The Effects of Light Duration and Intensity with Supplemental Light-Emitting Diode Lights on Grape Photosynthesis, Yield, and Fruit Quality

Abstract

‘Shine muscat’ grapevines must be cultivated in a protected facility to avoid insects and rain during the rainy season; however, this decreases the light intensity falling on the canopy of grapes. The use of light-emitting diodes (LEDs), with their high efficiency in converting electricity to light, is a useful method to supplement light for plant growth. This study was designed to primarily investigate the effect of the light duration and intensity of supplemental LED lights on grape growth. The photosynthetic and chlorophyll fluorescence measurements of leaves were used to evaluate the performance of photosynthesis. Grape yield and fruit quality were also investigated. Seven different light treatments were utilized to determine the proper light duration and intensity of supplemental LED lights. The results show that the supplemental LED light intensity with the photosynthetic photon flux density (PPFD) of 300 μmol/(m²·s) at 18:00–24:00 showed the highest grape yield, sugar–acid ratio, and economic benefit, with improvement values of 45.1%, 51.4%, and 23.6%, respectively, compared to unsupplemented control vines (CK). The difference between the net photosynthetic rate (P_n), the max net photosynthetic rate (P_max), and the leaf photosynthetic efficiency (α) between the treatments was negligible. Meanwhile, prolonging the light duration at night was more effective in improving the grape yield and fruit quality than increasing the light intensity in the daytime using supplemental LED lights. The results prove that the supplemental LED lights significantly optimized the light environment and improved grape yield and fruit quality.

1. Introduction

‘Shine muscat’ grapevines must be cultivated in a protected facility to avoid insects and rain in rainy regions, which can improve fruit quality and commodity competitiveness [1]. However, the covering and support structure of the facility block sunlight, leading to a decrease in light intensity [2]. Grapes are light-requiring heliophiles and need higher light intensity for growth [3]. Therefore, it is necessary to improve the light environment in the protected facility cultivation of ‘Shine muscat’ grapevines. The supplemented light technique is one of the best ways to solve this problem.

The supplemental light technique has been widely applied in agriculture [4]. Currently, high-pressure sodium lamps are widely used to supplement light in greenhouses; however, light-emitting diodes (LEDs) are being increasingly implemented as either a substitute or an addition [5]. Especially with the development of materials technology, LED lights have been suggested as having great potential for reducing greenhouse energy use as they are extremely efficient at converting electricity into light [6]. Compared to conventional lamps, the primary benefit of LEDs is their superior efficiency in transforming electrical energy into photosynthetically active light [6]. LEDs are 60% more efficient than conventional horticultural lamps, and their efficiency is anticipated to increase in the future [7]. However, the application of LEDs in the facility cultivation of ‘Shine muscat’ is relatively scarce. It is necessary to study the influence of different LED supplementary light environments on grape growth and fruit formation.

The light environment includes light intensity, period, and quality [6]. Grapes are photophiles; light duration and light intensity are the most important factors for supplemental light. The saturation point of grapevine is about 700~1200 μmol photons/m²/s. The photosynthesis of leaves decreases when the lower photosynthetically active radiation is absorbed by leaves, leading to reduced accumulation of carbon products and fruit yield [4]. At the same time, low light intensity hinders flower bud formation and development by regulating hormone metabolism, e.g., auxin and cytokinin content and distribution [8]. Supplemental light can improve the light environment and increase the photosynthetic rate and related metabolic activity. However, excessive light causes photoinhibition in plants, which affects plant growth and also increases production costs and waste of energy [9]. Therefore, it is important to determine the appropriate supplemental light intensity for protected grape cultivation. Light duration is the hours of lighting. In particular, when the light intensity is above the light compensation point, the light duration significantly affects the formation of grape yield and quality [1]. Increasing light duration with supplemental light technology is used to promote early flowering and improve the photosynthetic rate of plants [10]. However, long supplementary light durations may lead to a decrease in the photosynthetic rate and related fluorescence parameters [11]. Therefore, supplemental lighting duration is also very important in improving grape production efficiency.

The aim of this research is as follows: to identify the mechanism by which light duration and intensity affect grape photosynthesis, yield, and fruit quality. The suitable supplemental light duration and intensity were determined to provide a theoretical and practical basis for the efficient cultivation of the ‘Shine muscat’ grape.

2. Materials and Methods

2.1. Plant Material and Growth Conditions

‘Shine muscat’ vines grafted with ‘3309m’ rootstock were cultivated in a greenhouse. The gap between individual vines and adjacent rows was 3 m. Young shoots were arranged parallel to the ground using a pergola support structure (‘inverted-L’-shaped horizontal framework) (Figure 1B). Fruit thinning, canopy management, and defoliation were uniformly applied across all treatments, adhering to standard industry methods.

A total of 160 two-year-old ‘Shine Muscat’ grapevines were selected for testing. The experiments were conducted from March to July 2024 at the Baiyun Experimental Station (23°23′ N, 113°26′ E; 20 m above sea level), Guangdong Academy of Agricultural Sciences, in Guangzhou, China. The experimental vineyard has sandy-textured soil, with a bulk density of 1.42 g/cm³, a field water-holding capacity of 25.35%, a pH level of approximately 7.3, and an electrical conductivity of 1.2 mS/cm.

2.2. Experimental Design

The experiment was conducted using a fully randomized design, with five grapevines per LED lighting treatment as replicates (Figure 1A). The supplemental LEDs were installed 0.3 m above the grapevine canopy. The supplemental light spectra were compound LEDs, including red LEDs (peak at 660 nm, with a halfwave width of ±11 nm) and blue LEDs (peak at 460 nm, with a halfwave width of ±12 nm), with a red–blue ratio of 1:1 (Figure 1C). The following were the supplemental light conditions for grapevines: (1) 6:00–9:00 and 15:00–18:00 supplemented LED with a photosynthetic photon flux density (PPFD) of 50 μmol/(m²·s); (2) 6:00–9:00 and 15:00–18:00 supplemented LED with a PPFD of 100 μmol/(m²·s); (3) 6:00–9:00 and 15:00–18:00 supplemented LED with a PPFD of 200 μmol/(m²·s); (4) 18:00–24:00 supplemented LED with a PPFD of 100 μmol/(m²·s); (5) 18:00–24:00 supplemented LED with a PPFD of 300 μmol/(m²·s); and (6) 18:00–24:00 supplemented LED with a PPFD of 500 μmol/(m²·s) (seen as

Table 1. Specified period and length of time for each treatment.

Supplemental Light Time hh:mm	Intensity of Supplemental Light [μmol·m−2·s−1]	Naming
6:00–9:00, 15:00–18:00	50	50-Day
	100	100-Day
	200	200-Day
18:00–24:00	100	100-Night
	300	300-Night
	500	500-Night
Unsupplemented	0	CK

Note: The location where the intensity of supplemental light measured is 0.3 m under the LEDs.

). The intensity of supplemental light was measured on the grapevine canopy, which was 0.3 m below the LEDs. All of the treatments were performed in different plots isolated from one another with a 6 m distance cultivated with grapevines. For each grapevine, 24 shoots were supplemented with an LED light strip (Lnled, Guangzhou, China) and compared with unsupplemented control vines (CK). Supplementary light was applied from the period of full flowering to the stage of fruit ripening. The spectral distribution of the LED light strips was measured using a spectroradiometer (Avaspec-2048CL, Avates, Apeldoorn, The Netherlands). The distribution of the photon flux density of the supplemental LED on the grape shoots is shown in Figure 1D. The indoor and outdoor air temperature, humidity, concentration of carbon dioxide, and the PPFD above the grapevine canopy were monitored with a meteorological observation station.

2.3. Leaf Biochemical Component Measurements

One day before harvest, the uppermost fully expanded leaves behind the LED were collected to measure their chlorophyll content. Next, 0.2 g leaves near the fruiting location were placed into 10 mL of 95% ethanol, and absorbance was recorded at 665, 649, and 470 nm using a UV–Vis spectrophotometer (UV-1800, Shimadzu, Tokyo, Japan). Chlorophyll levels were determined using equations proposed by Lichtenthaler, H.K. and Buschmann, C. [12].

2.4. Light Response Curve and Chlorophyll Fluorescence Measurements

Light response curve and chlorophyll fluorescence measurements were taken on a sunny day during the maturity of fruit in June. Four plants from each treatment were randomly selected, and measurements were performed on the third leaf at the beginning of the base, from 09:00 to 11:00 a.m. The measurements were performed with a Li-6800 (LI-COR, Lincoln, NE, USA). During data collection, CO₂R was maintained at 400 μmol/mol, and the gas flow was regulated at 500 μmol/s to sustain a reference RH of 50%. For the light response curve assessment, the photosynthetic photon flux density (PPFD) followed a gradient of [1600, 1200, 1000, 900, 600, 400, 200, 100, 50, and 0 μmol/(m²·s)], with readings taken once photosynthesis stabilized. The minimal (F₀) and maximal (F_m) fluorescence were recorded to determine the maximum quantum efficiency of PSII photochemistry (F_v/F_m) [13].

2.5. Grape Yield and Fruit Quality Measurements

Five ‘Shine Muscat’ grape clusters were randomly chosen from each treatment to evaluate fruit weight and quality at both the cluster and plant levels. The total soluble solids (TSS) content of the juice was measured using a refractometer (Master-M, Atago, Tokyo, Japan), while total acidity was determined via NaOH titration, followed by pH assessment [1]. Deseeded pulps were homogenized (X700, Disrad, Shanghai, China) for 1 min after the addition of 50 mg/L Vc, centrifuged at 10,000× g for 10min at 4 °C (Centrifuge 5425R, Eppendorf, Hamburg, Germany), and filtered on cellulose paper, and the pulp juice was obtained for volatile analysis. The pulp juice was adjusted to pH 3.9 to obtain the same matrix as soon as possible, and the volume of juice was measured to calculate the concentrations of aromatic compounds expressed in μg/kg [14]. Headspace solid-phase microextraction and gas chromatography–mass spectrometry were used to qualitatively and quantitatively analyze the volatile components of the fruits [15,16]. The characteristic substances and aroma characteristics were identified and analyzed based on odor activity value aroma profiles and aroma notes. All measurements were performed in triplicate.

2.6. Calculations and Statistics

Daily light integral (DLI) describes the rate at which photosynthetically active radiation is delivered over a 24 h period [17], mol·m⁻²·d⁻¹.

$$DLI=P\times T\times 3600\times {10}^{-6}$$

(1)

where P is the photosynthetic photon flux density received by the canopy (μmol·m⁻²·s⁻¹) and T is the time (h).

The rectangular hyperbola function was fitted to steady-state P_n light response data [18]:

$$P_{\mathrm{n}}={\displaystyle \frac{\alpha \left(1-\beta I_{\mathrm{a}}\right)I_{\mathrm{a}}}{\left(1+\gamma I_{\mathrm{a}}\right)}}-R_{\mathrm{d}}$$

(2)

where P_n is the net leaf photosynthetic rate (μmol·m⁻²·s⁻¹); α is the initial slope of the irradiance response curve of photosynthesis when irradiance approaches zero (μmol·μmol⁻¹ photons); I_a is estimated from the incident PPFD multiplied by the absorptance coefficient of single leaf (μmol·m⁻²·s⁻¹); R_d is dark respiration (μmol·m⁻²·s⁻¹); and β and γ are coefficients that are independent of I_a.

At the same time, the saturation light intensity (I_sat) and the maximum net photosynthetic rate (P_max) of the plant are calculated:

$$I_{\mathrm{sat}}={\displaystyle \frac{\sqrt{\left(\beta +\gamma \right)/\beta }-1}{\gamma }}$$

(3)

$$P_{\max }=\alpha {\left({\displaystyle \frac{\sqrt{\beta +\gamma }-\sqrt{\beta }}{\gamma }}\right)}^2-R_d$$

(4)

The chlorophyll use efficiency of maximum leaf photosynthesis (P_max/Chl) was calculated as the P_max divided by chlorophyll content.

Maximum quantum efficiency of PSII (F_v/F_m), potential quantum efficiency of PSII (F_v/F₀), and PSII operating efficiency (Φ_PSII) were calculated according to the method of Baker [13].

The impact of the treatments on the studied plants was assessed via an analysis of variance (ANOVA), followed by a least significant difference (LSD) test at a 5% significance level, performed using SPSS 23.0 software (IBM), to determine statistical significance. Bar and line charts were generated and refined using the educational version of Origin 2024 (Origin Lab). Multiple comparisons were conducted with Duncan’s new multiple-range test.

3. Results

3.1. PPFD Received by the Canopy of Grapevines

The photosynthetic photon flux density (PPFD) and DLI received by the canopy of grapevines of CK are shown in Figure 2A. The average DLI received by the canopy of grapevines of CK during the 45 days after full flowering was 31.6 mol·m⁻²·d⁻¹ (Figure 2A) because it was cloudy and rainy in April and May in South China. However, it was sunny after June, with the average DLI received by the canopy of grapevines of CK during the 46~86 days after full flowering being 49.5 mol·m⁻²·d⁻¹. Meanwhile, regarding the results of the light response curve in Section 3.2.2, the light compensation point of leaves in the treatments was 13.3~27.7 μmol·m⁻²·s⁻¹. The effective light time was defined and the daily light time was the PPFD above the max light compensation point of 27.7 μmol·m⁻²·s⁻¹ (Figure 2A). It can be seen from Figure 2A that the average effective light time of the 20 days after full flowering was 10.1 h, 1.7 h lower than that of later times.

The cumulative value of DLI and effective light time from the period of full flowering to the stage of fruit ripening of each treatment is shown in Figure 2B. The light duration and intensity were adjusted by controlling the supplemental time and intensity of the LEDs in the canopy of grapevines. Later chapters will discuss its impact on grape photosynthesis, yield, and fruit quality. Meanwhile, the power consumption of LEDs used in the treatments was recorded and shown in Figure 2B.

3.2. Leaf Physiological Properties

3.2.1. Leaf Biochemical Components

Compared to CK, grapevine leaves cultivated under supplemental LED lighting exhibited a relatively higher chlorophyll content (Figure 3A–D). The chlorophyll content increased significantly when the supplemental light intensity increased from 0 to 100 μmol·m⁻²·s⁻¹ (Figure 3A). However, the chlorophyll content did not increase significantly when the supplemental light intensity increased from 100 to 500 μmol·m⁻²·s⁻¹. Meanwhile, the total chlorophyll content (Figure 3A), carotenoid content (Figure 3C), and carotenoid/Chl(a + b) ratio (Figure 3D) of leaves of grapevines grown under supplemental light in the daytime was higher than that of grapevines with supplementary light at night. The total chlorophyll content of the leaves of grapevines of 100-Day is 2.26 mg·g⁻¹, which is 4.6% higher than that of 100-Night (Figure 3A). Differences in chlorophyll a/b ratio between treatments were negligible (Figure 3B).

3.2.2. Light Response Curve and Related Parameters

The influence of supplemental lighting on the light response curve (Figure 4), the light compensation point (I_c), and saturation light intensity (I_sat) (Figure 5), along with other photosynthetic parameters (

Table 2. Light response curve fitting parameters of grapevines under different treatments (n= 3).

Treatment	α	Pmax [μmol·m2·s−1]	Rd [μmol·m−2·s−1]	Pmax/Chl [μmol·g·m−2·s−1·mg−1]
CK	0.042 ± 0.002 a	4.81 ± 2.64 a	0.95 ± 0.15 a	3.59 ± 1.81 a
50-Day	0.038 ± 0.008 a	6.36 ± 1.55 a	0.75 ± 0.21 a	3.90 ± 1.78 a
100-Day	0.036 ± 0.005 a	6.12 ± 1.97 a	0.39 ± 0.13 a	2.72 ± 0.72 a
200-Day	0.046 ± 0.009 a	7.50 ± 3.17 a	0.75 ± 0.22 a	3.26 ± 0.67 a
100-Night	0.047 ± 0.011 a	7.30 ± 2.33 a	0.74 ± 0.13 a	3.36 ± 1.44 a
300-Night	0.056 ± 0.010 a	8.04 ± 0.25 a	0.88 ± 0.10 a	3.73 ± 1.30 a
500-Night	0.050 ± 0.012 a	5.20 ± 2.36 a	0.88 ± 0.25 a	2.46 ± 0.06 a

Note: Letters show statistically significant differences (p < 0.05).

), was examined on the adaxial side of the leaf. Leaves under supplemental light treatment showed higher P_n when the absorbed PPFD was above 200 μmol·m⁻²·s⁻¹ (Figure 4). P_n increased as the supplementary light intensity increased from 0~300 μmol·m⁻²·s⁻¹. However, the P_n of leaves subjected to 500-Night treatment was lower than that of the other treatments. The difference in P_n between 100-Day and 100-Night was negligible.

The light compensation point (I_c) in the treatments was 13.3~27.7 μmol·m⁻²·s⁻¹ (Figure 5A). The I_c of CK and 50-Day was significantly higher than the other treatments (Figure 5A). The saturation light intensity (I_sat) in the treatments was 680.2~1143.9 μmol·m⁻²·s⁻¹ (Figure 5B). The I_sat of 500-Night was significantly lower than the other treatments (Figure 5B). Differences in α, P_max, R_d, and P_max/Chl between treatments were negligible (Table 2).

3.2.3. Leaf Chlorophyll Fluorescence Properties

The leaves of grapevines subjected to supplemental light had higher F₀ (Figure 6A) and F_m (Figure 6B) of leaf chlorophyll than the CK. Meanwhile, leaves subjected to the treatments showed obviously higher F_v/F₀ (Figure 6C) and F_v/F_m (Figure 6D) than CK. There was no significant difference in Φ_NO, Φ_PSII, and Φ_NPQ between the treatments and CK (Figure 7). The energy allocation pattern of PSII among the treatments was largely identical.

3.3. Yield and Fruit Quality

3.3.1. Grape Yield

Applying supplemental lighting to the grapevine canopy enhances the fruit yield of ‘Shine Muscat’ grapes (Figure 8). The yield per 667 m² was calculated by multiplying the fruit weight per grape by 82 plants. The fruit yield of grapes grown under supplemental light at night was higher than that of in the daytime. In particular, the grape yield of 300-Night and 500-Night treatments increased the yield most significantly, increasing by 45.1% and 36.8% compared to CK (Figure 8B), respectively.

Figure 8C shows the gross value of grapes per 667 m², the cost of power consumption of the LED per 667 m², and the benefit increased by LED supplemental light per 667 m². The gross value was obtained with the market price of ‘Shine muscat’ of 5.6 USD/kg. The cost of power consumption of the LED in the grape growing season was also calculated with the price of agricultural electricity of 0.084 USD/kWh. The benefit increased by LED supplemental light was the difference between the gross value and the cost of power consumption of the LED relative to that of CK, which was not considered the initial cost of the LED due to LED prices being unstable and becoming cheaper with the development of material technology.

The grape gross value of 300-Night and 500-Night treatments was 7236.0 USD/667 m² and 7221.9 USD/667 m², which was higher by 45.1% and 44.7% than that of CK (Figure 8C), respectively. The cost of power consumption of the LEDs in 500-Night treatment during the grape growing season was the highest at 1777.1 USD/667 m² with the power consumption of 21,156 kWh (Figure 2B), which was 1.7 times higher than that of the 300-Night treatment (Figure 8C). Lastly, 300-Night treatment has the highest benefit of 23.6% compared to CK (Figure 8C).

3.3.2. Fruit Quality

Grapevines exposed to supplemental light exhibited an increase in glucose total soluble solids but a decrease in total acidity (

Table 3. Response of grape fruit to different treatments (n = 5).

Treatment	Vitamin C [mg/100g]	Total Acidity [g/kg]	Total Soluble Solid [%]	Glucose [g/100g]
CK	4.81 ± 0.18 cd	3.66 ± 0.16 a	15.83 ± 0.14 a	14.43 ± 0.10 b
50-Day	5.45 ± 0.16 b	3.20 ± 0.19 b	16.14 ± 0.46 a	14.79 ± 0.50 b
100-Day	5.95 ± 0.24 a	3.01 ± 0.15 bc	16.38 ± 0.22 a	14.80 ± 0.51 b
200-Day	4.42 ± 0.11 d	2.86 ± 0.07 c	16.86 ± 0.05 a	14.98 ± 0.11 b
100-Night	4.34 ± 0.10 d	2.84 ± 0.06 c	16.46 ± 0.86 a	15.13 ± 0.42 b
300-Night	3.85 ± 0.05 e	2.88 ± 0.07 c	16.82 ± 0.06 a	17.30 ± 0.36 a
500-Night	5.02 ± 0.24 bc	3.18 ± 0.06 b	16.38 ± 0.42 a	16.32 ± 0.09 a

Note: Letters show statistically significant differences (p < 0.05).

). The vitamin C level of grapes grown under supplemental light in the daytime was higher than at night. The sugar–acid ratio served as a key parameter of fruit quality, calculated by dividing the total soluble solids content by the total acidity of the grape. As can be seen from Figure 9, grapes grown in a supplemental light environment showed a significantly higher sugar–acid ratio than CK. The sugar–acid ratio of 300-Night treatment was obviously higher than that of the other treatments.

3.3.3. Aroma Characteristics

‘Shine-Muscat’ grapes grown in a supplemental light environment showed a higher concentration of trans-2-Hexena (Figure 10A). The alpha-Terpineol (Figure 10B) and linalool (Figure 10C) levels following the 50-Day and 100-Day treatments were higher than the other treatments. Differences in 6,10-dimethylundeca-5,9-dien-2-one between treatments were negligible (Figure 10D). It is worth noting that the aroma contents of grapes grown under supplemental light at night were lower than that of in the daytime.

4. Discussion

4.1. Leaf Physiological Properties in Response to Supplemental Light

Grapevines thrive in light, and their photosynthetic rate rises as light intensity increases, provided that it remains below the saturation point [4]. Differences in the photosynthetic rate between treatments were negligible (Figure 4). The light response curve in this study was measured from 09:00 to 11:00 a.m. The nighttime supplemental light was off during this time (Table 1). Differences in supplemental light intensity in the daytime were not obvious. This was in accordance with the results of Ref. [4].

The I_sat of leaves in the treatments was 680.2~1566.9 μmol·m⁻²·s⁻¹. The stronger the supplementary light intensity, the lower the I_sat (Figure 5B). This indicated that in the absence of incident light on the leaves, grapevines are always in a state of hunger, so the light saturation point of these plants is highest. Meanwhile, the maximum net photosynthetic rate is the net photosynthetic rate of leaves at the light saturation point, which can be used to express the photosynthetic ability of plants under strong light and reflect the photosynthetic potential of plants [19]. Differences in P_max between treatments were negligible (Table 2). This means that the supplementary light intensity does not reach the saturation light intensity.

The net photosynthetic rate is negative when the light intensity is lower than the light compensation point (Figure 4). Through analysis of the light response, the light compensation point of the treatments was 13.3~27.7 μmol·m⁻²·s⁻¹ (Figure 5A). The light saturation point of grape leaves was higher under low-light-intensity conditions (Figure 5A). The light intensity of supplemental light must be within the range of the light compensation point and saturation light intensity. Differences in the light response curve and related parameters of leaves between increasing light duration and intensity with supplemental LEDs were negligible (Table 2).

Chlorophyll a is the main component of the photosynthetic reaction center complex, which mainly absorbs red and blue light [20]. Chlorophyll b is the component of the light-harvesting pigment–protein complex, which mainly absorbs blue light [21]. The red–blue ratio for supplemental LED light in this study was 1:1 (Figure 1). The total chlorophyll content of treatments was higher than that of CK (Figure 3). The chlorophyll a/b ratio can reflect the adaptability of plants to light intensity in the growing environment, and the difference in chlorophyll a/b ratio between treatments was negligible (Figure 3B). Supplemental light quality has a greater effect on the chlorophyll a/b ratio than supplemental light intensity [22].

F₀ represents the lowest fluorescence intensity of the photosynthetic system when all PS Ⅱ centers remain open during dark adaptation, originating from antenna chlorophyll. Fm denotes the peak fluorescence intensity when all PSII centers are fully closed under dark-adapted conditions, serving as the standard maximum fluorescence [23]. A significant increase in F₀ and a slight decrease in F_m cause a decrease in F_v/F_m, which is considered an indicator of photoinhibition [24]. In this study, the F₀ and F_m of the treatments showed the same trend; the difference in F_v/F_m (Figure 6D) between treatments was negligible. The values of chlorophyll fluorescence, chlorophyll content, and α all verified that the leaves of the grapevines were not photo-inhibited and the light system was not damaged under a supplementary light environment [25]. Meanwhile, there was no significant difference in Φ_NO, Φ_PSII, and Φ_NPQ between the treatments and CK (Figure 7), which once again proved that the light adaptability and self-protection ability of ‘Shine muscat’ grapevines are strong [26].

Grapes are light-requiring heliophiles and long-day plants. The intensity of supplemental light was low, with it being lower than the saturation light intensity of 1143.9 μmol·m⁻²·s⁻¹. According to the results of the total chlorophyll (Figure 3A) and chlorophyll a/b ratio (Figure 3B), there were negligible differences between the treatments. This indicated that the chlorophyll content and structure of chloroplasts were not significantly changed by low-intensity supplemental light. This was also proved by the leaf chlorophyll fluorescence properties (Figure 6). In general, the photosynthetic system of the leaf was significantly affected when the plants encountered a stressful environment [27], such as high temperatures, low light, etc.

4.2. Effects of Supplemental Light on Grape Yield and Fruit Quality

The influence of light duration, in which the light intensity was above the light compensation point, on grape yield and fruit quality cannot be ignored. Light duration and light intensity both directly affect the accumulation of photosynthetic products in grapes and then affect the formation of grape yield and quality [28]. The contribution value of light duration and light intensity on grape yield and fruit quality is the focus of this study.

Supplemental light at night can significantly improve the grape yield in the daytime. Prolonging the illumination time is beneficial to the accumulation of photosynthetic products [29]. In this study, the grape yield of 100-Night treatment was 6.0% higher than that of 100-Day treatment (Figure 8A). At the same time, enhancing light intensity on the grapevine canopy can boost both fruit yield and quality [29,30]. A similar trend was noted in this study, where additional lighting on the canopy led to a higher grape yield (Figure 8) and improved fruit quality (Table 3). Supplemental light at night means prolonging the light duration. And the synthesis time of grape leaf assimilates increased. This indicated that supplemental light at night can prolong the time of sugar metabolism in the source (leaves) and increase surge transportation to the sink (fruits) [11]. However, supplemental light in the day can increase surge metabolism in the leaves, but the capacity of surge transportation from the leaves to fruits is limited and cannot be higher than a certain value for grapes. Meanwhile, the sunlight intensity and duration are insufficient in Southern China, which results in reduced sugar accumulation and decreased content in grapes. Increasing canopy light supplementation aids in the movement of photosynthetic products from foliage to berries, leading to enhanced sugar accumulation [1]. In addition, a relationship between light and berry weight was observed [31]. This accounts for the significantly increased berry weight observed under supplemental lighting in the present study (Figure 8). The grape yield of 300-Night and 500-Night treatments was significantly higher than the other treatments (Figure 8A).

Supplemental light at night can slightly improve fruit quality compared to that in the daytime. In this study, the sugar–acid ratio of 100-Night treatment was 7.3% higher than that of 100-Day treatment (Figure 9). Organic acids exhibit greater sensitivity to environmental conditions than other physicochemical properties [32]. Supplemental light can improve the light intensity and cumulative DLI on grapevines, and there was a significant difference in the sugar–acid ratio between the treatments and control (Figure 9). Meanwhile, the sugar–acid ratio of 300-Night treatment was significantly higher than the other treatments (Figure 9).

Comprehensive analysis of the grape yield and fruit quality, supplemented with light at night with the light intensity of 300 μmol·m⁻²·s⁻¹, has great advantages. However, the aroma contents, e.g., alpha-Terpineol and linalool, were obviously lower than those of other treatments (Figure 10). Earlier research indicated that the accumulation of sugars, acids, and aromatic compounds occurs independently [33]. The study confirmed that a high sugar–acid ratio does not necessarily mean high aroma content.

4.3. Analysis of the Energy Consumption of LEDs

‘Shine muscat’ grapevines must be cultivated in protected facilities to avoid insects and rain, which results in utilizing supplemental light technology. Light-emitting diodes (LEDs) are considered highly promising for lowering greenhouse energy consumption due to their exceptional efficiency in converting electricity into light [6]. In this study, the power consumption (Figure 2B) and cost (Figure 8B) of the LED treatments was recorded and calculated. The power consumption of 300-Night treatment was 12693.6 kWh per 667 m² (Figure 2B), and the cost of power consumption was USD 1066.2 with the price of agricultural electricity at 0.084 USD/kWh (Figure 8B). The grape gross value of 300-Night treatment was 7236.0 USD/667 m² (Figure 8B). The benefit of 300-Night treatment increased by LED supplemental light was 23.6%, which was the highest of all treatments. This type of analysis is essential for farmers and policymakers to determine the most suitable lighting technology based on their regional climate and energy conditions, ensuring optimal and eco-friendly energy utilization.

The total carbon emissions can be calculated with the method of Ref. [34]. The carbon emissions generated by the supplemented LED mainly come from the energy consumption of lamps. The total carbon emissions of the supplemental LED with the photosynthetic photon flux density (PPFD) of 300 μmol/(m²·s) was 18 mg·m⁻²·s⁻¹. Of note, 55.6 kg of CO₂ was produced during the entire grape growth period. However, the CO₂ assimilation gain of grapevines was not considered, which will reduce carbon emissions. In the future, intermittent supplemental light can be used to balance energy consumption and grape cultivation.

5. Conclusions

This study demonstrates that prolonging light duration at night was more effective in improving grape yield and fruit quality than increasing light intensity in the daytime with supplemental LED lights. The chlorophyll content of the leaves of grapevines grown in the supplemental light environment was higher than that of the control. The light compensation point and the saturation light intensity of the leaves were decreased by the supplemental light. However, differences in other photosynthetic parameters, e.g., P_n, α, P_max, R_d, and P_max/Chl, between treatments were negligible. Grape yield increased significantly when the supplementary light intensity was higher than 200 μmol/(m²·s). The light intensity of the supplemental LED with the PPFD of 300 μmol/(m²·s) at 18:00-24:00 had the highest grape yield, sugar–acid ratio, and economic benefit with improvement values of 45.1%, 51.4%, and 23.6% compared to CK, respectively. However, supplemental light does not obviously increase the aroma contents of grapes. In the future, the market price of grapes needs to be considered to further determine the intensity and duration of supplementary lighting from an economic point of view. Moreover, intermittent supplemental light can be used to balance energy consumption and grape cultivation.