Hemp (Cannabis sativa L.) leaf photosynthesis in relation to nitrogen content and temperature: implications for hemp as a bio‐economically sustainable crop

Abstract

Hemp (Cannabis sativa L.) may be a suitable crop for the bio-economy as it requires low inputs while producing a high and valuable biomass yield. With the aim of understanding the physiological basis of hemp's high resource-use efficiency and yield potential, photosynthesis was analysed on leaves exposed to a range of nitrogen and temperature levels. Light-saturated net photosynthesis rate (A max ) increased with an increase in leaf nitrogen up to 31.2 AE 1.9 lmol m À2 s À1 at 25°C. The A max initially increased with an increase in leaf temperature (T L ), levelled off at 25-35°C and decreased when T L became higher than 35°C. Based on a C 3 leaf photosynthesis model, we estimated mesophyll conductance (g m ), efficiency of converting incident irradiance into linear electron transport under limiting light (j 2LL ), linear electron transport capacity (J max ), Rubisco carboxylation capacity (V cmax ), triose phosphate utilization capacity (T p ) and day respiration (R d ), using data obtained from gas exchange and chlorophyll fluorescence measurements at different leaf positions and various levels of incident irradiance, CO 2 and O 2 . The effects of leaf nitrogen and temperature on photosynthesis parameters were consistent at different leaf positions and among different growth environments except for j 2LL , which was higher for plants grown in the glasshouse than for those grown outdoors. Model analysis showed that compared with cotton and kenaf, hemp has higher photosynthetic capacity when leaf nitrogen is <2.0 g N m À2. The high photosynthetic capacity measured in this study, especially at low nitrogen level, provides additional evidence that hemp can be grown as a sustainable bioenergy crop over a wide range of climatic and agronomic conditions.

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

“…Bio-economically sustainable agronomy requires effective use of scarce nitrogen and water resources. While hemp is considered as a bio-economically sustainable crop (Finnan and Styles, 2013 ; Amaducci et al, 2015 ; Tang et al, 2017b ), its water- and nitrogen-use efficiencies have not been well addressed so far. As photosynthesis is the most important physiological process determining crop water- and nitrogen-use efficiencies, this study combined experimental and modeling analyses to assess the balance between canopy photosynthetic carbon gain and its water and nitrogen costs under different nitrogen and water regimes.…”

“…Two of the containers per N treatment were supplied with sufficient water (measured as the amount of transpired water in the previous day) during the measurement while the water supply for the other two was halved. This cycle of measurements started 49 days after sowing when the 6th−8th pair of leaves appeared, the same leaf stages at which gas exchange at leaf level was assessed (Tang et al, 2017b ). The second cycle of canopy gas exchange assessment (CAN2 hereafter) aimed to assess the response of canopy gas exchange to prolonged water shortage.…”

“…The values of model input parameters required for leaf photosynthesis were presented in Tang et al ( 2017b ) for the same hemp cultivar and are summarized in Supplementary Table S1 . The canopy related parameters LAI, SLN, k L (for diffuse light) and k n were derived in this study.…”

“…The most important physiological process determining crop resource use efficiency is photosynthesis, at both leaf and canopy levels. With the aim of understanding the physiological basis of hemp's resource use efficiency, photosynthesis physiology of hemp was assessed in our previous study (Tang et al, 2017b ) on leaves exposed to a range of nitrogen and temperature levels. Correlations between leaf photosynthesis and canopy photosynthesis are not always significant (Linderson et al, 2012 ; Tomás et al, 2012 ), because the latter is also affected by canopy size and the profile of resource distribution.…”

“…The objective of this study was to experimentally assess hemp PWUE c and PNUE c in relation to nitrogen and water availabilities. To that end, we parameterized a canopy photosynthesis model (Yin and van Laar, 2005 ; Yin and Struik, 2017 ), with leaf-level parameters estimated from our previous study for hemp (Tang et al, 2017b ). This model was used for a dual-purpose: (i) to correct gas exchange measurements within different canopy chambers, and (ii) to assess the main components of hemp PWUE c and PNUE c in order to provide supporting information for efficient use of water and nitrogen resources.…”