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Quinoa (Chenopodium quinoa Willd.) is an annual plant belonging to the Amaranthaceae family. The total accumulated incident photosynthetic active radiation (PAR) in a plant is a function of the site, year, sowing date, and phenology, whereas the amount of PAR intercepted by the plant is dependent on its leaf area index (LAI) and light extinction coefficient (KPAR). This study investigated changes in KPAR and radiation use efficiency (RUE) levels in quinoa before and after inflorescence (fruit set) color change under various temperature and light conditions through the sowing date. This study was conducted in the Jihad Keshavarzi allocated field of Khalilabad city in Khorasan Razavi province, Iran, using a randomized complete block design with three replications over two seasons, 2017–2018 and 2018–2019. Treatments included eight and seven sowing dates in the first and second season years, respectively. The results showed that the mean of KPAR was 0.66 and 0.31 in the first and second years, respectively. Also RUE in vegetative stages of development was higher than during the grain‐filling period. The mean of RUE was 2.60 ± 0.06 and 0.78 ± 0.17g/MJ in the first year before and after changing the color of the inflorescence (fruit set), respectively; these values were 2.12 ± 0.04 and 1.01 ± 0.61 g/MJ in the second year. The results indicated the base (TBRUE), lower optimum (TP1RUE), upper optimum, and ceiling temperatures of quinoa for RUE were determined to be 3, 18.8, 25.5, and 35°C, respectively.

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