Originally Posted by From The Article
Plant Culture and Treatments
Cannabis [Cannabis sativa L. ‘WP:Med (Wappa)’] cuttings were taken at a length of ≈13 cm and with three fully-expanded leaves from stock plants. Cuttings were taken from the ends of axial limbs and cut at a 45˚ angle. Each cutting was rooted in a 5.7 cm wide, 5.7 cm tall peat-based pot (Jiffy Products N.B. Ltd., NB, Canada) containing Pro-Mix PG Organic growing substrate (Premier Tech ) and arranged in trays at a density of 266 plants/m2. The substrate was soaked in a solution of ‘Spurt’ liquid organic fertilizer (2.0N–0.0P–0.83K; EZ-GRO Inc.) at a rate that supplied 123 mg N·L-1.
The experiment was a full factorial completely randomized design with four factors (rooting hormone, leaf number, cutting position and leaf tip removal), two levels per factor and 10 replications per factor combination.
For leaf number, cuttings had either one fully-expanded leaf removed (two leaves remaining) or were left with three leaves.
For cutting position, cuttings from terminal shoots were taken from either an apical position (node 10 and higher) or a basal position (below node 10).
For the leaf tip removal treatment, a portion of leaf tips (≈1/3 of the leaf area) was removed from the fully expanded leaves or the leaves were left uncut.
For the rooting hormone factor, the base (≈5 cm) of each stem was dipped in either 0.2% indole-3-butyric acid gel (synthetic rooting hormone; EZ-GRO Inc.) or in a 0.2% willow extract rooting gel (organic rooting hormone; EZ-GRO Inc.).
Trays were randomly arranged in a walk-in growth chamber (Conviron ATC60; Controlled Environments Ltd., Winnipeg, MB, Canada) and cuttings were misted with reverse osmosis (RO) water once, when they were placed in the chamber. From days 0 to 4 after cuttings were placed in the substrate (DAP), RH was maintained at 95% (s.d. ±1.3), reduced to 80% (s.d ±1.3%) for 5-8 DAP and to 60% (s.d. ±1.5%) for 9-12 DAP. Temperature was maintained at 24 ˚C (s.d.±0.04˚C) (day/night) for the entire period. Fluorescent lighting (Philips Lighting, Markham, ON, Canada) was used to maintain an 18-hour photoperiod. Photosynthetically active radiation (PAR) at canopy level was maintained at 50µmol·m-2s-1 (s.d. ±0.6 µmol·m-2s-1) for 0-4 DAP 1-5, 80µmol·m-2s-1 (s.d. ±0.7 µmol·m-2s-1) for 6-8 DAP and 115µmol·m-2s-1 (s.d. ±0.5 µmol·m-2s-1) for 10-12 DAP.
Rooting Assessment and Harvest
The bottom of the trays was observed daily from 7 DAP onwards for protruding roots, and cuttings were harvested at 12 DAP when approximately more than 50% of the cuttings showed visible roots at the bottom of the tray. Rooting success rate was measured on a binomial scale in which any visible adventitious root formation was considered rooted. Rooting success was calculated as the percentage of cuttings with roots in each treatment. Successfully rooted cuttings were assigned to either of two classifications based on degree of adventitious rooting: A root quality index (RQI) score of ‘1’ or ‘2’ was assigned by a third party without knowledge of the applied treatments based on a visual reference (Fig 1). Before RQI measurements, the substrate was washed from rooted cuttings with RO water and ratings were determined by a third party without knowledge of the applied treatments.
Rooting hormone had the greatest effect on both rooting success rate and root quality. The synthetic hormone delivered a 2.1 times higher rooting success rate (84% vs. 40%;χ2 = 39.0,P <0.0001) and 1.6 times higher root quality (1.6 vs. 1.0;χ2 = 41.1,P < 0.0001) than the organic hormone.[/b]
Removing leaf tips had the second greatest effect on rooting success rate. When leaf tips were removed, rooting success rate was lowered from 71% to 53% (χ2 = 9.8,P = 0.0018), though there was no effect on root quality.
Leaf number had no effect on rooting success rate, but rooted cuttings with three leaves had 15% higher root quality than those with two (1.5 vs.1.3;χ2 = 4.3,P = 0.038).
Cutting position did not influence rooting success rate or root quality. There was however, an interactive effect between cutting position and leaf tip removal on rooting success rate (Fig. 4). When leaf tips were removed, cuttings of basal origin had lower rooting success rate than apical cuttings (43% vs. 63%;χ2 = 5.7,P = 0.0169).
Rooting success was similar between cuttings with two and three leaves, suggesting that two leaves may provide sufficient carbohydrates, auxin and rooting co-factors (Haissig 1974) for successful rooting in cannabis. In the propagation of stem cuttings, increased photosynthetic surface area and resultant carbohydrate supply generally increased rooting success rate until another factor such as evapotranspiration stress became limiting (Davis and Potter 1989). Cuttings with three leaves showed no signs of wilting or other indications of evapotranspiration stress. It is likely that both two- and three-leaf treatments exhibited little evapotranspiration induced water stress in the stable, high humidity provided in this trial. It is estimated that under conditions of lower or less stable humidity, fewer leaves would deliver improved rooting success rate as these cutting would have a lower demand for humidity. Notably, three leaves increased root quality over two. The observed quality improvement was likely caused by the additional carbohydrates, rooting co-factors, endogenous auxin (Haissig 1974) or a combination of these factors provided by the additional foliage.
Further study is required to evaluate the effect of each of these factors and their interactions on cannabis stem cuttings to discern their relative importance.
Based on this finding, it is recommended that cannabis cuttings be taken with three or potentially more leaves (to improve rooting quality) so long as humidity during propagation can be adequately maintained.
It was expected that cutting leaf tips would have a similar effect to reducing leaf number since the source and thus potentially the amount of photosynthetic material was manipulated; however, cutting leaf tips reduced rooting success and had no effect on rooting quality.
Both cutting leaf tips and leaf number alter surface area for evapotranspiration and photosynthesis; however, there was a notable difference in the effects of these treatments. Leaf cutting influenced rooting success rate while leaf number influenced root quality. Further study is necessary to discern the reason for these differing effects. Based on these findings, it is recommended that leaf tips not be cut in cannabis cuttings; and, if less leaf material is desired to conserve space in the propagation environment or to prevent evapotranspiration stress, then fewer whole leaves be used instead.
There was no indication that basal cuttings had improved rooting success rate or quality over apical cuttings. Similar results were found in stem cuttings of fever tea (Soundy et al. 2008) and may be attributed to the lack of distinct stages of maturation in these herbaceous plants in contrast to most hardwood species (Schreiber and Kawase 1975; Morgan and McWilliams 1976; Husen and Pal 2006). These findings suggested that in cannabis, cutting position does not play an important role in rooting.
There was an interactive effect between leaf tip removal and cutting position. Cuttings from basal positions with two leaves had lower rooting success rate than any other combination of these two factors. In general, basal cuttings had smaller leaves (general observation without measurement) than apical cuttings which, through cutting of leaf tips, were left with less overall leaf surface area. The reduction in photosynthesis resulting from the smaller
leaf area might have then resulted in the lower rooting success rate.