Agronomy and photosynthesis physiology of hemp (Cannabis sativa L.)

Hemp (Cannabis sativa L.) is a sustainable high-yielding crop that delivers valuable fibres, seeds and psychoactive substances. However, there is a lack of field experimental data on the cultivation of hemp because its production was largely abandoned in the last century. Hemp is now considered as an ideal crop to produce innovative biomaterials, and in particular, the dual-purpose hemp production (fibre + seed) is now the norm in European countries, driven by the shift of a rapidly expanding market for hemp seeds coupled with lower quality fibre requirements for innovative biomaterials. This study brought new information on the agronomy and photosynthesis physiology for the resurging production of hemp, particularly for dual-purpose production in Europe. The effects of important agronomic factors, i.e. cultivar, planting density, and nitrogen fertilization, on the performance of the hemp crop were investigated under contrasting European environments. Based on the experimental data, for dual-purpose hemp production, a planting density of 90–150 plants m-2 is recommended for a monoecious cultivar that gives a long vegetative phase while leaving enough time for seed growth. A nitrogen fertilization rate of 60 kg N ha-1 was generally sufficient in the tested environments whereas further optimization of nitrogen fertilization requires accurate and precise assessment of plant nutritional status. To facilitate assessing plant nutritional status, a critical nitrogen dilution curve was determined for hemp. The responses of leaf photosynthesis to nitrogen content and temperature were quantified using a biochemical model of C3 leaf photosynthesis, based on a complete set of photosynthetic measurements for hemp leaves. Then, by combining measurements and modelling, an upscaling was made from the leaf to the canopy level to analyse hemp’s photosynthetic nitrogen-use efficiency (NUE) and water-use efficiency (WUE) in response to water and nitrogen supply. The effect of nitrogen supply level on hemp’s NUE and WUE was largely determined by its effect on canopy size or leaf area index (LAI). The effect of short-term water stress on WUE and NUE was reflected in the stomatal regulation, whereas long-term water stress enhanced leaf senescence, reduced LAI but retained total canopy nitrogen content, and thus resulted in a further increase in WUE. Findings in this thesis provided an improved understanding of the agronomy and photosynthesis physiology of hemp, particularly in relation to the dual-purpose production of hemp in Europe. Such understanding not only provides additional evidence that hemp can be grown as a sustainable crop over a wide range of climatic and agronomic conditions, but also provides essential information for parameterizing crop growth models. Prospects for further research were discussed in view of using the findings in this thesis in combination with a crop growth model to develop strategies for optimization of hemp cultivation and breeding.</p

Presentation_1_Water- and Nitrogen-Use Efficiencies of Hemp (Cannabis sativa L.) Based on Whole-Canopy Measurements and Modeling.PDF

<p>Interest in hemp (Cannabis sativa L.) as a crop for the biobased economy is growing worldwide because hemp produces a high and valuable biomass while requiring low inputs. To understand the physiological basis of hemp's resource-use efficiency, canopy gas exchange was assessed using a chamber technique on canopies exposed to a range of nitrogen (N) and water levels. Since canopy transpiration and carbon assimilation were very sensitive to variations in microclimate among canopy chambers, observations were adjusted for microclimatic differences using a physiological canopy model, with leaf-level parameters estimated for hemp from our previous study. Canopy photosynthetic water-use efficiency (PWUE_c), defined as the ratio of gross canopy photosynthesis to canopy transpiration, ranged from 4.0 mmol CO₂ (mol H₂O)⁻¹ to 7.5 mmol CO₂ (mol H₂O)⁻¹. Canopy photosynthetic nitrogen-use efficiency (PNUE_c), the ratio of the gross canopy photosynthesis to canopy leaf-N content, ranged from 0.3 mol CO₂ d⁻¹ (g N)⁻¹ to 0.7 mol CO₂ d⁻¹ (g N)⁻¹. The effect of N-input levels on PWUE_c and PNUE_c was largely determined by the N effect on canopy size or leaf area index (LAI), whereas the effect of water-input levels differed between short- and long-term stresses. The effect of short-term water stress was reflected by stomatal regulation. The long-term stress increased leaf senescence, decreased LAI but retained total canopy N content; however, the increased average leaf-N could not compensate for the lost LAI, leading to a decreased PNUE_c. Although hemp is known as a resource-use efficient crop, its final biomass yield and nitrogen use efficiency may be restricted by water limitation during growth. Our results also suggest that crop models should take stress-induced senescence into account in addition to stomatal effects if crops experience a prolonged water stress during growth.</p

Water-and nitrogen-use efficiencies of hemp (Cannabis sativa L.) based on whole-canopy measurements and modeling

Interest in hemp (Cannabis sativa L.) as a crop for the biobased economy is growing worldwide because hemp produces a high and valuable biomass while requiring low inputs. To understand the physiological basis of hemp\u2019s resource-use efficiency, canopy gas exchange was assessed using a chamber technique on canopies exposed to a range of nitrogen (N) and water levels. Since canopy transpiration and carbon assimilation were very sensitive to variations in microclimate among canopy chambers, observations were adjusted for microclimatic differences using a physiological canopy model, with leaf-level parameters estimated for hemp from our previous study. Canopy photosynthetic water-use efficiency (PWUEc), defined as the ratio of gross canopy photosynthesis to canopy transpiration, ranged from 4.0 mmol CO2(mol H2O) 121to 7.5 mmol CO2(mol H2O) 121. Canopy photosynthetic nitrogen-use efficiency (PNUEc), the ratio of the gross canopy photosynthesis to canopy leaf-N content, ranged from 0.3mol CO2d 121(g N) 121to 0.7mol CO2d 121(g N) 121. The effect of N-input levels on PWUEcand PNUEcwas largely determined by the N effect on canopy size or leaf area index (LAI), whereas the effect of water-input levels differed between short-and long-term stresses. The effect of short-term water stress was reflected by stomatal regulation. The long-term stress increased leaf senescence, decreased LAI but retained total canopy N content; however, the increased average leaf-N could not compensate for the lost LAI, leading to a decreased PNUEc. Although hemp is known as a resource-use efficient crop, its final biomass yield and nitrogen use efficiency may be restricted by water limitation during growth. Our results also suggest that crop models should take stress-induced senescence into account in addition to stomatal effects if crops experience a prolonged water stress during growth

Effects of Gibberellin Pre-Treatment on Seed Germination and Seedling Physiology Characteristics in Industrial Hemp under Drought Stress Condition

The present study aimed to explore the effects of exogenous gibberellins (GAs) on seed germination and subsequent seedling growth of hemp (Cannabis sativa L.) under drought stress. Seeds of two industrial hemp cultivars i.e., ‘Yunma 1’, (YM) and ‘Bamahuoma’, (BM) were treated with different concentrations of GA3 solution (0, 200, 400, 600, 800 mg/L) at 20 °C for 8 h. The effect of pre-treatment was assessed on germination characteristics and physiological indexes on subsequent exposure to drought stress using 20% (m/v) polyethylene glycol (PEG) for 7 days. The results revealed that seed germination in hemp was sensitive to drought stress, as the germination indexes (germination rate and germination potential) decreased significantly, and seedling growth (hypocotyl length and radicle length) was impeded under 20% PEG-6000 condition. GA3 pre-treatment affected germination rate, germination potential, hypocotyl length and radicle length. With increasing GA3 concentration, these indexes first increased and then decreased. For seedling physiology characteristics in hemp, GA3-pretreatment remarkedly increased the osmotic regulating substances (soluble sugar and soluble protein contents) and the activities of antioxidant enzymes (SOD, superoxide dismutase and POD, peroxidase), while sharply decreased the lipid peroxidation (malondialdehyde, MDA) in seedlings grown under PEG-6000 induced drought stress. These results suggested that seeds pre-treated with GA3 could enhance the drought tolerance of hempseeds, and the optimal effect of GA3 for seed pre-treatment of YM and BM could be obtained when the concentration of GA3 solution reached 400 mg/L and 600 mg/L, respectively