Longer Photoperiods with the Same Daily Light Integral Improve Growth of Rudbeckia Seedlings in a Greenhouse

Abstract

Supplemental light can increase growth and accelerate production of greenhouse crops, but it can be expensive if not provided in a way that promotes efficient use of the light. Dimmable light-emitting diode (LED) fixtures have the potential to reduce lighting costs because the output can be precisely controlled to meet crop needs. Because light is used more efficiently to drive photosynthesis at lower photosynthetic photon flux densities (PPFDs), we hypothesized that providing Rudbeckia fulgida var. sullivantii ‘Goldsturm’ seedlings with the same daily light integral (DLI), spread out over a longer photoperiod and at lower PPFDs, should improve growth. A DLI of 12 mol·m−2·d−1 was provided in a greenhouse over 12, 15, 18, or 21-hour photoperiods from a combination of sunlight and supplemental light from LEDs, using adaptive lighting control. Plants grown without supplemental light had an ≈12-hour photoperiod and received an average DLI of 5 mol·m−2·d−1, ≈58% less light than the four lighting treatments. Lengthening the photoperiod from 12 to 21 hours increased shoot dry mass (30%), root dry mass (24%), plant height (14%), leaf area (16%), and chlorophyll content index (48%), and decreased specific leaf area (26%). There was no significant effect of photoperiod on root mass fraction or compactness. Growth parameters of plants without supplemental light were 26% to 90% smaller compared with those in the 12-hour photoperiod treatment. Treatment effects on canopy size, seen as early as 2 weeks into the study, were correlated with final shoot dry mass. Longer photoperiods did not induce a shade-avoidance response, based on specific leaf area and compactness data. The 24% increase in root dry mass for the plants in the 21-hour photoperiod suggests that cropping cycles can be shortened by 1 to 2 weeks compared with the 12-hour photoperiod. This could result in more crop turns per year and increased profits. In addition, fewer lights would be needed for adequate growth, reducing the capital cost of the lighting system.

Increasing Growth of Lettuce and Mizuna under Sole-Source LED Lighting Using Longer Photoperiods with the Same Daily Light Integral

“…Lower PPFDs combined with longer photoperiods increased CCI, canopy light interception, and Φ PSII . The observed increase in growth under lower PPFDs and longer photoperiods is consistent with prior studies, which found that when DLI is held constant, plants generally produce more biomass when grown under longer photoperiods [7][8][9]11,12,39,40]. The observation that spreading a given DLI over a longer photoperiod with a lower instantaneous PPFD has clear benefits to growers; first, the initial capital expenses can be reduced because fewer lighting fixtures are needed to provide the lower PPFD.…”

“…Taking advantage of dimmable LED fixtures, we previously conducted greenhouse trials where plants in different treatments all received the same DLI, but spread out over photoperiods ranging from 12 to 21 h. The shoot and root dry mass of Rudbeckia fulgida var. sullivantii seedlings increased by 30 and 24%, respectively, as the photoperiod increased from 12 to 21 h [11], while lettuce shoot dry mass increased by 28% [12]. Under sole-source lighting, the effect of longer photoperiods, with the same DLI, on the growth of lettuce may be greater when that DLI is higher [13], consistent with the finding that the total daily electron transport rate through photosystem II responds more strongly to longer photoperiods when the DLI is higher [14].…”

Supplemental Far-Red Light Stimulates Lettuce Growth: Disentangling Morphological and Physiological Effects

“…This is consistent with a study that improved radiation capture by replacing blue light with far-red light, which increased the biomass of kale and lettuce due to increased leaf size [ 29 ]. Improving canopy size early in the plant’s growth and development is of benefit to growers, as seedlings capture little supplemental light due to their small size [ 54 ]. A larger canopy size allows plants to capture more light to drive growth, as light interception is a major determining factor for carbon gain.…”

Canopy Size and Light Use Efficiency Explain Growth Differences between Lettuce and Mizuna in Vertical Farms

“…Due to their small canopy size, seedlings capture only a small fraction of the provided light. Therefore, increased PCS at early stages can increase light capture and growth of seedlings [ 11 ]. A previous study compared different lettuce cultivars and found that early PCS was a good predictor of final shoot biomass [ 10 ].…”

“…Combining those data with PPFD data allows for estimation of the total incident light over the course of the growing cycle. Projected canopy size is also valuable to make crop growth predictions, since PCS early in the growing cycle may be correlated with the final dry weight of the crop, as we have previously shown in lettuce ( Lactuca sativa ) [ 6 , 10 ] and black-eyed susan ( Rudbeckia fulgida ) [ 11 ].…”