Evaluating the competition of tumble pigweed (Amaranthus albus L.), common purslane (Portulaca oleracea L.) and common millet (Panicum miliaceum L.) competition under salinity stress

The cultivation of crops in most parts of the world, in addition to competing with different weed species, is affected by a variety of environmental stresses, including salinity stress. The changes in the growth rate and production of crops under the influence of salinity stress and competition with weeds have been extensively studied. However, what is currently less investigated is the competition of several species of weeds with crops and the competition of crops with weeds under salt stress. Therefore, the aim of this experiment was to evaluate the competition between tumble pigweed, common purslaneand and common millet under soil salinity stress. This study was conducted as a factorial experiment based on a randomized complete block design with three replications. Experimental treatments consisted of two rates of soil salinity (3 and 10 dS.m-1), and different ratios of interference of the three species (tumble pigweed, common purslaneand, millet) including 0-0 -100, 0-25-75, 12.5-12.5-75, and 25-0-75, 50-0-50, 25-25-50, 0-50-50, 75-0-25, 37.5-37.5-25, 0-75-25, 0-100-0, 50-50-0 and 0-100-0. The desired density in each pot was constant and equal to 8 plants. After preparing the soil and passing through a 2 mm sieve, fertilizer and saline solutions were added to the soil and the soil was transferred to 5 kg pots. Then all three plants were planted and attained to intended density at the four leaf stage. At the end of the experiment, each plant species was separately harvested from each pot and were weighed after complete drying. Finally, for analyzing the results of the experiment, relative yield indicators, total relative yield (Land Equal Ratio), relative crowding and agressivity indices were used. The results showed that increasing soil salinity decreased the dry matter produced by common millet, tumble pigweed and common purslane by 19.5, 4.7 and 27.8 percent, respectively. Comapring the relative yield of millet with those of tumble pigweed and common purslane and also both weeds together by different ratios of planting showed that these values were more than predicted ones under either 3 or 10 dS.m-1 salinity level, which indicates the weak competitive ability of these two weeds against millet. The results of the relative crowding index of species showed that in the ratio of 75% of millet and 12.5% tumble pigweed and 12.5% of common purslane, the greatest rate of relative crowding index was achieved for millet under salinity level of 3 dS.m-1. Also, the greatest relative crowding index of tumble pigweed and common purslane under either salinity level was observed with the cultivated ratio of 75% for both weeds and 25% for millet, suggesting an increased competitive ability of these two weed species through increasing their density even under a higher level of soil salinity. By evaluating the agressivity index between tumble pigweed and purselane, it was found that this value was negative for purselane under the salinity level of 3 dS.m-1. Therefore, it seems that tumble pigweed is more competitive than the common purslane where both weed species grow together. In general, the results of this study showed that in conditions of salinity of 3 and 10 dS.m-1, common millet had more competitive ability than the two studied weeds, but with increasing salinity level, competitive ability of millet was reduced, while it increased weeds competitiveness. Therefore, in the situation of high weed densities, we may expect a greater damage to crops under salinity stress.