Production and quality of industrial hemp (Cannabis sativa L.) in response to water regimes

Drought and waterlogging stresses are major environmental constraints, which severely limit crop production in Australia, and worldwide. Both water deficiency and waterlogging stresses influence crop physiological and biochemical mechanisms and thereby crop growth and production. Responses of hemp (Cannabis sativa L.) to adverse soil moisture conditions are poorly understood and rarely studied, either within or outside Australia. Accordingly, this study investigated the effect of different soil water contents, ranging from water limiting to waterlogging conditions, on physiological, growth and yield responses of hemp. Data generated from the glasshouse and field experiments were subsequently used to generate model functions for predicting responses to water stress. Two field experiments were conducted in southeast Tasmania with different water supplies, to produce different water deficiency conditions. The experiments' findings demonstrated that hemp was adversely affected by water deficiency stress. Here, the reduction of leaf gas exchange, Photosystem II photochemical capacity (Fv/Fm; Ratio of variable leaf fluorescence to maximal leaf fluorescence), leaf CO2 assimilation rate, leaf transpiration rate, leaf stomatal conductance, leaf chlorophyll content (SPAD value), plant height and stem dimension, leaf area index (LAI), total above-ground dry weight (TADW), bark yield, seed yield and quality, thousand seed weight (TSW) and water-use efficiency (WUE) were observed with decreasing water supply. The measured parameters were significantly lower in the least irrigated treatment, compared with the highest irrigated treatment. Biomass and yield reduction were also occurred due to changes in plant phenology, leading to early flowering and shortening of the seed filling duration in stressed plants. In two field experiments undertaken in 2018/2019 and 2019/2020, it was also demonstrated the optimum water supply lies in between 1186 mm and 906 mm at this site and season for hemp growing. The experiments conducted in control conditions also showed reduction of photosynthetic characteristics, plant growth and yield when water application was less frequent or when irrigation interval increased beyond 10 days. The decreasing soil moisture also caused changes in plant phenology; in particular, flowering and seed maturation occurred earlier in most stressed plants compared with other treatments. A series of waterlogging experiments were conducted in controlled conditions in a glasshouse to evaluate the impact of waterlogging on the morphology, physiology, growth and yield responses of hemp. The experiments included treatments with different waterlogging durations, different plant growth stages (vegetative; six true leaf pair and flower initiation) at which waterlogging was imposed, multiple waterlogging events with different recovery periods. These experiments revealed that hemp was very sensitive to waterlogging duration, which has not previously been reported in the scientific literature. In this study, hemp did not survive twelve days of continuous waterlogging. Although plants exposed to waterlogging durations of three and six survived, there were significant reductions in plant photosynthesis, growth and yield attributes. This study also identifies that hemp was more sensitive to waterlogging during the vegetative phase, compared with the reproductive phase, and that increasing the number of waterlogging events and decreasing the gap between waterlogging events caused negative influences on plant growth and yield production. The recent resurgence in hemp production internationally is generating interest in the development modelling capability for cannabis. With this in mind, data from the experiments were used to develop simple statistical models for predicting yield (bark and seed) from water supply (that is, rainfall plus irrigation). Further, water stress functions were developed for potential incorporation in the APSIM farming system model. Overall, this study concludes that hemp is sensitive to water deficiency and waterlogging stresses. Therefore, maintaining a balanced water supply is vital to avoid detrimental effects caused by waterlogging and water deficit conditions. This has implications for the selection of growing regions that specifically avoid both low water holding capacity soils (sands), and slow draining soils (clays) in low lying and areas with winter dominate rainfall while ensuring adequate provision of water resources for supplementary irrigation.