+9.4% Yield Boost in Romaine Lettuce Trial

AT A GLANCE</h2

ABOUT THE PROJECT

Discover the future of agriculture with SFCC student Lydia Steinhoff’s groundbreaking study, guided by Professor Shultz and Manager Casas. Explore the growth of ‘Coastal Star’ romaine lettuce under UbiGro Inner 650 luminescent quantum dot (QD) greenhouse film versus a control film at Santa Fe Community College Research Greenhouse in 2022. Special thanks to UbiQD, Inc. for vital support.

Uncover the potential of Controlled Environment Agriculture and advanced greenhouse cover in revolutionizing crop cultivation. Ready to shape the future? Join us on this journey. College Research Greenhouse in 2022.

EXPERIMENT

UbiGro QD greenhouse films emitting at 650 nm were installed above a 254 ft2 (23.6 m2) nutrient film technique (NFT) system inside the Santa Fe Community College Research Greenhouse (SolaWrap cover, 83% PAR transmission). An identical neighboring NFT area in the greenhouse was chosen to serve as the control group, over which a clear polyethylene film (K50 Clear 6 mil, RKW Klerks) was installed to balance the light intensities and diffusivities between the two areas (see Table 1 describing the haze and transmission of the QD film and the polyethylene control film). A reflective mylar barrier was hung between to the two areas to prevent light mixing between treatment areas. A Watchdog Plant Growth Micro Station with four quantum light sensors was installed to measure daily light integral (DLI) on each side of the experiment using five-minute measurement spacings. Crops were grown with Calcium Nitrate, Magnesium Sulfate, and Potassium Nitrate-rich nutrient salt solutions with targets of EC 1.7 and pH 5.8. In order to better achieve proper light intensity for lettuce, shade curtains were deployed over the crops in both treatments.

Table 1. Optical properties of experimental films. Haze and PAR transmission of the QD film and the polyethylene control film.

Over an 11-week period spanning July to October 2022, two seven-week crops were grown. Each crop consisted of 72 plants, including 36 plants per experimental group. Seeds were sown in Oasis Rootcubes on an ebb-and-flow seedling table inside the greenhouse. Each pair of plant groups was transplanted into the NFT system, under the QD film and control films, at approximately three weeks after sowing. The harvest times ranged from 45 days after sowing (DAS) to 50 DAS. The first harvest was completed on September 8, 2022, and the final harvest was completed on October 6, 2022.

HARVEST DATA

Due to labor scheduling, critical activities including sowing and harvesting were conducted ±4 days from the target nominal DAS, so days after treatment (DAT) were kept consistent between experimental repeats. The maturity time for this lettuce cultivar is 57 DAS, according to seed purveryer Johnny’s Seeds; however, this 57-day maturity time can vary with sunlight intensity, seasonality, climate, and other variables, which can be better controlled in a greenhouse. These maturities at harvest represent typical harvest times for a commercial greenhouse grower.

Table 2. Average wet weights. Harvested wet weights, and % changes for lettuce harvests across experimental repeats. Positive % change values indicate greater performance under the treatment.

Average wet weights for each harvest were compared across both crops, and are shown in Table 2 and in the form of bar plots in Figure 1. Harvest data outside of three standard deviations from the mean were consdired outliers; no data qualified as outliers by this definition, so all data collected were included in the analysis. Plants grown under the QD film exhibited larger wet weights in each experimental repeat. Greater yield differences (i.e., increases) were observed under the QD film treatment when plants overall were larger as in the first experimental repeat where +11% more fresh weight was accumulated under the QD film.

The larger growth results represent the most impactful results of this experiment, as a commercial grower would grow out their crop to a marketable wet weight of ~100 to 150 g and would realize the yield benefit in terms of greater revenue (if selling by weight or size) or faster grow cycles (if selling by head count). This yield boost for the mature crop translated to the ability to harvest the same size crop 2-3 days earlier, which would compound to approximately one extra harvest cycle annually.

Figure 1. Average wet weight. Average wet weights measured at 28 DAT in each experimental repeat and overall, with 1 standard deviation error bars.

In this study, planting density was cut to approximately half-size from previous lettuce experiments in this NFT system to reduce shading by neighboring plants and thereby decrease variance; however, large variances remained, and larger variances were present in the treatment group in both experimental repeats. The persistent variance could have resulted from a number of factors, including the difference in cumulative light integral between experimental repeats. Harvest data from one week earlier indicated a lower magnitude of the treatment effects even one week earlier than these harvest data collected at maturity; more data on a larger scale could further clarify this result.

Table 3. Average dry weights. Harvested dry weights and % changes for lettuce harvests across experimental repeats. Positive % change values indicate greater performance under the treatment.

Average dry weights for each harvest were compared across both crops, and are shown in Table 3 and in the form of bar plots in Figure 2. Harvest data outside of three standard deviations from the mean were considered outliers; no data qualified as outliers by this definition, so all data collected were included in the analysis. Plants grown under the QD film exhibited larger wet weights in each experimental repeat.

Figure 2. Average dry weight. Average dry weights measured at 28 DAT in each experimental repeat and overall, with 1 standard deviation error bars.

Relatively greater increases in yield were observed under the QD film treatment when plants overall were smaller as in the second experimental repeat where +4.8% more dry weight accumulated under the QD film. Interestingly, the trend in dry weight is the opposite of the trend in wet weight as overall plant size increases. This greater improvement in dry weight at smaller overall sizes could be due to a greater proportion of the plant being water weight as the plants increase in overall size, indicating that the QD film may facilitate the input of water into the plant as it grows.

Daily Light Integral

Light intensity balance was important to quantify in this trial in order to isolate the effects of spectral quality. The two sides of the trial must balance light intensity as closely as possible in order to provide comparable results. Daily light integral (DLI) describes the number of photons in the photosynthetically active range (PAR, 400-700 nm) that are delivered per unit area over a 24-hour period, and is the typical cumulative metric for measuring light intensity over crops. Average DLI values for the QD film and control groups are plotted below in Table 4.

Table 4. Average treatment DLIs. Average DLIs measured during treatment periods below the films across experimental repeats. Positive % change values indicate greater light intensity under the treatment.

Seasonal and daily geometric changes in weather and sun angle interacting with the infrastructure of the greenhouse caused DLI differences to vary as the season changed from summer to winter. The first experimental repeat experienced a higher average DLI of 13.7 mol m-2 d-1 throughout the treatment period than the second experimental repeat with only 11.6 mol m-2 d-1. While the average DLIs under the shade curtains were suboptimal for lettuce growth, these DLIs were more appropriate for healthy lettuce growth than the average DLIs measured above the films during these periods (28.2 and 27.1 mol m-2 d-1 for repeats 1 and 2, respectively).
As is the case in all experiments studying spectral quality, it was important in this study to isolate the effect of spectral differences from the effect of light intensity.

In a light-intensity regime optimal for a specific crop, keeping the PPFD balanced within a 5% difference between experimental groups is ideal. During the first experimental repeat, the plants experienced +2.4% greater DLI under the QD film, whereas the plants experienced equivalent DLI under both films during the second repeat. Overall the experiment was tighly balanced with only +1.2% greated DLI under the QD film across both repeats. Balancing the light intensities so closely enabled this study to isolate the effect of spectral quality from light intensity with a high confidence.

Conclusion

This study exhibited that UbiGro Inner 650 quantum dot luminescent greenhouse films applied over Coastal Star romaine lettuce increased yields in both wet weight (+9.4% average increase) and dry weight (+4.4% average increase), allowing growers to achieve faster crop turnover (~2-3 days quicker) and higher productivity in the greenhouse. This greater productivity enables growers to squeeze in another crop cycle per year or come to market with larger crops, depending on their preference.
Achieving statistically significant results is challenging with biological variance and limited scope, so larger commercial trials are needed to clarify and further validate the results observed here.