Auxin, a plant hormone used in propagation to stimulate adventitious roots in cuttings, is the chemical compound Indole-3-Acetic acid (IAA). This naturally occurring hormone has been synthesized from the amino acid L-tryptophan, which is present in young leaves and developing seeds. In 1935 the synthetic compound, indole-3-butyric acid (IBA), was discovered and is now known to occur naturally in maize and other genera (Hartmann et al,. 2002). The polar characteristics of auxin moving from the leaf tips to the base was first hypothesized in 1882 by Sachs, and confirmed in 1929 by Went when he discovered that leaf extracts of Acalypha plants applied to cut Acalypha tissue induced root formation (Hartmann et al., 2002). Therefore, auxin is a hormone activated due to a wounding response when the severance of the cutting from the mother plant occurs. After detachment, auxin accumulates in the base of the stem and promotes adventitious roots formation.
Root zone heating is also a standard industry practice to hasten and improve the rooting of cuttings. By using pipes that circulate hot water or heat mats that warm the root system, the grower is saving on energy costs while mitigating foliage disease. The additional heat increases metabolic processes which in turn further stimulate the formation of adventitious roots. To determine the effectiveness of these practices, various cultivars of Chrysanthemum were subjected to certain variables that illustrated their rooting success after propagation through terminal cuttings. Both the use of auxin and root zone heat have a bell shaped curve that optimizes production success. Extremely low and high levels of each can negatively impact the formation of adventitious roots that are essential for efficient production. The objective of this lab was to establish optimal K-IBA concentrations for rooting, coupled with the presence or absence of root zone heat, with the hypothesis being Chrysanthemum cuttings have a more prolific adventitious root formation when using liquid K-IBA at 2500ppm with the addition of root zone heat.
Materials and Methods
On September 20th, 2016 in the student greenhouse at Longwood Gardens, five Chrysanthemum stock plants were used to collect a total of 480 terminal cuttings. These cultivars were; ‘Autumn Eyes’, ‘Kimmie’, ‘Sozan’, ‘Flair’ and ‘Frosty Time’. Four cuttings of the same cultivar were stuck in 5” Azalea pots that contained a substrate of 50% perlite and 50% Pro Mix HP Mycorrhizae soilless media by volume. The substrate was tested on the same date and indicated a pH of 6.8, while the electrical conductivity registered at 0.27 millisiemens. Before placing four cuttings in individual pots, each was dipped in various concentrations of either liquid powder K-IBA. The controlled group was dipped in ionized water while the other variables are as follow; IBA was applied as talc at concentrations of 0, 4000, and 8000ppm while the liquid IBA was applied at concentrations of 0, 2,500, and 5,000ppm. One group of talc powder and liquid auxin was then put on benches with heat mats and the other on benches without heat mats.
Cuttings in each treatment were placed on either a heat mat set to 85° F, or a regular bench with no additional root zone heat. A total of 480 cuttings were equally distributed over the 12 treatments. The cuttings were then placed under mist on a ten-minute interval that lasted 40 seconds. Greenhouse temperatures were set to a heating point of 60° F during the evening and a cooling point of 55° F during the day. The experiment concluded on October 9th, 2016 during which time root numbers were counted, root lengths were measured, and rooting percentage determined.
The results indicated certain treatments to Chrysanthemum cuttings produced more favorable rooting than others. Overall, table 4 shows ‘Kimmie’ and ‘Autumn Eyes’ had the best rooting percentage at 93.8%. Despite the high rooting percentage, C. ‘Kimmie’ had significantly less number of roots of 25.5 and ‘Autumn Eyes’ had the most roots per cutting at 39.8 as indicated by table 2. It was also evident that cuttings with the application of the liquid K-IBA rooting hormone at 2500ppm had the best results of number of roots per cutting. However, cuttings with no application of powder or liquid K-IBA produced the longest roots as can be seen in table 3. As the concentration of liquid and powder K-IBA was increased, adventitious root number and root length decreased, but this change was not always significant. The cuttings dipped in 8000ppm talc powder and 5000ppm liquid K-IBA produced the least amount of roots and the shortest length of roots. In addition, the application of root zone heating throughout the various concentrations of liquid and powder K-IBA produced compared to zero root zone heat produced the best results as exemplified in tables 2-4.
While the data shows optimal propagation methods for rooting of Chrysanthemum cuttings, other methods may have improved adventitious root number and length, as well as rooting percentage. Prestorage of Chrysanthemum cuttings for twelve days at 50° F enhances the rooting response compared with nonstored cuttings (Hartmann et al., 2002). Another method for better rooted cuttings could be to increase the amount of CO2. Walla and Kristofferson grew Chrysanthemum and Euphorbia stock plants at different CO2 concentrations and found that for certain cultivars, most cuttings were produced at 900ppm CO2 (Jackson 1986). The increase of CO2 at 900ppm showed greater numbers of longer roots than concentrations of 300ppm and 1800ppm CO2 (Jackson 1986).
Similarly, the data collected showed optimal levels of auxin concentrations for root number and length. Cuttings dipped in liquid auxin at 5000ppm and talc powder at 8000ppm significantly reduced rooting percentage. Although the absence of auxin produced the highest rooting percentage, adventitious root formation was still visible. In a commercial operation, the rate of adventitious root production shorten the time to transplant, reducing costs of propagation and therefore increasing profits. While the data showed best rooting practices based on variables like cultivar and auxin concentrations, it did not take into account substrate type, stock plant environmental factors, or other cultural conditions for the cuttings. Therefore, a clear understanding of auxin biosynthesis will ultimately have many significant impacts on agriculture and will also greatly extend our knowledge of fundamental plant biology (Somerville and Meyerowitz 2002).
Overall the data indicated that rooting percentage, root number, and root length is optimized when cuttings are treated with liquid auxin at concentrations of 2500ppm. Despite root zone heat slightly optimizing these results, its effects were not necessary for healthy adventitious root formation. These results confirmed the hypothesis that rooting percentage, root number and root length are best produced when Chrysanthemum cuttings are treated with 2500ppm liquid auxin and root zone heat.
Hartmann, Kester, Davies & Geneve. (2002). Plant Propagation (7th ed.). Upper Saddle River, NJ: Pearson Education.
Jackson, M. B. (1986). New root formation in plants and cuttings. Dordrecht: M. Nijhoff.
Somerville, C. R., & Meyerowitz, E. M. (2002). The Arabidopsis book. Rockville, MD: American Society of Plant Biologists.