Effect of Deficit Irrigation at Different Stages on Physiological Traits and Water Use Efficiency of Roselle (Hibiscus sabdariffa L.) under Two Planting Methods

The increasing demand for medicinal plants in traditional medicine as well as the pharmaceutical industry has created the need for some plants to be grown commercially, but the lack of soil moisture poses a serious threat to their production. Planting method can affect the emergence and growth rate of crop and lead to decrease water consumption and increase irrigation water efficiency as yield increases. Roselle (Hibiscus sabdariffa L.) is one of the drought tolerant plants. Saving water consumption by cultivating drought tolerant plants has been proposed as a strategy to combat drought. It should be noted, however, that irrigation without proper planning can reduce the growth and production of crops. In this regard, determining time of deficit irrigation with minimum damage is an appropriate solution that achieves optimum yield while saving water consumption. The aim of this study was to investigate the effect of different irrigation regimes and planting method on some physiological traits, yield and water use efficiency of Roselle plant.

The experiment was conducted 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. Irrigation (in seven levels i.e. I100: 100% crop water requirement (CWR), I180: 80% CWR from 8-leaf stage to end of flowering, I280: 80% CWR from 8-leaf stage to beginning of flowering, I380: 80% CWR from beginning to end of flowering, I160: 60% CWR from 8-leaf stage to end of flowering, I260: 60% CWR from 8-leaf stage to  beginning of flowering, I360: 60% CWR from beginning to end of flowering) and planting method (in two levels i.e. direct and indirect) were in 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.In order to measure physiological traits, sampling from the youngest leaves of plants under stress and control conditions was carried out at the maximum flowering stage. Physiological traits including chlorophyll a, chlorophyll b, carotenoid, leaf relative water content and proline content were measured using these leaves. At the harvest time, two square meters of each plot were harvested. Then, seed and calyx yield were measured and using the measured amount of water consumed, water use efficiency based on seed and calyx were calculated.Data were analyzed with SAS software; obtained averages compared with using Tukey's Studentized Range (HSD) Test at the 5% probability level.

The results showed leaf relative water content was affected only by irrigation and planting method and seed and calyx yields were affected only by irrigation. The leaf relative water content in seedling planting method was higher than direct planting. The highest relative leaf water content (84.66%), seed (667.54 kg ha-1) and calyx yield (392.73 kg ha-1) were obtained in I100 treatment that for yield had no significant difference with I801 treatment. The lowest these traits were also obtained in I360 treatment.The interaction effect between irrigation and planting method was significant on chlorophyll a, chlorophyll b, carotenoids, leaf proline content, seed and calyx water use efficiency. In most irrigation levels, transplanting improved these traits compared to direct sowing. I100 and transplanting treatment had the highest chlorophyll a (0.31 μmol g-1 fw), chlorophyll b (0.16 μmol g-1 fw) and carotenoids (0.56 μmol g-1 fw) and I360 and transplanting treatment had the highest leaf proline content (7.95 μmol g-1 fw). The highest seed (0.15 kg m-3) and calyx (0.08 kg m-3) water use efficiency were obtained in I160 and transplanting treatment and I180 and transplanting treatment was located in the second level. I360 and direct planting method had the lowest seed (0.06 kg m-3) and calyx (0.03 kg m-3) water use efficiency.

The results of this study revealed that deficit irrigation at 80% of the crop water requirement from 8-leaf stage to the end of flowering along with transplanting method was more effective in saving irrigation water with a good yield of the Roselle plant compare to treatment of 100% of the crop water requirement.