Physiological Challenges in Developing Hemp for the Florida Seed and Fiber Industry Under Current Climate Pressure

Hannah Mather

Overview

Industrial hemp has once again become a popular multi-use crop, grown for its stem and seed to produce a wide array of products. Projections of increasing temperatures, partially induced by climate change, have encouraged industries to consider hemp as a low carbon and renewable resource. However, past legal and social limitations on research and production have halted breeding efforts to improve horticultural traits and further develop the crop. The 2018 Farm Bill lifted restrictions on the growth and production of industrial hemp in the United States, with the University of Florida launching a pilot project in 2019. Many Northern hemp varieties are not adapted to the typical Florida growing regions in conjunction with increasing temperature averages, the latter being further exacerbated by current global warming trends. A lack of research and development of traits associated with flowering time, seed traits, and thermotolerance limits the current production potential under the current Florida environment. This information bottleneck fuels a great demand for improvement in many seed and fiber-based traits for the state market. This review aims to discuss current physiological information and challenges within the Florida hemp seed and fiber industry, as well as outline research and breeding goals to target these obstacles within the crop.

Introduction

Cannabis sativa, also referred to as hemp, has a long and rich history of use, especially within the United States. Although the true origin of hemp remains undetermined due to extensive human movement over millennia, the general center of origin is assumed to be Central and Southwest Asia. With use recorded back to 8000 and 4000 BCE in Japan and China, hemp played important roles as a source of seed, food, fiber in ancient civilizations (Tancig, et al., 2021). Hemp arrived in the U.S through the 1545 Spanish colonization and became an important fiber crop in early US colonies by the 1600s, with peak production occurring in the mid-1800s (Ash, 1948; Fike, 2016; Tancig, et al., 2021). Hemp fiber production began to decrease in the mid 1800s due to the development of synthetic fibers and the introduction of alternative fiber crops, specifically cotton and tobacco (Roth, et al., 2018). Shortly after, a series of federal legislation and legal blocks prohibited the production of hemp in the U.S, simultaneously halting hemp research (Tancig, et al., 2021). The 1937 Marijuana Tax Act and the 1970 Controlled Substances Act created many legal challenges in the possession and cultivation of Cannabis sativa crops, leading to a half-century pause in the research and development of hemp for industrial purposes (Blare, et al., 2022; Tancig, et al., 2021).

Hemp production in Florida, experimental or not, was prohibited until the 2014 Farm Bill (Tancig, et al., 2021). Recently, traditionally Florida grown cash crops are facing novel challenges such as disease or pest pressure, as well as market competition, which drives many farmers to consider growing hemp as a new commodity crop (Blare, et al., 2022). The 2017 and 2018 Farm Bills initiated hemp pilot projects for two universities, University of Florida and Florida Agricultural and Mechanical University, as well as opportunities for state farmers to begin researching and growing hemp as an agricultural commodity once again. A USDA approved hemp cultivation plan for Florida received approval in 2020, with a projected $20-$30 billion industry potential, and information on management and cultivar selection is openly available to the public (Blare, et al., 2022; Mylavarapu et al., 2020). Hemp is often legally characterized in two groups, industrial hemp and medical cannabis. The distinguishing factor is the dry matter concentration of the psychoactive compound delta-9-tetrahydrocannabinal (THC). Industrial hemp contains less than 0.3% THC on a dry-weight basis and medical cannabis contains greater concentrations. This is an important legal parameter that industrial growers must abide by. Despite these two classifications, both types can interbreed and are considered all one species (Lancig, et al., 2021).

Applications for Hemp

Hemp can be used for many modern purposes today, but hemp was also vital in early civilizations for gathering food, fishing, or other critical advancements such as animal raising (Tancig, et al., 2021). An advantageous aspect of hemp is the unique economic value and use of each of the different plant parts that can be harvested. Fiber products from the stem as well as seeds have uses in food, cosmetics, construction and automobile composites, biofuel and industrial purposes (Ahmed et al., 2022; Blare, et al., 2022).

Hemp Seed and Oil

Hemp seeds are botanically classified as an achene fruit with high quality oil. This high quality and concentration of oil classifies hemp as an oilseed rather than a grain, and the whole seed or seed segments may be utilized. Despite being considered an oilseed, grain-like use of hemp as a food source has been observed for thousands of years and continues to be an important market today (Kaur, et al., 2021). As a great source of all nine essential amino acids, protein, a high ratio of omega-6 to omega-3 fatty acids, antioxidants, and minerals (potassium, sodium, magnesium, calcium, iron, phosphorous, and zinc), hemp seeds are increasing in popularity (Blare, et al., 2022; Callaway, 2004; Leizer, et al., 2000; Leonard, et al., 2019; Kaur, et al., 2021). A 2010 study determined that hempseed had nearly the same amount of protein as soybean as well as high amounts of Vitamin E and minerals (Rodriguez-Leyva, et al., 2010), further supporting hemp’s nutritional profile. Flours made from hemp seed also contain more fiber and balanced protein than corn or wheat flour (Kaur, et al., 2021). Due to the high quality of protein and necessary fatty acids, the use of hemp seed in animal feed has also been suggested (Antunovic, et al., 2019). Hemp oil contains around 90% unsaturated fatty acids and is often used in cosmetics products such as makeup, soaps, and lotions (Devi and Khanam, 2019; Kaur, et al., 2021).

Hemp Fiber

Hemp varieties have long been cultivated and selected for fiber production, historically being an extremely strong and durable use of the plant (Adesina et al., 2020). Attention is turning back to natural fiber sources like hemp to combat high carbon emissions from man-made products due to the hemp’s competitive physical qualities, carbon sequestering ability, and renewability (Ahmed et al., 2022). Hemp fiber is often harvested in two forms; 1.) long phloem fibers referred to as ‘bast’ from the outer stem and 2.) short, stiff xylem fibers of the inner stem referred to as ‘hurd’ (Cherney and Small, 2016). Each fiber type has unique qualities for different purposes. Bast fibers are often harvested for automotive and paper textiles, while hurd fibers are often used in bedding or for construction as fiberboard or in composites like ‘hemp-crete’ (Adesina et al., 2020; Cherney and Small, 2016). The product, market, and regional environmental conditions all heavily influence the hemp cultivar that is grown. While the yield quality of a dual-purpose crop is lower than crops grown for either bast or hurd fiber, the economic advantage of being able to harvest multiple parts of the plant is appealing to many growers (Adesina et al., 2020). Other common uses for hemp fiber range from clothing, shoes, paper, as well as more industrial textiles like rope and nets. More recent innovations include bio-fuel applications and bio-composites, like previously mentioned hemp-crete or hemp-based plastics, which are becoming increasingly popular in the automotive, construction, and plastic industries (Adesina et al., 2020; Blare, et al., 2022; Karche, 2019; Roth, et al., 2018).

Challenges in Florida Cultivation

While Florida growers are looking to hemp as a potential commodity crop, the novelty of the industry comes with challenges due to lack of research in specific areas regarding local effect on flowering habits, seed yield and quality, and the current negative impacts global warming has on these hemp research areas.

Flowering Habits

Harvesting hemp plants for the highest quality fiber requires harvest timed during peak flowering phase. As flowering begins, nutrient and resource allocation switches from the stem/leaves to flowers/seeds which reduces the maximum quality potential of the fiber portions of hemp (Salentijn et al., 2019). Harvesting hemp for fiber past the optimum flowering window results in a major decrease in fiber quality and yield. Therefore, being able to accurately predict the initiation of flowering time enables for the best harvest for peak fiber yield, strength, and quality (Schluttenhofer et al., 2017; Zhang et al., 2021).

Hemp is generally considered as a quantitative short-day flowering plant, focusing on vegetative growth during the long days of summer and initiating flowering when nights exceed 12 hours (Petit, et al., 2020). Relatively short daylengths present one of the biggest challenges in cultivating hemp in subtropical and tropical regions like Florida since fiber quality is strongly influenced by flowering time (Petit et al., 2020; Zhang, et al., 2021). Many northern adapted hemp varieties display short juvenile phases under Florida conditions, often flowering before the desired window (Kelly-Begazo and Brym, 2019; Zhang et al., 2021). Despite this challenge, there is likely a large variability in day length sensitivity due to Cannabis sp. wide adaptation to many climates and latitudes (Zhang et al., 2018; Zhang et al., 2021).

Juvenile phase length and photosensitive development phase of a particular cultivar have been demonstrated to be largely dependent on the geographic or latitudinal origin, yet genetics are hypothesized to play a large role as well (Zhang, et al., 2021). Understanding basic requirements of the juvenile phase as well as the photoperiod sensitivity is crucial to selecting the right hemp cultivar for the desired region and production market. Selecting hemp cultivars that display late transitions from vegetative to flowering under short days is key to induce vegetative and stem growth for increased fiber biomass yield (Zhang, et al., 2021). Important gene pathways involved with hemp flowering have been investigated, with ‘photoperiodic pathway’ and ‘temperature pathway’ playing crucial roles in flowering time (Salentijn et al., 2019). A possible solution is breeding and utilizing ‘autoflowering’ types, in which transition from vegetative to flowering stages is induced by length of vegetative period rather than photoperiodic requirements (Elias et al., 2020).

While photoperiod presents the main challenge in timing hemp flowering under Florida environments, other economical fiber traits such as low THC content, high fiber quality, and resistance to lodging must continue to be at the forefront of research priorities (Williams, 2020). Due to the complex nature of flowering traits, further understanding and mapping of diverse populations under different Florida conditions is required to elucidate the underlying mechanisms. Using this underlying knowledge, selecting hemp for fiber within the desired region of production with these flowering traits in mind provides better assessments of elite germplasm.

Seed Development, Yield, and Quality

While recent modern cultivation has improved some key economic traits, many wild type traits are still expressed in hemp seed development and yield. A general inconsistency in hemp seed yield may partially be due to each genotype’s ambiguous history, or due to genotype and environmental interactions. Physiological challenges currently being faced in Florida production include indeterminate seed set patterns, seed quality inconsistency, and a high tendency of seed shattering.

Breeding for crops that are highly uniform and have seeds that mature within the desired window are key traits of interest (Williams, 2020). In the naturally dioecious hemp plant, a large proportion of males within a population will result in seed yield reduction since female plants are predominantly desired (Petit et al., 2020). Despite these differences, hemp plants generally display indeterminate flowering patterns which in turn result in a more continuous, yet uneven seed set and development over the growing season (Elias et al., 2020). Seeds found at the top of the plant are often more mature than those near the bottom, as female flowers start developing at the central apex of the plant (Bouloc, 2013; Elias et al., 2020). Due to the uneven flowering and maturation of seeds, harvest results in an inconsistency of high- and low-quality seeds due to their respective developmental phase at harvest (Elias et al., 2020). A consistent seed quality including desirable oil, protein, and fatty acid components as well as good vigor and viability is paramount to seed producers (Elias et al., 2020; Ferfuia et al., 2021). Selecting hemp with uniform flowering habits, or the use of autoflowers as previously mentioned, are achievable breeding goals.

Seed shattering is another undesirable trait that remains within most hemp cultivars. Seed shattering is also associated with inconsistent timing of inflorescence maturity within the plant, again an adaptation that historically allowed for extended periods of seed dispersal for the plant (Schluttenhofer et al., 2017). Growers often sacrifice the time it would take for all seeds to fully mature, and instead harvest at 70% maturity to reduce seed yield loss prior and during harvest (Williams and Mundell, 2016). However, selecting traits that reduce seed shattering like strong abscission zone walls or limited bract seed release could potentially increase yields by 15% by allowing full maturation and limited loss (Schluttenhofer et al., 2017). Regional development and cultivation of hemp varieties for elite seed and seed oil characteristics like uniform high quality seed set, as well as resistance to seed shattering, will be highly demand to improve current production (Schluttenhofer et al., 2017).

Climate Change: Impact of High Temperatures on Current Issues

There are many traits in hemp with room for improvement, however global warming adds complexity to the situation for plant breeders. Florida cultivation temperatures can, and often do, exceed hemps optimum and maximum temperatures of 29°C and 41°C, respectively, which highly influences plant development (Petit et al., 2020). Aside from previously discussed research gaps in hemp flowering and seed development, increasing temperatures reveal unique physiological obstacles that must be addressed by plant breeders.

Flowering Habits

Stressors such as high temperature, associated with climate change, can combine with photoperiod to jointly influence the flowering time in hemp (Hall, et al., 2012). Flowering in hemp can already be unpredictable, and high temperatures induce several changes to the plant development. High temperatures are very critical in the juvenile stage of hemp, and work in conjunction with flowering genes to transition from vegetative to reproductive (Salentijn et al., 2019). High temperatures can also reduce the duration between flower primordia development and full flowering, thus reducing the overall time to flowering (Amaducci, et al., 2008). This rapid transition from vegetative to flowering stages means the duration normally spent allowing the stem or leaves to accumulate biomass is significantly reduced, limiting the potential for both fiber production and peak flower set to produce seed. Identifying hemp with a slow maturation rate and a late tendency to flower by employing thermotolerant mechanisms will provide elite germplasm for research and production.

Seed Set and Development

Even if the plant escapes high temperatures during vegetative stages, current inconsistencies in hemp seed yield and quality can be partially attributed to high temperatures during seed filling, as seen in several other crops like rice, maize, soybean, and sunflower. Seed development directly follows the flowering stage, and high temperatures during this transition have demonstrated negative effects on seed formation, development, and quality such as oil accumulation; all of which reduce yield (Ferfuia et al., 2021). Research and breeding goals must focus on stabilizing and maintaining a stable grain fill despite high temperatures.

Legal Implications

The ability of high temperatures, as induced by global warming, to impact a hemp plant’s legal standing in a field remains to be a key challenge to not only Florida growers, but global producers as well. For legal production, the concentration of THC in dry weight matter must remain below 0.3%. However, genetic differences and stress inducing factors such as high temperature, high humidity, or nutritional limitations can result in higher concentrations than expected (Kelly-Begazo and Brym, 2019). This means under Florida conditions growers must be especially careful to select varieties that do not produce high amounts of THC that would result in crop disposal, even under high temperature conditions. To provide growers with a selection of hemp to grow, breeders must select and breed for hemp that displays thermotolerant characteristics in the various Florida regions, as hemp selected for thermotolerance in Northern regions will likely do poorly in Florida’s especially hot and humid environment.

Conclusion

Hemp products have a wide range of applications within many industries, with the added advantage of being more environmentally sustainable than alternative man-made materials. To maximize the potential of the Florida industrial hemp industry, elite varieties must be developed and selected for their specific market use and region of cultivation. To do so, certain characteristics that are critical to fiber and seed production must be identified, elucidated, and incorporated into cultivar development. Currently traits involving flowering habits, seed quality, and thermotolerance are paramount in maintaining hemp germplasm well adapted to Florida production regions. Photoperiod presents one of the largest challenges in subtropical hemp cultivation, and research to further understand flowering mechanisms and develop autoflowering hemp would be the most beneficial to Florida growers. Hemp for seed production requires more development in the uniformity of inflorescence formation and seed development, as well as resistance to seed shattering, to increase the quality and yield of seed harvests. Lastly, increasing global temperatures associated with climate change places additional pressure on these research areas, as breeders see negative impacts in similar traits of other more developed and domesticated crops. The Florida seed and fiber hemp industry provides promise to growers looking for a new crop, and since research restrictions have been lifted, programs are currently investigating these breeding bottlenecks.

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