Abstract
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 (PWUEc), 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 (PNUEc), 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 PWUEc and PNUEc 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 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.
Highlights
The pressures of climate change, natural resource scarcity and environmental pollution have fuelled interest in bioeconomically sustainable agronomy that requires effective use of scarcely available resources
The present study focuses on the scaling up of hemp photosynthesis from leaf to canopy and on analysing canopy photosynthetic water-use efficiency (PWUEc) and canopy photosynthetic nitrogen-use efficiency (PNUEc)
In the N60 plots where weed competition was negligible in the field experiment in 2014, leaf area index (LAI) was on average 3.2 and 4.8 m2 m−2 at linear growth stage and full flowering, respectively; specific leaf nitrogen (SLN) was on average 0.97 and 0.67 g N (m2 leaf)−1, respectively
Summary
The pressures of climate change, natural resource scarcity and environmental pollution have fuelled interest in bioeconomically sustainable agronomy that requires effective use of scarcely available resources. Hemp is a high-yielding multi-purpose crop that requires low inputs (Struik et al, 2000; Tang et al, 2016, 2017a) and has a positive impact on the environment (Bouloc and van der Werf, 2013; Barth and Carus, 2015). Its stems contain high-quality cellulose (De Meijer and van der Werf, 1994); high added-value compounds can be recovered from the female inflorescence and from threshing residues (Bertoli et al, 2010; Calzolari et al, 2017) after harvesting the seeds, that contain healthy oil (Leizer et al, 2000). Little attention has been paid to understanding the physiological basis of the high resource-use efficiency of hemp
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