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Abstract
Cannabis (Cannabis Sativa L.) is now legally produced in many regions worldwide. Cannabis flourishes under high light intensities (LI); making it an expensive commodity to grow in controlled environments, despite its exceptionally high market value. It is commonly believed that cannabis secondary metabolite levels may be enhanced both by increasing LI and by exposing crops to ultraviolet radiation (UV). However, there is sparse scientific evidence to guide cultivators. Therefore, the impact of LI and UV on yield and quality must be elucidated to enable cultivators to optimize their lighting protocols. We explored the effects of LI, ranging from 350 to 1400 μmol m-2 s-1 and supplemental UV spectra on cannabis yield and potency. There were no spectrum effects on inflorescence yield, but harvest index under UVA+UVB was reduced slightly (1.6%) vs. the control. Inflorescence yield increased linearly from 19.4 to 57.4 g/plant and harvest index increased from 0.565 to 0.627, as LI increased from 350 to 1400 μmol m-2 s-1. Although there were no UV spectrum effects on total equivalent Δ9-tetrahydrocannabinol (T-THC) content in leaves, the neutral form, THC, was 30% higher in UVA+UVB vs. control. While there were no LI effects on inflorescence T-THC content, the content of the acid form (THCA) increased by 20% and total terpenes content decreased by 20% as LI increased from 350 to 1400 μmol m-2 s-1. High LI can substantially increase cannabis yield and quality, but we found no commercially-relevant benefits of adding supplemental UV radiation to indoor cannabis production.
Discussion
Two of the dominant phytogenic factors that affect profitability in commercial drug-type cannabis production are marketable yield (i.e., mature, unfertilized female inflorescences) and the secondary metabolite content (e.g., content of cannabinoids and terpenes) in these marketable tissues. A primary objective of this study was to explore proof of concept for the potential for UV radiation treatments for increasing cannabinoid content, particularly THC, in a modern indoor-grown cannabis genotype. The genotype used in this study was typical of Type-I (i.e., drug-type) cannabis (de Meijer et al. 1992); with > 20% THC (i.e., > 200 mg g-1) and no detectable CBD in the inflorescence tissue (Table 1). A low amount of cannabigerol (CBG, the chemical precursor to THC and CBD) was detected. The ratio of T-THC to total equivalent CBG (T-CBG) was ≈ 18, possibly indicating the potential to increase the T-THC content by ≈ 5% (i.e., from ≈ 200 to 210 mg g-1). There was also no cannabinol (CBN) – a natural THC breakdown product – detected; which, along with the high ratio of T-THC to T-CBG, indicated that the plants were near peak maturity at harvest (Aizpurua-Olaizola et al. 2016).
i wykopalisko powiązane.
z trollem w od ponad połowy. Dawne dzieje 2009 r. Inne czasy inne forum... Z tekstem gościa proleglizacyjnego z Kanady. Chyba był to jeden z aktywistów. Tyt. pierwotny "optyka marihuany".
Pi razy oko wnioski z artu. Stary 2021.
wychodzi chyba na to według tych danych.
liście nieco więcej trichomow, tyle że ich się nie zbiera. Nieznaczny spadek plonu. Różnica ilości sumy THC thca pomijalna. Więcej wolnej formy THC przy suplementacji.
liniowy wzrost plonu pod wyższymi wartościami strumienia światła.
ocena opłacalności suplementu uv _ nie opłacalne.
wzrost jakości przy wysokich strumieniach światła. Zadziwia liniowy wzrost plonu. Pytanie o lai i k w uprawie. ??
czyli ? Panele LED z dodatkiem uva dają realnie raczej nic ( tym bardziej że to uva ). Efekt jest bardziej marketingowy i nakierowany na wiarę, że jest o niebo lepiej.
suplement uvb według tych danych branych na wiarę nie jest wart włożonej w niego kasy. Albo się wierzy w to albo nie. Kolejne dane.
pozdro
i następnie. Wnioski te same
UV Radiation Suppresses Cannabis Growth and Yield
While increasing UV radiation exposure suppressed overall vegetative plant growth (e.g., height and growth index) in both cultivars, the responses were more severe in LT than BW. However, these are in contrast with the UV-induced reductions in foliar biomass, which were substantially greater in BW. This was particularly surprising given that there were no consequent reductions in total inflorescence biomass in BW. In fact, despite some leaf senescence observed in both cultivars, harvest index – which is the ratio of inflorescence DW to total aboveground DW – went up by ≈10% in BW and went down by ≈10% in LT as UV-PFD increased from lowest to highest. Under low UV exposure, the harvest index for both cultivars was ≈0.6, which was similar to a different cultivar grown under the same PPFD in the same production system without UV (Rodriguez-Morrison et al., 2021a). Given that there were no UV exposure effects on inflorescence DW in BW, earlier and/or elevated foliar senescence in BW may have contributed to its relatively elevated harvest index.
Reduced aboveground biomass and lower yields are commonly observed effects of UV radiation on some other plant species (Teramura et al., 1990; Fiscus and Booker, 1995; Caldwell et al., 2003; Liu et al., 2005). The UV-induced alterations in leaf morphology and physiology probably contributed to the general reductions in growth and overall biomass in both cultivars. For example, reduced leaf area is a typical response to radiative stresses such as high PAR intensity and UV exposure (Wargent and Jordan, 2013; Poorter et al., 2019). In the present study, the reductions in individual leaf size, total foliar biomass, and leaf-level NCER with increasing UV exposure, would have limited the plants’ capacity to convert PAR into biomass (Kakani et al., 2003; Zlatev et al., 2012).
Total inflorescence DW and the proportion of that DW which is comprised of apical tissues are two major considerations for commercial cannabis production. The apical proportion may be of particular interest since these tissues are normally considered premium quality due to their relatively large size and potentially higher cannabinoid concentrations compared to higher-order (i.e., on lower branches) inflorescences (Namdar et al., 2018). Despite the UV-induced limitations to biomass accumulation seen in both cultivars, increasing UV exposure only reduced inflorescence DW in LT. Within this context, the various growth habits of common indoor-grown cannabis cultivars may influence their yield responses to UV stress. In the present study, BW and LT had disparate whole-plant reproductive macro-morphology (i.e., the distribution of inflorescence biomass within the canopy) under normal indoor conditions. For example, under minimum UV exposure, the apical inflorescence comprised 24% of the total inflorescence DW in LT compared to only 11% in BW. Apparently, growth habit may have predisposed BW’s mitigation of UV-induced yield reductions by partitioning relatively more inflorescence biomass to positions farther away (i.e., more protected from the UV) from the top of the plant. However, while this may be a self-protective response to reduce UV exposure to reproductively important (from an ecological sense) tissues, it still came at commercially-objectionable reductions in inflorescence quality, such as visually unappealing morphology (Figure 10).
To prevent UV-induced yield losses, such as are reported in the present study, it is conceivable that cannabis plants could be exposed to UV only after the majority of vegetative growth has completed [i.e., a few weeks after the visual appearance of inflorescences (Potter, 2014)]. This strategy would shorten the accumulated period of exposure to UV stress and may minimize some UV-induced foliar acclimations that could inhibit biomass accumulation. However, there is a risk that later-term UV exposure might also sufficiently stress unacclimated foliar tissues to provoke rapid-onset whole-plant senescence before the inflorescences reach optimum maturity. This strategy warrants further exploration.
UV Radiation Alters the Secondary Metabolite Composition of Cannabis Inflorescences
The most economically relevant cannabinoids (i.e., Δ9-THC and CBD) are predominantly found in their acid forms in mature female inflorescence tissues, which are converted to the psychoactive and medicinal neutral forms through decarboxylation (Eichler et al., 2012; Zou and Kumar, 2018). The neutral forms also exist in relatively low quantities in the fresh inflorescences and tend to increase in proportion to the acid forms as the inflorescences mature (Aizpurua-Olaizola et al., 2016). While the Δ9-THC concentration increased in BW with increasing UV-PFD, it was a relatively small proportion of the Δ9-THCeq; maximized at 3.3% at the highest UV-PFD. Further, CBN was undetectable in the inflorescences, which is an indicator that the crops were not past peak maturity at the time of harvest since Δ9-THC naturally degrades to CBN (Russo, 2007). There were no UV-induced enhancements to Δ9-THCeq, CBDeq, and CBGeq in either cultivar. These results are consistent with a recent study that found no UV treatment effects on Δ9-THCeq content in a Δ9-THC-dominant cultivar (Llewellyn et al., 2021), but contrast with studies on older genotypes (Pate, 1983; Lydon et al., 1987). For example, Lydon et al. (1987) found that inflorescence Δ9-THC concentrations increased linearly from 32 to 25 mg⋅g–1 in greenhouse-grown cannabis as UV exposure increased from their no-UV control up to biologically-effective UV doses (based on Caldwell, 1971) of 13.4 kJ⋅m–2⋅d–1. These contrasting results may be due to the disparate growing conditions (both before and during UV exposure), plant age at the time of UV exposure, and the relative magnitude of cannabinoid concentrations. Further, while the proportional increases in Δ9-THC content (28%) presented in Lydon et al. (1987) appeared to be substantial, the magnitude of their increase (i.e., only 7 mg⋅g–1) is probably inconsequential in the context of cannabinoid composition in modern genotypes which can have Δ9-THC concentrations that exceed 200 mg⋅g–1 (Dujourdy and Besacier, 2017).
Pate (1983) reported an increase in the ratio of Δ9-THC to CBD in inflorescence tissues of cannabis ecotypes grown in global positions with naturally higher UV exposures, which suggests that the production of Δ9-THC may be upregulated and CBD downregulated as adaptations (i.e., over multiple generations) to the localized environment. However, the results of the present study do not support this trend, at least as an acclimation response to UV stress of a single generation. Additionally, De Meijer et al. (2003) showed that cannabinoid profiles are largely genetically predetermined (e.g., a CBD-dominant cultivar is lacking the genetic predisposition to generate abundant Δ9-THC). This favors the concept that the upregulation of Δ9-THC under UV stress may be an adaptive response (i.e., over generations) rather than an acclimation response (i.e., during a single production cycle). Over the past few decades, there have been radical increases in inflorescence cannabinoid concentrations, which is often attributed to intensive breeding programs (Chouvy, 2015; Dujourdy and Besacier, 2017; Aliferis and Bernard-Perron, 2020) and the “sinsemilla” cultivation method that eliminates seeds and chiefly produces high potency female inflorescences (ElSohly et al., 2016). Thus, these factors may have a larger impact on cannabis inflorescence cannabinoid composition in indoor production than environmental factors such as UV stress.
While cannabinoids comprise the primary psychoactive and medicinal compounds in cannabis inflorescences, volatile terpenes are also economically valuable; both for the aromas that influence consumer preference and potential medicinal properties (Nuutinen, 2018; Booth and Bohlmann, 2019). UV exposure equivocally altered the terpene composition in the present study, with disparate responses within the different terpenes and between cultivars. However, total terpene concentrations in both cultivars decreased linearly with increasing UV exposure, which would tend to depreciate the overall quality of aromas and extracts (McPartland and Russo, 2001; Nuutinen, 2018).
While UV exposure did not result in any economically relevant increases in cannabinoid or terpene concentrations in cannabis inflorescences under the conditions of the present study, UV radiation has been shown to increase concentrations of UV-absorbing secondary metabolites (e.g., flavonoids and phenolic compounds) in many species (Huché-Thélier et al., 2016; Robson et al., 2019), including economically important essential oil producing crops (Schreiner et al., 2012; Neugart and Schreiner, 2018). However, UV-induced increases in secondary metabolite concentrations are often concurrent with biomass reductions (Fiscus and Booker, 1995; Caldwell et al., 2003). This paradox must be evaluated when considering the use of UV radiation to manipulate secondary metabolite composition in indoor cannabis production, since the simultaneous yield reduction may offset any improvements in secondary metabolite composition.
Compared to the UV spectra employed in most other studies, the biologically effective doses in the present study were dramatically higher for a given photon flux density due to the very short peak wavelength of the UV LEDs. In fact, ≈70% of the UV photon flux were at wavelengths below 290 nm, and thus outside of the solar spectrum that plants would naturally be exposed and adapted to Nikiforos et al. (2011). Therefore, cannabis may respond dramatically differently to UV from slightly longer wavelength LEDs (e.g., 300 to 315 nm).
Implications for UV Use in Indoor Cannabis Production and Future Research Directions
This study provided insight into the sensitivity of cannabis to relatively short-wavelength UVB radiation (including a small proportion of UVC) and long-term UV exposure. Increasing UV exposure levels generally had negative impacts on cannabis plant growth, yield, quality, and secondary metabolite composition. The plants exhibited primarily distress-type responses to UV radiation, even at low exposure levels; no amount of UV exposure resulted in substantial increases of cannabinoid concentrations. While none of the UV exposure levels in the present study would have been commercially beneficial, results from studies in other species (Huché-Thélier et al., 2016; Neugart and Schreiner, 2018; Höll et al., 2019; Robson et al., 2019) indicate a strong potential for there being UV treatment protocols – as yet unidentified through rigorous scientific investigation and reporting – that could enhance secondary metabolite concentrations in cannabis. Further research is required to determine if there is a combination of UV spectrum, intensity and time of application that would have commercially beneficial effects in cannabis production. The range of tested cannabis cultivars should also be expanded to cover a broader range of chemotypes and growth habits.
When making the decision to utilize UV wavelengths (as with any production technology) in indoor cannabis production, the positive crop outcomes must outweigh factors related to the cost of deploying the technology including infrastructure and energy costs, fixture lifespan, and health risks that UV radiation could pose to employes. While UVB LEDs in particular (Kusuma et al., 2020) and UV lighting technologies in general are much less energy efficient than modern horticultural PAR fixtures (Nelson and Bugbee, 2014; Radetsky, 2018), UV fluence rates are also typically many times lower than the PAR spectrum. The functional lifespans of UVB LEDs are currently much lower (Kebbi et al., 2020) than common horticultural LEDs (Kusuma et al., 2020); potentially leading to relatively rapid degradation in fluence rates over time. Given that plant responses in the present study were closely tied to the UV exposure level, fixture degradation could lead to inconsistencies between sequential crops, which is an important parameter in the indoor cannabis production industry.
Overall, it is still possible that the alternate UV treatment protocols may have more positive results in the controlled environment production of modern, drug-type cannabis cultivars; for example: longer wavelength and less energetic spectra (Hikosara et al., 2010) and shorter-term (e.g., proximal to harvest maturity) exposure (Johnson et al., 1999; Martínez-Lüscher et al., 2013; Huarancca Reyes et al., 2018; Dou et al., 2019). Future research could seek to promote eustress responses in cannabis secondary metabolite concentrations while minimizing distress responses (e.g., yield reductions) by using less energetic UV spectra and/or different daily exposure protocols than were used in the present study. The effects of cannabis plants grown under different lighting histories should also be investigated to determine the ideal developmental stage for UV exposure to achieve the desired effects in both yield and quality.
Conclusion
Long-term exposure of various intensities of relatively short-wavelength UV radiation had generally negative impacts on cannabis growth, yield, and inflorescence quality. By studying two cultivars with similar cannabinoid profiles, we found some differences in phenotypic plasticity in the temporal dynamics in morphology, physiology, yield, and quality responses to UV exposure level. For the first time this paper described the visible symptoms caused by UVB stress on indoor cannabis plants. Importantly, as it was applied in this study, UV radiation provoked substantially reduced yield in one cultivar, reduced inflorescence quality in both cultivars, and had no commercially relevant benefits to inflorescence secondary metabolite composition. Therefore, potential for UV radiation to enhance cannabinoid concentrations must still be confirmed before UV can be used as a tool in cannabis production.
Data Availability Statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Author Contributions
VR-M and DL performed the experiment and collected and analyzed the data. VR-M, DL, and YZ wrote and revised the manuscript. All authors contributed to the experimental design and approved the final manuscript.
Funding
This work was funded by Natural Sciences and Engineering Research Council of Canada (CRDPJ 533527 – 18). Green Relief Inc. provided the research facility, cannabis plants, experimental materials, and logistical support.
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.
Acknowledgments
We thank Derek Bravo, Tim Moffat, and Madeline Baker for technical support throughout the experiment. We also thank Angus Footman and Erica Emery for their logistical support. This article was first published as a preprint (Rodriguez-Morrison et al., 2021b).
Supplementary Material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpls.2021.725078/full#supplementary-material
Abbreviations
NCER, net carbon dioxide exchange rate; PPFD, photosynthetic photon flux; PFD, photon flux density; CCI, chlorophyll content index; SLW, specific leaf weight; LED, light-emitting diode; DLI, daily light integral; PAR, photosynthetically active radiation; DW, dry weight; SD, standard deviation; Δ9-THC, Δ9-tetrahydrocannabinol; Δ9-THCA, Δ9-tetrahydrocannabinolic acid; CBD, cannabidiol; CBDA, cannabidiolic acid; CBG, cannabigerol; CBGA, cannabigerolic acid; CBN, cannabinol; UV, ultraviolet; UVA, ultraviolet-A; UVB, ultraviolet-B; UVC, ultraviolet-C; Fv/Fm, variable to maximum chlorophyll fluorescence; TLI, total light integral; LT, ‘Low Tide’; BW, ‘Breaking Wave’; CB, culture basin; NIE, no increase in extent; NI, not investigated; UDL, under detection limit; UV-PFD, photon flux density of ultra-violet radiation.
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Ciekawe. Wątek z wątpliwościami być z roku 2009.
26.czerwec.2009.
Abstract
Cannabis (Cannabis Sativa L.) is now legally produced in many regions worldwide. Cannabis flourishes under high light intensities (LI); making it an expensive commodity to grow in controlled environments, despite its exceptionally high market value. It is commonly believed that cannabis secondary metabolite levels may be enhanced both by increasing LI and by exposing crops to ultraviolet radiation (UV). However, there is sparse scientific evidence to guide cultivators. Therefore, the impact of LI and UV on yield and quality must be elucidated to enable cultivators to optimize their lighting protocols. We explored the effects of LI, ranging from 350 to 1400 μmol m-2 s-1 and supplemental UV spectra on cannabis yield and potency. There were no spectrum effects on inflorescence yield, but harvest index under UVA+UVB was reduced slightly (1.6%) vs. the control. Inflorescence yield increased linearly from 19.4 to 57.4 g/plant and harvest index increased from 0.565 to 0.627, as LI increased from 350 to 1400 μmol m-2 s-1. Although there were no UV spectrum effects on total equivalent Δ9-tetrahydrocannabinol (T-THC) content in leaves, the neutral form, THC, was 30% higher in UVA+UVB vs. control. While there were no LI effects on inflorescence T-THC content, the content of the acid form (THCA) increased by 20% and total terpenes content decreased by 20% as LI increased from 350 to 1400 μmol m-2 s-1. High LI can substantially increase cannabis yield and quality, but we found no commercially-relevant benefits of adding supplemental UV radiation to indoor cannabis production.
Discussion
Two of the dominant phytogenic factors that affect profitability in commercial drug-type cannabis production are marketable yield (i.e., mature, unfertilized female inflorescences) and the secondary metabolite content (e.g., content of cannabinoids and terpenes) in these marketable tissues. A primary objective of this study was to explore proof of concept for the potential for UV radiation treatments for increasing cannabinoid content, particularly THC, in a modern indoor-grown cannabis genotype. The genotype used in this study was typical of Type-I (i.e., drug-type) cannabis (de Meijer et al. 1992); with > 20% THC (i.e., > 200 mg g-1) and no detectable CBD in the inflorescence tissue (Table 1). A low amount of cannabigerol (CBG, the chemical precursor to THC and CBD) was detected. The ratio of T-THC to total equivalent CBG (T-CBG) was ≈ 18, possibly indicating the potential to increase the T-THC content by ≈ 5% (i.e., from ≈ 200 to 210 mg g-1). There was also no cannabinol (CBN) – a natural THC breakdown product – detected; which, along with the high ratio of T-THC to T-CBG, indicated that the plants were near peak maturity at harvest (Aizpurua-Olaizola et al. 2016).
i wykopalisko powiązane.
THC a UVB
Jest to artykuł o wpływie promieniowania uvb na produkcje THC zdaje się że natężenie tego światłą ma istotny wpływ na zawartość tego składnika psychoaktywnego w naszych roślinkach :) Niestety nie mogę zająć się teraz tłumaczeniem tego tekstu z powodu natłoku zajęć, dlatego serdecznie zachęcam...
forum.haszysz.com
z trollem w od ponad połowy. Dawne dzieje 2009 r. Inne czasy inne forum... Z tekstem gościa proleglizacyjnego z Kanady. Chyba był to jeden z aktywistów. Tyt. pierwotny "optyka marihuany".
Pi razy oko wnioski z artu. Stary 2021.
wychodzi chyba na to według tych danych.
liście nieco więcej trichomow, tyle że ich się nie zbiera. Nieznaczny spadek plonu. Różnica ilości sumy THC thca pomijalna. Więcej wolnej formy THC przy suplementacji.
liniowy wzrost plonu pod wyższymi wartościami strumienia światła.
ocena opłacalności suplementu uv _ nie opłacalne.
wzrost jakości przy wysokich strumieniach światła. Zadziwia liniowy wzrost plonu. Pytanie o lai i k w uprawie. ??
czyli ? Panele LED z dodatkiem uva dają realnie raczej nic ( tym bardziej że to uva ). Efekt jest bardziej marketingowy i nakierowany na wiarę, że jest o niebo lepiej.
suplement uvb według tych danych branych na wiarę nie jest wart włożonej w niego kasy. Albo się wierzy w to albo nie. Kolejne dane.
pozdro
i następnie. Wnioski te same
Frontiers | Cannabis Inflorescence Yield and Cannabinoid Concentration Are Not Increased With Exposure to Short-Wavelength Ultraviolet-B Radiation
Before ultraviolet (UV) radiation can be used as a horticultural management tool in commercial Cannabis sativa (cannabis) production, the effects of UV on ca...
www.frontiersin.org
UV Radiation Suppresses Cannabis Growth and Yield
While increasing UV radiation exposure suppressed overall vegetative plant growth (e.g., height and growth index) in both cultivars, the responses were more severe in LT than BW. However, these are in contrast with the UV-induced reductions in foliar biomass, which were substantially greater in BW. This was particularly surprising given that there were no consequent reductions in total inflorescence biomass in BW. In fact, despite some leaf senescence observed in both cultivars, harvest index – which is the ratio of inflorescence DW to total aboveground DW – went up by ≈10% in BW and went down by ≈10% in LT as UV-PFD increased from lowest to highest. Under low UV exposure, the harvest index for both cultivars was ≈0.6, which was similar to a different cultivar grown under the same PPFD in the same production system without UV (Rodriguez-Morrison et al., 2021a). Given that there were no UV exposure effects on inflorescence DW in BW, earlier and/or elevated foliar senescence in BW may have contributed to its relatively elevated harvest index.
Reduced aboveground biomass and lower yields are commonly observed effects of UV radiation on some other plant species (Teramura et al., 1990; Fiscus and Booker, 1995; Caldwell et al., 2003; Liu et al., 2005). The UV-induced alterations in leaf morphology and physiology probably contributed to the general reductions in growth and overall biomass in both cultivars. For example, reduced leaf area is a typical response to radiative stresses such as high PAR intensity and UV exposure (Wargent and Jordan, 2013; Poorter et al., 2019). In the present study, the reductions in individual leaf size, total foliar biomass, and leaf-level NCER with increasing UV exposure, would have limited the plants’ capacity to convert PAR into biomass (Kakani et al., 2003; Zlatev et al., 2012).
Total inflorescence DW and the proportion of that DW which is comprised of apical tissues are two major considerations for commercial cannabis production. The apical proportion may be of particular interest since these tissues are normally considered premium quality due to their relatively large size and potentially higher cannabinoid concentrations compared to higher-order (i.e., on lower branches) inflorescences (Namdar et al., 2018). Despite the UV-induced limitations to biomass accumulation seen in both cultivars, increasing UV exposure only reduced inflorescence DW in LT. Within this context, the various growth habits of common indoor-grown cannabis cultivars may influence their yield responses to UV stress. In the present study, BW and LT had disparate whole-plant reproductive macro-morphology (i.e., the distribution of inflorescence biomass within the canopy) under normal indoor conditions. For example, under minimum UV exposure, the apical inflorescence comprised 24% of the total inflorescence DW in LT compared to only 11% in BW. Apparently, growth habit may have predisposed BW’s mitigation of UV-induced yield reductions by partitioning relatively more inflorescence biomass to positions farther away (i.e., more protected from the UV) from the top of the plant. However, while this may be a self-protective response to reduce UV exposure to reproductively important (from an ecological sense) tissues, it still came at commercially-objectionable reductions in inflorescence quality, such as visually unappealing morphology (Figure 10).
To prevent UV-induced yield losses, such as are reported in the present study, it is conceivable that cannabis plants could be exposed to UV only after the majority of vegetative growth has completed [i.e., a few weeks after the visual appearance of inflorescences (Potter, 2014)]. This strategy would shorten the accumulated period of exposure to UV stress and may minimize some UV-induced foliar acclimations that could inhibit biomass accumulation. However, there is a risk that later-term UV exposure might also sufficiently stress unacclimated foliar tissues to provoke rapid-onset whole-plant senescence before the inflorescences reach optimum maturity. This strategy warrants further exploration.
UV Radiation Alters the Secondary Metabolite Composition of Cannabis Inflorescences
The most economically relevant cannabinoids (i.e., Δ9-THC and CBD) are predominantly found in their acid forms in mature female inflorescence tissues, which are converted to the psychoactive and medicinal neutral forms through decarboxylation (Eichler et al., 2012; Zou and Kumar, 2018). The neutral forms also exist in relatively low quantities in the fresh inflorescences and tend to increase in proportion to the acid forms as the inflorescences mature (Aizpurua-Olaizola et al., 2016). While the Δ9-THC concentration increased in BW with increasing UV-PFD, it was a relatively small proportion of the Δ9-THCeq; maximized at 3.3% at the highest UV-PFD. Further, CBN was undetectable in the inflorescences, which is an indicator that the crops were not past peak maturity at the time of harvest since Δ9-THC naturally degrades to CBN (Russo, 2007). There were no UV-induced enhancements to Δ9-THCeq, CBDeq, and CBGeq in either cultivar. These results are consistent with a recent study that found no UV treatment effects on Δ9-THCeq content in a Δ9-THC-dominant cultivar (Llewellyn et al., 2021), but contrast with studies on older genotypes (Pate, 1983; Lydon et al., 1987). For example, Lydon et al. (1987) found that inflorescence Δ9-THC concentrations increased linearly from 32 to 25 mg⋅g–1 in greenhouse-grown cannabis as UV exposure increased from their no-UV control up to biologically-effective UV doses (based on Caldwell, 1971) of 13.4 kJ⋅m–2⋅d–1. These contrasting results may be due to the disparate growing conditions (both before and during UV exposure), plant age at the time of UV exposure, and the relative magnitude of cannabinoid concentrations. Further, while the proportional increases in Δ9-THC content (28%) presented in Lydon et al. (1987) appeared to be substantial, the magnitude of their increase (i.e., only 7 mg⋅g–1) is probably inconsequential in the context of cannabinoid composition in modern genotypes which can have Δ9-THC concentrations that exceed 200 mg⋅g–1 (Dujourdy and Besacier, 2017).
Pate (1983) reported an increase in the ratio of Δ9-THC to CBD in inflorescence tissues of cannabis ecotypes grown in global positions with naturally higher UV exposures, which suggests that the production of Δ9-THC may be upregulated and CBD downregulated as adaptations (i.e., over multiple generations) to the localized environment. However, the results of the present study do not support this trend, at least as an acclimation response to UV stress of a single generation. Additionally, De Meijer et al. (2003) showed that cannabinoid profiles are largely genetically predetermined (e.g., a CBD-dominant cultivar is lacking the genetic predisposition to generate abundant Δ9-THC). This favors the concept that the upregulation of Δ9-THC under UV stress may be an adaptive response (i.e., over generations) rather than an acclimation response (i.e., during a single production cycle). Over the past few decades, there have been radical increases in inflorescence cannabinoid concentrations, which is often attributed to intensive breeding programs (Chouvy, 2015; Dujourdy and Besacier, 2017; Aliferis and Bernard-Perron, 2020) and the “sinsemilla” cultivation method that eliminates seeds and chiefly produces high potency female inflorescences (ElSohly et al., 2016). Thus, these factors may have a larger impact on cannabis inflorescence cannabinoid composition in indoor production than environmental factors such as UV stress.
While cannabinoids comprise the primary psychoactive and medicinal compounds in cannabis inflorescences, volatile terpenes are also economically valuable; both for the aromas that influence consumer preference and potential medicinal properties (Nuutinen, 2018; Booth and Bohlmann, 2019). UV exposure equivocally altered the terpene composition in the present study, with disparate responses within the different terpenes and between cultivars. However, total terpene concentrations in both cultivars decreased linearly with increasing UV exposure, which would tend to depreciate the overall quality of aromas and extracts (McPartland and Russo, 2001; Nuutinen, 2018).
While UV exposure did not result in any economically relevant increases in cannabinoid or terpene concentrations in cannabis inflorescences under the conditions of the present study, UV radiation has been shown to increase concentrations of UV-absorbing secondary metabolites (e.g., flavonoids and phenolic compounds) in many species (Huché-Thélier et al., 2016; Robson et al., 2019), including economically important essential oil producing crops (Schreiner et al., 2012; Neugart and Schreiner, 2018). However, UV-induced increases in secondary metabolite concentrations are often concurrent with biomass reductions (Fiscus and Booker, 1995; Caldwell et al., 2003). This paradox must be evaluated when considering the use of UV radiation to manipulate secondary metabolite composition in indoor cannabis production, since the simultaneous yield reduction may offset any improvements in secondary metabolite composition.
Compared to the UV spectra employed in most other studies, the biologically effective doses in the present study were dramatically higher for a given photon flux density due to the very short peak wavelength of the UV LEDs. In fact, ≈70% of the UV photon flux were at wavelengths below 290 nm, and thus outside of the solar spectrum that plants would naturally be exposed and adapted to Nikiforos et al. (2011). Therefore, cannabis may respond dramatically differently to UV from slightly longer wavelength LEDs (e.g., 300 to 315 nm).
Implications for UV Use in Indoor Cannabis Production and Future Research Directions
This study provided insight into the sensitivity of cannabis to relatively short-wavelength UVB radiation (including a small proportion of UVC) and long-term UV exposure. Increasing UV exposure levels generally had negative impacts on cannabis plant growth, yield, quality, and secondary metabolite composition. The plants exhibited primarily distress-type responses to UV radiation, even at low exposure levels; no amount of UV exposure resulted in substantial increases of cannabinoid concentrations. While none of the UV exposure levels in the present study would have been commercially beneficial, results from studies in other species (Huché-Thélier et al., 2016; Neugart and Schreiner, 2018; Höll et al., 2019; Robson et al., 2019) indicate a strong potential for there being UV treatment protocols – as yet unidentified through rigorous scientific investigation and reporting – that could enhance secondary metabolite concentrations in cannabis. Further research is required to determine if there is a combination of UV spectrum, intensity and time of application that would have commercially beneficial effects in cannabis production. The range of tested cannabis cultivars should also be expanded to cover a broader range of chemotypes and growth habits.
When making the decision to utilize UV wavelengths (as with any production technology) in indoor cannabis production, the positive crop outcomes must outweigh factors related to the cost of deploying the technology including infrastructure and energy costs, fixture lifespan, and health risks that UV radiation could pose to employes. While UVB LEDs in particular (Kusuma et al., 2020) and UV lighting technologies in general are much less energy efficient than modern horticultural PAR fixtures (Nelson and Bugbee, 2014; Radetsky, 2018), UV fluence rates are also typically many times lower than the PAR spectrum. The functional lifespans of UVB LEDs are currently much lower (Kebbi et al., 2020) than common horticultural LEDs (Kusuma et al., 2020); potentially leading to relatively rapid degradation in fluence rates over time. Given that plant responses in the present study were closely tied to the UV exposure level, fixture degradation could lead to inconsistencies between sequential crops, which is an important parameter in the indoor cannabis production industry.
Overall, it is still possible that the alternate UV treatment protocols may have more positive results in the controlled environment production of modern, drug-type cannabis cultivars; for example: longer wavelength and less energetic spectra (Hikosara et al., 2010) and shorter-term (e.g., proximal to harvest maturity) exposure (Johnson et al., 1999; Martínez-Lüscher et al., 2013; Huarancca Reyes et al., 2018; Dou et al., 2019). Future research could seek to promote eustress responses in cannabis secondary metabolite concentrations while minimizing distress responses (e.g., yield reductions) by using less energetic UV spectra and/or different daily exposure protocols than were used in the present study. The effects of cannabis plants grown under different lighting histories should also be investigated to determine the ideal developmental stage for UV exposure to achieve the desired effects in both yield and quality.
Conclusion
Long-term exposure of various intensities of relatively short-wavelength UV radiation had generally negative impacts on cannabis growth, yield, and inflorescence quality. By studying two cultivars with similar cannabinoid profiles, we found some differences in phenotypic plasticity in the temporal dynamics in morphology, physiology, yield, and quality responses to UV exposure level. For the first time this paper described the visible symptoms caused by UVB stress on indoor cannabis plants. Importantly, as it was applied in this study, UV radiation provoked substantially reduced yield in one cultivar, reduced inflorescence quality in both cultivars, and had no commercially relevant benefits to inflorescence secondary metabolite composition. Therefore, potential for UV radiation to enhance cannabinoid concentrations must still be confirmed before UV can be used as a tool in cannabis production.
Data Availability Statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Author Contributions
VR-M and DL performed the experiment and collected and analyzed the data. VR-M, DL, and YZ wrote and revised the manuscript. All authors contributed to the experimental design and approved the final manuscript.
Funding
This work was funded by Natural Sciences and Engineering Research Council of Canada (CRDPJ 533527 – 18). Green Relief Inc. provided the research facility, cannabis plants, experimental materials, and logistical support.
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.
Acknowledgments
We thank Derek Bravo, Tim Moffat, and Madeline Baker for technical support throughout the experiment. We also thank Angus Footman and Erica Emery for their logistical support. This article was first published as a preprint (Rodriguez-Morrison et al., 2021b).
Supplementary Material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpls.2021.725078/full#supplementary-material
Abbreviations
NCER, net carbon dioxide exchange rate; PPFD, photosynthetic photon flux; PFD, photon flux density; CCI, chlorophyll content index; SLW, specific leaf weight; LED, light-emitting diode; DLI, daily light integral; PAR, photosynthetically active radiation; DW, dry weight; SD, standard deviation; Δ9-THC, Δ9-tetrahydrocannabinol; Δ9-THCA, Δ9-tetrahydrocannabinolic acid; CBD, cannabidiol; CBDA, cannabidiolic acid; CBG, cannabigerol; CBGA, cannabigerolic acid; CBN, cannabinol; UV, ultraviolet; UVA, ultraviolet-A; UVB, ultraviolet-B; UVC, ultraviolet-C; Fv/Fm, variable to maximum chlorophyll fluorescence; TLI, total light integral; LT, ‘Low Tide’; BW, ‘Breaking Wave’; CB, culture basin; NIE, no increase in extent; NI, not investigated; UDL, under detection limit; UV-PFD, photon flux density of ultra-violet radiation.
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Ciekawe. Wątek z wątpliwościami być z roku 2009.
26.czerwec.2009.
dekarboksylacja
A nie da się wydzielić postów nie na temat -> przenieść ich do innego tematu: scalić z istniejącym wątkiem: etc . ITP.
a wracając do: tego: // skoro już off to na całego ;D//
kilka pyt:
Jaki efekt na wybarwienie liści maja wysokie dawki UV. Czy odmiany o liściach ciemno zielono wybarwionych - robią się jaśniejsze ? / cześć odmian ma bardziej wyraźna różnice pomiędzy "kolorem" wierzchniej i spodniej strony liścia / Mi się wydaje, ze bardziej dotyczy to właśnie takich sztuk które są bardziej "afgani"
Następne: czy może liście - nie jaśnieją a wręcz przeciwnie: i pojawia się więcej "ciemnego" i czerwieniejącego zabarwienia u części ras.
Kolejne pyt: jak bardzo różnią się rośliny tej samej odmiany rosnące z i bez UV podczas wzrostu weg: - Czy występuje "efekt" zmniejszenia się powierzchni liści + ich zauważalne zgrubienie. Czy rośliny pozostają bardziej zwarte i odrobinę silniej się rozgałęziają.
+ Czy przy stosowanych dawkach obserwuje się zauważalnie spowolnienie wzrostu. // w porównaniu np: do roślin testowych nie rosnących pod UV//
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