## 3 Phytotoxic Effects of Phenolic Compounds
### Dihydrohalcones--The Most Abundant Apple Phenolic Compounds
#### 3.4.2 Phytotoxic Potential of Dihydrochalcones  
Apple dihydrochalcones that enter the soil through root exudation or the decomposition of plant material may exhibit inhibitory effects on other plant species, including weeds. A significant challenge in studying such alleopathic interactions in the rhizosphere lies in the complexity of isolating and identifying root-exuded compounds, as well as the intricate nature of rhizosphere ecosystems and the activity of microorganisms that can modulate the effects of allelbochemicals. To eliminate factors affecting the quality and quantity of exuded allelbochemicals, a model system of an in vitro culture of genetically transformed apple roots was recently established in several apple cultivars [31]. UHPLC mass spectrometry confirmed that genetic transformation did not alter the levels of major specialized metabolites in apple roots such as phlorizin, glycosylated andglycosated flavonoids, and phenolic acids compared to non-transformed apple roots. The established in vitro culture system of transformed apple roots thus provides a controlled environment for studying the apple's allelopathic potential in the absence of microorganisms and other disruptive factors from the rhizosphere [31].  
The phytotoxic effects of apple root exudates were evaluated using the model plant _A. thaliana_. The findings revealed that apple root exudates did not inhibit seed germination but exerted significant phytotoxic effects on seedling growth and development, reducing root and shoot growth by 35.1 and 62.1%, respectively [31]. Notably, the exudates had the most pronounced impact on lateral root formation, decreasing the number of lateral roots by 74% after 10 days of treatment. The authors demonstrated that root growth inhibition in seedlings is associated with disrupted expression of cyclin-dependent kinase (CDK) genes [31], which play a central role in cell cycle regulation and the evolutionarily conserved mechanism of cell division in eukaryotes [242]. CDKs belong to a family of serine/threonine protein kinases, whose substrate specificity is determined by binding to the corresponding cyclin (CYC) and whose activity is regulated by a series of CDK activators and inhibitors. Treatment of _A. thaliana_ seedlings with apple root exudates for 10 days reduced the expression levels of cyclin-dependent kinases _CDKA1;1_ and _CDKB2;1_ and cyclins _CYCA3;1_ and _CYCB2;4_ in shoots, while gene transcript levels in roots were unaffected [31]. The analysis of specialized metabolites in the apple root exudates identified the presence of dihydrohalcones, including phlorizin (8.01 ng mL\({}^{-1}\); 0.017 \(\upmu\)M) and phloretin (7.49 ng mL\({}^{-1}\); 0.027 \(\upmu\)M), alongside caffeic acid (14.58 ng mL\({}^{-1}\); 0.08 \(\upmu\)M) and chlorogenic acid (8.82 ng mL\({}^{-1}\); 0.025 \(\upmu\)M).  
Based on these findings and the existing literature data on phloretin's role in ARD, as well as its known multifunctionality and reactivity, further research investigated its phytotoxic effects on species beyond the _Malus_ genus. [32; 33; 34].