Responses of Physiological Indices of Forage Sorghum under Different Plant Populations in Various Nitrogen Fertilizer Treatments

In order to evaluate physiological indices of forage sorghum in different plant densities and nitrogen levels, an experiment was conducted at Research Farm, Faculty of Agriculture, Islamic Azad University, Isfahan (Khorasgan) Branch, Isfahan, Iran. The main plots were plant densities, namely, 250000, 300000, 350000, and 400000 plants per ha, and four levels of nitrogen, namely, 0, 80, 160 and 240 kg N/ha were sub-plots. The field was under cultivation of barley during the previous winter. For all plant density treatments, from 25 days after plantation until 95 days after planting, the total dry matter trend increased gradually. The highest total dry matter was observed in 95 days after plantation which was related to 400000 plants per ha and 240 KgN/ha. Study the trend of variances of crop growth rate showed that in all treatments, the crop growth rate was low in the beginning of sampling, thereafter increased considerably up to 60 days after planting with a peak in 60 days after planting, then showed a declining trend after that. In all of plant density treatments, RGR decrease during plant growth and reached to a zero at 75-85 days after planting, and it reached into negative after these days until harvesting time. In different plant nitrogen levels, RGR trends also decrease during plant growth and reached to a zero at 40-60 days after planting. The maximum LAI was obtained for 400000 plants per ha and 160 kgN/ha. Study of trend of net assimilation ratio (NAR) showed that in all treatments, the NAR was stable in the beginning of sampling, thereafter showed a declining trend that toward zero (90-95 days after planting).