Effects of Led Supplementary Lighting and NPK Fertilization on Fruit Quality of Melon (Cucumis melo L.) Grown in Plastic House

Melon plants cultivar ‘#120’ (orange flesh) were grown in 2017 in a plastic house located at the experimental farm of the Department of Plant Production Technology, Faculty of Agricultural Technology, King Mongkut's Institute of Technology Ladkrabang (KMITL), Thailand. The plastic house was covered with 0.15 mm-thick polyethylene plastic film. Seeds were sown in polystyrene transplant flats filled with peat moss. When seedlings had two to three true leaves (about two weeks from sowing), they were transplanted individually into 20 × 33 cm black polyethylene bags filled with 3 : 1 garden soil: cow manure mixture. Before transplanting, the soil mix in bags were saturated with water and allowed to drain through small horizontal slits about 4 cm from the ground. Nine bags were used for each treatment, each bag representing one replicate and all were taken for measurement of responses throughout the experiment. The bags were spaced at 0.4 × 0.6 m as usually practiced. Other cultural management practices, such as trellising, irrigation, pesticide application and weed control, followed commercial recommendations for plastic house-grown melons.

A 3 × 3 factorial experiment in completely randomized design (CRD) with nine replications was used, with LED supplementary lighting as factor A and fertilizer application as factor B. Lighting treatments were: natural day light (NDL) as control; NDL plus LED1 (red 630 nm, red 660 nm, blue 450 nm, blue 460 nm, white 14000 K, UV 410 nm and IR 730 nm) and NDL plus LED2 (red 630 nm, red 660 nm, blue 450 nm and blue 460 nm). For LED1, APL-G-300W-03 lamps (Forest Grower, China) were used, while for LED2, APL-G-F4-75X4W lamps (Forest Grower, China). LED supplementary lighting was applied every day for 12 h from 6:00 pm to 6:00 am, starting one week after transplanting and ending one week before harvest. Fertilizer application used complete fertilizer 15–15–15 (N–P₂O₅–K₂O) at 5, 7, and 9 g per plant. The fertilizer was applied weekly starting after transplanting and until a week before harvesting. Commercial practice used 7 g 15–15–15 per plant applied weekly similar to the present study.

The plants flowered after 20–27 days from transplanting. Hand pollination was done and one fruit was maintained per plant. At commercial maturity (about 50 days from fruit setting), the fruits were harvested for quality evaluation.

Plant growth was monitored weekly in terms of height increment and leaf chlorophyll content. Plant height was determined starting on the second week after transplanting up to two weeks before harvest. Chlorophyll content was non-destructively measured using SPAD-502 chlorophyll meter (Minolta Camera Co., Japan) starting at fruit set (about four weeks after transplanting) up to two weeks before harvest. The results were the average of readings taken from three index leaves (lower and upper leaf surface reading) near the developing fruit, which were used throughout the measurement period.

Fruit quality was evaluated in terms of physical and chemical attributes. Fruit size was measured by taking the diameter at the middle or equatorial part of the fruit using a Vernier caliper. Fruit mass was taken using a digital weighing scale (Adventurer, OHAUS Corp., USA). Fruit volume was determined by displacement method by placing the fruit in a container filled to the rim with water and measuring the spilled water with a graduated cylinder. Fruit peel and flesh thickness were measured using a Vernier caliper. Fruit firmness was determined using a firmness tester (Atago TR-53025, Italy). Fruit peel color was measured using a Hunterlab Colorimeter (ColorFlex, Hunter Lab, USA) taking the average of five readings of L*, a* and b* values from the top, middle and bottom parts of each fruit. As chemical attributes, total soluble solids (TSS) content, titratable acidity (TA) and pH were determined. TSS was measured using a refractometer (Model PAL-1, Tokyo, Japan). For TA analysis, 10 g of pulp tissue was homogenized with 40 ml of distilled water. The homogenate was filtered through cotton wool. To the 5 ml of filtrate one to two drops of 0.1% phenolphthalein indicator was added and titrated using 0.1 N NaOH to an endpoint of faint pink color (pH 8.1). The results were expressed as percentage malic acid per 100 g fresh weight. pH was measured using a digital pH meter (Model HI2213, Hanna Instruments, USA).

The results were analyzed by performing analysis of variance (ANOVA) and treatment means comparison by the Tukey's honest significance test (HSD; p < 0.05) using Statistix 8 software (Analytical Software, USA).

The plants grew more rapidly with LED supplementary lighting as compared with the natural day light control (Table 1). LED-treated plants were significantly higher than the control throughout the whole experiment. LED2 was more effective than LED1 in stimulation of plant growth. Plant height also increased with increasing the amount of NPK fertilization. Differences between the three NPK treatments were significant at each term of observation. Interaction effect was significant throughout the observation period as some treatments did not follow the main effect of LED lighting and NPK application. For example, at 14 days from transplanting, plants fertilized with 5 g and 7 g NPK had statistically comparable height under LED1 or LED2 conditions. At 63rd day from transplanting, heights of plants exposed to LED1 or LED2 and fertilized with 7 g were comparable to that of plants fertilized with 5 g or 9 g but between 5 g and 9 g, differences were significant.

Chlorophyll content generally decreased with the fruit development and maturation (Table 2). LED lighting maintained higher chlorophyll content except at 49 and 56 weeks from transplanting, in which chlorophyll contents from all treatments were statistically comparable. LED2 was most effective in maintaining high chlorophyll content. LED1 resulted in significantly higher chlorophyll content than the control only at 63rd week. Plants fertilized with 9 g had significantly higher chlorophyll content than those fertilized with 5 g through the experiment. Significant interaction effect was obtained except at 49 and 56 weeks from transplanting. At 28th week from transplanting, only LED2 lighting plus 9 g of fertilizer resulted in significantly higher chlorophyll as compared with the control plus 5 g fertilizer. At 35th and 42nd day from transplanting, both 7 g and 9 g of fertilizer under LED2 lighting resulted in significantly higher chlorophyll content than the control with 5 g. At 63rd week, the three NPK treatments under LED2 lights had comparable chlorophyll contents but significantly higher than that of the control with 5 g fertilizer.

The results are in agreement with previous findings, particularly those reporting on the promotive effect of red-blue LED (LED2) on plant biomass and chlorophyll development (Li & Kubota 2009; Yorio et al. 2011; Li et al. 2012). In melon, Cui et al. (2017) found that exposure to red-blue LEDs resulted in increased plant height and maximum net photosynthetic rate. In other crops – tomato, cucumber, pepper and strawberry, blue (450–470 nm) plus red (620–660 nm) LEDs increased plant biomass and photosynthetic capacity of the plants (Hernández & Kubota 2012; Samuolienė et al. 2012a; Piovene et al. 2015). It was reported that plants are sensitive to radiation wavelengths from UV (280–400 nm) to far-red (700–800 nm), with blue and red wavelengths as the most effective regions influencing overall plant growth and yield (Folta & Carvalho 2015). These radiations are primarily absorbed by the chlorophyll pigments, which initiate the photosynthetic process (Ouzounis et al. 2015; Huché-Thélier et al. 2016). However, the entire spectrum of light is not beneficial for plants; UV and infrared radiations could adversely affect growth processes (Morrow 2008; Mitchell et al. 2012). This may account for the lower effectiveness of LED1 (red-blue with white, UV and IR components) than LED2 in promoting growth and chlorophyll development. On the other hand, NPK fertilization increased plant height and generally favored chlorophyll synthesis. Increasing the NPK level from the recommended rate of 7 g NPK 15–15–15 per plant to 9 g also increased plant height and chlorophyll content. In an earlier study on melon, application of the recommended fertilizer rate in combination with organic fertilizer was the most effective in increasing plant growth parameters than using 50% or 75% lower than the recommended dose (Shafeek et al. 2015). Inorganic NPK fertilizers are more easily available to plants then organic ones.

Fruit size, mass and volume increased with LED supplementary lighting (Table 3). LED2 was more effective than LED1 in bringing this effect. With LED2 lighting, fruit size increased by about 3 cm, fruit mass by about 1,400 g and fruit volume by about 1,300 ml relative to the control. LED1 caused smaller but significant increases in fruit size, mass and volume compared to the control. Fruit size, mass and volume also increased with increasing NPK level but differences were significant only for fruit mass. It was highest in plants fertilized with 9 g and lowest with 5 g. Interaction effect of LED lighting and NPK fertilization was significant only for fruit mass and volume due to deviation from the main effect of LED or NPK treatment. For fruit mass, the effects of 7 g and 9 g in the control (no LED) and 5 g and 7 g under LED1 and LED2 were not significant. For fruit volume, the effects of the three NPK treatments under each of the lighting treatment did not significantly differ.

Peel and flesh thickness differed significantly with LED lighting and with NPK fertilization (Table 3). The values were highest in LED2 and lowest in the control corresponding to their effect on fruit size. The effect of LED1 and LED2 was comparable for peel thickness, while for flesh thickness, LED1 effect was inferior compared to that of LED2. Significant interaction effect was obtained for both parameters as only 5 g fertilizer for peel thickness and 5 g and 7 g for flesh thickness in the control (no LED) significantly differed from the effect of LED2 regardless of NPK level.

Fruit size and mass are the main determinants of economic yield of melon. The results showed that LED lighting, particularly LED2, remarkably increased fruit yield as compared with the control and the increased fruit size was accompanied by the increase in thickness of the peel and flesh. This effect can be attributed to improved vegetative growth – plant height and chlorophyll content, which could have increased photosynthetic capacity and assimilates production affecting fruit development. LED2 was more effective than LED1 in improving growth and correspondingly in increasing fruit yield. The use of red-blue LED was also found to increase the yield of other crops due to increased rates of photosynthesis (Shimazaki et al. 2007; Dong et al. 2014; Sabzalian et al. 2014; Piovene et al. 2015; Choi et al. 2016) and the increase in fruit yield was due to the increased fruit size rather than increased number of fruit per plant as in the case of strawberry (Nadalini et al. 2017). On the other hand, the increased fruit yield in response to NPK fertilization was consistent with earlier results in melon (Damarany & Farag 1994; El-Desuki et al. 2000; Shafeek et al. 2015) and other cucurbits – watermelon (Ojo et al. 2014), pumpkin (Oloyede & Adebooye 2013). As regards the combined effect of NPK fertilization and LED lighting, we did not find direct evidence in the literature. Overall, the results of the present study demonstrated that LED2 combined with the highest NPK level (9 g NPK per plant) produced the biggest and heaviest fruit resulted from the most vigorous vegetative growth measured in terms of plant height and chlorophyll content. Without LED combined with the lowest NPK level, fruit yield and vegetative growth were the lowest among treatments.

Fruit produced under LED2 lighting were lighter in color measured as higher lightness (L*) values compared to that of fruit under LED1 and control treatments (Table 4). The effects of NPK fertilization and the interaction effect of LED and NPK treatments were not significant. The a* values (green to red) were lower in fruit under LED2 comparing with LED1 and control. Fertilization with 9 g NPK significantly decreased a* values, while 7 g had comparable effect as that of 5 g. However, this effect was obtained only under no LED treatment, while under LED1 or LED2 conditions, the effect of the three NPK levels did not differ significantly. This accounted for the significant interaction effect of LED and NPK treatments. The b* values (blue to yellow) were not significantly affected by LED and NPK treatments but the interaction effect was significant and the application of 7 g under LED1 condition resulted in significantly higher b* values than that of 5 g under LED2 condition.

The results indicate that LED2 exposure and high NPK fertilization caused a less saturated color of the fruit expressed with a lower a* and higher L* values compared to that of the other lighting and NPK treatments. Fruit color is the first quality attribute perceived by consumers at the point of purchase and a lighter color is desired for melons in Thailand. In tomato, colorimetric data on a* and b* coordinates that are correlated with lycopene content and red color showed that LED lighting reduced red color (lower a* and b* values) compared to the control (no LED) (Dzakovich et al. 2015). In strawberry, LED lighting particularly blue LED also reduced a* and b* values (Nadalini et al. 2017). However, this is a negative quality factor as red color (high a* values) is desired for the fruit. In contrast, LED lighting increased the anthocyanin content of strawberry, which is mainly responsible for the red fruit color (Kadomura-Ishikawa et al. 2013), while promoted red color development in tomato (Xu et al. 2012). The inconsistency of results could probably be due to the differences in light intensity and other growing conditions, which often was a problem when comparing the results of experiments conducted under different light parameters and cultural management systems (Lin et al. 2013).

LED1 and LED2 had comparable effect on flesh firmness, ranging from 51.3–57.5 N, which was significantly higher than that of the control – 44.1 N (Table 5). Application of 9 g NPK resulted in firmer fruit (55.4 N) than that of 5 g (47.3 N). At 7 g NPK, intermediate firmness values were obtained (50.3 N), which was not statistically different from that of the two other NPK levels. Interaction effect of LED and NPK treatments was significant as only LED2 combined with 9 g of fertilizer resulted in significantly higher firmness (65.5 N) than the control regardless of the NPK level (39.7–48.2 N).

Fruit firmness determines textural quality and susceptibility to physical damage due to handling hazards during the postharvest period. Firm flesh is a desired mouth feel quality in melons. The results showed that LED2 combined with 9 g NPK was the most effective in producing high firmness fruit. It is speculated that the favorable effect of LED2 and high NPK levels on vegetative growth increased the rates of photosynthesis and assimilated the production and utilization in metabolic processes, including those responsible for cellular integrity and rigidity (e.g., pectin metabolism) that contribute to the overall fruit firmness. In an earlier study, LED lighting was found to have no effect on fruit firmness (Nadalini et al. 2017).

LED lighting increased TSS content, with LED2 being more effective than LED1 (Table 5). TSS increased from 14.16°B in the control to 15.87°B in LED2. NPK application had no significant effect. Deviating from this main effect of LED and NPK treatments resulted in significant interaction effect. Only fruit from the control combined with 5 g fertilizer had markedly lower TSS content (13.47°B) than fruit from LED2 combined with 9 g (16.4°B). Concomitant with the increase in TSS, TA content decreased in response to LED lighting (Table 5). TA decreased from 0.6% in the control to 0.2% in LED2; LED1 had intermediate effect. NPK fertilization increased TA content but the effect of 7 g and 9 g NPK was statistically comparable. Under each of the lighting treatment, application of the 9 g decreased TA content relative to lower NPK levels. This accounted for the significant interaction effect. Juice pH was statistically similar among all treatments and ranged from about 6.0–6.6 (Table 5).

Light quality has a pronounced effect on the accumulation of various metabolites in plants (Bian et al. 2015). Increased accumulation of plant metabolites (e.g., soluble sugars) was observed in the presence of red-blue LED. Natural daylight supplemented with red-blue LEDs increased the soluble sugars and organic acids in different crops (Samuolienė et al. 2012b; Lin et al. 2013; Samuolienė et al. 2013; Dong et al. 2014; Choi et al. 2015; Xie et al. 2016; Hasan et al. 2017). In melon, Cui et al. (2017) showed red:blue LED to be the most effective in increasing soluble sugar content. The results of the present study for TSS conform to this finding but the decrease in TA content was in contrast to the general effect of LED on organic acid accumulation, which is not always associated with pH. Other investigators found no remarkable effect of LED lighting on fruit quality parameters such as TSS, TA, and pH; the authors concluded that LED supplementary lighting had neutral effect on fruit quality and can be used as alternative to other lighting supplements, such as HPS lamps (Dzakovich et al. 2015; Nadalini et al. 2017). On the other hand, NPK fertilization in melons is important to increase fruit yield and quality (El-Desuki et al. 2000). However, Shafeek et al. (2015) found that NPK application did not significantly affect total sugars and TSS content of melons. Similar results were obtained in the present study. The combined effect of NPK fertilization and LED lighting on melon fruit quality has not been so far reported. In this study, melon fruits produced under LED2 conditions combined with 9 g NPK 15–15–15 per plant were the sweetest (highest TSS) and had the lowest sourness or astringency (lowest TA content).