The potential of industrial hemp (Cannabis sativa L.) as an emerging drought resistant fibre crop

Periods of drought are projected to increase in intensity and frequency across many parts of the world, affecting freshwater availability for agriculture and limiting cropping productivity. Exploring innovative opportunities to utilise novel drought resistant crops should be prioritized to sustainably meet growing demand for food and fibre. The potential benefits of industrial hemp (Cannabis sativa L.) as a drought resistant fibre crop have been touted, but the underlying evidence base of such claims is conflicting. Hemp has several drought resistance traits that allow it to thrive under water deficit, including deep roots and effective stomatal regulation, but studies report varying results for water requirements and water use efficiency. In this context, we provide a comprehensive discussion of the current state of knowledge regarding fibre hemp water use in a range of environments and between varieties, highlighting physiological attributes that contribute to its drought resistance with a view to guiding and stimulating further research. With relatively low water requirements compared to other fibre crops, hemp shows great potential as a drought resistant crop, offering exciting possibilities to produce sustainable fibre in a changing climate.


Introduction
Drought -a transient, extreme environmental condition resulting from a period of low precipitation that leads to a shortage of water -is projected to increase in intensity and frequency in the future, with climate change triggering increasingly sporadic precipitation events (da Silva et al. 2011;Hoegh-Guldberg et al. 2018).The consequences of intensified drought patterns will be severe, affecting freshwater availability for agriculture and industry, damaging essential natural ecosystems, and reducing access to safe drinking water (Acevedo et al. 2022).Immediate and proactive adaptation measures within agriculture, such as the identification and adoption of drought resistant crops, are thus needed to maintain cropping productivity (da Silva et al. 2011;Satriani et al. 2021).
Vol:. ( 1234567890) Drought resistant crops can endure or adapt to dry conditions, allowing them to grow and reproduce during periods of decreased water availability.These crops have specific characteristics, such as deep root systems, effective stomatal regulation, and tissue osmotic adjustment mechanisms that act on both short-and long-term time frames to reduce water loss and use soil-stored water more efficiently (Gupta et al. 2020).Under-explored or alternative fibre crops that require less water to produce quality fibre and that are better suited to adverse environmental conditions may provide innovative opportunities for step changes in fibre production under drought conditions.
Here, we examine industrial hemp (Cannabis sativa L.) as a fibre crop that shows potential for cultivation in water-limited environments.While low water use and drought resistance in hemp has been touted (Rehman et al. 2013;García-Tejero et al. 2019;Satriani et al. 2021;Blandinières and Amaducci 2022;Gill et al. 2022), the scientific evidence is unclear, particularly as research and development has been limited by the stigma and regulations surrounding cannabis in the past.As reviews on fibre hemp water use and drought resistance are thus scarce, the objectives of this mini review are to: 1. Compile evidence for hemp water use allowing comparison with water use in other fibre crops; 2. Describe how hemp responds to water deficit; and 3. Identify and suggest key opportunities for future research.

Industrial hemp history, uses, and agronomy
Industrial hemp, an ancient herbaceous species originating from Central Asia, has been used for medicine and fibre for over 6,000 years (Amaducci et al. 2015), but its use declined in the twentieth century due to the introduction of synthetic fibres and strict regulations driven by the stigma surrounding cannabis (Żuk-Gołaszewska and Gołaszewski 2018).Recently, the fast-growing plant has attracted renewed interest owing to its many applications in agriculture, textiles, papermaking, cosmetics, bio-composites, construction, biofuels, and food industries (Irakli et al. 2019).Compared to medicinal strains of cannabis, which have high levels of the psychoactive compound Δ 9-tetrahydrocannabinol (THC), industrial hemp contains very low levels of THC, making it non-psychoactive.
Hemp provides one of the strongest available natural fibres.High quality bast fibre, from the secondary phloem in the stem, is a prized material used for rope and twine, textiles including fabrics and clothing, highquality paper, biodegradable plastics, construction and automotive components, and insulation (Pickering et al. 2007;Adesina et al. 2020).Hemp clothing is becoming increasingly popular as an alternative to cotton or linen.Hurd from the inner woody portion of the hemp stem was previously regarded as a by-product of bast fibre production but is now considered a valuable resource, being used for building materials, animal bedding and feed, textiles, and composite materials (Crini et al. 2020).In particular, hempcrete, made from a mixture of hemp hurds, water and lime, is emerging as an energy-efficient, sustainable alternative to traditional concretes and building materials, which currently contribute 5-8% of global greenhouse gas emissions (Teh et al. 2017;Jami et al. 2019).
While this review will focus on hemp production for fibre, hemp seeds are an excellent source of nutrition, containing all essential amino acids and fatty acids necessary to maintain healthy human life, as well as high protein and omega-3 contents (Matthäus and Brühl 2008;Leizer et al. 2015;Carus et al. 2016;Żuk-Gołaszewska and Gołaszewski 2018;Irakli et al. 2019;Schultz et al. 2020).Hemp is also a promising feedstock crop for the production of sustainable biofuels (Das et al. 2020).
Hemp, an annual C3 dicotyledonous angiosperm belonging to the Cannabaceae family, can be dioecious, with tall, slender male plants and bushier female plants, or monoecious (Amaducci et al. 2015).It is considered to be a high-yielding, fast-growing crop that requires relatively little fertiliser and herbicide (Struik et al. 2000).Owing to its high adaptive phenotypic plasticity, hemp responds readily to environmental cues, making it an ideal choice to cope with future climate changes (Small 2015).Despite the wide range of conditions in which hemp can be cultivated successfully, it is sensitive to poor soil structure, waterlogging, and sowing rate (Struik et al. 2000;Adamovics et al. 2017;Żuk-Gołaszewska and Gołaszewski 2018;García-Tejero et al. 2019).

Fibre hemp water requirements
Hemp growth occurs during summer, due to its shortday photoperiod requirements, when environments are driest and limitations in rainfall mean supplemental irrigation is often required.Although hemp is grown in a multitude of environments throughout Europe, Asia, Australia and North America (Amaducci et al. 2015;Adesina et al. 2020), most water studies have been conducted in Mediterranean environments in Southern Europe and USA using supplemental irrigation.These studies suggest that, overall, hemp requires 250-700 mm (approximately 2.5-7.0ML/ha) over the entire growing season for optimum yield, with 250-350 mm (2.5-3.5 ML/ha) required during the vegetative stage (Amaducci et al. 2000;Di Bari et al. 2004;Cherrett et al. 2005;Cosentino et al. 2013;Blandinières and Amaducci 2022).Without optimum water, hemp aboveground biomass is reported to decrease by up to 60% (Amaducci et al. 2000;Cosentino et al. 2013;Campbell et al. 2019), with reports of early maturity, reduced stem mass, and stunted plants (Adesina et al. 2020), particularly when water deficit occurs prior to successful establishment.After establishment, plants are better able to cope with reduced water availability, due to the development of deep root systems allowing access to deep soil water (Duque Schumacher et al. 2020;Blandinières and Amaducci 2022).Successful growth and limited yield penalties have been reported without continuous irrigation over the entire season, even in hot, dry conditions, if irrigation occurs consistently during establishment (Herppich et al. 2020).Although evapotranspiration demand and summer rainfall differ between environments and climates, seasonal water use between 220 and 450 mm is the most common range for fibre hemp (Table 1).
Furthermore, water requirements also vary among hemp varieties, likely due to large genetic variability arising from past breeding restrictions as a consequence of cannabis prohibition (Table 1).According to Babaei and Ajdanian (2020), while water stress reduced the yield of Iranian hemp varieties, significant variability was intrinsic among them.Flowering patterns and plant sex also impact water requirements, with fibre hemp requiring 250 mm of water for monoecious early genotypes and 450 mm for dioecious late genotypes over the growing season (Tang et al. 2018).There have been limited extensive trials that examine inter-varietal differences in hemp water use, presenting opportunities for research in a range of environments.

Comparison of fibre crop water requirements
The growing scarcity of freshwater resources highlights the need for sustainable solutions in irrigated agriculture (Acevedo et al. 2022).In this context, hemp presents a significant advantage to other fibre crops, with its low water requirements positioning it as a promising fibre option.Hemp has been estimated to require 70% less water than cotton (Schultz et al. 2020), which has water requirements of above 700 mm in subtropical or Mediterranean climates.Unlike cotton, hemp can also thrive under a highly variable precipitation pattern with successful growth reported under exclusively rainfed conditions (Herppich et al. 2020).Furthermore, hemp offers a distinct advantage over cotton as it produces fibres vegetatively, while cotton must be grown to the flowering stage.This shorter growth cycle facilitates earlier harvesting of hemp fibre, a significant benefit when facing prolonged drought periods.
While direct comparisons of hemp with bamboo, flax, and jute are more difficult, due to their differing regions of production and preferred climates, these plants are not as well-adapted to dry environments (Table 1).Bamboo, which produces a lignocellulosic fibre similar to bast, grows predominantly in tropical parts of Asia with very high rainfall of 1,000-2,000 mm (Dierick et al. 2010;Zhang et al. 2019).Jute and flax produce bast fibre like hemp, but both require more water for cultivation (Table 1).Jute requires greater than 1,000 mm, which is mostly supplied via monsoonal rainfall (Kundu 1956;Panigrahi et al. 1992), while flax is reported to require above 700 mm during a growing season in subtropical or continental climates (Bauer et al. 2015;Heller and Byczynska 2015).As many of the world's agricultural regions become increasingly arid in the future, hemp can therefore play a crucial role in producing quality fibre, in a relatively short timeframe, in dry climates in which other fibre crops would not survive.

Fibre hemp responses to water deficit
While fibre hemp water requirements are relatively well established, at least in Mediterranean environments (typified by warm, dry summers and cool, wet winters), less is known about how water deficit affects its physiology and morphology.Hemp is relatively drought resistant, but prolonged periods of water deficit can lead to multiple physiological changes which can have major yield ramifications. Short-term water stress reduces stomatal conductance, whereas long-term stress increases leaf senescence and reduces canopy photosynthetic nitrogen-use efficiency (Tang et al. 2018) (Fig. 1).According to Gao et al. (2018), most genes related to photosynthesis were downregulated in hemp under water stress.While downregulation is beneficial for decreasing cellular damage, these responses suppress plant growth-related processes, reducing biomass and yield, as observed in multiple studies (Table 2).When irrigation was reduced from 400 to 100 mm in field trials, yield decreased dramatically in Futura varieties (Scordia et al. 2022).Although not a typical fibre hemp variety, Gill et al. (2022) found that the Black Label cultivar showed reduced biomass, height, and total seed yield under water stress, but maintained filled seeds for reproduction (Fig. 1).Herppich et al. (2020) also reported differences in drought acclimation strategies between two high-yielding fibre varieties in German field trials, with Ivory employing an 'optimistic' strategy of high carbon gains over a short growth period and Santhica 27 producing higher yield using a 'pessimistic', longer term strategy.Although Ivory plants had larger leaf area and greater water use efficiency, early onset of senescence greatly reduced fibre yield and biomass production.
Hemp develops deep root systems that likely contribute to its drought resistance.Hemp roots penetrate to a depth of up to 2 m, with a long taproot allowing plants to reach the water table in some instances (Amaducci et al. 2008).Deep rooting is favorable in water-limited environments but root depth depends on soil depth and compaction, so drought combined with poor or shallow soil will limit drought resistance in hemp (Blandinières and Amaducci 2022).
The reductions in hemp biomass, growth, and seed yield under drought conditions are likely a consequence of decreased stomatal opening and photosynthetic rate.Gill et al. (2022) reported stomatal closure and reduction in photosynthesis under water stress, but biomass water-use efficiency did not increase (WUE).This contradicts previous studies stating that WUE in hemp increases under water stress and is likely because the water deficit was severe, limiting biomass production.Although hemp aboveground biomass and yield decrease under low water availability, this does not necessarily indicate critical crop failures (Blandinières and Amaducci 2022).Hemp has been reported to survive and reproduce at an exceptionally low soil gravimetric moisture content of 5%, at which point most other crops cannot survive (Gill et al. 2022).As discussed, cotton, bamboo, flax, and jute all require higher water availability for growth (Table 1) and therefore are unlikely to produce quality fibre at such a low level of water availability.Thus,  while hemp is a drought resistant crop, the length of exposure, timing, and intensity of the water deficit plus environmental and soil conditions, agronomical decisions, and genotypic traits will greatly influence this resistance.Drought resistance trait variation among different varieties, observed in a study by Sheldon et al. (2021), could help identify superior cultivars for dry environments.
Given the variability, more research is needed to identify the most drought resistant hemp varieties, particularly by comparing germplasm for their water-saving traits.

Next steps for fibre hemp in a drier climate
This mini review has highlighted areas of both wellresearched and underdeveloped fibre hemp literature.We suggest future studies involving: • Extensive varietal trials to establish vigorous, fastgrowing hemp lines that exhibit drought resistance without trade-offs in bast and hurd production; • Establishment of diversity sets from across regions where hemp might be grown and model strains that are prioritised for experiments to allow for benchmarking and validation across regions and experiments; • Identification of an agreed key set of, and screening for, physiological traits associated with drought tolerance (eg.photosynthetic traits, stomatal density, root length, osmotic adjustment) using a genome wide association study (GWAS) to find target genes for breeding; • Plant water uptake dynamics at the whole plant level to describe the vulnerability of hemp to cavitation and embolism, as well as plant hydraulic conductances; • Exploration of physiological traits (eg.leaf wilting, leaf area adjustment, osmotic adjustment) and anatomical traits (eg.leaf structure, xylem structure, cuticle composition) that might confer drought resistance in hemp; and • Investigation of the role of plant microbe interactions (eg.mycorrhiza) in enhancing water uptake and water use in hemp.
Water treatments differ between studies but are always presented with greatest water availability at the top and least at the bottom, for each reference.WUE is the water use efficiency (g of biomass produced per litre of water).nr = data not reported, ETc = crop evapotranspiration, SAW = soil available water.Assuming full cover irrigation or rainfall, 100 mm is equivalent to 1ML/ha

Conclusions
Addressing the challenges presented by exacerbated drought conditions while ensuring sufficient cropping productivity will require a combination of strategies.Hemp has the potential to grow under limited or variable water, with relatively low water requirements of 220 mm to 450 mm compared to other fibre crops.As hemp research has been restricted in the past, future studies must investigate the physiological and molecular mechanisms underlying hemp drought resistance as well as identify genetic traits that can improve its WUE and that can be effectively targeted by breeding programs.While challenges exist in the widespread adoption of hemp, it shows great potential as a drought resistant crop, offering exciting possibilities to produce sustainable fibre in a changing climate.
Futura 75 WUE increased from 2.73 kg m −3 to 3.45 kg m −3 when water stressed (Cosentino et al. 2013), while Tang et al. (2018) found that photosynthetic (intrinsic) WUE, defined as the ratio of net CO 2 assimilation rate to transpiration (Lamaoui et al. 2018), ranged from 4.0 mmol CO 2 (mol H 2 O) −1 to 7.5 mmol CO 2 (mol H 2 O) −1 under water stress.Reported differences may be attributed to variable water stress intensity and duration, as well as environmental conditions.As cotton WUE averages 1 kg m −3 (Himanshu et al. 2023), hemp WUE is up to 3.5 times greater than cotton.Long-term water stress increases leaf senescence and reduces canopy photosynthetic nitrogen-use efficiency (Tang et al. 2018), so while hemp can withstand some water deficits, its biomass and seed are still restricted by water limitation during growth.

Fig. 1
Fig. 1 Schematic diagram of the current understanding of industrial hemp waterstress responses.Conflicting responses or those that lack evidence are indicated by a question mark

Table 1
The reported water requirements of common fibre crops, including industrial hemp, with units standardised in mm

Table 2
Responses to water deficit in industrial hemp varieties