Investigating the Effect of Different Levels of Irrigation and Humic Acid Foliar Application on the Physiological Characteristics and Indigo of Indigofera tinctoria L.

IntroductionIndigo (Indigofera tinctoria L.) is a flowering plant belonging to the Fabaceae family. This plant has a high ability to grow again after harvesting, and the low sensitivity of this plant to the attack of pests and diseases in hot weather conditions is one of the reasons for increasing the yield of this plant. Global climate changes have led to changes in rainfall patterns and it is predicted that periods of insufficient rainfall will lead to drought with a gradual decrease in water available to the soil of plants, causing an impact on the growth and development or early death of the plant. The unreasonable long-term use of chemical inputs, including fertilizers and pesticides, has increased environmental pollution and in some cases has had a negative impact on the yield and quality of agricultural products. One solution is to use the principles of sustainable agriculture, especially the use of organic inputs in agricultural ecosystems. Humic acid (HA) is an organic biostimulant that significantly affects plant growth and increases crop yield. The purpose of the experiment is to investigate the effect of irrigation intervals and foliar spraying of humic acid on the physiological characteristics and indigo content of Indigofera tinctoria.Materials and MethodsThis research was carried out as split plot layout based on randomized complete block design with three replications during 2019 in the agricultural research institute of Zabol Research Institute in Zahak city of Sistan and Baluchistan province in Iran. The experimental treatments were irrigation intervals of 7, 9 and 11 days as the main factor and five levels of humic acid foliar application as the sub-plot (no foliar application, foliar application with a concentration of 20, 30, 40 and 50 liters per hectare). Irrigation treatments were carried out two months after the planting date. Humic acid foliar application was started after three months from the date of planting in the flowering stage, every 15 days. The foliar spraying was done at the end of the day and at sunset with a back pump sprayer and with the same pressure on the bushes. In order to calculate the leaf yield by removing the marginal effect, the crop was harvested in the last week of November 2019 and kept in the warehouse at room temperature for 23 days. Then the dry leaves were separated and the weight of the leaves was obtained in each plot and its yield was calculated in terms of tons per hectare. Wilson's method was used to prepare ash and extract to measure the concentration of iron, zinc, phosphorus and potassium elements. The amount of photosynthetic pigments was calculated by Arnon's method. To measure guaiacol peroxidase enzyme by Fielding and Hall method, calculate ascorbate peroxidase by Yoshimura method, catalase enzyme by Beers and Caesar method, proline amount in leaves was measured by Bates method. Also, the indigo content was measured based on the method provided by Stoker et al. (1998) and Sales et al. (2006).Results and DiscussionThe results of analysis of variance showed that the effect of different levels of irrigation on chlorophyll a, catalase, iron and zinc was significant. The effect of different levels of humic acid foliar application on chlorophyll a and b, carotenoid, ascorbate peroxidase, catalase, guaiacol peroxidase, iron, zinc, potassium, phosphorus and indigo content was significant. Also, the results of the analysis of variance showed that the interaction effect of irrigation in foliar application of carotenoid humic acid, ascorbate peroxidase, catalase, guaiacol peroxidase, iron, phosphorus and indigo content was significant. The highest (39.32 mg.g-1 fresh weight) and the lowest (24.49 mg.g-1 fresh weight) content of indigo were obtained at the 7-day irrigation interval under the conditions of foliar spraying of 50 liters per hectare and the 11-day irrigation interval under the conditions of no humic acid applicationConclusionThe results of investigating the effect of reducing different levels of irrigation and increasing humic acid foliar spraying on indigo showed that the amount of chlorophyll a and b and zinc increased. Also, the increase in humic acid caused an increase in potassium and blood. At each level of irrigation, it was found that the amount of carotenoid and indigo content increased with the increase of humic acid foliar application. Some other physiological characteristics of indigo such as proline, ascorbate peroxidase, catalase and guaiacol peroxidase showed a decrease in each irrigation level with increasing humic acid foliar application. In future researches, it is possible to consider the comparison of soil application and foliar application of humic acid. In general, for the production of vesme leaves, 9-day irrigation interval in foliar application of 40 L ha-1 of humic acid is suitable; and In order to obtain more indigo content, an 11-day irrigation interval in foliar application of 40 L ha-1 of humic acid seems appropriate.