Optimizing Cutting Physiology, Morphology, and Rooting with Supplemental Irradiance and Root-zone Heating
Propagative young plant production for the U.S. alone has a wholesale value of over $585 million. The goals of propagators include producing high-quality rooted cuttings by maximizing root growth, total mass, root-to-shoot mass ratio, and stem diameter while minimizing production time and stem elongation. Previous research indicated growth and development of adventitious roots is strongly influenced by root-zone temperature. We have revealed the importance of photosynthetic light (Lopez and Runkle, 2008; Currey, Hutchinson, and Lopez, 2012), and that subsequent growth and development of rooted cuttings is impacted by the environmental conditions experienced during propagation (Hutchinson, Currey, and Lopez, 2012). The specific objectives of this ongoing research are to identify the growth, morphological, and physiological responses of cuttings (herbaceous annuals, perennials, and ornamental grasses) to varying photosynthetic daily light integrals (DLIs) and supplemental light (SL) quality, temperature, and nutrition. We have shown that increasing the DLI during propagation to ≈10 to 12 mol∙m–2∙d–1 with SL from high-pressure sodium (HPS) lamps increased root and shoot growth of nine herbaceous annuals by up to 1100% and 380%, respectively (Currey, Hutchinson, and Lopez, 2012). Increasing the DLI during root development enhanced biomass accumulation and altered biomass allocation by increasing the root mass ratio by up to 200% (Currey and Lopez, 2012), while starch concentrations increased by up to 950%. Additionally, net photosynthesis (Pn) of cuttings increased by 150% and 250% as DLI increased by ≈6 or 12 mol∙m–2∙d–1, respectively. There were few significant morphological or physiological differences among rooted cuttings propagated under HPS lamps and light-emitting diodes (LEDs) of different light quality; as well as for flowering plants grown from cuttings propagated under the different SL qualities (Currey and Lopez, 2013). Lastly, by using SL during propagation, a one acre Indiana GH can reduce its heating costs by up to 25% ($11,000) during three weeks of propagation and 27% ($6,800) during finishing, respectively. [Funding Provided by the USDA-SCRI, Fred C. Gloeckner Foundation, Four Star and Pleasant View Gardens, and the Indiana Flower Growers Association.
Light-emitting Diodes for Supplemental and Sole-source Lighting
Foliage of red leaf lettuce and ornamental foliage, such as purple fountain grass (Pennisetum setaceum), is often green and not visually appealing to consumers when produced under the low-light GH conditions found in the northern latitudes. Our objectives were to quantify the effect of end-of-production (EOP; prior to harvest or shipping) LED SL of different qualities and intensities on foliage color of these crops. Using a colorimeter we determined that EOP SL providing 100 μmol·m–2·s–1 of 100:0, 0:100, or 50:50 (%) red:blue (R:B) light for ≥5 d resulted darker foliage (↓L*) and change from green to red (↑a*) and yellow to blue (↑b*) (Owen and Lopez, in press). Therefore, growers could place their crops under high-intensity R and B LEDs for 5 to 14 days prior to shipping to enhance anthocyanin synthesis and thus foliage color and consumer appeal.
We have also been working to quantify the impact of light quantity, quality, and duration from LEDs to identify production strategies for GH and indoor plant factory producers of ornamental young plants and microgreens. For the majority of the bedding species studied, seedlings grown under LED SL with a light ratio of 85:15 R:B light were generally more compact with a larger stem diameter, higher sturdiness quotient, and higher relative chlorophyll content than those under HPS lamps (Randall and Lopez, 2013). Alternatively, LEDs may be used for sole-source photosynthetic lighting (SSL) to grow seedlings and microgreens in indoor high-density multi-layer controlled environments. Recently we have determined that seedlings grown under a light ratio 87:13 R:B light from LED SSL providing a DLI of 10.5 mol∙m–2∙d–1 were of similar or greater quality compared to those under SL; indicating that LED SSL could be used as an alternative to traditional GH seedling production (Randall and Lopez, in press). Additionally, we have determined that SSL providing a DLI of 18 mol∙m–2∙d–1 and light ratios of 87:13 R:B or 84:7:9 R:FR:B, significantly increased total anthocyanins of kohlrabi microgreens compared with those grown under 74:18:8 R:G:B light. However, under lower DLIs, mustard microgreens had higher alpha- and beta-carotene and lutein levels (Gerovac, Craver, Kopsell, and Lopez, submitted). The results from these studies can help Brassica microgreens growers select light qualities and intensities from LEDs to achieve preferred growth characteristics and increase pigments and phytochemicals associated with human health benefits. [Funding Provided by the USDA-SCRI, Philips Lighting, and Hort Americas].
Using High Tunnels and Root-zone Heating to Produce Cold-tolerant Bedding Plants
Today, energy for heating accounts for 10 to 30% of the total operating cost of commercial greenhouses and these costs are expected to increase significantly in the near future. High-tunnels (HTs) and root-zone heating (RZH) are energy-efficient technologies used for protected and controlled environment production of specialty crops. However, limited research-based information is available regarding HTs (Currey, Mattson, and Lopez, 2014) or RZH for energy-efficient bedding plant production. The specific objectives of this research were to quantify the effects of: 1) transplant dates and row covers in an unheated HT compared to a heated GH on growth and development of cold-tolerant bedding plants and 2) five RZH temperatures in combination with reduced GH air temperature compared to a commercial control on growth and development of cold-tolerant, - intermediate, and -sensitive bedding plants and poinsettia. In one study comparing growth in the HT to the GH, dianthus and petunia transplanted in week 13 were 33% and 47% shorter and had 51% and 31% more visible buds, respectively (Gerovac, Mattson, and Lopez, in press). However, there was a one week delay in time to flower (TTF) for both species in the HT, compared to the GH. In a second study, as RZH temperature set-points increased, TTF of all cold-tolerant and -intermediate species decreased; however, TTF was delayed in all RZH treatments with reduced air temperature compared to the commercial control. For example, TTF of petunia and marigold was delayed by 4 d when grown with a RZH set point of 27 °C (Gerovac and Lopez, in press). Overall, the results obtained from these experiments indicate that HTs and RZH can be used for bedding plant production and could reduce energy costs. Further research into these two objectives is ongoing within our program. [Funding provided by the USDA Specialty Crop Block Grants, Purdue Agriculture Mission Orientated Grant, and the College of Agriculture Research Initiative].
Reduced Temperature Finishing of Poinsettias
The potted holiday poinsettia is a high-input, floriculture crop that is produced in GHs throughout the U.S. from July to December. In 2007, numerous GH operations producing poinsettias were forced to stop growing their crop or went out of business due to a large increase in energy costs. Brian Krug from the University of New Hampshire and I worked with poinsettia breeders to find cultivars that were tolerant of reduced temperature finishing. We selected eight red poinsettia cultivars based on their early response attributes (initiate and finish within 6 to 8 weeks), moderate to high-vigor, and naturally large bracts. We determined that time to anthesis increased by 2 to 15 d and 15 to 22 d, as finishing temperature was reduced during the day/night by 3/2 and 4/6 °C (12 h/ 12 h), respectively (Camberato, Krug, and Lopez, 2012). Based on these results, growers can now save between 15 to 30% by starting the crop one to two weeks earlier, when heating is not necessary. In mid-October, they can begin finishing the crop at 20/13 ºC or 21/17 ºC. Additionally, we determined that poinsettias grown under reduced temperature finishing were of higher-quality (more intense color and longer post-harvest life) and reduced need for chemical growth regulators (Lopez and Krug, 2009). In 2013, we received the Alex Laurie Leading Floriculture Publication Award from AmericanHort for addressing a significant grower challenge. Currently, we are investigating whether growers can further lower their air temperature set points by using RZH. [Funding Provided by the Fred C. Gloeckner Foundation, the American Floral Endowment (AFE), Ecke, Syngenta Flowers, Dummen, and Selecta].
Plant Growth Regulators on Greenhouse Crops
Plant growth regulators (PGRs) are an important tool in ornamental plant production. For many containerized crops, plant height or stem length must be controlled to produce a plant that is both aesthetically appealing when sold and meets size specifications for shipping and/or display. In order to meet these requirements PGRs are applied to control stem elongation. A number of commercialized chemical retardants, and several in development, are available for this purpose. Additionally, there are a variety of application methods (sprays, drenches, liner dips, bulb soaks) that interact with other factors (timing, growing substrate composition, etc.) that affect chemical efficacy. Our goal is to evaluate and develop successful protocols for applying PGRs with a variety of methods to curb excessive stem elongation. In addition to controlling stem elongation, PGRs can be used to increase branching or reduce water loss. As new products are developed, we evaluate the chemicals on important crops to determine what will be successful for a commercial ornamental crop producer. [Funding Provided by FINE Americas, SePro, OHP, Syngenta, and Valent].