2. Results

2.1. Effect of potato glycoalkaloids onF. solaniultrastructure

The controls without potato glycoalkaloid treatment presented thin and uniform cell walls, complete structure, hyphae surrounded by a continuous outer layer, intact internal tissue structure, no extravasation of cell contents, normal development, and clearly visible vacuoles, mitochondria, and other organelles (Fig. 1A). However, after treatment with EC50 potato glycoalkaloids, the fungal cell walls became thinner and irregular, cell internal structure was disrupted, and some membrane structures were incomplete (Fig. 1B). Furthermore, after treatment with 2 g·mL−1 potato glycoalkaloids, cell walls of mycelia had a bubble shape, outer layer components were altered, cell wall structure was incomplete, the outer cell membrane was discontinuous, structure of mitochondria and other organelles was not obvious, and extracellular inclusions were exuded (Fig. 1C). In contrast, following treatment with methanol, the cell wall structure was clear, mitochondria and ribosomes were clearly visible, and the vacuolar structure was intact, indicating that methanol did not affect antibacterial activity of potato glycoalkaloids (Fig. 1D). These findings indicated that potato glycoalkaloids destroyed the cell surface morphology of F. solani , severely damaging the cytoplasm, mitochondria, and other organelles, as well as increasing the cell membrane permeability and so causing leakage of cell contents.

2.2. Effect of potato glycoalkaloids onF. solani cell membrane permeability

Figure 2 shows the effects of potato glycoalkaloids on the cell membrane permeability of F. solani . The relative cell membrane permeability increased with time in both the control and treatment groups. However, the relative cell membrane permeability was significantly higher for the treatment compared with the control group. In both the control and treatment groups, the relative cell membrane permeability rapidly increased during 1–4 h, with a greater increase for the treatment than the control group. After 4 h, the difference in relative cell membrane permeability quickly broadened between the two groups, indicating that the F. solani cell membrane damage caused by potato glycoalkaloids at EC50 concentration intensified at 4 h. Subsequently, the relative cell membrane permeability of the control group gradually decreased during 4–9 h, whereas that of the treatment group increased by 25.2% at 9 h, and remained stable at 8 and 9 h.

2.3. Effect of potato glycoalkaloids on soluble protein in F. solani

The soluble protein leakage in the treatment group exhibited an upward trend with time, which significantly differed from that in the control group (Fig. 3). During 0–6 h, soluble protein leakage in the treatment group increased from 66.50 to 169.51 μg∙mL−1, which was much higher than that in the control group. However, during 6–24 h, the soluble protein leakage in the treatment group gradually decreased with time, possibly due to consumption of soluble proteins by newly formed F. solani cells for growth. After 24 h, the soluble protein leakage in the treatment and control groups gradually increased and stabilized.

2.4. Effect of potato glycoalkaloids on soluble sugar in F. solani

Soluble sugar leakage in the treatment group significantly differed from that in the control with time (Fig. 4). During 0–24 h, soluble sugar leakage in the treatment group increased from 117.4 to 132.5 μg∙mL−1, which was much higher than that in the control group. However, after 24 h, leakage in the treatment group decreased with time owing to consumption of soluble sugar in the extracellular fluid by newly formed F. solani cells for growth.

2.5. Effect of potato glycoalkaloids on reducing sugar content in F. solani

The reducing sugar content in the extracellular fluid significantly differed between the treatment and control group with time (Fig. 5). In both groups, the reducing sugar content in the extracellular fluid sharply decreased during 0–2 h, but gradually reduced during 2–72 h, indicating that potato glycoalkaloids significantly decreased the absorption and utilization of reducing sugar by F. solani.

2.6. Effect of potato glycoalkaloids on fat content in F. solani

The fat content in the extracellular fluid of both the treatment and control groups increased during 0–48 h, with significantly higher fat leakage for the treatment compared with the control group (Fig. 6). The fat content in the extracellular fluid of the treatment group increased by 54.20% and 52.07%, compared with sterile water and methanol control groups at 48 h, respectively, indicating that potato glycoalkaloids damaged the cell structure and caused fat content leakage from F. solani plasma membrane. However, after 48 h, fat content in the extracellular fluid of both treatment and control groups showed a downward trend, possibly resulting from a weakening effect of potato glycoalkaloids or a self-remediation mechanism of F. solani .