Paper referenced:
Tsuruyama, J., & Shibuya, T. (2018). Growth and flowering responses of seed-propagated strawberry seedlings to different photoperiods in controlled environment chambers. HortTechnology, 28(4), 453–458. https://doi.org/10.21273/HORTTECH04061-18
Introduction
Although other crops are often in the spotlight when it comes to growing food in greenhouses, strawberry is gaining popularity and for good reason. It can be grown in greenhouses through the cold winter months in temperate climates to make local, fresh, high quality fruit available when not much else is. To maximize fruit set and profitability, starting the production cycle with high quality transplants is a necessity. Transplants, also known as plug or tray plants, can be produced from seed, rooted runner tips, or field dug bare root crowns, though, use of field dug plants can introduce pests and diseases into the greenhouse. Seed propagated hybrid strawberry cultivars suited for greenhouse production have been developed, leading to increased adoption of this technique in Europe and Japan.
Controlled environment technology presents strawberry plug producers with the tools needed to provide growers with high-quality transplants due to the tremendous level of control over environmental conditions such as light quality and quantity, humidity, and temperature. This high degree of control is advantageous because greenhouse grown plugs produced for August/September transplant can experience high temperatures and variable conditions, which can delay flowering and fruit production. However, with the use of an indoor controlled environment facility, plants can be grown under optimal conditions no matter the weather outside. In addition to temperature, photoperiod (the amount of time plants are exposed to light) as well as light intensity, can affect the growth and flowering of strawberries. Due to this, determining the optimal photoperiod for indoor plug production could lead to enhanced quality of transplants.
Methods
In this study the authors use two-seed propagated cultivars, one European (‘Elan’) and one Japanese (‘Yotsuboshi’), to produce tray plants for mid-August transplant. Both cultivars are long-day strawberry types which generally meaning flowering is promoted by long light periods.
To start the experiment, once seedlings had germinated and grown two true leaves, at 23 days old, they were replanted into larger trays for the light treatment phase. Next, groups of seedlings from both cultivars were subjected to different propagation systems. Four groups were grown in a growth chamber with blue/red LED lighting, this allowed the researchers to control the conditions the plants experienced, while the control group was grown in a greenhouse. The growth chambers were maintained at 25°C (77°F), but had different photoperiods and light intensities. The photoperiods tested in growth chambers were 8, 12, 16, and 24 hours. To ensure all plants received the same total amount of light, the light intensities were proportionally adjusted based on photoperiod, so the shortest photoperiod had the highest intensity and the longest had the lowest. The control plants were subjected to summer greenhouse conditions, moderated by shading during the day and air conditioning at night. The greenhouse average photoperiod was 13.6 hours with day temperatures around 30°C and night temperatures around 23°C for an average temperature of 26.8°C (80.2°F). All plants were grown in their respective treatment conditions for 38 days. After which, 10 plants per cultivar of each treatment were measured for dry mass, leaf area, leaf number, and length of the longest petiole to assess plant growth. Using pre- and post-treatment leaf area and dry mass, the relative growth rate (increase in mass), net assimilation rate (photosynthesis efficiency), and leaf area ratio were calculated.
After 38 days, when they had 6-7 leaves, plants were transplanted into a different greenhouse for the flower emergence trial, which lasted 110 days (from mid-August to late November). Plants were checked daily for flower bud emergence. Temperatures started high around 40°C (104°F) in August but slowly cooled as time progressed to a more typical strawberry production temperature range (25-10°C).
Results and Discussion
In both cultivars long-day, low intensity lighting out performed short-day and greenhouse conditions regarding plant mass, leaf area, petiole length, relative growth rate, and net assimilation rate, indicating enhanced photosynthetic efficiency. This suggests that plants were able to use the steady low amount of light over long periods more efficiently than high amounts of light over short periods or the summer greenhouse conditions, which exceeded the ideal growing temperatures for strawberry. Thus suggesting that using a controlled environment system with low intensity long-day lighting was more effective for plant growth than the greenhouse control.
Regarding how long it took plants to flower once transplanted into a fruit production greenhouse, for Elan, long-day conditions nearly halved time to flower compared to greenhouse control and short day photoperiods. This suggests that the long-day low intensity light treatments were effective for inducing flowers earlier than the summer greenhouse or short day conditions.
In Yotsuboshi however, photoperiod treatments did not have an effect on time to flower. Yet, the greenhouse control flowered slightly sooner than the photoperiod treatments, which may be due to transplant shock that the controlled environment plants experienced. These results demonstrate what other studies have found in that cultivars can react differently to the same conditions even if they are the same photoperiod type. Thus, these results suggest that more research is needed into which factors and their levels affect Yotsuboshi flowering to better understand the cultivar’s flower emergence.
Overall this study demonstrates that using low intensity LED lighting in controlled environment settings for long-day seed-propagated strawberry tray plants is a viable alternative to summer greenhouse production.
After reading a little bit more about ‘Elan’ I found out that with a 15 hour or more photoperiod it produces flowers more readily. I’m now wondering if the greenhouse control for ‘Elan’ would have seen different results if supplemental lighting had been used to increase the photoperiod length.
For future research, any ideas about what other conditions, besides photoperiod, could be used as treatments that might bolster the controlled environment transplant growth compared to the greenhouse? Especially for ‘Yotsuboshi’?
It is interesting to consider different factors that may affect strawberry growth in a controlled environment. As I thought about this and searched for factors that could support transplant growth, I found an article that was recently published this year in Frontiers that describes how ultra-violet (UV) radiation affects strawberry firmness and color – providing information on the impact of UV radiation on fruit quality and flavonoid contents. While not specific to the Japanese cultivar ‘Yotsuboshi’, it could serve as useful information for possible future studies.
You can find the article I’m referencing here: https://doi.org/10.3389/fpls.2021.611893
I think that air temperature effects on flowering needs to be investigated further for ‘Yotsuboshi’. The ‘Yotsuboshi’ greenhouse control treatment flowered before all plants that received photoperiod treatments inside the growth chamber. I think this difference in flowering initiation observed between the greenhouse and growth chamber plants could have involved air temperature and not just photoperiod. The growth chamber plants were maintained at 77 degrees Fahrenheit, while the greenhouse experienced an average of 80.2 degrees Fahrenheit. I think that increased temperature in the greenhouse would increase metabolic activity in the strawberries, resulting in a quicker maturation. There is a concept known as Growing Degree Days, which predicts plant developmental milestones based off of the total heat the plant experiences. The strawberries received the light treatments (and differences in average temperature) for 38 days, so perhaps the temperature difference accumulated during that time period and resulted in earlier flowering.
Here is an extension article from Purdue which describes Growing Degree Days further: https://www.agry.purdue.edu/ext/corn/news/articles.01/Corn_GDD_Calc-0423.pdf
In my opinion, the experimental design as well as statistical analysis parts had a few issues that could be improved. Firstly, the researchers decided to use 38 seedlings of “Elan” grown under sunlight versus 25 seedlings from the remaining treatments and cultivars for transplantation. The rationale behind the unequal sample size was not mentioned or justified. Moreover, some of the plants died after transplantation, which were missing data and contributed to more treatments that were imbalanced. However, it was good that the authors’ choice of statistical tests, ANOVA and Tukey’s posthoc tests are robust tests that can handle unequal sample sizes. Secondly, the experiment was only conducted once, which is unusual and reduced the convincingness of the study. The authors also failed to mention specific experimental design with details about randomization, blocking, and sampling approach.
I would be curious to see advice from these authors for greenhouse/indoor production growers based on their results. Because one variety, Elan, has greater flower bud initiation from long-photoperiod exposure, would that variety be recommended over Yotsuboshi for indoor growth? Or are there a number of factors that would make Yotsuboshi a better variety for growers despite its lower flower bud initiation? The conclusions simply state that the benefits of long photoperiods are cultivar dependent, but I would be interested to know which (if either) approach would be better for indoor strawberry growth: higher-flowering cultivars (regardless of photoperiods), or cultivars that respond better to longer photoperiods.