Leading with light

Art: © med photo studio | Adobestock

Petunia is the top selling flowering bedding plant in the United States. Greenhouse growers may use a variety of strategies to make sure their plants flower on schedule and reach marketable size including plant growth regulators, adjusting day and night temperature, and cultivar selection to target the market demands. However, some growers still have delayed flowering of some petunia cultivars. One important environmental factor that may affect flowering time is lighting. First and foremost, petunia is a long day plant meaning that long-days (short nights) trigger flower initiation and development. Most varieties on the market are facultative long day, i.e., long days are not required for flowering but speed it up. Growers trying to initiate flowers during naturally short days of winter can use day-extension lighting (supplemental lighting to extend the hours of daylight) or night interruption lighting (low intensity lighting from 10 p.m. to 2 a.m. to break the night into two short periods).

However, petunia also responds to supplemental light with higher quality and earlier flowering. In terms of supplemental light, both quantity and quality are important. Light quantity is measured two ways: instantaneous light (units: µmol·m^-2·d^-1 ) and the total amount received in a day, i.e. the daily light integral (DLI, units: mol·m^-2·d^-1 ). Light quality refers to the spectrum or color of light (i.e., how much or what percentage of the light is blue light, red light, etc.). Traditionally growers used high intensity discharge (HID) lights like high pressure sodiu (HPS) lights. However, more growers are considering light-emitting diode (LED) fixtures in which the spectrum delivered to the plant can be selected. One of the least understood spectra is far-red (FR), which is 700-750 nm or beyond red (which is 600-700 nm light). FR radiation is poorly visible to our eyes but perceived by plants. Past research has shown FR radiation can promote flowering in certain long day bedding plants but can also have the unwanted effect of increasing plant height. However, little information is available on practical FR application on greenhouse petunias. The effect of light quantity and quality during young plant production of petunias may be especially important for subsequent flowering and size of plants. We conducted an experiment to determine how DLI and additional FR radiation applied at liner/transplant stage affects the flowering and overall growth of petunia plants.

The study used three vegetatively propagated petunia cultivars: ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’. Plants were started from cuttings and rooted for three weeks in a propagation greenhouse before being moved to the experimental greenhouse for lighting treatments. Foliar spray of Collate was applied at week 2. Rooted liners then received different lighting treatments for four weeks. Greenhouse sections were setup with the same environmental conditions but different DLI targets: 6, 9, and 12 mol·m^-2·d^-1 (in the winter study) and with an additional treatment of 15 mol·m^-2·d^-1(in the summer study). Light was controlled through high pressure sodium supplemental lighting and use of retractable shade curtains. In each greenhouse section, two growing benches were used: one without supplemental FR and one with FR bars (FGI 730 NM Far Red LED Grow Light, Forever Green Indoors, Seattle, WA) providing 30 μmol·m^-2·d^-1 additional FR for 16 hours per day. The three cultivars were randomly placed on each growing bench and grown on for four weeks under the lighting treatments. Liners were spaced at week 4 (one week after the start of the lighting treatment). At week 7, 10 representative liners from each cultivar and each treatment were destructively harvested for data collection on plant height, number of branches, number of buds and open flowers, shoot fresh weight (FW) and dry weight (DW). Then, another ten representative liners from each cultivar and each treatment were selected and transplanted to 4.5-inch pots and grown in a common environment with 15 mol·m^-2·d^-1 DLI for another three weeks. A Bonzi drench was applied at week 9 when plant canopy fully covered the soil surface. The finished plants were destructively harvested at week 10 and similar data were collected. The experiment was conducted twice, once in summer and once in winter. Morning DROP (lowering the temperature for four hours at daybreak to 66° F) was applied to offset warmer days in the summer experiment. The summer crop cycle averaged 71/66° F day/night with average daily temperature (24-hour period) of 69° F. The winter crop cycle had a 63/58° F day/night temperature with average daily temperature of 61° F.

At the end of the liner production stage (seven-week period, four weeks of lighting treatments), we observed that liners grown under 6 mol·m-2·d-1 were stretched to an extent that the plants are not sellable. For 9 mol·m^-2·d^-1 and above, plant height was acceptable. FR radiation had some promoting effect on the plant height which was especially apparent under low DLIs. In both crop cycles, higher DLI resulted in greater biomass (shoot FW/DW) for all cultivars, but FR had very limited promoting effect. Besides, root FW/DW, which is a sign of plant health, increased with higher DLI.

At the end of the finishing period, plants were also evaluated. We observed similar results of plant height as we saw at the end of the liner production stage. Plants with 6 mol·m^-2·d^-1 had overall unacceptable quality (too tall, poor branching). Similar to the liner stage, we observed that finished plants had only subtle response to FR in terms of shoot FW/DW. Higher DLI promoted earlier flowering in both summer and winter crop cycles. The level of promoting effect was cultivar dependent and it was more significant in winter (as cuttings from our summer crop cycle had already initiated flowers). For example, in the summer crop cycle, Bermuda Beach flowered four days earlier under 15 mol·m

^-2·d^-1vs. 6 mol·m-2·d

^-1, but in the winter crop cycle, it flowered two weeks earlier under 12 mol·m^-2·d^-1 vs. 6 mol·m^-2·d^-1. We did not observe significant effect of FR on flowering time in most cases, but for Bermuda Beach in the winter crop cycle, FR promoted earlier flower for up to five days. Besides earlier flowering, higher DLI also resulted in more flower buds and more open flowers at harvest. For example, Bermuda Beach had an average of 7 flower buds under 6 mol·m-2·d-1 vs. 17 flower buds under 15 mol·m^-2·d^-1. FR also had limited effect on the number of flower buds.

In conclusion, higher DLI at the liner stage had positive effect not only on flower development, including earlier flowering, more flower buds and more open flowers at harvest, but also on plant biomass accumulation and shorter/compact plants. Most of these results were carried over to the finished plants. Therefore, DLI treatments at liner stage were important for the future development of the petunia crop. Far-red in general had little effect on petunia flowering, yield and morphology in this greenhouse study. We think this is because sunlight already contains 20% FR and HPS lights also have some far-red, so the applied FR had minimal additional responses. One thing worth noticing is that liners grown under 6 mol·m-2·d-1 stretched to an extent that the plants were not marketable. All plants that received 9 mol·m^-2·d^-1 DLI or greater had an acceptable level of plant quality (shape, weight, and flower number) with increased DLI having even greater quality. Further research should be conducted with higher level of FR or under a background of LED lights without FR, but the current study showed 30 μmol·m^-2·d^-1 for 16 hours daily did not benefit the petunia cultivars we evaluated under greenhouse conditions.

Jiaqi Xia is a PhD candidate (jx338@cornell.edu) and Neil Mattson is professor and greenhouse extension specialist in the School of Integrative Plant Science at Cornell University (nsm47@cornell.edu). The authors would like to thank Pleasant View Gardens and Proven Winners for plant material used in the study. This work was supported by the USDA ARS Floriculture & Nursery Research Initiative and by USDA-NIFA-SCRI Award Number # 2018-51181-28365. More information is available at www.hortlamp.org