Citation: Dongmei Lyu, Rachel Backer, Donald L. Smith,Three plant growth-promoting rhizobacteria alter morphological development, physiology, and flower yield of Cannabis sativa L., Industrial Crops and Products, Volume 178, 2022, 114583, ISSN 0926-6690 https://doi.org/10.1016/j.indcrop.2022.114583.
The addition of plant growth-promoting rhizobacteria (PGPR) has been shown to alter development of both plant primary growth and secondary metabolite production. PGPRs can increase plant growth, can help buffer plant homeostasis through stress conditions, and can help plants better contend with pests and pathogens. The exact mechanism how PGPRs alter plant growth is dependent on many factors and can vary from inoculant to inoculant. For example, PGPRs can act as phosphate solubilizers, making more phosphorous available to the plant. PGPRs can also alter plant growth by lowering the pH of soil or by producing plant hormones.
Lyu et. al., (2022) set out to better understand the affect of PGPR addition to Cannabis plants. Using three different PGPRs, Bacillus, Pseudomonas, and Mucilaginibacter, the authors conducted two separate studies. First, they evaluated the benefit of microbe addition during early root development; for the second experiment the effect of PGPR on physiological parameters as well as yield were evaluated.
For the first experiment, the authors report an increase in root surface area when standardized to initial cutting weight. This standardization is important because it accounts for differences in beginning size of cuttings. While increases were reported for all three PGPRs, the only treatment that produced a significantly larger root area was the Pseudomonas sp. While there were general trends towards increases in length, average root diameter, and root volume, none of the microbes tested had a significant impact on these parameters.
For the second experiment, the authors report no significant change in photosynthesis rates for plants inoculated with PGPRs on the last day of vegetative growth. Importantly, during the midpoint of flowering PGPR plants inoculated with Pseudomonas and Mucilaginibacter had significantly higher rates of photosynthesis than the PGPR-lacking control and plants inoculated with Bacillus. In particular, the highest rates of photosynthesis were measured from plants inoculated with Pseudomonas bacteria. At the end of the flowering stage, plants inoculated with Bacillus or Mucilaginibacter had significantly greater floral numbers than the control and had greater axillary growth rate than the control. Finally, dry yield was significantly higher in plants inoculated with Mucilaginibacter and Pseudomonas, with Pseudomonas inoculated plants producing the highest dry floral yield.
The authors demonstrate that inoculation with PGPRs can significantly increase dry floral weight. The authors also demonstrate that not all PGPRs increase dry floral weight. What does that mean for a grower? The increases in dry floral weight were approximately 7% for Mucilaginibacter and 11% for Pseudomonas; meaning if your plant yielded 100 g of dry floral tissue the addition of either bacteria would result in a 7g or 11g increase, respectively.
This study is promising for growers. The use of PGPRs has also been linked to improved stress tolerance and secondary metabolite production in other plants. Questions that still need to be investigated include are single strain inoculants better than mixtures, are PGPRs still beneficial in outdoor systems, how does soil fertility influence PGPR effectiveness, and many more. Whether the increase is enough to warrant the use is a personal choice each grower will have to make, however PGPRs may offer affordable ways to boost yield.
*Disclaimer: All work above is original to The Potanist and expresses our opinions and interpretations. We encourage growers to reach out before applying any perceived recommendations. Advice can be contextual, please consider your system and understand there is no “magic bullet” treatment.
