Floral hemp (Cannabis sativa L.) responses to nitrogen fertilization under field conditions in the high desert

Abstract

While most studies on floral hemp (Cannabis sativa L.) concur that additions of nitrogen (N) increase plant growth, the performance of floral hemp is heavily influenced by environmental conditions, management and cultivar selection. In regions with a short growing season, the availability of soil N may determine plant developmental rates, final inflorescence biomass and cannabinoid concentrations, but no studies have addressed this for field-grown hemp under high-desert conditions. This field study evaluated the effect of no supplemental N and N fertilization at 90 kg ha-1 on three hemp cultivars (Berry Blossom, Red Bordeaux, and Tahoe Cinco) in Northern Nevada. N increased plant height, canopy cover, stem diameter and shoot biomass, but other physiological parameters were dependent on cultivar. For instance, inflorescence biomass and inflorescence-to-shoot ratio in Red Bordeaux was not affected by N fertilization. Similarly, cannabinoid concentrations were affected by timing of harvest and cultivar but not by N treatment. We evaluated the use of a SPAD meter for ease of determining leaf N deficiency, and correlations with leaf chlorophyll content showed that the SPAD meter was a reliable tool in two cultivars but not in Tahoe Cinco. N treatment increased overall CBD yield, which was driven by increases in inflorescence biomass. Tahoe Cinco was the best CBD yielding cultivar, as it maintained a high inflorescence-to-shoot ratio regardless of N treatment. Our study suggests that even though hemp may have a positive response to soil N management, adjustments based on genotype by environment interaction should be aimed at maximizing cannabinoid yield either by increasing biomass and/or CBD concentrations as long as THC levels are within the permissible <0.3% for U.S. industrial hemp cultivation.

Fig 1

Soil canopy cover (A) and plant height (B) for N+ (n) and control (c) treatments of cultivars Berry Blossom (BB), Red Bordeaux (RB), and Tahoe Cinco (TC) grown under field conditions. Values are mean ± standard error (A and B: n = 8). Means followed by different letters are statistically different at P<0.05. (A) N treatment P<0.001, Cultivar P<0.001. (B) N treatment P<0.001, Cultivar P<0.068, N treatment x Cultivar P = 0.049.

Fig 2

SPAD leaf values (A) and total leaf chlorophyll (B) for N+ (n) and control (c) treatments of cultivars Berry Blossom (BB), Red Bordeaux (RB), and Tahoe Cinco (TC) grown under field conditions. Values are mean ± standard error (A: n = 120; B: n = 24). Means followed by different letters are statistically different at P<0.05. (A) N treatment P<0.001, Cultivar P<0.001, N treatment x Cultivar P = 0.008. (B) N treatment P<0.001, Cultivar P<0.001.

Fig 3

Total leaf nitrogen (A) and leaf δ¹³C (B) for N+ (n) and control (c) treatments of cultivars Berry Blossom (BB), Red Bordeaux (RB), and Tahoe Cinco (TC) grown under field conditions. Values are mean ± standard error (A: n = 24; B: n = 24). Means followed by different letters are statistically different at P<0.05. (A) N treatment P<0.001, Cultivar P<0.014. (B) N treatment P<0.001, Cultivar P<0.001, N treatment x Cultivar P<0.001.

Fig 4

Fv’/Fm’ (A) and ΦPSII (B) leaf values for N+ (n) and control (c) treatments of cultivars Berry Blossom (BB), Red Bordeaux (RB) and Tahoe Cinco (TC) grown under field conditions. Values are mean ± standard error (A: n = 120; B: n = 120). Means followed by different letters are statistically different at P<0.05. (A) N treatment P = 0.357, Cultivar P = 0.007, N treatment x Cultivar P = 0.170. (B) N treatment, P = 0.094, Cultivar P = 0.177, N treatment x Cultivar P = 0.053.

Fig 5

CBD concentration (A) and THC concentration (B) of inflorescence at 10% and 90% pistil dieback for N+ (n) and control (c) treatments of cultivars Berry Blossom (BB), Red Bordeaux (RB) and Tahoe Cinco (TC) grown under field conditions. Values are mean ± standard error (A and B: n = 3–4). Means at 10% pistil dieback did not significantly differ. At 90% pistil dieback, means followed by different letters are statistically different at P<0.05. Compact letter display for pistil dieback at 10% and 90% were analyzed separately (90%: a-d) (A) Pistil dieback P<0.001, N treatment P = 0.472, Cultivar P = 0.029, Pistil dieback x Cultivar P<0.001, Pistil dieback x N treatment P = 0.028, N treatment x Cultivar P = 0.881. (B) Pistil dieback P<0.001, N treatment P = 0.461, Cultivar P = 0.015, Pistil dieback x Cultivar P<0.001, Pistil dieback x N treatment P = 0.018, N treatment x Cultivar P = 0.879.

Fig 6

CBD yield (A) and CBD-to-THC ratio (B) at 10% and 90% pistil dieback for N+ (n) and control (c) treatments of cultivars Berry Blossom (BB), Red Bordeaux (RB) and Tahoe Cinco (TC) grown under field conditions. Values are mean ± standard error (A and B: n = 3–4). Means followed by different letters are statistically different at P<0.05. Compact letter display for pistil dieback at 10% and 90% were analyzed separately (10%: x-z; 90%: a-d). (A) N treatment P<0.001, Cultivar P = 0.002, N treatment x Cultivar P = 0.002. (B) Pistil dieback P<0.001, N treatment P = 0.001, Cultivar P<0.001, N Treatment x Cultivar P = 0.053, Pistil dieback x N Treatment P = 0.592, Pistil dieback x Cultivar P = 0.357.

Fig 7. Correlation between SPAD, leaf chlorophyll content, leaf N content, SLA, SLN, Fv’/Fm’, and Φ_PSII.