Nitrogen source matters: "High NH4/NO3 ratio reduces cannabinoids, terpenoids, and yield"

"The nitrogen (N) form supplied to the plant, ammonium (NH4+) or nitrate (NO3–), is a major factor determining the impact of N nutrition on plant function and metabolic responses," says Prof. Nirit Bernstein of the Volcani Center in Israel. Recently, Prof. Bernstein received the International Agronomy Award from the American Society of Agronomy for "vision and outstanding scientific breakthroughs in cannabis plant science and agronomy that benefits the cannabis industry worldwide." For her recent nitrogen study, the researchers hypothesized that the ratio of NH4/NO3 supplied to cannabis plants affects the physiological function and the biosynthesis of cannabinoids and terpenoids, which are major factors in the cannabis industry. To evaluate the hypothesis, they examined the impact of five supply ratios of NH4/NO3 (0, 10, 30, 50, and 100% N-NH4+, under a uniform level of 200 mg L–1 N) on plant response. 

Method 
The plants were grown in pots under controlled environment conditions. Medical cannabis plants of the cultivar "Annapurna 2", a certified cultivar for commercial medical use in Israel, were used as a model plant in this study. Plants were propagated from rooted cuttings in coconut fiber plugs. During the first week after planting (during the week of vegetative growth) before the initiation of the nutritional treatments, a uniform fertigation regime was practiced under a long-photoperiod (18/6 h light/dark) in a controlled environment growing room. The plants were irrigated daily to allow 30% drainage; the fertigation followed the optimal N and K regimes recently developed by the researchers for the vegetative growth phase.

Uniform plants were then selected for the experiment, and randomly divided into treatment groups of increasing N-NH4+ supply: 0, 10, 30, 50, and 100% N-NH4+ (and a corresponding decrease in N-NO3– supply), five replicated plants per treatment. The remaining N was supplied as N-NO3–, to a final uniform level of 200 mg L–1 N in all treatments. This level was previously demonstrated to be within the optimal range for medical cannabis cultivation. Mineral nutrients were supplied dissolved in the irrigation solution at each irrigation, from final (pre-mixed) solutions. At the last week before harvest the plants were irrigated with distilled water without fertilizers as is routinely practiced in the commercial cultivation of medical cannabis. N was supplied in a constant concentration of 14.3 mM [e.g., 200 mg L–1 (ppm)]. The treatments included different ratios of N-NH4+/N-NO3–: 0, 10, 30, 50, and 100% of the total N. 

"Dramatic impact"
The results revealed high sensitivity of cannabinoid and terpenoid concentrations and plant function to NH4/NO3 ratio, thus supporting the established hypothesis. The increase in NH4 supply generally caused an adverse response: Secondary metabolite production, inflorescence yield, plant height, inflorescence length, transpiration and photosynthesis rates, stomatal conductance, and chlorophyll content were highest under NO3 nutrition when no NH4 was supplied.
Ratios of 10–30% NH4 did not substantially impair secondary metabolism and plant function but produced smaller inflorescences and lower inflorescence yield compared with only NO3 nutrition. Under a level of 50% NH4, the plants demonstrated toxicity symptoms, which appeared only at the late stages of plant maturation, and 100% NH4 induced substantial plant damage, resulting in plant death.

This study demonstrates a dramatic impact of N form on cannabis plant function and production, with a 46% decrease in inflorescence yield with an increase in NH4 supply from 0 to 50%. Yet, moderate levels of 10–30% NH4 are suitable for medical cannabis cultivation, as they do not damage plant function and show only little adverse influence on yield and cannabinoid production. Higher NH4/NO3 ratios containing above 30% NH4 are not recommended since they increase the potential for severe and fatal NH4 toxicity damage.

To read the complete study, go to www.frontiersin.org