Effect of Planting Methods and Water Consumption on Quantitative and Qualitative Traits of Roselle (Hibiscus sabdariffa L.)

IntroductionRoselle (Hibiscus sabdariffa L.) is a medicinal plant belonging to the Malvaceae family that has been used as medicine since ancient times. Sepals of roselle are a source of anthocyanin, vitamin C and other antioxidants, such as flavonoids (hibiscetine, gossypetine, and sadderetine). Due to the fact that drought is one of the prominent geographical features of Iran, saving water by cultivating drought-resistant crops can be considered as a solution to deal with drought. However, it should be noted that applying deficit irrigation without careful planning can lead to a reduction in quantitative or even qualitative yield. Therefore, due to the medicinal importance of roselle and its resistance to drought, the aim of this study was to investigate the effect of different irrigation regimes and planting method on some quantitative and qualitative traits of roselle plant.Materials and MethodsThe experiment was carried out as split plot based on randomized complete blocks design with three replications at the Research Farm of Kashmar Agricultural and Natural Resources Research Station, during two growth seasons 2016 and 2017. Treatments included seven levels of Irrigation (I1: 100% crop water requirement (CWR), I2: 80% CWR from 8-leaf stage to end of flowering, I3: 80% CWR from 8-leaf stage to beginning of flowering, I4: 80% CWR from beginning to end of flowering, I5: 60% CWR from 8-leaf stage to end of flowering, I6: 60% CWR from 8-leaf stage to beginning of flowering, I7: 60% CWR from beginning to end of flowering) and two level of planting method (direct and indirect) as main plots and sub plots, respectively. In the transplanting method, the seeds were sown in the transplant tray on March 10, 2016 and 2017 and transplanted to the farm in four-leaf stage. Direct cultivation was also carried out in April, in 2.5 × 7 meter plots at a depth of 1.5 cm. At the harvest time, two square meters of each plot were harvested to measure number seed and sepal yield, biological yield, and harvest index. In addition, qualitative traits including vitamin C, anthocyanin, total phenol, seed protein and oil were measured. Data from the two field experiments were analyzed with SAS software; obtained averages compared with using Tukey's Studentized Range (HSD) Test at the 5% probability level.Results and DiscussionThe results showed that the interaction effect between irrigation and planting method was significant on biological yield, vitamin C, total phenol, seed protein and oil. The highest seed oil (21.25%) was obtained in I1 and transplanting treatment. I7 and direct planting method treatment had the highest total phenol (41.96 mg g dry weight-1) and seed protein (31.32%). The maximum biological yield (9064.33 kg ha-1) and vitamin C (32.35 mg 100 ml-1) were observed in I1 and direct planting method and I4 and direct planting method, respectively. There was no significant difference in any of the irrigation levels between the two planting methods except for the total phenol at two levels of I5 and I6, which its amount was higher in the direct planting method than transplanting. Seed yield, sepal yield, seed harvest index, sepal harvest index, and anthocyanin were only affected by the main effect of irrigation. As the intensity of stress increased, the yield and harvest index decreased and the amount of anthocyanin increased. The highest seed (667.54 kg ha-1) and sepal yield (392.73 kg ha-1) were obtained in I1 treatment, which had no significant difference with I2 treatment. The lowest seed and sepal yield were also obtained in I7 treatment. According to the harvest index, the only I7 treatment showed a significant difference with other treatments. So that the lowest of seed (7.85%) and sepal harvest index (4.5%) were obtained in this treatment.ConclusionThe results of this study revealed that water stress had the negative effect on the sepal, seed, and biological yield and number of fruit per roselle plant in both transplanting and direct sowing methods. However, with increasing drought stress, the plant's quality traits, including vitamin C, anthocyanin, total phenol, and seed protein improved. The extent of drought effect varied depending on the growth stage of the plant; however, there was no significant difference in seed and sepal yield obtained in stress-free and irrigation at 80% water requirement from 8-leaf stage to end of flowering treatments. The most significant impact was observed during the sensitive growth stage of the plant, particularly from early to late flowering, under drought conditions. Additionally, the results indicated that the transplanting method did not significantly enhance the studied traits at various irrigation levels compared to direct cultivation. Based on the climatic conditions of the study area, irrigating at 80% of the water requirement from the eighth leaf stage to late flowering of the roselle plant appears to be a viable strategy for achieving satisfactory quantitative and qualitative performance while conserving water resources.