Light Spectrum Variably Affects the Acclimatization of Grafted Watermelon Seedlings While Maintaining Fruit Quality

Abstract

In many countries of Europe and Eastern Asia, watermelon production is mainly based on the use of grafted seedlings. Upon grafting, seedlings undergo a period of healing where artificial lighting is provided by light-emitting diodes in controlled chambers in order to accelerate and improve the healing process. The objective of our study was to test the effect of light quality on the final product (i.e., seedlings ready for transplanting) in the nursery, as well as to evaluate the possible implications on fruit quality after field cultivation. Narrow-band blue (B) and red (R) wavelengths, 64–36% R-B (36B), 76–24% R-B (24B), 88–12% R-B (12B), and 83–12% R-B plus 5% far-red (12B+FR) wavelengths were tested. 12B+FR enhanced the root dry weight, root architecture, and maximum photosynthetic rate, while RB combinations generally showed better root system development with increased blue portion. R light induced inferior root dry weight and quality indices (root/shoot and shoot–dry–weight/length ratios), lower gas exchange parameters, and chlorophyll content, but high shoot length and leaf area. B light led to inferior root architecture, lower stem diameter, leaf area, and maximum photosynthetic rate. Both R and B wavelengths showed decreased concentration of macronutrients and trace elements. After field cultivation, fruit quality (i.e., morphology and color), and valuable nutritive characteristics (i.e., phenolics, carotenoids, lycopene, antioxidants) maintained high quality irrespective of light treatments. Overall, 12B+FR performed well in almost all qualitative parameters including the morphology, the root development, and photosynthesis, while also maintaining high fruit quality.

A Light Recipe including Far-Red Wavelength during Healing of Grafted Watermelon Seedlings Enhances the Floral Development and Yield Earliness

“…From DAT 0, stems were significantly narrower under the influence of FL and B, an effect which carried on up to DAT 14 (Figure 1A). This is in accordance with a previous study of our group which showed that monochromatic B limited the stem diameter of acclimatized watermelon seedlings compared to red-containing LEDs [14], while FL also induced narrow stem development of the final product (unpublished observation). Stem diameter has been proposed and has widely been used as an indicator of seedling quality in vegetable species, such as tomato, pepper, eggplant [15], cucumber [16], and watermelon [17].…”

“…Compared to 2018, our 2021 watermelons produced about 30% greater total phenolics (0.12-0.15 mg/g in 2018 versus 0.20-0.23 mg/g in 2021) and about 50% greater antioxidant capacity displayed by FRAP (35-40 µg/g in 2018 versus 76-87 µg/g in 2021), pointing to the harsh climatic conditions which acted positively towards watermelon fruit quality. In the same study in 2018, proper morphology, root development, photosynthesis, and high fruit quality were noted on grafted watermelon plants that were treated with blue + far-red LED light during the healing stage [14].…”

“…Javanmardi and Emami [22] reported greater total soluble solids in tomato fruits treated with blue light at the seedling stage [22], but this was not evident in our case. In another study of our group [14], watermelon transplants treated with varying spectral compositions of red and blue light, and then cultivated on the field in 2018, maintained fruit quality and high quality of important nutritive characteristics. Compared to 2018, our 2021 watermelons produced about 30% greater total phenolics (0.12-0.15 mg/g in 2018 versus 0.20-0.23 mg/g in 2021) and about 50% greater antioxidant capacity displayed by FRAP (35-40 µg/g in 2018 versus 76-87 µg/g in 2021), pointing to the harsh climatic conditions which acted positively towards watermelon fruit quality.…”

The Use of High-Quality Watermelon Seedlings Is Prerequisite to Limit the Transplanting Shock and Achieve Yield Earliness

“…This compound plays a significant role against prostate cancer [26]. In another experiment [27], watermelon fruits in Greece, in 2018, which had milder summer temperatures, showed lower total phenolic content and antioxidant activity compared those in the present study. This was also observed in another study of our group which resulted in greater fruit quality in 2021 crops compared with that of 2018 crops [28].…”

“…This compound plays a significant role against prostate cancer [26]. In another experiment [27], watermelon fruits in Greece, in 2018, which had milder summer temperatures, showed lower total phenolic content and Strikingly, Optimum plants reached 82%, 93%, and 100% of their total yield from flowers that bloomed on DAT 30, DAT 32, and DAT 34, compared to DAT 36, respectively. The respective percentages for Acceptable plants were 36%, 73%, and 94%.…”