Polyploidisation is a very common phenomenon in the plant kingdom and plays a key role in plant evolution and breeding. It promotes speciation and the extension of biodiversity. It is estimated that approximately 47% of flowering plant species are polyploids, derived from two or more diploid ancestral species. In natural populations, the predominant methods of whole-genome multiplication are somatic cell polyploidisation, meiotic cell polyploidisation, or endoreduplication. The formation and maintenance of polyploidy is accompanied by a series of epigenetic and gene expression changes, leading to alterations in the structural, physiological, and biochemical characteristics of polyploids relative to diploids. This article provides information on the mechanisms of formation of natural and synthetic polyploids. It presents a number of examples of the effects of polyploidisation on the composition and content of secondary metabolites of polyploids, providing evidence of the importance of the phenomenon in plant adaptation to the environment, improvement of wild species, and crops. It aims to gather and systematise knowledge on the effects of polyploidisation on plant physiological traits, including stomatal conductance (Gs), transpiration rate (Tr), light saturation point (LSP), as well as the most important photosynthetic parameters determining biomass accumulation. The text also presents the latest findings on the adaptation of polyploids to biotic and abiotic stresses and explains the basic mechanisms of epigenetic changes determining resistance to selected stress factors.

Hop (Humulus lupulus) is cultivated for industrial purposes, primarily for brewing, but also for pharmaceutical and cosmetic applications. The hop cones are valued for their secondary metabolites, including bitter acids and prenylflavonoids. The objective of the presented study was to investigate the impact of reducing nitrogen fertilisation on the activity of genes involved in the production of bitter acids and xanthohumol in cultivars Lubelski (aroma) and Magnat (bitter) grown in the field. Furthermore, the content of these compounds was determined. The results demonstrated that the majority of studied genes were expressed at a similar level despite the reduction in the nitrogen dose by 25–30%. Nitrogen dose influenced the activity of genes BCAT2, HS1, and PAL. Moreover, the differences in gene expression were observed between cultivars and developmental stages. The reduction in nitrogen dose did not result in a reduction in secondary metabolite content. The content of alpha and beta acids in Magnat was approximately 14.8% and 3.2%, respectively, while in Lubelski it was approximately 7.7% and 5.8%, respectively. The content of xanthohumol in both cultivars was approximately 0.7%.

Nornicotine is a secondary metabolite formed in tobacco leaves by the oxidative N-demethylation (conversion) of nicotine. Its high level is undesirable because this alkaloid is a precursor of N-nitrosonornicotine, which has been shown to have carcinogenic properties. The aim of the study was to assess the nicotine and nornicotine content in four successive generations of ten tobacco cultivars/breeding lines. The possibility of reducing potentially harmful compounds in the cultivars/breeding lines was also determined. The study was conducted as field experiments between the years 2014 and 2018. The alkaloid content in the leaves was determined by the gas chromatography/ mass spectrometry (GC/MS) method. The systematic assessment of the alkaloid profile of tobacco and eliminating converter plants in four successive generations, particularly within breeding lines characterized by a wide conversion range, made it possible to reduce the nornicotine content and, thus, the potentially carcinogenic compounds in the leaves. Three lines, ZD2, TNX1, and WGLB with a stable conversion rate of ≤3% and low content of nornicotine were obtained. Furthermore, the morphological traits of the isogenic lines ZD2, TNX1 and WGLB, which exhibit markedly different conversion capacity were evaluated. The greenhouse experiment showed that there were significant differences in some morphological traits. The non-converting lines TNX1 and ZD2 produced longer and wider 9th and 15th leaves than the converting analogues. A relationship has been identified between the traits that determine the phenotype of tobacco cultivars/lines and their ability to convert nicotine to nornicotine.