Effects of Light Spectra on Morphology, Gaseous Exchange, and Antioxidant Capacity of Industrial Hemp

Abstract

Introduction

Industrial hemp (Cannabis sativa L.) is an ancient and versatile crop valued for its uses in food, fiber, and medicinal industry (Rehman et al., 2021a). Environmental stress lead to slower growth of hemp (Jiang et al., 2021). Therefore, it is necessary to find out most favorable conditions for optimal production of industrial hemp.

Light is one of the essential environmental factors for plant growth and development. Varying light quality affects the plant growth (Rehman et al., 2017, 2020). During photosynthesis, green plants capture light energy and transform it into chemical energy (Fukuda et al., 2008). However, shifting wavelengths can affect plant morphology, anatomy, and physiology, identified by phytochromes (Kami et al., 2010; Saleem et al., 2019). For example, The blue and red light wavelengths are identified to affect many plant physiological processes during growth and development, mainly photosynthesis. (Li et al., 2020). In a previous study, blue light increased the rate of germination, leaves number per plant, number of roots, and frequency of stomata and pigment contents in Stevia rebaudiana Bertoni (Simlat et al., 2016). Blue light have an effect on chlorophyll biosynthesis, plant height, and stomatal opening (Heo et al., 2002; Jao et al., 2005; Shimazaki et al., 2007). Tomato grown under BL showed high yield and quality of fruit with resistance to disease (Xu et al., 2012). Blue light can also promote the accumulation of phenylpropanoid-based compounds without affecting the plant morpho-anatomical traits, while red light alters the plant morphology and physiology without showing a positive effect on secondary metabolism (Landi et al., 2020). Red light wavelength promote stem growth and flowering (Rehman et al., 2017). According to Naznin et al. (2019) Blue and red light proportion is important improve growth, pigment, and antioxidant capacity in vegetable plants under controlled conditions but the proportion of blue light with red light is species dependent. Furthermore, previous literature suggests species-specific response toward light stress in plants (Cope and Bugbee, 2013; Rehman et al., 2020) and this response of plants to light is mediated by different photoreceptors. For example, cucumber is more responsive to spectral distribution than tomato in greenhouse (Trouwborst et al., 2010; Hernández and Kubota, 2012).

Light being an important environmental factor also influences the gas exchange characteristics of a plant. For example, guard cells respond directly to blue light (Mott, 2009). Red light showed higher photosynthetic activity in B. nivea (Rehman et al., 2020). Red light increases stomatal conductance, net photosynthesis, intercellular CO₂, and transpiration rate, while blue light reduce these gas exchange parameters (Saleem et al., 2020). Reduction in gas exchange attributes under blue light revealed a reduction in photosynthesis associated with stomatal closure, thus reducing transpiration rate and intercellular CO₂ (Hogewoning et al., 2010).

Stressful environmental conditions trigger excessive production of reactive oxygen species (ROS), such as superoxide radical (O²⁻), hydrogen peroxide H₂O₂, hydroxyl radicals (OH), and singlet oxygen (O₂; Maurya et al., 2020; Turan, 2020; Rehman et al., 2021b). According to Asada and Takahashi (1987), 1 % of oxygen is diverted to produce ROS in plants. ROS can damage cellular components through oxidation of carbohydrates, lipids, proteins, and DNA which cause plant death (Shah et al., 2019; Bhuyan et al., 2020). To avoid this loss, plants regulate the production of ROS by recruiting enzymic and non-enzymic antioxidants (Kurutas, 2016; Sachdev et al., 2021). Antioxidant enzymes activity under varying light spectra is more complex for example callus grown in dark showed higher antioxidant enzymes activities (Adil et al., 2019). Exposure of higher proportion of blue light with red light caused an increase in antioxidant activity in lettuce (Son and Oh, 2013). Blue light showed significantly greater antioxidant enzymes activity in Anoectochilus roxburghii (Ye et al., 2017). Similar observations of higher SOD and CAT activity in tomato leaves were recorded under blue light as compared to red light (Kim et al., 2013). Conversely, the antioxidant activity in Rubus hongnoensis was promoted under red light treatment (Oh et al., 2021). Accumulation of MDA and proline contents in leaf indicate oxidative stress (Deng et al., 2012; Rehman et al., 2019). Proline protects the plants from stresses as well as helps to recover from stress (Hayat et al., 2012). Above literature suggests that light quality evoke diverse morpho-physiological response in plants with contrasting results. Therefore, impacts of different color light wavelengths on plant response and their underlying mechanisms remain elusive and required further investigation.

Present study was designed to investigate the effects of light colors on industrial hemp growth, gaseous exchange, and antioxidant capacity with an objective to find the best suitable light spectrum for optimized hemp production. Findings of present study would provide an improved understanding of the physiological and photosynthetic responses of hemp to light quality.

Materials and Methods

Results

Influence of Light Quality on Chlorophyll Content and Gas Exchange

Chlorophyll change (SPAD unit) was determined in industrial hemp leaves grown under different color light-emitting diodes (Figure 1). Blue light improved the Chl contents by 7.4%; however, Chl contents were declined by 16.8 and 10.9% under RL and RBL, respectively, as compared to WL. Blue light promoted gas exchange traits in industrial hemp as represented by significant increase in net photosynthesis (Pn), stomatal conductance (Gs), transpiration rate (Tr), and intercellular CO₂ (Ci) by 13.3, 44.1, and 12.8 and 8.8%, respectively, as compared to WL. Conversely, RL caused significant decrease in net photosynthesis (47.3%), stomatal conductance (54.2%), transpiration rate (52.7%), and intercellular CO₂ (43.9%) compared with the WL. Furthermore, mix light RBL also showed decreased in gas exchange trait compared with WL (Figure 2).

Figure 1

Effects of differential light quality on SPAD (Soil Plant Analysis Development) value of industrial hemp grown under different color light-emitting diodes. Bars indicated the mean ± SD (n = 3). Different letters on bars indicated significant difference between treatments at p ≤ 0.05. Different abbreviations used in the figure are as follows: WL, white light LED (control); BL, blue light LED, RL, red light LED; RBL, 50% red and 50% blue Light LED.

Figure 2

Effects of differential light quality on net photosynthesis (A), stomatal conductance (B), transpiration rate (C), and intercellular CO₂ concentration (D) in industrial hemp grown under different color light-emitting diodes. Bars indicated the mean ± SD (n = 3). Different letters on bars indicated significant difference between treatments at p ≤ 0.05. Different abbreviations used in the figure are as follows: WL, white light LED (control); BL, blue light LED, RL, red light LED; RBL, 50% red and 50% blue Light LED.

Correlation and Principal Component Analysis

Correlation between different studied parameters of plant growth, chlorophyll content, gas exchange, and antioxidant capacity of industrial hemp grown under different colors of LEDs is shown in Figure 4. Correlation showed shoot fresh biomass and shoot dry biomass, plant height, leaf number per plant, stem diameter, and root length were positively correlated with chlorophyll content, Pn, Gs, Tr, and Ci, while negatively correlated with malondialdehyde (MDA), and proline (Pro) contents and SOD and POD activities in leaves of industrial hemp. This correlation suggested a close relation between observed parameters of hemp. The loading plots of principal component analysis (PCA) to evaluate the effect of different light wavelengths on different studied attributes of C. sativa are presented in Figure 5.

Figure 4

Correlation of different studied parameters in industrial hemp. Different abbreviations used in the figure are as follows: SFM, shoot fresh weight; SDM, shoot dry weight; NL, number of leaves per plant; PH, plant height; SD, stem diameter; RL, root length, SPAD, Chlorophyll; Pn, net photosynthesis; Gs, stomatal conductance; Tr, transpiration rate; C _i, intercellular CO₂ concentration; SOD, superoxide dismutase; POD, peroxidase; MDA, malondialdehyde; Pro, proline.

Figure 5

Loading plots of principal component analysis (PCA) on different studied attributes of industrial hemp grown under different color light-emitting diodes. Different abbreviations used in the figure are as follows: SFM, shoot fresh weight; SDM, shoot dry weight; NL, number of leaves per plant; PH, plant height; SD, stem diameter; RL, root length, SPAD, Chlorophyll; Pn, net photosynthesis; Gs, stomatal conductance; Tr, transpiration rate; C _i, intercellular CO₂ concentration; SOD, superoxide dismutase; POD, peroxidase; MDA, malondialdehyde; Pro, proline.

Discussion

Light is essential for plant growth and development (Kim et al., 2002). Plants respond to light variations for the completion of their life cycle (Ye et al., 2017). However, different colors of light influenced differently on plant growth (Fukuda et al., 2008). Seed germination, photosynthesis, biomass accumulation, stomatal opening, and closing can be optimized by adjusting light wavelengths (Taiz and Zeiger, 2002; Pinho, 2008; Vänninen et al., 2010; Saleem et al., 2019, 2020; Rehman et al., 2020).

The light-emitting diode (LED) is today’s most energy-efficient and rapidly developing lighting technology. Present study investigates the effects of different color LEDs (White, blue, red, 50% red with 50% blue mix light) on the growth, chlorophyll content, gaseous exchange, and antioxidant capacity of industrial hemp, to screen out the best suitable light spectrum higher growth of plants. Our results showed that BL significantly increase the industrial hemp growth in terms of shoot fresh and dry biomass, plant height, number of leaves per plant, stem diameter, and root length. Similar results were reported by Oana et al. (2019) that BL determined a high level of rate and fresh weight of hemp sprouts. In another study, Nanya et al. (2012) reported that stem height in tomato seedlings depend on proportion of BL. Reduction in stem height and improvement in plant extension growth was observed in cucumber grown under blue radiation which, shows a species-specific response (Hernández and Kubota, 2012). Red light and RBL reduced the growth of plants as shown by low shoot fresh biomass and shoot dry biomass, leaf number per plant, plant height, stem diameter, and root length compared with control (Tables 1, ​,2).2). Goins et al. (1997) and Yorio et al. (2001) also reported a lower dry weight accumulation in wheat, spinach, lettuce, and radish grown under red light compared with white fluorescent tubes. Thus an optimized light spectrum develop the value and quality of cannabis (Magagnini et al., 2018).

The blue light has been associated with leaf characteristics (Hogewoning et al., 2010). Chloroplast is an organelle where chlorophyll pigment captures light energy and converts in energy storing molecules (Kirchhoff, 2019). However, ultrastructure of chloroplast is influenced by light exposure (Chen et al., 2018). Our results revealed higher chlorophyll content in C. sativa leaves under BL. Similar results of higher chlorophyll content in lettuce leaves under blue light in comparison with red light were reported by (Kobayashi et al., 2013). In the current study, boost in industrial hemp biomass grown under BL might associated with higher photosynthetic rate in leaves. Blue light influenced gas exchange traits (Figure 2) and signifying an increase in photosynthesis in hemp plants. However, different species showed different photosynthetic responses under BL, and this response also vary with an increase or decrease in BL portion within a spectrum. Conversely, Matsuda et al. (2004) reported reduced photosynthetic rate in rice grown under RL, which is similar to our results. Blue light also regulates the opening and closing of stomata. Stomatal function affects the photosynthesis rate by exchange of CO₂ and H₂O between atmosphere and plant leaf, and the influence on stomata physiology is a well-known process regulated by BL (Lanoue et al., 2017).

Reactive oxygen species (ROS) are produced in plant cells under abiotic stress conditions. However, ROS production varies to a great extent with species of plants, their genotypes, level of stress tolerance, and the stress duration (Hasanuzzaman et al., 2020). In present study, industrial hemp grown under RL and RBL exhibit the high SOD and POD activities, while BL showed declined activity of antioxidant enzymes as compared to control (Figure 3). In contrary, a previous study on S. rebaudiana Bertoni showed higher POD activity when grown under BL however, RL exhibit reverse trend (Simlat et al., 2016). Uneven antioxidative response might be due to changes in protein functions in various tissues of the plants. Consequently, ROS scavenging enzymes SOD and POD are involved in the mechanisms of protection of protoplasm and cell integrity (Rehem et al., 2012). Furthermore, current research demonstrates that the plants under RL, RBL exhibit higher accumulation of MDA and proline contents, represent an oxidative damage to lipid membranes (Figure 5), while BL helps to reduce oxidative stress in industrial hemp plants. Furthermore, it was reported that hemp grown using LEDs had higher concentrations of CBD, THCV, CBG, and cannabinoids (Magagnini et al., 2018). However, the mechanisms underlying the effect of blue wavelengths on the cannabinoid pathways will require further research. Present results further confirmed that blue light benefits industrial hemp production.

Conclusion

The results of this experiment confirm that blue light has a significant promotive effect on industrial hemp growth, biomass, gas exchange characteristics, and chlorophyll content. Moreover, blue light reduce oxidative stress on plants. However, red light and red blue mix light lessen the plant growth and photosynthesis along with higher MDA and proline contents accumulation in hemp leaves, which may damage cellular organelle membranes. Furthermore, findings of present study showed a close link between quality of light and investigated morphophysiological attributes in industrial hemp. Thus blue light can successfully be used for hemp production in industry. Keeping in view the economic significance, present technique is beneficial due to its low-cost over large-scale cultivation. However, potential of blue light spectrum for industrial hemp production should be tested at higher level.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

LL and GD: conceptualization, supervision, and funding acquisition. XL, XC, and RW: methodology and formal analysis. LZ, MD, and MR: data analysis. MR: writing – original draft preparation. SF: writing – review and editing. All authors contributed to the article and approved the submitted version.

Funding

This research was supported by a grant from the National Natural Science Foundation of China (31760403), Yunnan Provincial Joint Fund for Local Colleges and Universities (202001BA070001-129, 2018FH001-025, and 202001BA070001-163), the Yunnan Applied Basic Research Projects (202101AT070434), and CARS-16-E10.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

References