vInvestigating the Effect of Phosphorus Fertilizer Levels and Density of Wild Mustard (Sinapis arvensis L.) on Morphophysiological Traits of Safflower (Carthamus tinctorius L.)

Oilseeds are very important as the raw material for the production of vegetable oil (one of the basic needs of the society in the food field). Therefore, achieving any success in increasing the amount of production and supplying as many of these products as possible to meet the domestic needs of the country is considered a valuable and great success. Safflower, with the scientific name Carthamus tinctorius L., is an annual long-day plant from the chicory family. Wild mustard (Sinapis arvensis L.), which is also called Brassica kaber in some literatures, is one of the most important weeds belonging to the Brassicaceae family. Competition can perhaps be considered the most important biological interference factor effective in determining crops productivity.Effective management of weeds in agricultural systems is very decisive. Extensive and repeated use of herbicides has led to the emergence of resistant weed biotypes, which has often increased the cost of control. It has also caused some concerns about the negative environmental effects of herbicides. When the competition is for light, the competitive ability of the species is first determined by the morphological traits. The response of crop height to weed competition is related to the density and intensity of competition and the type of weeds and can be positive or negative.

An split plots experiment based on randomized complete block design with three replication was carried out in the Research Farm of University of Birjand during the 2018-2019 cropping year. The treatments include four level of phosphorus fertilizer (0, 25, 50 and 75 kg P 2O5 ha-1) as the main plot and the four wild mustard densities (0, 7, 14 and 28 pl m-2) as subplots. Plant growth characteristics were measured from 150 to 210 days after planting (DAP) in five stages at 15-day intervals. Also, at the harvest maturity, the yield and yield components of the crop were determined.

According to the results, the highest (35.48 g m-2) and lowest (28.23 g m-2) safflower leaf dry weights were obtained from control (no-mustard) and 28 pl m-2 mustard densities at 165 DAP, respectively. The highest (1.67) and lowest (1.19) leaf area index of safflower were achieved at 210 DAP using 25 and 50 kg P 2O5 ha-1, respectively. Based on these results, it can be concluded that the most effective level of phosphorus on the improvement of safflower growth traits (e.g. leaf area and dry weight and stem dry weight) and its competitive ability was 25 kg P2O5 ha-1, although the its effects were not significant for many traits, and as phosphorus levels increased, the competition shifted more in favor of wild mustard. In accordance with these results, and probably due to luxury consumption by weeds, it has been observed that when weed density is high, adding fertilizer leads to superiority of weed growth over crop (Blackshaw et al., 2008). Also, the highest leaf area index of mustard (0.63) was obtained at a density of 56 mustard pl m-2 at 165 DAP and the lowest one of (0.26) was observed at a density of 14 pl m-2 at the same time. It was also observed that the height and leaf area index of wild mustard were the highest in all measuring stages at higher weed densities, indicating the competitiveness of the weed. The increase in weed density had a negative impact on the safflower, although insignificant in many cases, which could be the result of the competitive effect of mustard for resources such as radiation and nutrients (Wright et al., 1999).

In general, the increase in weed density had a positive effect on the yield and its components in wild mustard and a negative effect on the safflower, and under these condition, the application of more than 25 kg P2O5 ha-1 does not have a positive effect on the crop. Thus, it seems that the revision in weed management and the use of fertilizer as an agronomic strategy can be effective in reducing the crop losses caused by the presence of high densities of weeds (Clements et al., 2014).