Effects of supplemental lighting using HPS and LED lamps with different light spectra on growth and yield of the cucumber (Cucumis sativus L.) during winter cultivation in greenhouse

Cucumbers are grown intensively in greenhouses in soilless systems all over the year because of the high demand of the market. During winter months, owing to lack of natural light, yield and fruit quality are usually insufficient and cultivation is not economically profitable. Cucumbers, due to their high demand for heat and good cultivation technology (soilless cultures, well-equipped greenhouses) cannot be grown without supplemental lighting the whole year. Traditionally used sodium lamps (high-pressure sodium [HPS]) in greenhouse crops are usually the main source of light in winter months, although light emitting diodes (LEDs) are becoming more and more common in horticulture (Massa et al., 2008; Morrow, 2008; Mitchell et al., 2012). The LED lamps using chips on board (COB) technology are known to be a very efficient source of light

Because of greater efficiency of LED lamps and a possibility of light spectra regulation, this kind of supplemental lighting is more often used not only in research but also in production (Mitchell et. al., 2012, Pattinson et al., 2016; Virṧile et al., 2017; Samoulienė et. al., 2019). However, it appears that the most important factor conducive to spreading the technology of LED application in horticulture is high electrical efficiency, especially in LED lamps constructed in COB. Light intensity and quality are exceptionally important factors for many horticultural plants grown in the northern hemisphere during winter months (Blom and Ingratta, 1984). Even though other growth factors such as cultivation time, good initial plant quality, microclimate in a greenhouse and fertigation are within the optimal range, yield and fruit quality are influenced significantly by light availability (Kowalczyk et al., 2018). Positive effects of LED lighting for vegetables growth and yield were shown by many researchers (Massa et al., 2008; Gajc-Wolska et al. 2013; Olle and Virsille 2013; Kowalczyk et al., 2018; Kowalczyk et al., 2020). Light penetration from the top of plant canopy to the lower leaves is often obstructed in case of high-growing plants like tomatoes, cucumbers and bell-peppers in greenhouse cultivation. Therefore, the application of LEDs with a special construction that enables to use them as interlights mounted between the plant rows is recently being developed. Pettersen et al. (2008) showed that this type of supplemental lighting increased yield and quality of fruits via increased photosynthesis of medium and bottom leaves of cucumbers.

The aim of the experiment was to evaluate the influence of two light sources (HPS and LEDs) and different light spectra characteristics on growth and yield of cucumber ‘Pacto’ F₁ during winter cultivation.

The experiment was performed in two separate climate-controlled greenhouse compartments of the Research Institute of Pomology and Floriculture in Skierniewice, Poland, from December 2018 until March 2019. Four-week-old seedlings of cucumber (Cucumis sativus L.) ‘Pacto’ F₁ raised in mineral wool cubes 10 × 10 cm were planted on wool mats (Grotop Master, 100 × 15 × 10) with four plants on each one. Five different supplemental light treatments were applied: I – HPS + LED (top light with HPS and LEDs as interlighting), II – only HPS as top light, III – LED R (LED COB type with an increased level of red band), IV – LED W (LED COB type, white), V – LED B (LED COB type with an increased level of blue spectrum). Lamps (LED and HPS) spectra characteristics are presented in Table 1. LED COB lamps with possibility or spectra light and intensity regulation were prepared by Spectrolight (Łodz), LEDs for interlighting were prepared by Instytut Elektrotechniki (Warsaw). Light intensity and spectra characteristics were measured using Lighting Passport Spectrometer. Light treatments: HPS + LEDs and HPS were grown in one greenhouse compartment and the other three: LED R, LED W and LED B in the second compartment in analogous climatic conditions. Plants were illuminated for 18 h (from 5 am to 11 pm), setting the threshold value (on and off) at 130 W. The photosynthetically active radiation at the plant level at planting was 120 ± 20 mmol · m⁻² · s⁻¹. The plants were drip-irrigated with a complete nutrient solution. They were fertilised according to the plant requirements using a complete nutrient solution (EC - electroconductivity 2.1–2.3, pH 5.5–5.9), contained (mg · dm⁻¹) NPK (nitrogen, pottasium and phosphorus) at 200, 31.5 and 260, respectively, and microelements. The irrigation frequency was controlled using the weighting system and humidity sensors fixed onto the growing mats. The temperature in the greenhouse compartments was kept at 22–24°C/18 day/night, relative air humidity at 60–70%. Plants were grown without CO₂ supplementation. The climate control in the greenhouse was controlled using Priva system. The first fruit buds (five to six on each plant) were removed. Regular fruit harvest started after 4 weeks of cultivation. The assessment of the effect of lighting on early yield and quality of cucumbers was completed after 8 weeks of cropping. According to the length of harvested fruits, they were divided into two groups: shorter and longer than 20 cm. The fruit was harvested, measured and weighed every 2–3 days, when fruits reached 180–230 g. The results are presented on figures as total yield of fruits during 8 weeks of evaluation, yield of fruits longer than 20 cm and the percentage of long fruits (>20 cm) in total yield of cucumbers. Fruit quality was expressed as total soluble salt (TSS) and vitamin C content was evaluated twice (March 7 and 14, 2019). TSS was measured with refractometer HI 9601 (Hanna Instr.) and vitamin C with Tilmann's reagent. Differences between the obtained results were presented graphically and statistical differences between the treatments were assessed using a standard error (SE). There were 16 plants in each treatment and each plant was considered as repetition. The calculation was performed using Statistica 14.0 (StatSoft Inc., USA).

The first harvest crop of cucumbers was obtained on 25 January 2019 (fourth week of the year). After 8 weeks of cropping, the obtained results showed that cucumber plants illuminated exclusively with sodium lamps (HPS) gave 3,590 g from one plant during that period, while using HPS as top lighting and additional light as inter-rows 3,890 g (Figure 1). The yield of plants illuminated exclusively by LEDs lamps varied depending on the variant of the spectrum applied and was respective for LED R (red light supplementation), LED W (without additional supplementation) and LED B (blue spectrum supplementation), 3,300 g, 3,900 g and 3,250 g. The differences between fruit yield from I treatment (HPS and LED as interlights) and IV treatment (LED W) were not significant. Treatments of cucumbers with LED B (blue spectrum supplementation), with the lowest ratio B:R (2.66), gave lower fruit yield compared with other light treatments.

Figure 1

The obtained results indicated that the yield of ‘Pacto’ F₁ cucumber illuminated by HPS lamps and at the same time inter-row LED lamps were similar to LED W lamps (i.e. without additional supplementation in the range of red and blue spectrum). Comparing the fruit quality (fruit length), the highest yield of fruit longer than 20 cm at harvest, the best results were observed in case of treatment I – with HPS and LEDs (Figure 2). The lowest yield of fruit longer than 20 cm gave plants grown with LED B and LED R. However, comparing the percentage of fruits longer than 20 cm in the total yield, the best results were observed in case of I treatment – HPS and LED interlighting and subsequently of treatment V – LED B – supplemented with B wavelength (Figure 3). The positive effects of using LEDs as interlights for cucumbers were also observed by Kowalczyk et al. (2018), Hao et al. (2012), Hovi-Pekkanen and Tahvonen (2008), Hovi et al. (2004). High yield and good fruit quality of cucumber ‘Svyatogor’ F₁ grown with LED as top light with additional two lines of LED inter-row light lighting were also shown by Kowalczyk et al. (2020). It seems the cucumber being a thermophilic plant has the best conditions for growing and fruit development with HPS (emitting heat) used as a top light and additionally with LED interlights, which may improve photosynthetical activity of medium and bottom leaves significantly. This phenomenon in cucumbers was observed by Sarkka et al. (2017), Pettersen et al. (2008), Trouwborst et al. (2010). The weekly yields of fruits on each light treatment are presented in Figure 4.

Figure 2

Figure 3

Figure 4

The application of LED as interlights together with HPS as top light (treatment I) gave the highest vitamin C content in cucumber fruit and the highest TSS readings, comparing with other light treatments in both dates of fruit evaluation (Table 2). Higher level of vitamin C in all treatments observed at the second date of measurements can be influenced by better access to light of fruits being closer to the top of plants and of course higher natural light intensity in the mid-March period. Grzelakowska et al. (2013) and Wierzbicka and Kuskowska (2002) showed that vitamin C content in cucumber fruits can vary according to climate condition during cultivation, especially when plants are grown at low temperature. Migut et al. (2018) obtained exactly the same content of vitamin C (2.8 mg · 100 g⁻¹) in cucumber fruits as in the present study shown in treatment I (HPS as top light and LED as interlights) in the second date of evaluation. In case of TSS of cucumber fruits, the values varied according to the different light treatments from 2.97 Brix to 3.13°Brix. These results are similar to these obtained on cucumbers by Reddy et al. (2018), which varied between 2.8 and 2.9°Brix and slightly lower than was shown by Mulla et al. (2018), who obtained 3.9° Brix. The results of the present study confirm the other research on cucumber during winter cultivation than HPS as top light and LED as interlighting can increase the yield, gave higher percentage of long fruits and significantly can improve fruit quality expressed as vitamin C content and TSS in fruit flesh. Interlighting LED lamps are also beneficial for other greenhouse crops like the tomato (Menard et al., 2006; Gómez and Mitthel, 2014), and bell-peppers (Joshia et al., 2019).