Extinction coefficients and radiation use efficiency of barley under different irrigation regimes and sowing dates

The aim of this experiment was to determine the hourly extinction coefficient and radiation use efficiency (RUE) of winter barley under various sowing date and irrigation regimes. Field experiments were performed at the Experimental Research Station at the College of Agriculture of Shiraz University in Iran over two consecutive years: 2012 and 2013. The study used an experimental design arranged in split plots based on a randomized complete block design with three replicates. There were four irrigation level treatments in the main plot, consisting of the full crop Irrigation requirement (W1), 75% and 50% of full irrigation (W2 and W3) and a dry land treatment (rain-fed, W4). The four sowing dates consisted of October 23rd (T1), November 6th and 22nd (T2 and T3) and December 6th (T4) which were considered the subplots. Two types of radiation adjustment were applied in order to evaluate the effect of air temperature changes on photosynthesis. The hourly extinction coefficient was 0.584 based on Photo-synthetically Active Radiation (PAR), and the values of RUE ranged from 0.73 to 2.17 g MJ−1 and 0.85 to 1.94 g MJ−1 in 2012 and 2013, respectively. The variations in air temperature and environmental conditions caused a significant difference among the RUE values in 2012 and 2013. The maximum values of RUE were obtained in W1, which were significantly higher than other irrigation treatments. However, the RUE decreased significantly under irrigation level reductions. It is concluded that delaying in sowing date decreases the RUE values and the reduction in Growing Degree Day (GDD) before barley dormancy can effectively change the RUE. Therefore, to achieve a suitable value of RUE, the value of the GDD should be more than 120°-days before barley dormancy under any irrigation regime.