Optimization of cow manure and plant density in peppermint (Mentha piperita L.) as a medicinal plant using response surface methodolgy

Optimization of fertilizer and plant density are management approaches to improve resources efficiency and decline environmental pollutions. Response surface methodology (RSM) is defined as a set of mathematical and statistical techniques that are used to develop, to improve or to optimize a product by using Central composite designs. In this work, optimization of cow manure and plant density on biomass yield and essential oil percentage of peppermint as a medicinal plant species using central composite design for RSM was evaluated.

An experiment was conducted using central composite design with 13 treatments and two replications at the Research Field of Ferdowsi University of Mashhad during the growing season of 2017-2018. The treatments were allocated based on low and high levels of plant density (10 and 15 plants.m-2, respectively) and cow manure (0 and 70 t.ha-1, respectively). Fresh weight of stem, dried weight of stem, fresh weight of leaves, fresh weight of leaves, total dried weight and essential oil percentage in two sampling times and two cuttings were measured as dependent variables and changes of these variables were evaluated by a regression model. Lack-of-fit test was used to evaluate the quality of the fitted model. The adequacy of the model was tested by analysis of variance. The full quadratic polynomial equation was tested to determine the significance of the model and the component of the model (linear, squared and first-order interaction terms) and the quality of the fitted model was judged using the determination coefficient (R2).

The results showed that effect of linear and square components were significant on all studied characteristics in two sampling times. Interaction between the two factors of manure and plant density was significant on the studied traits only for the second sampling stage. Lack of fit had no significant effect on the studied traits. The full square model for the response variables gave insignificant lack-of-fit indicating that the data were satisfactorily explained. In the first and the second cuttings, R2 ranges for biomass yield and biomass yield+ essential oil were 70.39-84.62% and 27.36-63.06%, respectively. The highest observed and predicted values of biomass yield were recorded in 12.5 plants.m-2+ 35 t.ha-1 cow manure (755.13 g.m-2) and 15 plants.m-2+ 35 t.ha-1 cow manure (319.92 g.m-2), respectively. The mean yield of biomass yield in the second cutting was higher than the first cutting. Optimum levels of plant density and cow manure and desirability for biomass yield were calculated with 10 plants.m-2+23 t.ha-1 and d= 0.94 and the parameters for biomass yield, and essential oil percentage were with 10 plants.m-2, 49 t.ha-1 and d= 0.96, respectively.

In general, it seems that resource use optimization using RSM may be suitable cropping approach for sustainable production of medicinal plants in agroecosystems.