## 3 Phytotoxic Effects of Phenolic Compounds
### Dihydrohalcones--The Most Abundant Apple Phenolic Compounds
#### 3.4.1 Can Dihydrochalcones Be Considered Allelichemicals? This question was first raised by [PERSON] in 1959 [226], who attributed the presence of dihydrohalcones to the development of Apple Replant Disease (ARD). This disease manifests as stunted growth of both tree and root systems, alongside poor fruiting performance in seedlings planted on sites of previous apple orchards [227]. Examination of the root systems of ARD-affected trees has revealed the degradation of epidermal cells and cortical tissues, as well as a significant reduction in the development of lateral roots [228; 229]. Additionally, a pronounced reduction in the number of functional root hairs has been noted [230]. Although the challenge of establishing apple orchards on soil previously used for apple cultivation has been documented for over two centuries, the etiology of ARD remains highly controversial. The involvement of phenolic compounds in the development of ARD remains a significant question, given the numerous but still controversial pieces of evidence regarding the influence of rhizosphere microorganisms and nematodes [231]. Although [PERSON] hypothesized that phlorizin and phloretin, detected in apple orchard soils, might play a role in ARD symptom development [226; 232], their auto-allelopathic activity remained largely unexplored for decades. However, [PERSON] et al. [233] and [PERSON] et al. [234] demonstrated that root exudates from wild apple species _Malus pumila_ and _Malus prunifolia_ inhibit the seed germination and seedling development of their own species. In addition, the results of [PERSON] et al. [235] imply that dihydrochalcones released into the soil via root exudation or decomposition of plant residues (fallen leaves, fruits, bark, or roots) could be the key factors in ARD symptom development. [PERSON] et al. [236] observed that treating Fuji apple seedlings with a mixture of soil and shredded roots from the M26 rootstock variety resulted in a significant reduction in Chl content, root mass, and total plant biomass after four months. Ultra-high-performance liquid chromatography (UHPLC) mass spectrometry analysis revealed a substantial increase in the levels of phlorizin and phloretin in the treated mixture compared to the control substrate. [PERSON] et al. [237] suggested that low concentrations of phlorizin (1.0 uM) stimulated growth in _Malus lupehensis_ Rehd. seedlings, while higher concentrations (1-4 mM) exerted inhibitory effects, causing ultrastructural changes in the chloroplasts and mitochondria of leaf and/or root cells. High concentrations of phlorizin also elevated malondialdehydehyde (MDA) content, an indicator of membrane lipid peroxidation, as well as the activity of the antioxidant enzymes SOD, POX, and CAT. Furthermore, elevated phlorizin concentrations were associated with reduced transpiration and photosynthesis rates [237]. Similar effects of phlorizin were observed in seedlings of the wild apple species _M. micromulus_, where increased activities of SOD, POX, and CAT, as well as elevated levels of MDA and proline, were recorded in proportion to the applied concentration and treatment duration [238]. [PERSON] et al. [239] reported a reduction in respiration rates and the activity of enzymes involved in the tricarboxylic acid cycle in roots of _M. lupehensis_ treated with 4.0 mM phlorizin. Additionally, phloretin and phlorizin, implemented at concentrations detected in orchard soils, increased the expression levels of proteins associated with defense mechanisms in root cells of _M. lupehensis_, as reported by [PERSON] et al. [240]. Treatments with these compounds upregulated genes related to defense system-related and free radical-scavenging proteins in the roots, indicating that phlorizin and phloretin could be involved in the onset of ARD. Recently, [PERSON] et al. [241] analyzed the correlation of _Fusarium_ and _Mortierella_ species and phlorizin content in the soil and soil organic matter with ARD severity.