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

Abstract

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.