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Cannabis flowering time lapse videos

Interested to see what your next strain will really look like when it’s grown? This collection of Dutch Passion time lapse cannabis grow videos allows you to see each strain grown from seed to harvest. This allows you to observe growth rates at various points during cultivation and see precisely how tall, bushy and heavy these strains become. Sit back, relax, and enjoy watching some of Dutch Passion’s most popular autoflower seeds and photoperiod feminised seeds grown to harvest!

Auto Cinderella Jack time lapse video

With independent laboratory measured THC levels of over 25%, Auto Cinderella Jack is quite possibly the world’s strongest autoflower. A perfect choice for any grower wanting a guaranteed killer harvest of insanely potent buds.

For pure potency, you won’t find many more powerful autoflower strains! Just like the rest of the Dutch Passion autoflower seed collection, she is easy to grow under a wide range of grow conditions and grow techniques. This hybrid strain is equally at home indoors or outdoors. Indoors she takes around 75 days to grow from seed to harvest.

Auto Blackberry Kush time lapse video

If you like heavy harvests with some beautiful blue and deep purple hues to your buds then Auto Blackberry Kush is just what you’re looking for! With very high THC levels around (and perhaps above) 20% this is also a supremely potent strain that can satisfy even the most experienced smoker.

Auto Blackberry Kush may be one of the most visually stunning autoflower seeds you have ever grown. She could also be the quickest!, sometimes ready to harvest in as little as 9 weeks after planting the seeds. Pay attention to humidity towards the end of bloom, these heavy dense blooms benefit from lower humidity levels and good air circulation.

Auto Orange Bud time lapse video

Auto Orange Bud is part of the exclusive Dutch Passion Extremely High THC seed collection. This is reserved only for those exceptional strains which can approach 25% THC in good grow conditions. On top of that you get to enjoy the world famous original tangy Orange Bud taste of Mandarin and Orange.

The high powered anti-anxiety high combines with a mouth watering zesty flavour of freshly picked citrus fruit. Home grown cannabis simply doesn’t get much better than this! Auto Orange Bud is just as easy to grow as the rest of the Dutch Passion autoflower strains. Expect a straightforward, easy grow and a harvest around 10-11 weeks after seed germination.

Auto Lemon Kix time lapse video

Another member of the elite Extremely High THC seed collection, Auto Lemon Kix is one of the most potent autoflower strains currently available. The mind-numbingly powerful THC levels suit the more experienced cannabis user aiming for maximum potency.

One of the best features of Auto Lemon Kix is the unique, sweet multi-layered candied fruit flavours. It really is an unusually rich and delicious taste. Combined with the sky-high THC levels, the deliciously sweet taste completes the perfect all-round package. She’s easy to grow with heavy yields. What are you waiting for?

Auto Glueberry OG time lapse video

Auto Glueberry OG is part of the USA Special cannabis seed collection. She combines very high THC levels with exceptionally heavy harvests. She can take a week or so longer than other auto strains (typically 12 weeks from autoflower seed to harvest) but the wait will reward you with real XXL yields.

Auto Glueberry OG seeds suit the grower with the space to accommodate her tendency to grow tall and wide. She responds well to LST and training if you need to control her growth. In the right hands this strain is superior to most high-performance photoperiod strains in terms of yield and potency

Auto Banana Blaze time lapse video

If you like sweet, tropical fruity banana flavoured buds with heavy yields then Auto Banana Blaze is a perfect selection for your grow room. This auto strain suits the grower needing a plant with numerous chunky blooms, fast growth and no complicated grow room maintenance. Auto Banana Blaze reaches around a metre tall and is ready to harvest in about 11 weeks.

Auto Banana Blaze is a good auto to grow in the natural way, no special grow techniques are necessary to get the best out of this lady!

Auto Colorado Cookies time lapse video

Auto Colorado Cookies uses connoisseur quality parent genetics from Auto Blueberry and Girl Scout Cookies. This strain shows remarkable consistency, with all the plants generally showing similar XL growth structure and the same consistently very high THC levels. If you want predictable excellence from your grow room then Auto Colorado Cookies is a great way to achieve it.

It isn’t recommended to top/FIM this auto since you will lose out on the colossal main bloom. When harvesting, we recommend waiting for a few more amber trichomes than usual. This allows the buds to really deliver their very best high!

Auto Brooklyn Sunrise time lapse video

Auto Brooklyn Sunrise is a hybrid USA auto strain which takes around 11 weeks to grow from autoflower seed to harvest. The genetics come from a New York elite AK clone combined with Diesel. It’s a powerfully fuel flavoured strain with a fast acting and hugely pleasurable high.

The main advice when growing Auto Brooklyn Sunrise seeds is to go gently with the nutrients. This strain is happier with feeds on the light side. All in all, this is a real champion auto that delivers an enviable all round performance, combining speed, potency and yield with a complex but truly delightful diesel-taste.

Passion Fruit time lapse video

For any strain to carry the ‘Passion’ name you know it has to be exceptional! Passion Fruit is just that, a top-shelf photoperiod feminised strain made by hybridising original Orange Bud with Sweet Pink Grapefruit. The taste is citrus with pure Passion Fruit. THC levels are very high and bloom time is around 8 weeks.

In terms of grow latitude, Passion Fruit can handle cooler temperatures well. Cool night temperatures can produce some beautiful autumnal blue/purple hues in the buds and leaves. Not only does this increase bag appeal, this can also increase resin levels making her even more potent and sticky!

Auto Night Queen time lapse video

Auto Night Queen is a compact indica autoflower strain which takes around 10 weeks to grow from seed to harvest indoors. The short, stocky growth profile suits the indoor grower with limited headroom. The highly potent buds suit the experienced smoker who can handle very high THC levels and heavy indica effects!

These tough Afghani Kush genetics can take a beating. This makes her easy to grow in a range of indoor conditions. She also does well outdoors. The buds are particularly hard, dense and resinous. One small nugget goes a long way!

Auto Blueberry time lapse video

Auto Blueberry is a real connoisseurs delight. The sweet fruity blueberry taste is complemented by a smooth, powerful and long lasting high. What’s more, she is perhaps the fastest autoflower seed in the Dutch Passion collection. Some phenotypes are ready to harvest just 8-9 weeks after autoflower seed germination. Cannabis cultivation doesn’t get much faster than that!

With a short life cycle, in order to get the most from Auto Blueberry a fast growth system such as a Hydroponics method can deliver good results. Coco fibre may also help deliver large plants. Enjoy the great taste and the blissfully enjoyable high from this repeat customer favourite!

Auto Mazar time lapse video

Auto Mazar is a classic indica autoflower strain with cannabis cup winning Afghani Kush genetics. She delivers a powerful effect, a strong high with a powerful body stone which medical users adore. The taste is rich, deep and hash scented. Yields can be exceptionally high.

The best taste comes from organic growing, but the use of hydroponics allows the experienced grower to push these classic cannabis genetics right to their limit. Auto Mazar seeds are equally at home indoors as well as outdoors.

Auto StarRyder time lapse video

Auto StarRyder is another fast autoflower strain. She can be ready to harvest around 9 weeks after autoflower seed germination. She delivers heavy, rapid harvests of THC rich cannabis indoors but is also very highly rated by outdoor growers. This is a tough, robust but fine quality autoflower.

Auto StarRyder grows in an Xmas tree structure with plenty of side branches. These should yield heavily in good grow conditions. The plants often tend to remain on the compact side, making her a convenient indoor and outdoor strain.

Forest Dream time lapse video

Forest Dream is an outdoor variety developed in The Netherlands for growers with climates similar to the Northern European average. She is often ready to harvest in early October and uses robust, proven sativa genetics.

Forest Dream is easy to grow outdoors with good weather resistance. She usually reaches around 2m tall outdoors but can reach 3-4m in good conditions. A reliable and consistent choice for any guerrilla grower!

Watch more cannabis-related videos

These time lapse grow videos show you the type of result you can expect from Dutch Passion cannabis seeds. Great care is taken with the development of our prize-winning cannabis genetics, therefore we aim to give growers the best information about the type of grow they can expect.

Time-lapse videos are a great way to show you how the plants grow. Serious growers may also be interested in checking out the full Dutch Passion archive of grow reviews. You can find them here…

What was your favourite time lapse video? Please let us know your suggestions for new cannabis time lapse videos in the comments below. And don’t forget to check out our online cannabis seedshop. You can buy the best autoflower and feminised cannabis seeds there as well as keep track of our latest promotions.

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When to Switch From Vegetative Growth to Flowering

Making the switch from vegetative growth to flowering is a pivotal moment as a grower. Too early and your yield will be lower and your plant might suffer health problems. Too late and you could end up with overgrowth or burned buds.

There are a lot of factors that will affect your decision to make the switch, from plant age to strain, and unless you’re growing an auto-flowering plant – you have complete control over when to make the switch happen.

Our guide will help you learn how and when to switch to flowering stage from vegetative growth for your individual grow set up.

Cannabis Growth Cycle

Before we get into the various conditions that will help you know when to make the switch from vegetative growth to flowering stage, here’s a quick reminder of the basic growth cycle of your cannabis plants with ( extremely! ) rough timescales:

  • Germination/Seed: 1 – 2 weeks
  • Seedling: 2 – 3 weeks
  • Vegetative: 2 – 8 weeks
  • Flowering: 6 – 10 weeks
  • Harvesting: time to reap your rewards!

As we’ve mentioned above, strains will have a huge effect on these timeframes ( which we’ll explain further later on ) so it’s absolutely worth making sure you do your research on your chosen strain before you grow.

How to Make the Switch From Vegetative Stage to Flowering Stage

It’s all about your light schedules. When your plant is in its vegetative state, it will need more light to flourish, a minimum of 18 hours a day during this growth period. Most growers will use an 18-6 light schedule, although some choose a 24-0 light schedule.

When you’ve decided it’s time to make the switch from vegetative to flowering, cut the lights to force your plant into flowering. A 12-12 light schedule will do the trick.

But how do you know when it’s time? We’ve broken down all the factors you need to bear in mind below.


We briefly touched on this above but there are real genetic differences between indica and sativa and they both behave very differently during the flowering stage.

Indica strains are known to generally produce bushy, short plants comparatively to sativa. They’ll only gain around a quarter to half of their height during the flowering stage, so to maximize your yield it’s best to extend your vegetative stage.

Sativa, on the other hand, is known for being taller and they grow a lot more in height throughout the flowering stage, with some sativa strains even doubling in height from the start of flowering to harvest.

If you’ve picked a hybrid strain, do your research or ask another experienced grower for their advice.

Clones or Seeds

If you don’t give your plant enough time to establish a solid root system, you’re far more likely to have issues and complications in the flowering stage.

Clones can shoot up in height quickly, so you might need to switch based on this alone. Seedlings can be flowered earlier if your growth method requires it, but they’ll still need 2 to 3 weeks before making the switch.

Age Is Just a Number

Some growers will swear by a full two months for the vegetative stage, but this isn’t always true. Especially for clones, as soon as they have the root system mentioned above, you can switch to flowering if that’s what your growing method dictates. However, it’s worth remembering that mistakes in the flowering stage are far more difficult to recover from, so ensure you have a healthy plant before the switch.

A lot of where a plant’s age will factor into your decision is how it fits in with your growing method and whether you are looking to maximize your yield or speed up your growth cycle. Plants can be kept in vegetative stage for up to 60 days if you’re looking to create the largest yield possible.

Fairly often though, grow space limits this anyway, so it’s often advisable to switch to flowering stage before your plant has grown too large for your space.

Plant Height

Only you know how much space you have to grow. The longer your cannabis plants are kept in a vegetative state, the taller they’ll become. If you leave them too long, you’re going to end up with an overgrown plant for your space, which will lead to your canopy sitting too close to the lights and burning.

How close to letting your plant get to the light will depend on your light fixtures. If you have high powered lights, you’ll need to keep your plant’s canopy further away from them. A general rule of thumb is to never have your plants closer than 30cm to your light fixtures.

Growing Methods

Whether you call it a growing method or training your plant, all this will affect when to make the switch from vegetative growth to flowering. Generally, high-stress methods mean your plant will need more time to establish itself in vegetative growth, while low-stress methods mean you can make the switch to flowering earlier.

If you’re growing multiple plants and using the sea of green (SOG) method, you want your plants to flower as early as possible so they only produce one large bud. As you’ll generally be growing several indica strains in close-knit quarters here, aim to make the switch to flowering when your plants have reached a height of approximately 15cm to 30cm depending on your grow space. As this is an amplified version of 12-12 from seed, similar rules apply for this method.

With the screen of green (SCROG) method, where you lay a mesh screen horizontally above your plants, you need to leave your plants in vegetative growth until they’ve grown through the screen. This will depend on your individual grow space set up, but screens are generally between 30cm to 60cm above the base of the plants. To reach the screen, your plants will need to stay in vegetative growth for several more weeks than with the sea of green method.

High-Stress Methods

If you’re super cropping to produce larger yields, leave your plant in a vegetative state for longer as it will keep the height of your plant in check. Similarly, if you’re topping by removing the plant’s main stem as a seedling to grow more colas, this high-stress method demands you leave your plant in the vegetative growth period for much longer than other methods to ensure you have a healthy, established plant for flowering.

For those lollipopping, by removing the lower growth of the plant to focus energy on the canopy of the plant, your switch will be based on height and grow space. For sativa strains as they’ll continue shooting up when flowering, around 30cm to 45cm is common. For shorter indica strains, leaving them in vegetative growth up to 100cm is common.

Outdoor Grows

For outdoor growers, typically your cannabis plant will flower of its own accord. Usually in line with the seasons, when days become shorter than 12 hours. Some climates don’t allow for this natural light cycle, so you might need to force your plant to switch from vegetative to flowering. The same rules apply here as indoors, all you need to do is reduce exposure to 12 hours of light a day.

Sometimes plants will struggle to make the switch even if the natural outdoor light has shortened to 12 hours. If this is the case for your cannabis plants, it might be due to man-made lights like garden lights or street lights.

Autoflowering Cannabis

If you’re growing an auto-flowering strain, your plant will start flowering regardless of the light schedule you put it on. This makes auto-flowering plants ideal for outdoor grows as long as the temperature remains hot enough.

Generally speaking, auto-flowering plants will switch from vegetative stage to flowering after around 3 to 4 weeks. Flowering stretches for auto-flowering plants will vary depending on the strain.

While you won’t need to worry about light cycles as much or when to make the switch, your plant will still yield more if it receives the recommended amount of light relative to its growth period. So ensuring your plant receives the full 18 hours of light in vegetative stage will help ensure you have a strong and healthy plant with a good yield later on.

The Flowering Stage

For most strains, the flowering period will be somewhere between 7 to 10 weeks, though there are some up to 12 weeks. Usually, you have a 2 to 3-week window on when to harvest for most plants, but leaving your plant longer will increase your yield as your plant bulks up.

Flowering Too Early

Although the vast majority of growers now use feminized seeds, it’s always worth making sure you’ve sexed your plant as early as possible and dealt with the male plants accordingly.

If you’re using a high-stress growth method, flowering too early may also cause your cannabis plant to become a hermaphrodite so this is always something to keep an eye on when routinely checking plant health. If you notice a hermaphrodite plant, one with male and female flowers, it’s often best to destroy it to limit its influence on other plants. If the plant only produces a few male flowers, these can be removed and the pollen sterilized with water.

Switching with Grobo

If you’re using a Grobo as your grow set up, then you’re in for a much easier time. As an automated grow box, Grobo uses deep water culture, a hydroponic technique, to grow the healthiest plants.

All you need to do is add your plant, pick your grow recipe from our hundreds of strains on our app, and your Grobo system will do the rest.

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Find your perfect automated grow box on our site. If you want some Grobo swag, check out our awesome hats !

Genetic Architecture of Flowering Time and Sex Determination in Hemp (Cannabis sativa L.): A Genome-Wide Association Study

The datasets presented in this study can be found in online repositories. RADseq data, selected informative SNP markers and the annotated transcriptomic data of the marihuana cultivar C. sativa ‘Purple Kush’ are deposited in the 4TU.ResearchData archive (doi: 10.4121/12826832 and doi: 10.4121/13121885.v1).


Flowering time and sex determination in hemp (Cannabis sativa L.) strongly influence fiber quality and seed production of this crop. The control of these traits is paramount for the breeding of new cultivars. Yet, little is known about the genetics underlying such complex traits and a better understanding requires in depth knowledge of the molecular mechanisms responsible for these traits. In this report, the genetic architecture of flowering time and sex determination in hemp was studied using a Genome-Wide Association Studies (GWAS) approach. Association studies were performed on a panel of 123 hemp accessions, tested in three contrasting environments, using a set of 600 K SNP markers. Altogether, eight QTLs were identified across environments; six for flowering time traits and two for sex determination. These QTLs covered genomic regions with 33 transcripts predicted to be involved in flowering and sex determination as well as a microRNA, miR156. Genes related to perception and transduction of light and transcription factors well-known to regulate flowering were identified in QTLs for flowering time traits. Transcription factors and genes involved in regulating the balance of phytohormones, specially auxins and gibberellic acid, were identified in QTLs for sex determination. Sex determination QTLs were associated with the development of male flowers in female plants and thus with the stability of sex determination in monecious plants. The present study elucidates relevant knowledge on the genetic mechanisms of flowering and sex determination traits in hemp, and provides new tools for hemp breeding.


Hemp (Cannabis sativa L.) is naturally a diecious species with distinct male- and female plants. The sexual phenotype of Cannabis frequently shows some flexibility leading to differentiation of hermaphrodite plants, also known as monecious phenotypes (Moliterni et al., 2004). The species is a short-day plant characterized by sexual dimorphism. True males can be recognized by their typical morphology characterized by slender stature, few leaves and hanging inflorescences carrying male flowers. Female plants produce female pistillate flowers in dense panicles heads interspaced with leafy bracts. Monecious plants morphology resembles that of female plants (Faux et al., 2016). Male plants mostly flower earlier than female plants (Bócsa and Karus, 1998; Struik et al., 2000) and die shortly after flowering, while female plants remain alive until seed maturation.

The post-emergent growing period of hemp is divided into a vegetative phase and a flower development phase. The vegetative phase is divided into a temperature-dependent basic vegetative phase and a daylength-dependent photoperiod induced phase (Lisson et al., 2000). Hemp is strongly sensitive to changes in the photoperiod regime and temperature input (Amaducci et al., 2008a, 2012; Salentijn et al., 2015, 2019; Petit et al., 2020b). Flowering time of hemp rapidly responds to these environmental variations to synchronize the flowering with the environment, especially with the daylength of the growing season (Amaducci et al., 2012; Salentijn et al., 2019). Daylength has a key influence on the timing of flowering in hemp (Lisson et al., 2000). Hemp flowering is generally inhibited during long-day photoperiod regimes and is induced promptly when the photoperiod changes to short-day regimes with a threshold of ∼10–12 h of uninterrupted darkness (critical photoperiod of ∼12–14 h of daylight) (Salentijn et al., 2019). However, Schaffner (1926) observed that hemp grown under 24 h daylength also flowered, but with a strong reduction of dry matter in the floral parts. Similarly, Van der Werf et al. (1994) reported hemp plants of cultivar Fedrina 74 and Kompolti Hybrid TC that flowered under continuous illumination (24 h). The initiation of flowering independently of the photoperiod is referred to as “autoflowering” or day neutral. Some cannabis cultivars, especially for cannabinoid production, such as C. sativa subsp. sativa var. spontanea, have been described as day neutral plants [reviewed in McPartland (2018)]. Consequently, all hemp cultivars are considered as quantitative short-day plants (Lisson et al., 2000), since they have different sensitivity to photoperiod. A recent study described large quantitative variation of flowering time in a panel of 123 accessions (Petit et al., 2020b). To simplify, breeders classify hemp cultivars in early, middle and late flowering cultivars (Amaducci et al., 2015; Salentijn et al., 2015). Despite the large variation in flowering time and the extreme phenotypes (day neutral), if the critical short-day regime is not reached within the growing season or if cultivars are very late flowering, hemp plants generally remain vegetative until harvesting time (Lisson et al., 2000; Amaducci et al., 2008a, b; Petit et al., 2020b). In hemp, temperature regime is of special importance during the basic vegetative phase (juvenile stage), since it is dependent on the temperature (Lisson et al., 2000). The completion of the basic vegetative phase requires a certain temperature input, before entering to the daylength-dependent photoperiod induced phase. Hemp growth requires a base air temperature of around 1°C, with an optimal temperature for growth of 29°C and a ceiling temperature 41°C. A range of 306–636°Cd (accumulated thermal time over a period with a base temperature of 1°C) are required for completion of the basic vegetative stage and beginning the daylength-dependent induced phase [references in Amaducci et al. (2008a, 2012) and Salentijn et al. (2019)].

Hemp is a diploid species with 9 pairs of homomorphic autosomal chromosomes and a pair of heteromorphic sex chromosomes: X and Y (2n = 20) (Moliterni et al., 2004). The haploid genome size is 818 Mbp for female plants and 843 Mbp for male plants (van Bakel et al., 2011). Sex determination system in diecious hemp has been well studied. Male plants carry the heterogametic sex (XY) and female plants the homogametic one (XX). However, despite the presence of specific sex chromosomes, the phenotypic expression of sex in hemp shows some flexibility. Some diecious hemp plants produce flowers of the opposite sex than the one determined by their chromosomal composition (Moliterni et al., 2004). Monecious hemp plants carry the homogametic sex (XX) and the ratio of female to male flowers in a single monecious plant is highly variable (Faux et al., 2014). This variation ranges from monecious plants that have predominantly male flowers to predominantly female flowers (Faux et al., 2013, 2014, 2016). Diecious hemp species abundantly exist in nature, while monecious plants have been developed from some mutants that were selected during the domestication of the crop. Monecious accessions tend to show a wide range in sex ratios within the crop, including unisexual plants, and may gradually return to natural dioecy after a few generations (Bócsa and Karus, 1998; Amaducci et al., 2008a; Faux et al., 2013, 2014; Faux and Bertin, 2014). Constant strict selection of monecious plants is therefore needed to maintain monoecy during the seed multiplication (Moliterni et al., 2004). The instability of the sexual phenotype across generations, and the quantitative nature of expression of the sex suggests that sex expression is a rather polygenic trait (Faux et al., 2013, 2014; Faux and Bertin, 2014).

Genetic variation of sex expression among cultivars in hemp has been observed (Moliterni et al., 2004; Faux et al., 2014, 2016). Moliterni et al. (2004) studied the differences in genetic expression between males and females of the diecious hemp cultivar ‘Fibranova.’ They identified polymorphic fragments of cDNA-AFLP markers that allowed to develop a male-specific SCAR marker for the early detection of male plants. Cloning and sequencing revealed that the mRNA of the AFLP markers belonged to genes involved in the activation of auxin-induced genes, probably involved in the female sex differentiation (Galoch, 1978). Faux et al. (2016) performed the first association mapping studies in three biparental hemp populations, two of diecious- and one of monecious hemp, using 71 AFLP markers. The study resulted in the identification of 5 QTLs associated with sex expression putatively located on sex chromosomes (the X chromosomes of monecious hemp and the X and Y chromosomes of diecious hemp). Four markers were found common between monecious and diecious populations, suggesting a partly common genetic basis. These results establish the prerequisite for further research on the genetic determination of sex in hemp using quantitative approaches. Furthermore, monecious accessions were partly linked to earliness, while diecious accessions to later flowering. This link suggested a partly common genetic variation and thus a partly common genetic basis underpinning the determination of sex-type (monecious/diecious) and flowering time (Faux et al., 2013; Petit et al., 2020b). Another link between sex-expression and flowering time is also found in the fact that male flowers are earlier than female flowers.

Many studies suggested that in hemp, epigenetic mechanisms could be associated with the control of sex determination, besides genetic factors [reviewed in Truta et al. (2007)]. The phenotypic outcome of sex expression in hemp might be controlled at the transcriptional- or post-transcriptional level, so without alterations in the DNA or affecting chromatin structures (Soldatova and Khryanin, 2010). The studies confirmed the role of endogenous phytohormones in the regulation of genetically determined sex type, as well as the possibility to modify it by exogenous external factors, which include ions, phytohormonal treatments, environmental factors such as photoperiod, etc [reviewed in Truta et al. (2007)]. The external factors exert a strong modifying effect, especially on the sex expression of monecious plants, in which both staminate and pistillate flowers are induced, in different proportions. The accumulation of Cu ++ and Zn ++ ions with zeatin induce feminization, whereas the accumulation of Pb ++ ions favors a masculinization effect (Chailakhyan and Khryanin, 1978; Freeman et al., 1980; Galoch, 2015). The phytohormones gibberellins, auxins, ethylene and cytokinins play a role on the expression of sex in many monecious and diecious systems (Truta et al., 2007). Generally, auxins and ethylene have feminizing effects (Heslop-Harrison, 1963; Galoch, 1978), whereas cytokinins and gibberellins have masculinizing effects, though contrasting effects have been observed for some species and experimental conditions [reviewed in Ainsworth (2000)]. For example, Chailakhyan and Khryanin (1978) and Galoch (1978), cited by Dellaporta and Calderon-Urrea (1993) also reported feminizing effects of 6-benzylaminopurine (cytokinin) in species of Vitis, Spinacia, and Cannabis. In species such as Mercurialis annua and Arabidopsis thaliana, auxins induce expression of genes which exert a direct control on the biogenesis of sexual organs. Examples of such auxin regulated genes are auxin responsive factor genes (arf genes) in tomato, arabidopsis and papaya, which are transcription factors that control female flower development [reviewed in Li et al. (2016)]. It is possible that the gibberellins act as repressors of female flower development. The ratio of the different phytohormones is very important in the phenotypic sex expression in hemp (Truta et al., 2007). Gibberellins are for instance known to promote masculinization in spinach [reviewed in Salentijn et al. (2019)]. West and Golenberg (2018) reported that external treatment of gibberellic acid (GA) in spinach affected the expression of the gibberellic acid insensitive gene (gai gene), which is a transcription factor of the DELLA family. The gai gene is highly expressed in female inflorescences. GAI transcription factor is a repressor of the expression of B-class homeotic genes, which are masculinizing factors. B-class genes stimulate male organ formation and simultaneously supress the development of female organs in the flowers. This study in spinach concluded that high levels of exogenous GA inhibit the expression of gai gene, which consequently release inhibition on the masculinizing factors. Furthermore, the effect of photoperiod on sex determination of hemp could be associated with the external regulation of GA biosynthesis by differences in light quality and daylength (Hedden and Phillips, 2000). Schaffner (1921) reported that female hemp plants grown under low light winter conditions produced male flowers, while those plants grown under spring normal conditions did not produce male flowers. Another environmental factor is the soil quality. For example, a nitrogen-rich nutrition induces masculinization [reviewed in Truta et al. (2007)].

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Hemp is a multi-purpose crop valuable for the production of fibers, cannabinoids, seeds and oils (Salentijn et al., 2015). It is particularly known to produce bast-fibers of large quality (Petit et al., 2020a, b). The transition from the vegetative to the flower development phases (flowering time) is of great relevance for hemp breeding, since maximum fiber quality occurs shortly after flowering (Meijer et al., 1995; Amaducci et al., 2008b, 2015). After this transition, the fiber quality decreases as the nutrient flow and the carbon partitioning is shifted from the stems, leaves and roots toward the development of flowers and seeds. Moreover, the flower development phase also coincides with the time-point of secondary bast fiber formation, which is characterized by intense lignification (Liu et al., 2015) and may cause a decrease in fiber quality (Mediavilla et al., 2001). Consequently, harvesting time is an important crop management factor influencing fiber quantity and quality in hemp (Westerhuis et al., 2009). The sex of the plants is another important factor influencing these traits. Large differences in the content and quality of fibers between diecious and monecious plants have been reported. A recent study showed that, monecious accessions produced on average more bast fibers, and higher levels of cellulose and mannan, while diecious accessions showed a stronger vigor in combination with later flowering and higher contents of xylans and lignin (Petit et al., 2020b). Additionally, as lignification intensifies around the onset of flowering and continues until seed maturation, female plants are more lignified than males (Liu et al., 2015). Male plants are also known to produce finer fibers than female but are more susceptible to pests (Amaducci et al., 2015). In addition, the proportion of males and females has an effect on the seed yield, as a large proportion of males is associated with seed yield reduction (Faux and Bertin, 2014). Furthermore, diecious hemp accessions are characterized by heterogeneity in fiber and seed production. In contrast, monecious accessions are more stable in both fiber and seed production because the plants develop more uniformly (Mandolino and Carboni, 2004; Salentijn et al., 2015). The differences in properties between diecious and monecious plants have traditionally led to a specialization of their applications, as sex determination of hemp strongly influences fiber quality and seed yield [reviewed in Salentijn et al., 2019]. As a consequence, diecious are mostly used for fiber production, while monecious are produced for dual purpose: fiber and seed production (Amaducci and Gusovius, 2010; Amaducci et al., 2015).

Hemp breeding goals targeted for control of flowering time and sex determination offer the potential for sustainable gains on hemp yield and quality of fibers. This will help to breed for cultivars better adapted to specific photoperiod regimes and with desired seed and/or fiber yields and qualities. For instance, early flowering under non-inductive (long-daylength) conditions and reduced sensitivity to photoperiod is interesting for hemp adaptations and essential for reproductive success and good fiber and seed yields in Northern European latitudes. Meanwhile, late flowering, with a prolonged vegetative stage is interesting for increasing fiber yield in low European latitudes (Salentijn et al., 2019). Maintenance of monoecy is essential to increase the homogeneity in dual production of fiber and seeds. This would reduce the efforts to eliminate female and male diecious plants sporadically occurring across generations (Hall et al., 2012; Amaducci et al., 2015; Salentijn et al., 2015). A recent study described large range of variation in flowering time traits and sex determination (monecious/diecious) in a panel of 123 hemp accessions (Petit et al., 2020b). The study described strong influence of genetic components with large heritability estimates for flowering time and sex determination. In addition, significant genotype-by-environment interactions (G × E) were reported. These interactions indicated significant heritable variation of flowering time sensitive to the environment. Nevertheless, little is known about the genetic and molecular mechanisms that control sex determination, the origin of high plasticity of sex expression, sexual dimorphism and flowering time of hemp. In addition, markers to maintain hemp monoecy or to select for specific flowering time phenotypes are not available.

The present study describes a GWAS approach to characterize the genetic architecture underpinning the length of the vegetative period, flowering time, and sex determination in hemp. A panel of 123 hemp accessions was used to measure these traits in three locations across Europe with contrasting photoperiod regimes. The hemp panel included a large variation of monecious and diecious genotypes and early and late flowering accessions (Petit et al., 2020b). Here, we present QTLs for flowering time traits and sex determination. Furthermore, candidate genes for the QTLs across locations were identified and will contribute to a better understanding of the molecular mechanisms regulating flowering and sex determination in hemp.

Materials and Methods

Plant Material

A panel of 123 hemp accessions was used in this study. The panel included a large diversity of accessions from various origins (Europe, China and Canada) and primarily used for different purposes. For specific information of each accession see Supplementary Table 1. The accessions of the panel included large variation in flowering time and in sex type, namely diecious, monecious and accessions with large range of variation in between (Petit et al., 2020b). Plants were grown in three locations across Europe with different photoperiod regimes; in Rovigo (CRA – Centro di ricerca cerealicoltura e colture industriale, Italy, 45°N 11°E), in Chèvrenolles, Neuville-sur-Sarthe (FNPC – Fédération Nationale des Producteurs de Chanvre, France, 48°N 0.2°E), and in Westerlee (VDS – VanDinter Semo BV, Netherlands, 53°N 6°E). Field testing was performed between April and September 2013. Three biological replicates were grown per accession and location in a randomized complete block design and the experimental units were plots of 1m 2 in CRA and VDS and of 1.5 m 2 in FNPC. In all three locations, the same sowing density was used to aim a density of 100 plants/m 2 .

Phenotyping of the GWAS Panel

Each plot was phenotyped for the presence of monecious, independently of the ratio male/female flowers, and diecious plants, independently of the number of male and female plants, according to Petit et al. (2020b). Sex determination was measured per plot assessing “1” when the plants were diecious, “2” when the plants were a mix of diecious and monecious and “3” when the plants were monecious. Data used in the GWAS analyses were provided as the mean between the three pots per accession in each location. Phenotypic values ranged from 1 to 3. Values close to 1 indicated that the accession had mostly diecious plants, values close to 2 indicated that the accession had a mix of diecious and monecious plants and values close to 3 indicated that the accession had mostly monecious plants. The choice of this phenotypic scale was done according to the aim of this research. This study aims to get insights into the sex determination of hemp; monecious (male + female flowers) vs. diecious (male or female flowers), instead of sex expression (male vs. female), as previously done in studies such as Sengbusch (1952) and Faux et al. (2014, 2016).

Flowering time traits were measured in 10 plants of the three middle rows from each plot. Data were provided as the mean per plot. Emergence of the plants was scored in one row per plot at day = N, N + 2, N + 4, N + 7, where N is the day of sowing. Beginning of flowering [FL_Begin in degree-days (the accumulated thermal time over a period with a base temperature of 1°C)] and full flowering (FL_Full in degree-days) were calculated relative to the day of emergence as follows: