To comprehensively understand the toxic effects and ecological risks of microplastics on major economic tree species, a pot experiment was conducted using polylactic acid (mPLA) microplastics as the test object to explore the effects of different concentrations (0.1%, 0.5%, 1%, 5%, 10%, w/w, mass fraction) of microplastics on the growth and physiological characteristics of mulberry trees. The study results showed that, compared with the control group, the biomass, total chlorophyll content, and net photosynthetic rate of mulberry trees in the mPLA treatment group were significantly reduced under high concentration (10%) treatment; the activities of SOD and CAT and the MDA content were significantly increased by 50.00%, 47.83%, and 60.87%, respectively, at a 10% concentration. The results indicate that the toxic effects of microplastic addition on mulberry trees are related to the type and concentration of microplastics. High concentrations of mPLA can damage the photosynthetic system of plants, affecting photosynthesis, causing oxidative damage and thus inhibiting the growth of mulberry plants.

Salt stress is a relative concept, excessive sodium ions can inhibit the normal growth and development of mulberry seedlings through osmotic stress, oxidative damage and ion imbalance. Calcium as a second messenger to regulate the process. Therefore, calcium should also be taken seriously in experimental design. The experiment was conducted by setting different salt concentrations (0, 50, 200 mmol L- 1) and exogenous calcium contents (0, 2.5, 5, 10, 20 mmol L- 1) on mulberry seedlings in hydroponics. The effects of exogenous calcium on nutrient accumulation, distribution and stoichiometric characteristics of mulberry seedlings under salt stress were studied. Results showed that salt stress reduced mulberry growth, but exogenous calcium reversed this effect, when the concentration of exogenous calcium was 10 mmol L- 1 the biomass and nutrient accumulation of mulberry seedlings could reach the maximum, and there are significant differences with other treatments (p < 0.05). And salt stress shifted the calcium absorption focus from leaves to trunks. Exogenous calcium mitigation on mulberry stoichiometry under salt stress shows initial decrease then increase, the N: P ratio showed that salt treatment could change the P stress to N stress in mulberry seedlings. Meanwhile, there was a significant positive correlation between the biomass of each organ with C: P and N: P (p < 0.01, 0.001), but a extremely significant negative correlation between the biomass with others stoichiometric ratios (p < 0.001). According to the results of leaf stoichiometric ratio and correlation analysis, the optimum application amount of exogenous calcium in mulberry seedlings under salt stress was 10 mmol L- 1. This research offers a theoretical foundation for salt-alkali soil fertilization and has significant implications for soil salinization management.

The economically adaptable mulberry (Morus alba L.) has a long history of grafting in China, yet the physiological mechanisms and advantages in drought tolerance remain unexplored. In our study, we investigated the responses of self-rooted 2X (diploid), 3X (triploid), and 4X (tetraploid) plants, as well as polyploid plants grafted onto diploid seedling rootstocks (2X/2X, 3X/2X, and 4X/2X) under drought stress. We found that self-rooted diploid plants exhibited the most severe phenotypic damage, lowest water retention, photosynthetic capacity, and the least effective osmotic stress adjustment compared to tetraploid and triploid plants. However, grafted diploid and triploid plants showed effective mitigation of drought-induced damage, with higher relative water content and improved soil water retention. Grafted plants also improved the photosystem response to drought stress through elevated photosynthetic potential, closed stomatal aperture, and faster recovery of chlorophyll biosynthesis in the leaves. Additionally, grafted plants altered osmotic protective compound levels, including starch, soluble sugar, and proline content, thereby enhancing drought resistance. Absolute quantification PCR indicated that the expression levels of proline synthesis-related genes in grafted plants were not influenced after drought stress, whereas they were significantly increased in self-rooted plants. Consequently, our findings support that self-rooted triploid and tetraploid mulberries exhibited superior drought resistance compared to diploid plants. Moreover, grafting onto seedling rootstocks enhanced tolerance against drought stress in diploid and triploid mulberry, but not in tetraploid. Our study provides valuable insights for a comprehensive analysis of physiological effects in response to drought stress between stem-roots and seedling rootstocks.