Optimal nitrogen rates and clonal effects on cannabinoid yields of medicinal cannabis

Abstract

Nitrogen (N) nutrition and germplasm of clones can influence biomass and cannabinoid concentration in medicinal cannabis. However, there are discrepancies on the optimal nitrogen (N) application rate at the flowering stage to achieve maximum yield and if, or how, this interacts with clones from different seed lines of the same genotype. This research examined the relationship between N application rate, concentration of cannabinoids and biomass yield of a CBD-type medicinal cannabis cultivar in clones propagated from five different seed lines (hereafter referred to as clones). Clonal rooted cuttings were propagated from five mother plants germinated from seeds of cultivar 'Tas1'. Five N levels (30, 90, 160, 240 and 400 mg/L N) were imposed at the start of the inflorescence period and continued until harvest eight weeks later. Some pollen contamination occurred during the trial so that seed biomass was assessed for each plant and included in statistical analysis. Weight of total biomass, leaves and inflorescence (from upper and lower canopy positions), N%, and cannabinoid concentrations were measured after the harvest. Results indicated that increasing N supply generated a clear upward trend in inflorescence biomass that peaked at 160 mg/L N after which it did not significantly change, while leaf biomass steadily increased with N. Delta9-tetrahydrocannabinol (THC) and cannabidiol (CBD) concentrations decreased significantly with increasing N concentration in leaves with a similar, but non-significant, trend for inflorescences. The CBD to THC ratio increased with increased N. Clone source was strongly correlated with cannabinoid concentration, but not leaf, inflorescence or total biomass, across all N treatments. Clones 13 and 27 developed greater cannabinoid concentrations relative to clones 18 and 26 irrespective of N treatment. Pollen contamination induced seed development that comprised up to 5% of inflorescence biomass dry weight but this did not significantly affect whole-plant biomass, N accumulation (N%), or cannabinoid concentration. These findings provide valuable insights for improving cannabinoid yield in this widely cultivated plant species.

Keywords: CBD; Fertigation; Inflorescence; Nitrogen nutrition; Pollen contamination; THC.

Fig. 1

N concentration, biomass, total CBD and total THC (%DW) in top and bottom inflorescences, leaves and total (average of inflorescences plus leaves). Each bar displays the average level of the outcome variable predicted from the corresponding model based on Eq. (1), with the clone effect removed. The error bars represent the average absolute difference (above or below) between any two treatments that would be notionally “statistically significant” according to a Tukey multiple comparison contrast analysis within the model (assuming a Bonferroni adjustment for the six treatments and a Type I error rate of 0.05). This enables coherent comparisons between treatment effects on an easy to interpret scale. For example, on the top left graph, the lower limit of the error bar for treatment 3 does not overlap with the top of coloured bar for treatment 2 in the same graph: thus the p-value is < 0.05 for the test of the null hypothesis that the difference in the effect of treatment 3 and treatment 2 on top inflorescence N concentration is truly zero. Stars indicate which treatments have statistically significant (p < 0.05) differences to treatment 1 (30 mg/L) in pairwise tests, after adjustment for multiple testing.

Fig. 2

N percentage (% DW) of biomass (x-axis) versus cannabis biomass, CBD and THC yield (grams), respectively. Colours identify clones. Treatment labels are given as adjacent text (N levels in terms of mg/L). The lines are estimated using models based on Eq. (2), but in each case lines are drawn using the maximally complex model (in terms of statistically significant x-variables from Eq. (2), according to p < 0.05), justified via a series of nested-model F-tests (see supplementary material, Table S3).

Fig. 3

N percentage (% DW) of biomass (x-axis) versus the ratio of CBD to THC. Colours identify clones. Treatment labels are given as adjacent text (N levels in terms of mg/L). The lines are estimated using models based on Eq. (2), but in each case lines are drawn using the maximally complex model (in terms of the statistical significance of the fixed-effect x-variables from Eq. (2), according to p < 0.05), justified via a series of nested-model F-tests.