اثر سطوح بقایای آفتابگردان (Helianthus annuus L.) بر خصوصیات جمعیت علف‌های‌هرز و عملکرد و اجزای عملکرد نخود (Cicer arietinum L.)

Introduction
Weeds deprive the crop plants from limited available nutrients, space, light, and moisture. Hence, the physiological activities and growth of crops are negatively affected in the presence of weeds. Ultimately, poor crop productivity is the result of weed-crop competition. Allelopathy is primarily based on the ability of certain plant species to produce secondary chemical compounds, which exert some sorts of biological ef fects on other organisms (Waller, 2004). It is biochemical interaction between individuals and ecological communities systems. Allelopathic compounds are released into the soil through root exudation, leaching by dews and rains, volatilization and decaying different plant tissues in soil (Rice, 1984). The transport of allelochemicals to target weed species is facilitated by microorganisms. Allelochemicals also, promote the activities of soil microbes, which pose a positive effect on crop plants. In many cases, these compounds inhibit the germination or growth of neighboring plants (Ebana et al., 1981) and affect plant populations (Chase et al., 1991).
Sunflower contains water-soluble allelochemicals that inhibit the germination and growth of other species (Sadeghi et al., 2010), and could be used in weed management programs. Orooji et al. (2008) evaluated allelopathic potential of sunflower on redroot pigweed and common lambsquarter in the laboratory and greenhouse experiments. Ashrafi et al. (2008b) studied the allelopathic effects of sunflower on germination and growth of wild barley.
Chickpea (Cicer arietinum L.) is the third most important legume worldwide. It is predominantly grown as a post-rainy season crop on conserved soil moisture and experiences progressive terminal drought stress with varying intensity. Chickpea is a weak competitor against weeds and availability of post–emergence herbicides particularly against broad-leaf weeds is limited (Solh & Palk, 1990).

Materials and Methods
In order to study the effect of sunflower residue levels on population, density, dry weight and diversity of weeds and yield components, biological yield and grain yield of chickpea (Cicer arietinum L.), an experiment was performed based on a randomized complete block design with four replications at the Agricultural Research Station, Ferdowsi University of Mashhad, Mashhad during 2013. Treatments included application of zero, 625, 1250, 1875 and 2500 kg.ha-1 sunflower residues. Traits such as density, dry weight and Shannon-Wiener diversity index of weeds at two sampling stages, height of main stem, distance of first pod to soil surface, number of branches, pods and 100-seed weight, biological yield and grain yield of chickpea were measured.

Results and Discussion
The results indicated that 14 weed species belong to 10 families were observed at two sampling stages. Poaceae was the most dominant family amongst these. The effect of sunflower residue levels was significant (p≤0.01) on weed density and dry weight and Shannon-Wiener diversity index at two sampling stages. Increasing the residue level considerably declined the density and dry weight of weeds. At the first and second sampling stages, the lowest Shannon-Wiener index was recorded at 2500 kg sunflower residue ha-1 with 0.2 and 0.03, respectively. Moreover, growth characteristics, yield components and yield of chickpea were significantly affected by sunflower residue levels (p≤0.01). The maximum biological yield and grain yield were observed in 2500 kg.ha-1 sunflower residue with 237.9 and 97.2 kg.ha-1, respectively. It seems that the application of increased residue levels of sunflower through decreasing weed growth and improvement of soil characteristics has resulted in an enhanced growth and yield of chickpea. So, in order to achieve the principles of sustainable agriculture, the application of sunflower residues could be considered for weed control and yield enhancement of chickpea as an important and valuable legume.

Conclusions
Allelopathic crops express their allelopathic activity through exudation of allelochemicals. Growing allelopathic crop may become an important way to suppress weeds, especially So the use of allelopathic plant residues as mulches are important ways that can be practiced for economical, environment friendly weed management in agricultural systems. The allelopathic potential of crops is desired to be strengthened using conventional and modern plant breeding techniques.