efficient in irradiation capacity and may have a higher photosynthesis rate when exposed to the high irradiation conditions. The average canola RUE in the current study was 3.80 and 3.63 g MJ −1 m −2 in 2014 and 2015, respectively. In general, the crop growth rate was higher in the first year than in the second year due to the fewer cloudy days and more incident radiation.CONCLUSION: Results indicated that duration of growth, crop growth rate, and harvest index were crucial for enhancing biomass and seed yield. Also, a relatively high correlation was found between the RUE and DLAI max . The cultivars that reached their maximum LAI later demonstrated higher RUE, and consequently had higher biological and seed yield. The results obtained could be used to develop an improved canola crop growth model and breeding programs.