Investigating the effect of some plant growth regulators and planting medium on the rooting and growth of Stevia (Stevia rebaudiana Bert.) cuttings

Stevia is a plant belongs to composite family, and the sweet leaves of this plant contain steviol glycoside, which has no calories, and it can be up to 300 times sweeter than sugar. The stevia plant propagated by seeds, cuttings and tissue culture. The most success method is by tissue culture, in comparison with others. Cuttings may be prepared from stems, roots or leaves. However, the most common method for propagation is taken from stem and use rooting hormones. Auxin compounds such as indole butyric acid (IBA) and naphthalene acetic acid (NAA) are among the most important plant growth regulators, which are widely used on stem cuttings to accelerate the formation of aberrant roots, increase the percentage of rooting and increase the number of roots. Each cutting is used so that some bases that have rooting problems can produce roots with the use of these compounds. The effect of growth regulators IBA and NAA and their combination on rooting and growth in different plants has been proven. Therefore, the aim of this study was to determine the best planting medium and the concentration of IBA and NAA on rooting characteristics in apical cuttings of Stevia.

This experiment was designed in the agronomy laboratory, Campus of Agriculture and Natural Resources, Razi University, Kermanshah, Iran, as a factorial in the form of a completely randomized design (CRD) in three replications. Factors include: soil + perlite + sand, soil + cocopeat + sand, soil + sawdust + sand, soil + cocopeat + perlite, soil + perlite + sawdust, soil + cocopeat + sawdust and soil + sand + cocopeat + perlite + sawdust and concentration of plant growth regulator: control (distilled water), IBA250 mg lit-1, IBA500 mg lit-1, NAA250 mg lit-1, NAA500 mg lit-1, IBA250 mg lit-1 + NAA250 mg lit-1 and commercial rooting powder (containing 0.2% IBA). The cuttings were taken on July 25, 2016 from the apical part, 10 cm long without branches and flowers, with uniform thickness and 4-6 leaves. A number of 10 cuttings for each pot were dipped in plant growth regulators for five seconds and planted after 10 minutes. Planting in pots were placed inside the germinator under conditions of temperature (between 20 and 22 degrees Celsius), humidity (96%) and controlled light (16 hours of light and 8 hours of darkness) to stimulate rooting. After four weeks, some characteristics of the cuttings including stem length, root number, rooting percentage, maximum root length, total root length per cutting, dry root weight per cutting, number of leaves and length of shoots were checked.

The results of variance analysis of the data showed that there was a simple and interaction effect between planting medium and growth regulators on all investigated traits, including shoot length, root number, maximum root length, root dry weight, total root length, number of leaves, and root percentage. were very significant (at the 1% level). The highest number of roots (55 roots per plant) was obtained in the IBA500 treatment and in the soil + cocopeat + sand substrate. However, the highest percentage of rooted cuttings (100%) was obtained in the NAA250 treatment and in the substrate of soil + sand + cocopeat + perlite + sawdust. The number of roots was obtained by counting the number of misplaced roots coming out of the cuttings, and their length and length were not considered. However, in the percentage of rooted cuttings, having even one root for a cutting and the survival of that cutting is meant in counting. Probably, the reason for the increase in the number of roots for each cutting in this treatment is due to the effect of IBA500 hormone in this substrate. In this condition, the level of IBA hormone was at its maximum level. IBA250 treatment and in soil + sand + cocopeat + perlite + sawdust substrate produced the highest root dry weight (74.7 mg per plant) and the highest total root length (145.8 cm). The highest root height (10.33 cm) was obtained using IBA250+NAA250 and in soil + sand + cocopeat + perlite + sawdust substrate. The traits of dry matter and the total length of the root are related, and it is obvious that the longer the length of the root, the greater the amount of dry matter. But the length or height of the root may not match with these two traits, which was the case in this experiment. The height of the root is the length of the longest root. The maximum shoot length or plant height was 12 cm in the IBA250 treatment and in soil + sand + cocopeat + perlite + sawdust. IBA250 treatment and soil + cocopit + sand had the highest number of leaves, 11.6 leaves per plant. These two traits (shoot length and number of leaves per plant) were at the highest value in two different planting beds in IBA250, which indicates the positive effect of IBA250.

In general, the result of this experiment showed that although the highest number of roots was obtained in the treatment of IBA500 and in soil + cocopeat + sand, but cuttings from the apical of the stevia stem were prepared in a mixture of substrates including soil + sand + cocopeat + perlite + sawdust and the use of NAA250 and IBA250 were successful. These results provide an opportunity to choose appropriate propagation methods for future studies.