To find more economical substrates than those used in the rooting of the mint species, this study conducted a rooting experiment with substrates mixed with peatmoss based on the low-cost saprolite (10t/about 150 thousand KRW) that is relatively easy to obtain in Korea. Total 5 types of substrates were used in the experiment: saprolite, peatmoss, and three mixed substrates with different ratios of saprolite (Sa):peatmoss (Pe) such as 75:25, 50:50, 25:75 v/v. The result of particle size analysis of saprolite showed that it was typical saprolite including 22.68±1.94% gravels (2 mm or bigger), and Blumenerde (Germany, GRAMOFLOR Co.) was used for peatmoss. 90 cutting slips, 30 each with 3 repeats for 3 treatment plots (10 per treatment as 1 repeat), were cut for each of the five types of substrates, adding up to total 450. The 105 cell plug tray was used, and management after cutting was done in accordance with custom. Two weeks after cutting, the rooting rate, number of roots for each individual, root length, and root fresh weight were measured. The ones longer than at least 1mm were used to measure root length. To measure root fresh weight, the roots were separated and placed on a paper towel, and dried by gently pressing for 5 seconds, and then measured by 0.001 g using an electronic scale. A new paper towel was used for every measurement to avoid weight difference caused by water absorption.

Considering the cost reduction in propagation by cutting on saprolite in Experiment 1, saprolite was used by itself as solid medium (using 105 cell plug tray), and water (tap water in Seoul) that can be easily applied in home gardening was used as non-solid medium to compare the rooting patterns of mints. The water medium was fully replaced twice a week. Three types of cutting slips were divided into top stem with the growing point (removing all except 2-3 leaves including the growing point) and low stem (stem right below the top part that is cut off, removing all except 2-3 leaves), and total 180 cutting slips were cut on 2 types of substrates, 5 each for 3 repeats. Two weeks later, the cutting slips were dug out or taken out from the water, and rooting rate, number of roots, root length, root width, and root fresh weight were measured as in Experiment 1. Root thickness was measured by 0.01 mm using a caliper.

To determine the effects of rooting promoter (Rootone) on rooting in solid medium saprolite, cutting slips were divided into top part and low part, and put into the 105 cell tray filled with saprolite. To keep the base of the cutting slips from drying, leveling was done for 1 hour and cutting slips were dust coated in a customary method using Dongbu Rootone (1-naphthylacetamide 0.4%), after which they were put into the cutting bed. Total 270 cutting slips were cut, with 10 each for 3 repeats in the top part and 5 each for 3 repeats in the low part per mint. The same treatment and management was done as before. Two weeks later, rooting rate, number of roots, root length, root width, and root fresh weight were measured.

Since there is not enough literature to review regarding the rooting patterns with the Rootone treatment in liquid medium water, a preliminary test is conducted regarding the method and time for Rootone treatment on cut areas. In the test, cutting slips were put into water right after dust coating with Rootone, and most of Rootone fell apart into the water. Thus, to come up with a way to keep Rootone from falling apart from the stem for 2 weeks, we did not put the cutting slips into the water immediately after dust coating. Instead, we left them for 5-10 minutes so that Rootone gets stuck to the plant base before putting in the water. Rootone did not come off from the cutting slips in the cutting bed after leaving for 10 minutes, and it remained that way in the water for 2 weeks during the experiment.

Based on this result, we cut the stems in 45 degrees and coated the cut areas with Rootone dust after 1 hour of leveling, and put the cutting slips into the cutting bed after 10 minutes so that the dust sticks to the stem. They were put in the same way as fixing the plants on the growth cabinet for deep flow culture (Park and Lee, 1999), fixing the cutting slips with a sponge for rooting. Total 90 cutting slips were cut, 5 each for 3 repeats. Two weeks after the treatment, rooting rate, no. of roots, root length, root width, and root fresh weight were measured.

Each item was repeated 3 times, and for statistical analysis, significance test (0.5%) was performed through between-treatment Turkey multiple range test using SAS Program.

We tested the effects of mixed substrates of saprolite and peatmoss on the rooting of 3 labiatae plants, and discovered that saprolite showed highest rooting rate and number of roots for peppermint and catmint, whereas rooting rate and number of roots were highest for spearmint in Sa:Pe 75:25, followed by saprolite. On the other hand, peatmoss showed the lowest rooting rate and number of roots (Table 1). Root length was high in Sa:Pe 75:25, 50:50 for both spearmint and peppermint. However, root fresh weight was 0.180 g in saprolite and 0.170 g in Sa:Pe 75:25 for spearmint, showing almost no difference, whereas for peppermint, root fresh weight was 0.310 g in saprolite and 0.230 g in Sa:Pe 75:25, showing a statistical difference. As for catmint, root fresh weight was 0.060 g in saprolite but 0.010g in Sa:Pe 75:25, also showing a statistical difference (Table 1). It has also been reported in a test on various mixtures of peatmoss with field soil made up of soil cement of reclaimed land that rosemary takes root best in mixed substrates than peatmoss alone (Han, 2001). The fact that number of roots, rooting rate and root fresh weight turned out to be higher in saprolite than other treatments suggests that important factors such as optimum moisture and void were better than other substrates. In general, saprolite in Korea is known to have a very high content of phosphoric acid (526 mg kg^-1) (Park et al., 2004), which implies that phosphoric acid may have had a positive effect on the initial formation of roots, and it has also been reported that phosphoric acid is highly effective for corn root growth (Mackay and Barber, 1986). As it has been reported that substrates with a lot of peatmoss rather hindered rooting due to high water holding capacity, moisture and oxygen perform a key role in rooting requirements, and thus the air permeability of the substrate determines the rooting of cutting slips (Park and Lee, 1999). Saprolite used in this experiment has 22.68±1.94% gravels at least the size of 2 mm, and higher percentage of void than other mixed substrates may have served as a positive factor for rooting. The difference in rooting depending on the substrate properties in cutting is greatly affected by the void percentage of soil, and high water holding capacity rather hinders rooting (O’Neil and Carrow, 1983). The root length was higher in the treatment mixed with peatmoss than saprolite due to the difference in soil compaction. The physical properties of soil change the root length or root shape of carrots (Olymbios and Schwabe, 1977). Therefore, due to the physical resistance in saprolite, root length may have been suppressed than in substrates mixed with peatmoss.

Greater significance of this experiment is that it provided the possibility to use saprolite instead of peatmoss substrate for cutting substrate, which is imported at a high price by herb or horticultural farms (Park and Lee, 1999). If farms that mass-produce herb seedlings in Korea can purchase unpolluted saprolite to enhance the competitive edge of herb seedlings by lowering the prices and achieve effective rooting, it can be a good idea to use saprolite by itself or mixed with other treatments for vegetative propagation of herbs.

Developing simpler and more convenient methods of herb rooting in home gardening will promote health through cultivation, with health effects such as aromatherapy in the broad sense. In this perspective, we conducted a rooting test using saprolite that brought good results in Experiment 1 as the solid medium (control medium), and water as the liquid medium.

The results showed that the number of roots was generally lower in water than saprolite for the top part of spearmint, but for the low part it increased to 15.3 in saprolite and 20.2 in water. Root length was longer in water for both spearmint and peppermint. For peppermint, both the top part and lower part showed a statistical difference in the number of roots in water (Table 2). As such, non-solid medium showed good effects on rooting and root length for spearmint and peppermint, whereas catmint did not show rooting on both the top part and lower part. As the Encyclopedia of Herbs by Bown introduces 25 types of mints and mentions that Mentha aquatica (watermint) shows good growth by the water (Bown, 1995), mints genetically tend to endure well underwater due to the differentiation of various species. On the other hand, catmint comes from a relatively dry area of Nepi near Rome and thus it has high drought tolerance but low moisture tolerance, thereby showing almost no rooting underwater. Spearmint and peppermint showed poor rooting on the lower part in saprolite, and this result is consistent with the study on parts of rosemary and lavender, which proved that top part stem cuttings including the growing point showed better rooting than the low part (Lee, 1999; Moon, 2015). This is generally due to the physiological properties of plants that have higher auxin content on the growing point than the lower part (Park and Lee, 1999).

In general, there is a difference in rooting depending on the amount of irrigation after cottage on solid medium due to the reaction between the moisture content difference in the substrate and the void (Park and Lee, 1999), and this may bring different results depending on the plants. But since the initial symptom of the leaves drying after cottage did not appear in water, it seems like a good method that can be applied to small home gardening.

To find a better way to promote rooting with saprolite that showed a significantly high rooting rate in Experiment 1, we experimented with Rootone treatment and obtained the following results. Both the top and low parts of the three types of cutting slips showed a remarkable increase in rooting in the treatment plot compared to control medium (Table 3, Fig. 1, 2). For spearmint, 15.3 top part cutting slips were rooted in control medium, while 41.1 cutting slips were rooted in Rootone treatment, which is three times more. The same results were found in low part cutting slips of spearmint and peppermint. For catmint that showed poor rooting in the previous experiment, 1.9 top part cutting slips in control medium increased to 7.1 in Rootone treatment, which is more than a three-fold increase.

As already well known, this is the result of rooting facilitated by Rootone, which is consistent with studies on Ficus (Kwack et al., 1989), oregano and marjoram (Kim, 2001), and rosemary (Han, 2001). This may be due to the fact that treating cutting slips with phytohormone (IBA) sends the substances required for the formation of adventitious roots to the base from the top part (Hartmann et al., 2010). Rooting on parts did not take root is a result of facilitating the generation of adventitious roots on tissues that are somewhat lignified, as the rooting promoter increases the auxin content in the plant cutting slips.

The fact that catmint, which shows poor rooting in general cutting, shows favorable rooting with Rootone treatment may serve as a good outcome that can be applied to herb farms or those with gardening as a hobby. Root length increased by more than 2-10 times after Rootone treatment, and root fresh weight also increased (Table 3). This is consistent with the finding of the rooting experiments (Im, 2011) on various herb species including woody herbs like rosemary (Han, 2001), herbaceous herbs like oregano and marjoram (Kim, 2001), and stevia (Chang, 2011), which discovered that Rootone promotes more effective root growth than other rooting promoters.

In particular, this study has great significance in that it provided the possibility that herb seedling farms can produce economical and strong seedlings if they use saprolite that is much lower in cost than peatmoss and perform cutting with Rootone.

There have been no rooting test results in the water thus far, and herbs in particular had no cases in Korea. Thus, this study is significant in providing results of facilitating underwater rooting more easily in home gardening.

Compared to 3.2 for spearmint and 16.2 for peppermint in the control group, there were 46.8 for spearmint and 59.6 for peppermint in Rootone treatment, showing significant results by 15 times for spearmint and 4 times for peppermint. Catmint, which showed no rooting at all in non-solid medium test in Experiment 2 above, showed 1.3 roots. In other words, number of roots, root length, root width, and root weight all showed a statistical difference in Rootone treatment compared to control medium (Table 4 and Fig. 3).

The effects of Rootone in many solid mediums that have been carried out thus far (Kwack et al., 1989; Han et al., 2001; Kim, 2001; Han, 2001; Moon, 2015) were also found in liquid medium like water. However, for water rooting, attempts in terms of home gardening seem more appropriate than commercial use, and as it has been reported that oxygen and temperature of rooting zone in cutting have a great impact on rooting (Gislerod, 1982), water rooting treatment in mass propagation may raise other technical issues such as dissolved oxygen and water temperature by period, and thus requires various approaches in the future.