castor (ricinus communis l.) and pigweed (amaranthus retroflexus l.) growth indices in terms of interference

introduction growth analysis has been widely used in breeding programs to identify the important plant developmental phases and components related to higher yield under a particular set of environmental conditions. castor bean (ricinus communis l.) is an important commercial crop. castor oil based by products is used in manufacturing of several commercially important commodities like surfactants, coatings, greases, pharmaceuticals, cosmetics, polyesters, polymers, etc. interference (interactive effects among species on inter-species populations) is one of the main issues on the eco-physiology of plant populations where weeds impose negative effects by approaching the plant to compete in light, water and nutrient elements availability and results in reduced growth and yield (shinggu et al., 2011). growth indices are useful for interpreting plant reactions to the crop and weed density. various reasons have been attributed for the low productivity among the most important is weed competition (radosevich, 1987). the aim of the present experiment was evaluating the interference effects of redroot pigweed on growth indices of castor bean in northwest of iran. materials and methods this experiment was conducted in urmia, iran (agricultural research of west azarbayjan, saatlo station (37°44´18״ n latitude and 45° 10´ 53״ e longitude, at 1338 m above sea level)) in 2012. the soil of the experimental field was sandy - loam, with ph of 7.2. competitive pattern of experiment was in two-factor based on a randomized complete block design (rcbd) with three replications arranged in four castor plant densities (3, 4, 5 and 6 plants.m-2) and four redroot pigweed densities (0, 5, 10 and 15 plants.m-2). redroot pigweed and castor seeds were simultaneously directly planted on the 22th may in 2012. redroot pigweed plants were weeded at the times related to the treatments level. irrigation and intercultural operations were performed whenever necessary. plots were 3m×5m with 60 cm between rows. seven times during plant growth stage castor plants were harvested from each plot considering marginal effects. the plants were transferred to the laboratory for evaluating of dry matter. excel (microsoft office, 2007) was used for drawing of diagrams. results and discussion the results showed that the highest dry matter (dm), leaf area index (lai), crop growth rate (cgr) and relative growth rate (rgr) of castor were observed in pure stands of castor. among pure stands, the highest and the lowest dm and cgr were achieved in castor densities of 5 and 3 plants.m-2, respectively. the highest (4.06) and lowest (0.90) of castor lai were observed in 6 plants.m-2 of castor density and 3 plants.m-2 of castor with 15 plants.m-2of pigweed at 1046 gdds, respectively. the maximum (0.317 g.m-2.g.d.d-1) and minimum (0.114 g.m-2. g.d.d-1) of crop growth rate for castor plants were achieved at the same gdds in 5 plants.m-2 of castor, and 3 plants.m-2 of castor with 15 plants.m-2 of pigweed, respectively. these parameters were decreased by increasing pigweed densities. therefore, it can be concluded that dm and cgr are decreased, due to increasing pigweed density; these parameters are slightly affected by castor density. measurement results of pigweed characteristics indicated reduction in lai, dm, weed rate growth (wrg) and rgr of pigweed in presence of castor bean. increasing in castor density caused a greater reduction in the mentioned characteristics whereas in the highest density of pigweed (15 plants.m-2), increasing in castor density caused 44, 40 and 134 % decrease in lai, dm, wrg, respectively. conclusion the results of this study showed that increasing plant density to 6 plant.m-2 increased lai, dm, cgr and rgr. therefore, it (6 plants.m-2 of castor bean density) can be recommended for reducing redroot pigweed damage in castor bean field. acknowledgments the authors acknowledge the financial support of the project byvice president for research and technology, urmia university, and west azarbaijan agricultural and natural resources research center, iran.