The Effect of Mechanical Stress on Some Morpho-Physiological and Biochemical Characteristics of Coleus blumei Benth.

Coleus (Coleus blumei Benth.) from Laminaceae family is an herbaceous, fast growing plant with colorful and decorative leaves which is used as an ornamental and pot plants. Producing short and compact potted plants is valuable in floral industry. A chemical procedure has long been used for this purpose. However, chemical procedure is environmentally unsafe, while it is expensive. Therefore, researches are carried out to find cheaper and safer methods. Several researches have been done on height control, such as genetic manipulation, temperature management, light quality manipulation, controlled water deficit and withholding nutrients. Controlled mechanical stresses are included the potential non-chemical methods. In the present research, using a laboratory vibration simulator, the effects of vibration stress with different frequencies and durations had been examined on growth control of coleus. The experiment was conducted at the research greenhouse of Lorestan University, Iran in 2017. Uniform rooted cuttings of Coleus were transplanted in plastic pots (15 cm diameter and height) that filled with equal proportion of soil, sand and cow manure. The experimental design was a factorial based on a completely randomized design with three replications. After plant establishment, the treatments were applied every morning, with 7.5, 10 and 12.5 Hz frequencies and 0, 5 and 10 min durations, using a laboratory vibration simulator. Treatments application lasted for four weeks and then, plant height, stem diameter, total number of leaves per plant and leaf area, total number and length of side shoots, root volume and length, leaf, stem and root fresh and dry weights, relative water content, electrolyte leakage and the amounts of photosynthetic pigments were assessed. The results showed that interaction effects of frequency and duration of mechanical stress were significant at the 0.01 level in plant height, the total number and length of side shoots, root volume, leaf, stem and root fresh and dry weights, relative water content, the amounts of photosynthetic pigments, while it was not significant on electrolyte leakage. The main effects of duration of mechanical stress were significant at the 0.01 level in total number of leaves per plant and leaf area. Mean comparisons showed that with increasing frequency and duration of mechanical stress, plant height, total number of leaves per plant and leaf area, the total number and length of side shoots, root volume and length, leaf, stem and root fresh and dry weights and relative water content decreased. The shortest plant height was found in plants that treated with 12.5 frequency for 10 min which was 31% shorter than control plants. The main effects of frequency and duration of mechanical stress were significant at the 0.01 level in stem diameter, while their interaction effect was not significant. Stem diameter reduced with increasing stress duration. The stress effect on stem diameter was similar in 7.5 and 10 Hz frequencies. However, stem diameter was lower in plants treated with 12.5 Hz. Mechanical stress decreased the total number of leaves per plant and leaf area and the effects of 5 and 10 min duration were similar. With increasing frequency and duration of mechanical stress, the amounts of photosynthetic pigments increased; so that, the highest amount of those were found in plants that treated with 12.5 Hz frequency for 10 min. According to the results of the present research, vibration stress could control plant growth in Coleus. Stress treatments with all frequencies and durations, decreased plant height and other growth parameters i.e. the total number of leaves per plant. Leaf area, root length, plant biomass and relative water content were also decreased, while, electrolyte leakage was not significantly affected. At the same time, mechanical stress increased the amounts of photosynthetic pigments. Plants treated with 7.5 Hz frequency for 5 min showed 16% shorter plant height compared with controls, while other plant growth characteristics including stem diameter, leaf area, electrolyte leakage, MDA content and the amounts of photosynthetic pigments were similar to those in control plants. Therefore, this treatment could be recommended as a good vibration treatment for practical use. However, based on the production conditions, other vibration frequencies and durations could be chosen. In total, vibration stress using a vibration simulator could be a proper method for producing short and compact pot plants due to its cheapness and being easy to use and an environmentally friendly method.