Optimization of Irrigation Intervals and Amount of Super Absorbent in Peppermint cultivation Using Response-Surface Methodology

Introduction

The lack of water in the agricultural sector in recent years and its continuation in the future is an undeniable reality in Iran, and a solution must be sought to deal with it. In order to cope with the effects of drought and water stress on plants, superabsorbent materials may be used. As a result of absorbing and storing water, these materials are able to change the amount or frequency of irrigation water. When it is used to cultivate medicinal plants, such as peppermint, this issue becomes even more critical. To ensure the optimal development of the root structure, aerial parts, and essential oil percentage of the plant, it is necessary to create optimal conditions in terms of providing moisture. It is time-consuming and costly to conduct multiple tests in order to achieve this objective. Thus, simulation and optimization models have been suggested as a solution to this problem. The Response Surface Methodology (RSM) is widely used for optimizing a variety of factors due to its speed and ease of use. The researcher must first prepare the data required for each combination of treatments outlined in this model. Following this, a statistical model is constructed based on the combination of treatments obtained. The next step is to determine the optimal condition for the independent variables so that the dependent variables approach their maximum, minimum, or target value. According to the review of sources, the RSM has been effective in determining the optimal values of factors in the agricultural sector. As a result, it can also be used to optimize the use of superabsorbent for peppermint plants. Due to the lack of research on this issue to date; this study was designed to optimize the use of superabsorbent in different rounds of irrigation to maximize the quantitative and qualitative traits of this medicinal plant.

Materials and Methods

The present research was conducted in the research greenhouse of the Agricultural Engineering Research Institute in Karaj in 2018-2019. Specifically, three irrigation intervals (2, 4, and 6 days) and three weight percent of Aquasource superabsorbent (zero, 1, and 2 weight percent of superabsorbent/soil) were tested. To conduct the experiment, certain amounts of superabsorbent were used in each pot according to the stated weight percentages. Prior to irrigation, the moisture content in each pot was measured using a Lutron Professional Soil Moisture Meter, and irrigation water was applied accordingly. Additionally, this device was calibrated in the laboratory environment before treatment was applied.

The central square design is one type of response surface method. In this method, independent variables are determined to determine the predicted dependent variable through an experimental design. This design considers the average levels of the factors as the central point. Using this method, the experimental treatments are displayed as +1, 0 and -1, which represent the highest, average and lowest levels of the independent variable, respectively.

Results and Discussion

Variance analysis showed that the regression model for water productivity was statistically significant at the five percent probability level and for the other traits studied at the one percent probability level. In contrast, quadratic regression was statistically significant only for root weight, root length, shoot weight, essential oil percentage, and water productivity traits. The regression of other traits was not statistically significant. Therefore, the RSM cannot be used to predict and optimize these traits. Based on the lack of significance of the lack of fit test, the results of regression analysis are reliable compared to variance analysis. The same results were reported by Mansoori et al. (1400). Their study examined the effects of different amounts of irrigation water and fertilizer on sugar beet plants, and found that there was no significant difference between variance analysis and regression analysis. The RSM was also confirmed to be effective in optimizing the traits studied. The overlapping map of the investigated parameters was prepared in the form of Figure (1) in order to determine the optimal limit and common surface. In the upper range of this map, which includes the most irrigation intervals and superabsorbent consumption; Parameters such as water productivity do not change much. This parameter reaches its maximum value in the center of the map. Generally, other parameters tend to reach the optimal limit within the range of the bottom right part of the map, which is highlighted in white. So, in order to optimize the parameters of root weight, root length, shoot weight, essential oil percentage, and water productivity, their target values were determined as 1.1 (kg/m2), 33 (cm), 2 (kg/m2), 3 (%), and 4 (kg/m3).

Conclusion

The effects of different irrigation intervals (3 levels of 2, 4 and 6 days) and Aquasource superabsorbent (3 levels of zero, 1, and 2% by weight of superabsorbent/soil) were examined on some peppermint plant traits. Results showed that the level-response model did not significantly differ from statistical methods. As a result, it is possible to trust the results obtained. The traits of root weight and length, shoot weight, essential oil percentage and water productivity were used in the Response surface methodology (RSM), and all other traits were found to increase with an increase in irrigation water, except for essential oil percentage and water productivity. In terms of essential oil percentage and water productivity, the reduction in irrigation intervals up to +0.8 had an upward trend and then it began to decline. Increasing the amount of superabsorbent negatively affected the characteristics of root weight and length, shoot weight, and essential oil percentage. As the amount of superabsorbent was increased to a range of -0.3, water productivity increased, and then the value of this parameter also decreased. As a result of these conditions, the RSM was effectively used to optimize irrigation interval and superabsorbent amount, and it was determined that the best conditions were obtained by utilizing a three-day irrigation interval with a superabsorbent concentration of 0.3%. Therefore, compliance with these conditions is recommended for peppermint cultivation.