Effects of Planting Date and Deficit Irrigation on Water Use Efficiency of Cumin (Cuminum cyminum L.) at two different Densities in Mashhad Conditions

Introduction Supplies of irrigation water are severely limited and water use efficiency (WUE) has become more vital, especially for valuable drought tolerant plants. Green cumin (Cuminum cyminum L.) is a medicinal and spices plant adapted to arid and semi-arid regions such as Iran. Planting date, irrigation and plant density can affect quality and quantity of cumin. Seed yield of cumin decreases by increasing water deficit, despite absorbing water even in very low water potentials. Changes in planting date can also affect water use efficiency due to rainfall and evapotranspiration (ET) differences between each season. Therefore, crop yield should increase in parallel with decreasing water consumption (or evapotranspiration) for optimal water use and increasing WUE. Additionally, different plant densities and late or early planting dates could affect morphological traits and grain yield. Materials and Methods In order to study the effects of planting date, deficit irrigation and plant density on yield, yield components and WUE of cumin, a field experiment was conducted as split-split plot based on randomized complete block design with three replications at the Agricultural Research Station, Ferdowsi University of Mashhad during growing season of 2017-2018. The main plots were irrigation levels including 100, 75 and 50% field capacity (FC). Three planting dates were studied in the split plots (December, March and April) and two plant densities (40 and 80 plant m-2) arranged as split-split plots. Soil moisture content was determined a day before and two days after each irrigation with TDR. Then, irrigation was done based on treatments during the growth period. Number of umbel per plant, number of seed per umbel, 100-seed weight, plant weight, seed yield and WUE were measured at the end of growing season. Analysis of variance were calculated using R software and Duncan's multiple range test was used at 5% probability level to compare the averages. Results and Discussion The results showed that the full irrigation (100% FC) of cumin in December with 80 plants m-2 had the highest number of umbel per plant, number of seed per umbel and seed yield, whereas, 100-seed weight and plant weight increased and WUE decreased with decreasing density (40 plants m-2) under 100%FC during December. The lowest seed yield were obtained from 75 and 50% FC in April with 40 plants m-2 (53.77% and 52.20% respectively) compared with December with 100% FC and 80 plants m-2. Seed yield were decreased by irrigation levels of 100 and 75% FC in March and April with 80 plants m-2 12.83, 29.24, 21.43 and 35.11%, respectively compared with 100% FC in December (80 plants m-2), despite the highest WUE. Therefore, increasing plant density with medium irrigation in late planting dates (March and April) can partially compensate decreasing cumin seed yield by increasing WUE. The correlation analysis showed that 100-seed weight and plant weight had significant negative correlation with WUE in full irrigation and two levels of water deficit. However, there was no significant correlation between WUE and seed yield under both levels of water deficit and full irrigation conditions. Conclusions The results of this study indicate that seed yield can be improved by increasing plant density when rainfall is proportional to plant water requirement in autumn and winter. In addition, increasing plant density in medium irrigation (75% FC) conditions in late planting dates (March and April) can increase WUE and produce high economic value.