Effects of Light Quantity and Quality on Horticultural Crops

1. Introduction

Light plays a fundamental role in the growth and development of plants. It is the primary energy source of photosynthesis, enabling the process of carbon assimilation in the chloroplast [1]. On the other hand, light is an environmental signal that stimulates physiological processes and affects the synthesis of secondary metabolites in horticultural crops [2]. The rate of photosynthesis determines biomass accumulation in plant tissues, and it is a major driver of plant yield. In contrast, secondary metabolites significantly impact the phytonutrient profile and nutritional quality of crops [3].

The lighting environment in which plants thrive is also characterized by quantitative and qualitative parameters. In horticulture, the quantitative measures of light are the photoperiod and the photosynthetic photon flux density (PPFD), which correspond to the number of incident photons of photosynthetically active radiation (PAR) per area and per time interval [4]. PAR is defined as a waveband ranging from 400 to 700 nm [5]. PPFD is expressed in µmol m⁻² s⁻¹.

The term “light quality” is associated with the spectral distribution of photon irradiance. Absorption spectra of various photoreceptors in higher plants cover a much broader wavelength range than PAR, spanning from 280 nm to 800 nm [6]. Photoreceptors sense the spectral differences in irradiance, trigger growth, and developmental processes, allowing plants to adapt to various environmental conditions. Light spectra in horticulture are often categorized by photon irradiance ratios [7]. The PAR waveband is divided into 100 nm wide wavelength intervals: B (blue, 400–500 nm), G (green, 500–600 nm), and R (red, 600–700 nm). Additional wavebands used in horticulture are the FR (far-red 700–800 nm), UV-A (315–400 nm), and UV-B (280–315 nm) wavebands. The full-spectrum white light spanning the PAR waveband is often abbreviated as W (400–700 nm). The quotient of photon irradiances, measured in two wavebands, e.g., the red/blue ratio (R/B), is regarded as a light quality attribute. Another often-used light parameter is the red/far-red (R/FR) photon ratio.

The quantity and quality of light are strongly correlated in nature. Daylight intensity and light color change with solar elevation, the altitude of the location, and meteorological conditions. Daylight intensity and spectral features, such as the R/B ratio or R/FR ratio, are not independent parameters and vary within a relatively narrow domain [8]. Shading nets [9] can reduce daylight intensity with a minor change in the spectral distribution of incident light. Supplementary lighting in greenhouses [10] extends the photoperiod and provides light treatments that are beneficial for crop growth and development [11]. LEDs, as a sole source of light, enable the complete separation of the quantitative and qualitative light parameters, enabling the testing of spectra that do not occur in nature [12].

This Special Issue, “Effects of Light Quantity and Quality on Horticultural Crops”, was launched to collate research results on the interactive response of plants to variations in light intensity and spectral distributions.

2. Overview of Published Articles

The publications in the Special Issue cover a broad range of lighting solutions for horticultural crops. The quantitative and qualitative light parameters, the investigated crop, and the measured effects are summarized in

Table 1. Main findings of the Special Issue in the order of contribution number.

#	Light Quantity	Light Quality	Crop	Effect
1	PPFD: 90, 180 µmol m−2 s−1 Photoperiod: 12 h	R/B: 2.1, 3.1, 5.0	lettuce	growth traits nutrient profile
2	PPFD: 160 µmol m−2 s−1 Photoperiod: 12 h	B, R, W, B + R + W	Ethiopian kale	growth traits nutrient profile
3	PPFD: 200 µmol m−2 s−1 Photoperiod: 12, 14, 16, 18 h	W	Chinese cabbage	bolting and flowering time, gibberellin conc.
4	Relative solar irradiance: 75–100% Photoperiod: 10–14 h (daytime)	Control: no shade Gray, blue, black	piquin pepper	phytochemical profile
5	PPFD: 33–390 µmol m−2 s−1 Photoperiod: 16 h	R/B: 2.01–2.78 R/FR: 2.57–4.27	pea	growth traits
6	Relative solar irradiance: 50%, 75%, 80%, 100% Photoperiod: (daytime)	Control: no shade Black shading nets	tomato	biomass, photosynthesis rate, metabolism
7	Relative solar irradiance: 25%, 50%, 75%, 100% Photoperiod: (daytime)	Control: no shade Black shading nets	blueberry	hormone and enzyme activities
8	PPFD: 80, 160 µmol m−2 s−1 Photoperiod: 14 h	R/B = 4.1	lemon basil	growth traits, phenolic content
9	Supplementary LED light PPFD: 40, 170 µmol m−2 s−1 Photoperiod: 16 h, 0.5 h EOD	R/B = 3 FR R/B = 3 + FR	tomato	crop yield and quality
10	Review paper covering a broad range of light parameters		pepper	phytochemical profile

in order of contribution number. Five papers (Contributions 1, 2, 3, 5 and 8) described experiments using LEDs as the sole source of light. In three contributions (4, 6, and 7), shading nets were employed to reduce the light intensity of natural daylight. In Contribution 9, LEDs were employed as supplementary lighting in a greenhouse, and the final publication (Contribution 10) is a review of the phytochemical profile of peppers (Capsicum fruits) affected by a broad range of lighting conditions.

A hot chili pepper variant, piquin pepper (Capsicum annuum L. var. glabriusculum) was the focus of Contribution 4. Two publications (6 and 9) dealt with tomato (Solanum lycopersicum L.) cultivars. Other horticultural crops explored in this Special Issue were lettuce (Lactuca sativa L.) (1), Ethiopian kale (Brassica carinata A. Braun) (2), non-heading Chinese cabbage (Brassica campestris spp. chinensis Makino) (3), pea (Pisum sativum L., cv. Kleine Rheinländerin) (5), blueberry (Ericaceae, Vaccinium) (7) and lemon basil (Ocimum citriodurum Vis.) (8).

2.1. LEDs as a Sole Source of Light

In Contribution 1, Hernández-Adasme et al. tested three different light spectra (B + W, R + W and R + B) at two PPFD levels enabling the interaction of light quantity and quality to be studied. The high PPFD increased the fresh weight of lettuce and total phenolic and flavonoid content relative to low levels. On the other hand, antioxidant activity decreased with the increase in light intensity. These results highlight the importance of controlling light intensity to optimize the nutrient profile of the horticultural crop.

Substrate and light quality interactions were revealed in Contribution 2. The effect of four different spectra (B, R, W, and B + R + W) at 160 µmol m⁻² s⁻¹ on kale microgreens was studied using three types of substrates. The research is a good example for design space screening and identifying the cultivation parameters, leading to high and affordable yields.

Contribution 3 is the only paper in this Special Issue that uses the photoperiod as a quantitative light parameter. Liu et al. measured the bolting and flowering time as a function of the photoperiod and determined the lighting conditions required to achieve the optimum stem morphology. The duration of light/dark time intervals were set at four different levels (12/12, 14/10, 16/8, and 18/6 h), whereas the PPFD was kept constant at 200 μmol·m⁻²·s⁻¹. Endogenous gibberellin concentrations measured in stem tips and young leaves indicated that the bolting and flowering of cabbage is regulated through the synthesis of gibberellin.

Contribution 5 by Balázs et al. is an outlier in a sense that the article presents a method of quantifying variations in lighting environments using pea microgreens as the sole test vehicle to demonstrate the effect of light intensity and light quality variations in a vertical farm. A broad PPFD range (33–390 µmol m⁻² s⁻¹) was established on the cultivation trays by adjusting the power of the LEDs and the distance between the LED luminaires and the illuminated crop. The fresh weight of pea seedlings exhibited strong correlations with local PPFD levels measured in a high-spatial-resolution experiment. The study highlighted that the local light intensity accounted for 31% of the fresh weight variations, and the rest of the noise was attributed to the differences among the individual plants.

The growth, yield, and phenolic content of lemon basil was investigated by Daud et al. (Contribution 8) under controlled environmental conditions. Plants were grown under mixed red and blue LED light, with an R/B ratio of 4.1. Two PPFD levels, 80 and 160 μmol·m⁻²·s⁻¹, were tested in the experiment. The electrical conductivity (EC) (concentration) of the nutrient solution was an additional factor set at four different levels. The experiment demonstrated that under low PPFD levels, plant development was limited by light availability, and there were minor differences in the fresh weights among the four different nutrient concentrations. The interaction between photon irradiance and EC became apparent at high PPFD. The maximum yield and the best phytochemical traits were measured in the same PPFD = 160 μmol·m⁻²·s⁻¹, EC = 2.6 mS cm⁻¹ treatment.

2.2. Shading Net Experiments

Colored shading nets affect several environmental parameters during crop growth: light intensity, spectral distribution of light, and microclimates, including air temperature and relative humidity. In the experiments of Jiménez-Viveros and Valiente-Banuet (Contribution 4), the phytochemical profile of piquin pepper cultivated under four shading conditions was investigated. The maximum reduction in light intensity was 25% in the case of black mesh relative to the control without shading. The air temperature reduction was 10% or less at the peak temperature during one day. This work highlighted the beneficial effect of shading on the fruit quality, but the relevance of the key conclusions is limited to growers in tropical/sub-tropical regions. In an additional review (Contribution 10), Jiménez-Viveros et al. summarized the effects of light on the nutritional properties of pepper.

In Contribution 7, An et al. studied the physiological response of blueberry in another shading net experiment. Anthocyanin content and key enzyme activities were measured in blueberry leaves cultivated under 25%, 50%, 75%, and 100% intensity of natural daylight. The endogenous hormone concentrations and enzyme activities positively correlated with light intensity. Anthocyanin concentration, however, exhibited a maximum at 75%. The molecular mechanism of anthocyanin synthesis through light intensity regulation was a key finding of this research.

The advantage of shaded tomato cultivation in warm tropical and sub-tropical climate was highlighted by Delgado-Vargas et al. in Contribution 6. Two tomato cultivars were grown under four solar irradiance conditions using three different types of shade mesh: 100% (without mesh crop cover), 80%, 75%, and 50%. The photosynthetic rate measurements revealed that open-field plants were exposed to the highest level of abiotic stress, limiting the growth and development of both cultivars. The photon irradiance exceeded the light saturation limit early in the morning and remained above the saturation level throughout the day. The shade nets were efficient in reducing the light intensity and decreasing the air temperature at the canopy, resulting in the alleviation of stress factors.

2.3. Supplementary LED Lighting

While designed shading was advantageous in tropical climates, the shade of the construction elements in a building integrated rooftop greenhouse created suboptimal lighting conditions for tomato cultivation in the Mediterranean area. Appolloni et al. tested three lighting strategies to supplement solar radiation and found 17% yield increase regardless of the type of treatment used (with or without far-red, during the whole day or at the end of the day). The paper investigated the economics of the supplementary lighting by estimating the specific production cost increase associated with the energy consumption of LED luminaires.

3. Conclusions

The ten publications in this Special Issue covered only a tiny proportion of the practical lighting challenges that horticulturalists are faced with. The lighting applications described ranged from shading nets to LEDs as a sole source of light. Several publications investigated the interactions between the lighting conditions and other environmental parameters, which are indispensable for optimizing closed indoor cultivation systems. The publications described methods to find the trade-off between the quantity and quality (nutritional profile) of the horticultural crop.

The use of qualitative attributes to describe the lighting environment significantly limits the transferability of experimental results. This limitation is particularly pronounced in shade net experiments. While relative solar irradiance allows for direct comparison of parallel tests differentiated by the shading net type only, the absence of absolute values for B, G, R, and FR wavebands hinders the direct comparison with results from other experiments conducted under different conditions. Future studies should incorporate absolute measurements of these wavebands as a function of time to enhance the comparability and robustness of findings across diverse experimental setups.