the effects of drought stress on yield, yield components and anti-oxidant of two garlic (allium sativum l.) ecotypes with different planting densities

introduction drought stress reduces plant growth by affecting various physiological and biochemical processes, such as photosynthesis, respiration, translocation, ion uptake, carbohydrates, nutrient metabolism and growth promoters. garlic (allium sativum l.) is an annual bulb crop that has been cultivated since ancient times and was used as a spice and condiment for many centuries. garlic is an important plant because of its pharmaceutical properties. the optimum yield of this bulb crop depends on well-managed irrigation, fertilization and cultivation practices. in the final and middle stages of growth, garlic is sensitive to water stress and low irrigation is unsuitable in these stages. this experiment was established to study the influence of drought stress and planting density on yield and its components and the non-enzymatic anti-oxidant content of two different garlic ecotypes. materials and methods this study was conducted in 2011-2012 in a farmland at the south east of semnan city. the experimental layout was a split-plot factorial with a randomized complete block design with three replications. the treatments were comprised of three factors: irrigation rates (60%, 80% and 100% of estimated crop evapotranspiration (etc)) as the main plot and the factorial combination of three levels of planting density (30, 40 and 50 plants.m-2) and two ecotypes (tabas and toroud) as the sub-plots. to estimate the crop water requirement, different meteorological parameters were collected from semnan weather station and were used based on fao-56 water irrigation calculation instructions. after harvesting, ten garlic plants were sampled randomly in each plot and bulb yield components were measured. to calculate the leaves anti-oxidant content, dpph method was used. the statistical significances of mean values were assessed by analysis of variance and lsd tests at p≤0.05. all calculations were performed using sas and mstat-c softwares. results and discussion drought stress decreased bulb yield and dry matter percentage significantly. planting density had significant effects on bulb yield and the yield of planting density of 50 plants m-2 were significantly higher than two other densities. the interaction of drought stress and ecotype factors affected the dry matter percentage. drought stress decreased fresh and dry weight, length of bulbs and the bulb diameter significantly. drought stress decreased fresh and dry weight, diameter, length and number of cloves significantly as well. drought stress is an important limiting factor at the initial phase of plant growth and establishment. it affects both elongation and expansion growth (shao et al., 2008). fresh and dry weight and length of cloves were significantly higher in toroud ecotype. in contrast, the number of cloves in the bulb was significantly higher in tabas. studying the correlation coefficients showed that the bulb yield per unit area was significantly and positively correlated with diameter, weight and length of cloves and bulbs and also the dry matter percentage at p≤0.001. the maximum correlation belonged to yield at the unit area and weight of the bulb (r=0.72). in other words, any bulb-weight-increasing factor did have the highest effect on increasing the yield per unit area as well. drought stress, increased leaf non-enzymatic anti-oxidant significantly. anti-oxidants plays significant roles in ros scavenging and influences cellular ros balance. activation of antioxidant system helps the plants to tolerate stress form induced damage. the effect of ecotype was significant on anti-oxidant content and the value were significantly higher in toroud ecotype. toroud ecotype showed resisting reactions against higher levels of drought stress by increasing the non-enzymatic anti-oxidant content and created tolerating mechanisms versus stress. conclusion drought stress reduced yield and yield parameters and increased non-enzymatic anti-oxidant content of garlic. the increment of anti-oxidant content showed the tolerance of garlic to drought stress. the maximum bulb yield was obtained at the highest planting density.