Evaluation of the inter- and intra-specific competition of tumble pigweed (Amaranthus albus L.), redroot pigweed (Amaranthus retroflexus L.) and grass pea (Lathyrus sativus L.) under drought stress

Grass pea is an annual plant which produces adequate yield in unfavorable conditions. South Khorasan is one of the areas where grass pea cultivation has been spread, but due to rainfall rate and high evaporation and transpiration in this region, plants are facing drought stress; On the other hand, Amaranthaceae family weed (pigweed and tumble pigweed) also among the most important weed in farm fields in this region of Iran. Since the studies of multi-species competition provide more detailed information about relationships circumstance between plants, the aim of this study is to evaluate the competitiveness of pigweed, tumble pigweed and grass pea under drought stress using experiments with replacement series. The experiment was carried out in a factorial based on RCBD with three replications. Experimental treatments consisted of two levels of water stress (80 and 50% of soil water field capacity), different weed interference ratios with grass pea (tumble pigweed + pigweed- grass pea) including (0-100, 75-25, 50-50, 25-75 and 100-0%), as well as different two weed interference ratios with each other (tumble pigweed - pigweed) containing (0-100, 50-50 and 100-0). Optimum density of eight plants per pot was fixed. Plants were cultivated after soil preparation and addition of chemical fertilizers and at four leave stage, the desired density was reached. At the end of the experiment, all plants were harvested from each species separately and weighed after drying. Finally, for results analyzing of the experiment, relative yield indices, total relative yield (LER) were used and relative competition Intensity index and aggressiveness index were used. The results showed that grass pea, pigweed and tumble pigweed dry matter, decreased with reducing soil water content of 80 to 50 % of field capacity by 33, 30 and 12 % respectively. Also, the results showed that predicted yield of tumble pigweed was close to the actual yield. The results of relative yields showed that in sowing ratio of 50-50% grass pea and pigweed, 50-50% of grass pea and tumble pigweed, also 50-50% of grass pea and both two weed together in 80% treatment of soil water field capacity, the relative yield of grass pea were 0.33, 0.36 0.39 respectively, that with drought stress increasing to 50% of soil water field capacity, increased by 36, 50 and 31% reached to 0.45, 0.54 and 0.51 respectively. Changes in relative yield curve of grass pea under 80% treatment of soil water field capacity irrigation, in competition with tumble pigweed, pigweed and both weeds together was convex; which indicates that with enough water in the soil, grass pea competitiveness is less than Amaranthaceae family weed. The results of relative competition Intensity index showed that numerical value of this index for grass pea, tumble pigweed and pigweed, in all sowing ratios is a positive value, which shows higher inter-specific competition than intra-specific competition in different sowing ratios. Also the results showed that in the sowing ratio of 50% grass pea with 25% tumble pigweed and 25% pigweed in 50% treatment of soil water field capacity, Grass pea aggressiveness index against both weed separately and together was positive value, which indicates a relative superiority of grass pea under drought stress in competition with these two weed. In generally, the results of this study showed that water stress reduced all three plants growth but, decline in tumble pigweed was lower. The results also showed that the competition formed between these three species was Inter-specific and have negative effects on all three plants. Although the aggressiveness index indicates that the grass pea was beaten in most sowing ratios, but with the increase of drought stress and water deficit, due to inter-specific competition between weed, competition between weed and grass pea has been reached to lowest level.