Within stock plant production systems, herbaceous perennial crops need to produce enough vegetative material to justify the cost of the space that they occupy on the greenhouse bench. Although harvesting cuttings before bloom time is not preferred (Walters, 1982), taking cuttings year-round is a challenging task because perennials flower at different times during the year. Studies have shown that the reproductive tissue on cuttings can inhibit root and vegetative development (Gibson and Cerveny, 2005); therefore, it is highly desirable to keep stock plants in a juvenile or vegetative state of development. Because the success of propagating vegetative cuttings will depend on the propagator’s judgment (Wells, 1971), producing stock plants with more available vegetative material during the growing season would certainly increase the propagator’s options.
To achieve target uniformity, perennial producers often prune their plants manually, or, although less common, the use of plant growth regulators to enhance vegetative plant growth and remove reproductive tissue (Preece and Read, 1993). However, manually pruning perennials to encourage vegetative growth can be labor intensive (Banko and Stefani, 1996) and add additional unwarranted expense (Holland et al., 2007). Applying plant growth regulators, on the other hand, is generally less labor intensive than manual pruning, although there is a chance it can cause phytotoxicity in certain crops (Meijón et al., 2009). Three specific plant growth regulators have been identified for their potential benefits in stock plant production: gibberellic acid, benzyladenine, and ethephon.
Gibberellic acid 4 and 7 (GA4+7) is a plant growth regulator that has been isolated from species of the fungus, Gibberella fujikuroi, and were first found in Japan in 1926 by E. Kurosawa (Salisbury and Ross, 1969). GA4+7 promotes growth primarily through cell enlargement that is uniform throughout plant tissue. GA4+7 also hydrolyzes starch into sugar, which, in turn, provides energy and encourages uptake of water by cells; cell wall elasticity is another result of GA activity in the cell (Moore, 1984). Therefore, the application of GA4+7 to promote plant growth could result in more available propagation material from stock plants.
Cytokinins, specifically benzyladenine, have been found to be involved in nearly all aspects of plant growth and development (Leopold and Kreidermann, 1975). Benzyladenines are known for cell enlargement, not cell elongation as with auxins and GA. They promote cell growth in all directions (Preece and Read, 1993), which can result in the decrease of apical dominance if cytokinin levels in the plant are elevated (Hartmann et al., 2002). Therefore, increased lateral growth could be used to obtain more propagation material from stock plants.
The third plant growth regulator, ethephon, is an ethylene inducer that enters the plant and breaks down into three molecules: phosphate, chloride, and ethylene. These molecules are released into the plant systems, effecting plant growth and reproductive development (Preece and Read, 1993). Ethephon is widely used to promote auxiliary shoot development without damage to the apical meristem (Hayashi et al., 2001). Therefore, an increase in branching and a decrease in flower development could result in significantly more vegetative growth on herbaceous perennial stock plants.
Herbaceous perennial responses to plant growth regulators can vary across cultural and environmental conditions (Cochran and Fulcher, 2013). Previous research that involved herbaceous perennial liners with applications of 300, 600, 900, or 1200 ppm benzyladenine showed increased branching on ‘Ruby Star’ coneflower (Echinacea) at rates as low as 300 ppm (Latimer et al., 2011), while ‘Silver Lode’ coral bells responded minimally to an application of 600 ppm benzyladenine for both plant height and width (Latimer and Freeborn, 2015). The wide range of possible plant responses indicates the importance of studying two determining factors: 1) the rate and application timing based on the target crop and 2) the effect on plant growth the producer is trying to achieve (Cochran and Fulcher, 2013).
The herbaceous perennials used in these studies were ‘Snow Angel’ coral bells (Heuchera sanguinea) and Orange Carpet hummingbird trumpet (Epilobium canum ssp. garrettii ‘PWWG01S’). During meetings with greenhouse and nursery operators who are propagating the two herbaceous perennial plants, they identified two main production problems with both ‘Snow Angel’ coral bells and Orange Carpet hummingbird trumpet: 1) the lack of vegetative propagation material from stock plants and 2) low rooting percentage rates during propagation. On the basis of previous research, it was determined that using plant growth regulators could resolve both problems.
The overall objective of these studies was to evaluate vegetative and floral development of ‘Snow Angel’ coral bells and Orange Carpet hummingbird trumpet after applications of three commercially available plant growth regulators with the active ingredients: GA, benzyladenine, or ethephon. The hypothesis of this study was that applications of plant growth regulators would result in increased vegetative propagation material from ‘Snow Angel’ coral bells and Orange Carpet hummingbird trumpet stock plants and not affect rooting success.
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