## 3 Phytotoxic Effects of Phenolic Compounds
### Dihydrohalcones--The Most Abundant Apple Phenolic Compounds
#### 3.4.3 Phloretin as a Plant Growth Inhibitor  
[PERSON] et al. [32] demonstrated that phloretin acts as a potent growth inhibitor that causes severe morphological abnormalities and agravitropic responses in _A. thaliana_ seedlings. The authors investigated phloretin's effects in concentrations ranging from 125 to 1500 \(\upmu\)M and observed significant dose- and time-dependent inhibition of seedling growth. Root development and primary root elongation were more strongly inhibited than leaf growth, with lateral root formation significantly suppressed, especially at concentrations above 500 \(\upmu\)M. High phloretin concentrations (750-1500 \(\upmu\)M) initially stimulated adventitious root formation, likely as a stress response, but prolonged exposure stunted their growth and induced root necrosis and browning [32]. In contrast to _A. thaliana_, where root growth inhibition gradually increased over time and peaked at the end of treatment (86.6% at 1500 \(\upmu\)M after 15 days of treatment), the inhibition in _C. bursa-pastoris_ seedlings was highest at the start (59.5% on day 5 with 1000 \(\upmu\)M) but remained stable or even decreased in the subsequent days [34]. Phloretin similarly suppressed lateral root development in _C. bursa-pastoris_, which was particularly evident at concentrations of 500 and 1000 \(\upmu\)M (41.5 and 41.6%, respectively, after 10 days of treatment). However, the inhibitory effect was less pronounced compared to _A. thaliana_, where reductions of 75.6 and 58.9% were observed at the same concentrations. The disparity in phloretin's phytotoxic effects between these two related species was further evident in the inhibition of shoot growth. In C. _bursa-pastoris_, shoot growth inhibition progressively increased over the treatment period, peaking at 29.7% in 14-day-old seedlings grown on media with 1000 uM phloretin. Under identical conditions, _A. thaliana_ seedlings exhibited nearly double the inhibitory effect, with a 48.1% reduction in shoot growth after 14 days at the same phloretin concentration. Phloretin treatment also disrupted the gravitropic response of _A. thaliana_ seedlings, leading to serious disturbances in root architecture and histology [32]. The frequency and type of deformities were dose- and time-dependent, with 500 uM phloretin inducing the most abnormalities. A significant reduction in the starch content of columella cells was also observed, disrupting gravitropic perception in treated seedlings. Histological analysis revealed disorganized columnella cells, shortened meristematic and elongation zones, and hypertrophied epidermal and cortical cells detaching from underlying layers [32]. Using histochemical staining, [PERSON] et al. [34] detected an increased presence of phenolic compounds, lipids, and pectin in the roots of _C. bursa-pastoris_ seedlings treated with 500 uM phloretin. The blue coloration observed in root cells following Toluidine Blue O staining indicated the accumulation of phloretin and/or its metabolic derivatives. As phloretin readily interacts with various biological molecules, plants tend to quickly metabolize phloretin into more stable, water-soluble glycosylated forms that can then be actively transported into vacuoles. The excess accumulation of lipids observed in the axial root tips and root hairs of treated seedlings suggests disrupted lipid degradation or inefficient energy utilization due to growth stagnation [34]. The authors highlighted that the accumulation of pectin in the root border cells presumably serves as a mechanism to inhibit phloretin uptake by binding to and reducing its concentration within the cells. It is well-established that covalent bonding, as well as multiple hydrogen bonds and hydrophobic interactions, can occur between polysaccharides, such as pectin, and polyphenols [243; 244; 245]. [PERSON] et al. [246] demonstrated direct interactions between phloretin and pectin polysaccharides, specifically arabians from sugar beet.