Abstract: Background, aim, and scope With the acceleration of the industrialization process, heavy metal pollution has become one of the hot issues in the world. Cd2+ is a more common heavy metal pollutant, so the effect of Cd2+ stress on plants has been widely concerned. Previous studies have shown that Cd2+ stress has toxic effects on many aspects of plant growth and development, including plant DNA and photosynthesis. So, it is important to explore the safe and effective methods to alleviate the toxic effects of Cd2+ pollution on plants, but the researches on these topic are still less. Nitric oxide (NO) acting as a new signaling molecule plays an important role in plant growth and development, defence responses and stress resistance. Previous studies have shown that NO can alleviate the toxic effects of some stresses on plants, such as drought stress, salt stress, hot stress and low temperature stress. However, whether and how NO alleviates the toxic effects of heavy metal stresses on plants are still less known. The aim of this paper is to explore whether and how NO alleviates the toxic effects of Cd2+ stress on plant DNA and photosynthesis. Materials and methods In this study, seedlings of mung bean (Phaseolus raditus L. cv. Qindou-20) were used as plant materials, grown in plant growth chambers under a 14 h photoperiod (300-3500 μmol·m-2·s-1), at a day/night temperature cycle of 25±2℃/20±2℃ and a relative humidity of 75% and incubated in 1/2 Hoagland solution. After 33 h of seedling growth, all the seedlings were divided into four groups and transferred to 1/2 Hoagland solution alone (CK), 1/2 Hoagland solution containing 50 μmol·L-1 NO donor sodium nitroprusside (SNP), 1/2 Hoagland solution containing 5 μmol·L-1 CdCl2 (Cd2+) or 1/2 Hoagland solution containing 50 μmol·L-1 SNP and 5 μmol·L-1 CdCl2 (SNP+Cd2+), respectively. When seedlings were treated for 3 d and 8 d, the net photosynthetic rate (Pn), stomatal conductance (Gs), and intercellular CO2 concentration (Ci) were measured with a portable CIRAS-2 type photosynthesis system (PP Systems) and stomatal limitation (Ls) was calculated with the formula Ls=(Ao-Ai)/Ao×100%, where Ao is the Pn when Ci is equal to the atmospheric CO2 concentration, and Ai is the Pn at the atmospheric CO2 concentration. DNA contents and DNA hyperchromicity were measured with TU-1800s UV spectrophotometer. Results Compared to CK, Cd2+ stress alone reduced DNA content, enhanced the degree of cross-linking between DNA chains in leaves and roots, decreased Pn, Gs and Ci and increased Ls in leaves. Compared to Cd2+ stress alone, the combined treatment of exogenous NO and Cd2+ increased DNA content and decreased the cross-linking between DNA chains in leaves and roots, while Pn and Gs were increased, and Ci and Ls were decreased in leaves. Discussion In consistent with the results of previous studies, our results in this paper also show that Cd2+ stress has the toxic effects on plant DNA and photosynthesis. Furthermore, our results clearly show that exogenous NO can effectively alleviate the toxic effects of Cd2+ stress on plant DNA and photosynthesis. In addition, although previous studies have paid much attention on the inhibitory mechanisms of Cd2+ stress on photosynthesis, it is still unclear which factor between stomatal and nonstomatal limitations plays more important role during different stages of Cd2+ stress-inhibited photosynthesis. Our results in this paper indicate that the stomatal limitation plays a more important role during the early stage of Cd2+ stress-induced inhibition of photosynthesis. Combined with the previous results, we suggest that under Cd2+ stress, the inhibition of photosynthesis in mung bean leaves is the results of both stomatal and nonstomatal limitations, but the stomatal limitation is dominant during early stage, nonstomatal limitation becomes the dominant one during late stage. Moreover, our results also show that exogenous NO alleviates the inhibitory effects of Cd2+ stress on photosynthesis mainly by improving the photosynthetic activity of seedling mesophyll cells. Conclusions (1) Exogenous NO can effectively alleviate the toxic effects of Cd2+ stress on DNA and photosynthesis in mung bean seedlings; (2) Stomatal limitation plays a more important role during the early stage of Cd2+ stress-inhibited photosynthesis; (3) NO alleviates the inhibitory effects of Cd2+ stress on photosynthesis mainly by improving the photosynthetic activity of mesophyll cells. Recommendations and perspectives Although our results show that NO can effectively alleviate the toxic effects of Cd2+ stress on DNA and photosynthesis, the detailed mechanisms underlining these processes are still unclear, which should be studied in the future.
Keywords: NO; Cd2+ stress; mung bean seedlings; DNA; photosynthesis; ; ; ;