Effect of SO2 wet deposition on morphology and light energy utilization in mulberry leaves

In order to explore the effects of SO2 wet deposition on the photosynthetic characteristics of leaves of mulberry seedlings, we exposed leaves to simulated SO2 wet deposition treatments. Two SO2 concentrations (100 and 50 mmol·L-1) were applied. The results showed that compared to 0 mmol·L-1 SO2 wet deposition (CK), the 100 mmol·L-1 treatment clearly damaged mulberry leaves. A wide range of effects were present in treated leaves: chlorosis; leaf blade edges withering; cell shrinkage; indistinct cell edges; decreased stomatal numbers; decreased leaf water content; reduced maximum net photosynthetic rate; significantly reduced light compensation point and light saturation point; and significantly high rates of dark respiration, photorespiration, and transpiration. Overall, light use efficiency by the mulberry leaves was reduced by the 100 mmol·L-1 SO2 treatment. In mulberry leaves treated with 50 mmol·L-1 SO2, the following effects were seen: leaf blade margins were slightly withered; cell volumes were less than those in leaves treated with 100 mmol·L-1 SO2; leaves were dark green; leaf water content decreased slightly; cell volume decreased, but cell density increased; stomatal numbers increased; light compensation point, light saturation point, dark respiration, CO2 compensation point, and water use efficiency decreased; the maximum net photosynthetic rate decreased; however, photorespiration and transpiration rates increased. We conclude that the mulberry could adjust to the treatment by reducing respiration consumption and increasing photorespiration and transpiration rates to adapt to the 50 mmol·L-1 SO2 stress. The curve parameters of chlorophyll fluorescence in response to light were similar between the two SO2 concentrations stress. The actual photochemical efficiency, photochemical quenching coefficient, and electron transfer rate decreased with the increase in light intensity; these values were significantly lower than those of CK in photon flux density conditions over 400 μmol·mol-1. Meanwhile, photochemical quenching coefficient and photochemical quenching were significantly higher than those of CK. Mulberry, therefore, has some resistance to SO2 stress through adapting leaf structures and modifying heat dispersion.