Even when growing indoors, pests and pathogens might find a way into the cultivation. To prevent such a dreadful scenario, the best way is to start with resilient and disease-free plantlets. To do that, micropropagation has been widely used as it ensures rapid production of disease-free plants as well as plant uniformity. However, in a recent study by Adel Zarej et al. published on In Vitro Cellular & Developmental Biology – Plant, the authors point out how such a method is not very suitable for large-scale propagation, because of the high degree of plant hyperhydricity, low growth rate, poor rooting, and acclimation efficiency. While micropropagation can be very useful for the preservation of genetic materials, none of the methods involving stem, cotyledon, and flower explants are feasible in a large-scale, high market demand setting.
What is photoautotrophic micropropagation?
As a solution, the authors propose photoautotrophic micropropagation (PAM): this refers to “propagation and growth of explant or plant under disease-free conditions on a medium containing no supplemental organic components including sugars, vitamins, and plant hormones.” While studies on the efficiencies of PAM have been conducted already, little is known in regard to standardized conditions under which PAM can be carried out more efficiently.
First, the researchers started looking into the amount of fertilizer necessary. The results of the study showed that the application of 300-mL fertilizer containing 5-mM MES provided balanced air and mineral nutrient content in the grow medium and support a better grow rate.
Cutting length and type of wounding
Then there is the question of how long cuttings need to be in order to root faster. To assess that, the authors cut explants of different lengths and compared them, seeing a better performance off longer cutting sizes compared to the shorter ones. They argue that this is probably a result of the greater amount of nutrients accumulated since there is no supplementary material for cuttings in PAM. On top of that, longer cuttings had a higher number of buds compared to the shorter ones; considering that buds and leaves contain root-promoting compounds that travel to the base of the cuttings, this would also explain why longer cuttings performed better under PAM conditions. The authors also addressed the type of wounding that needs to be carried out when getting cuttings from a plant. Even though the researchers compared different types of wounding, they noted that there was no significant change in the rooting success of their tested cultivars. However, they point out that there is a need for further microscopy work to better understand how wounding treatments could increase the rooting success.
Light intensity and CO2
When it comes to lighting and light intensity, the authors found that light intensity indeed influences rooting parameters in cuttings, but also that “CO2 concentration inside the culture vessel should be above the compensation point and is not being considered a limiting factor.” This is a key point in PAM, considering that explants rely only on light for the synthesis of primary and secondary metabolites. As they noted, “unlike conventional micropropagation, where the CO2 and transpiration are restricted, there is a line of evidence showing that high PPFD significantly enhances root initiation and normal root vascular development when the CO2 is not a limiting factor.” Thus, a PPFD of 150 μmol m−2 s −1 is required to boost the rooting success of PAM.
The authors then concluded that by following their method, they could “shorten the culture period by approximately 70%, solve physiological disorders, offer survival rate above 90%, and reduce costs by over 50% by using larger ventilated culture vessels and sugar-free medium.”