Strawberry Challenges
Strawberries are one of the most frequently grown crops in greenhouses, however, their production can be challenging due to their sensitivity to environmental factors such as light, water, CO2, temperature, and nutrients (Zhang et al., 2008; Zheng et al., 2007). Among these factors, light holds special significance, acting as the catalyst for photosynthesis through the photo-dissociation of water. The reduction in sunlight hours during winter can detrimentally impact the growth of strawberry plants.
Why now?
We have seen an increase of strawberry production in greenhouses due to the growing demand for fresh fruit and out-of-season production. On top of that, research indicates that greenhouse cultivation of strawberries, as opposed to field production, offers a greater potential for profitability (Cantliffe et al., 2008). Additionally, advancements in agricultural and horticultural practices, involving the manipulation of light types and photoperiod, have been implemented over several decades (Demotes-Mainard et al., 2016; Dueck et al., 2016; Folta and Childers, 2008). The behaviour of each Fragaria cultivar responds differently to changes in photoperiod and thermoperiod, emphasizing the necessity of adapting cultivars to local light and temperature conditions (Bradford et al., 2010).
The cultivation of high-quality strawberries involves a controlled flowering and early production phase, often impacted by a combination of low temperatures and a short photoperiod. Despite these conditions being suboptimal for fruit production, developmental cues, facilitated by blue and red lights through the plant's circadian clock, play a crucial role in steering the growth process (Choi et al., 2015).
Ideal Light Setup
Complementation of ambient light in greenhouses by either blue light-emitting diodes (LED) light or a combination of blue and red LED light significantly increases Strawberry fruit production. Interestingly, higher levels of organic acids and phytochemicals, including phenolic compounds, are found when strawberries have been cultivated in greenhouses with supplementary red LED light or a combination of blue and red LED lights (Choi et al., 2015).
Hidaka et al. investigated how the growth and yield of plants are influenced by two distinct commercially available light sources: LEDs and fluorescent lamps. Their experiment highlighted the effectiveness of supplemental lighting with higher irradiance LED for achieving high yields in strawberry cultivation. Plants under LED illumination exhibited photosynthetic rates significantly higher than those exposed to fluorescent light or left without supplemental lighting. The recorded photosynthetic photon flux density (PPFD) values exceeded 400 µmol m⁻² s⁻¹ at leaf heights ranging from 10 to 30 cm.
Enhanced Light Effects
This heightened photosynthesis facilitated robust plant growth, expanded leaf area, and enhanced leafy dry matter. There was notable improvement in average fruit weight, fruit quantity, and overall yield. The superior yields in LED-exposed plants were attributed to the comparatively higher light intensities of LED illumination compared to fluorescent lamps. Additionally, the sweetness, indicated by fruit soluble solids content (BRIX), saw an increase under LED lighting.
Strawberries exhibited greater growth in terms of plant height, leaf, runner, inflorescence, and crown characteristics when exposed to varying ratios of red to blue light. Also, there is a positive impact of red and blue light on the formation of strawberry runners, inflorescences, and crowns (Naznin et al., 2016). Applying supplementary lighting for 13 hours, commencing one hour before sunset, led to a significant 300% increase in flowering without compromising fruit quality in experimental conditions (Díaz-Galian et al., 2020). Although strawberry size declined over time in both treated and control groups, employing a comparable strategy in commercial greenhouses resulted in improved fruit quality.
Other Benefits
Light can also act as stress alleviator; in the study by Malekzadeh Shamsabad et al., it was found that using a modulation light spectrum, particularly using red and blue light, can effectively reduce the effects of salinity and alkalinity stresses. While white/yellow light enhances vegetative traits in non-stress conditions, exposure to additional blue, red, and a combination of blue and red light improves the plants' tolerance to stress conditions. This suggests that beyond enhancing yield and vegetative traits, the application of specific light spectra can be beneficial for alleviating abiotic stress in strawberries. The manipulation of light spectra influences the plants' resistance to stress by impacting element absorption and the performance of the photosynthetic apparatus. In conclusion, using red and blue light proves advantageous not only for improving overall plant growth but also for reducing the impact of abiotic stresses.
