Effect of Seeding Density on Radiation Absorption and Use Efficiency of Rice (Oryza sativa L.) Genotypes under Direct Seeding Conditions

Introduction: Effective use of sunlight is a great opportunity to improve crop productivity. In conditions without growth limiting factors, there is a linear relationship between radiation absorption and dry matter production in plants, that the slope of this relationship is radiation use efficiency. In addition to genotyping, radiation use efficiency is affected by crop management. Changes in density of rice varieties by affecting leaf area index and light extinction coefficient change the amount of absorption and distribution of light within the canopy and thus affect dry matter production. Therefore, the response of different rice cultivars to change in density is not same. According to this, the present study was carried out to investigate the radiation absorption and efficiency of use and also determine the light extinction coefficient in new and old rice cultivars at different direct seeding densities in Rasht climatic conditions. Material and Methods: The present experiment was carried out as a factorial based on randomized complete block design with three replications at research field of Rice Research Institute of Iran-Rasht. Treatments included five levels of densities (65, 80, 95, 110, and 125 kg ha-1) and three levels of cultivar (Taher (promising line), Anam and Hashemi). Sampling was performed once every two weeks, 25 days after emergence. Sampling was performed with 0.5×0.5 m quadrates to measure leaf area, dry weight, and radiation in different treatments. Results and Discussion: The results showed that the interaction effect of cultivar and density on radiation use efficiency, LAI, grain and biological yield were significant. Harvest index was affected only by density and light extinction coefficient was not affected by any density and cultivar treatments. The highest grain yield of Taher (6380 kg ha-1) and Anam cultivars (6100 kg ha-1) were obtained in 95 kg ha-1 density, while the highest yield of Hashemi cultivar (5490 kg.ha-1) was observed in 110 kg ha-1 density. Grain yield followed biological yield, and treatments with higher biological yield had higher grain yield. The highest leaf area index (5.63) was observed in Hashemi cultivar and 110 kg ha-1 density in 70 days after emergence. At this stage, the lowest leaf area index (4.21) was observed in Taher cultivar at 65 kg ha-1 density. Although, in the early stages of growth, higher densities had a higher rate of leaf area expansion, but the highest leaf area index did not differ significantly between 95, 110, and 125 kg ha-1 densities. The light extinction coefficient of this experiment varied from 0.4 to 0.5 in different cultivars and densities and Taher cultivar had the higher light extinction coefficient. Although, no significant difference was observed between treatments. The highest grain yield of Taher (6380 kg ha-1) and Anam (6100 kg ha-1) cultivars were obtained in 95 kg ha-1 density, but the highest yield of Hashemi (5490 kg ha-1) was obtained in 110 kg ha-1. In this study, the highest (3.06 g MJ-1 PAR) and the lowest (2.20 g MJ-1 PAR) radiation use efficiency were observed at 95 and 125 kg ha-1 of Taher cultivar, respectively. In all cultivars, the lowest radiation use efficiency was observed at density of 125 kg ha-1. Anam and Taher cultivars had the highest radiation absorption (773 and 852 MJ-1 PAR, respectively) during the growing season at 95 kg ha-1. Although, the highest radiation absorption of Hashemi (880 MJ-1 PAR) cultivar was obtained at 110 kg ha-1 density. Conclusions: The results showed that Taher cultivar had the highest grain and biological yield compared to Anam and Hashemi cultivars. Based on yield, the best density was 95 kg ha-1 for Anam and Taher cultivars and 110 kg ha-1 for Hashemi cultivar. Increasing the density of rice cultivars to the optimum level increased the radiation absorption and use efficiency. There was a positive correlation between radiation absorption and efficiency with biological and grain yield of rice cultivars.