Populus tremula x P. alba Microshoot Secondary Metabolism Response after Paenibacillus sp. Inoculation In Vitro

Bacterial biostimulants are an eco-friendly alternative to chemical fertilizers. However, before their introduction into open ecosystems, broad-scope studies need to be carried out. Paenibacillus sp. was shown to positively affect poplar root growth. It was hypothesized that alongside these improvements, the Paenibacillus sp. inoculant may affect its host's secondary metabolism. Populus tremula x P. alba microshoots were inoculated in vitro. Microshoots were tested for chlorophyll, carotenoid, total flavonoid (TFC), total phenol content (TPC) and free radical scavenging capacity during primary growth after 4, 6 and 8 weeks. The results showed that the inoculation decreased shoot phenolics and free radical scavenging capacity after 6 and 8 weeks. Chlorophyll b amounts increased after 6 and 8 weeks. Carotenoid content decreased after 6 weeks, while chlorophyll a and carotenoid levels increased after 8 weeks. Correlation and principal component analyses showed that the inoculant changed the way in which the photosynthesis pigment content relates to TPC, TFC and radical scavenging activity. Overall, these data suggest that the inoculant does statistically significantly affect Populus tree secondary metabolism in the later stages of the initial growth period. This effect may potentially be compensatory in nature.