Effect of drought stress and mouse barley (Hordeum murinum L.) competition on growth and yield of cumin (Cuminum cyminum L.) under greenhouse conditions

Introduction Cumin (Cuminum cyminum L.) is the most important medicinal plant in Iran that main part of its production costs spends on combat weeds. Given the difference among different species in water use patterns and techniques for moisture stress resistance, it may be feasible to manage weeds by irrigation practices (Zand et al., 2010). In an in-depth review of literature on the competition between weeds and crops for water, Patterson (1995) concluded that lower soil moisture sometimes benefits crops by weakening weeds’ competitiveness. It seems that cumin is of those crops that have a competitive advantage in arid climates. The growth of cumin so extensively accompanied with dryness that most farmers know it as a rain-fed crop (Kafi et al., 2002). Research on weeds of cumin farms in Southern Khorasan Province shows that false barley (Hordeum murinum L.) is the most prevailing narrow-leaf weed in the region (Amininasab, 2014). Seemingly, the competition between cumin and weeds has not been studied comprehensively. Therefore, it appeared necessary to carry out a study to assess the effect of drought stress on the competition between false barley and cumin.   Materials and Methods The study was carried out in the experimental greenhouse of Faculty of Agriculture in Birjand in February-March 2015 on the basis of a Randomized Complete Block Design with four replications. The studied factors including the density of false barley at four levels (0, 1, 3, and 5 plants per pot equivalent to 0, 26, 79, and 131 plants per m2) and soil moisture relative to field capacity (FC) at three levels of normal conditions (soil moisture = FC), moderate stress (soil moisture = 60% FC) and severe stress (soil moisture = 30% FC). Cumin density was five plants per pot (131 plants per m2) in all treatments. Plastic pots were used in the present study with capacity to hold 6 kg soil, mouth diameter of 22 cm, and depth of 21.5 cm. According to field capacity determined in the laboratory, the water required for 100%, 60% and 30% FC was 750, 450 and 225 cc, respectively. To keep the pre-determined moisture levels, all pots were weighed every day and the required water was added. The recorded growth and yield measures of cumin included the changes in height, leaf number, individual plant yield, total biomass, and seed number. Data were analyzed by DSAASTAT macro ver. 1.022 in MS-Excel Software Package. Also, means were compared by FLSD test at the 5% significance level, and the graphs were drawn by MS-Excel Software Package.   Results and discussion It was found that drought stress significantly affected the loss of some growth and yield measures of cumin including plant height, leaf number, total biomass, root length, and individual plant yield at the 1% level, whilst the effect of weed density and the interaction between weed density and drought stress was significant for total biomass, leaf number, and plant yield at the 1% level and for plant height at the 5% level. They were not significant for root length. According to the results, adequate soil moisture increased inter-species competition of false barley, and weed density had higher negative impact on cumin yield. In these conditions, weeds occupied whole pot space at the density of three plants per pot, whilst cumin yield showed more stability at different weed densities under drought stress of 30% FC. It can be said that when soil moisture was not limited, false barley exploited the favorable conditions better than cumin and reduced its growth by limiting the space and using nutrients and water. In total, it was revealed that the highest cumin yield was obtained from adequate moisture and no-weed conditions. Also, under the lack of soil moisture limitations, false barley at the density of three plants per pot (79 plants per m2) incurred the greatest damage to cumin. So, it seems necessary to irrigate cumin farms adequately and to remove false barleys, especially under adequate soil moisture. In addition, the full removal of false barleys after cumin harvest will be a suitable managerial approach.