To explore the mechanism of how lead (Pb) and cadmium (Cd) stress affects photosynthesis of mulberry (Morus alba L.), we looked at the effects of different concentrations of Pb and Cd stress (at 100 and 200 μmol L⁻¹), which are two heavy metal elements, on leaf chlorophyll (Chl), photosynthesis gas exchange, Chl fluorescence, and reactive oxygen species (ROS) metabolism in mulberry leaves. The results showed that higher concentrations of Pb and Cd reduced leaf Chl content, especially in Chl a where content was more sensitive than in Chl b. Under Pb and Cd stress, the photosynthetic carbon assimilation capacity of mulberry leaves was reduced, which was a consequence of combined limitations of stomatal and non-stomatal factors. The main non-stomatal factors were decreased photosystem II (PSII) and photosystem I (PSI) activity and carboxylation efficiency (CE). Damage to the donor side of the PSII reaction center was greater than the acceptor side. After being treated with 100 μmol L⁻¹ of Pb and Cd, mulberry leaves continued to be able to dissipate excess excitation energy by starting non-photochemical quenching (NPQ), but when Pb and Cd concentrations were increased to 200 μmol L⁻¹, the protection mechanism that depends on NPQ was impaired. Excessive excitation energy from chloroplasts promoted a great increase of ROS, such as superoxide anion (O₂^•-) and H₂O₂. Moreover, under high Pb and Cd stress, superoxide dismutase (SOD) and ascorbate peroxidase (APX) were also inhibited to some extent, and excessive ROS also resulted in a significantly higher degree of oxidative damage. Compared with Cd, the effect of Pb stress at the same concentration level displayed a significantly lower impact on Chl content, photosynthetic carbon assimilation, and stomatal conductance. Meanwhile, Pb stress mainly damaged activity of the oxygen-evolving complex (OEC) located on PSII donor side, but it reduced the electronic pressure on the PSII acceptor side and PSI. Furthermore, under Pb stress, the NPQ, SOD, and APX activity were all significantly higher than those under Cd stress. Thus under Pb stress, the degree of photoinhibition and oxidative damage of PSII and PSI in mulberry leaves were significantly lower than under Cd stress.

为了探讨铅（Pb）和镉（Cd）胁迫如何影响桑（Morus alba L.）光合作用的机制，我们研究了不同浓度的铅（Pb）和镉（Cd）胁迫（在100和200μmolL ^{-1下）的影响。}）是桑叶中叶绿素（Chl）上的两种重金属元素，光合作用交换，Chl荧光和活性氧（ROS）代谢。结果表明，较高的Pb和Cd浓度会降低叶片Chl的含量，尤其是在Chl a中，其含量比Chl b更为敏感。在铅和镉胁迫下，桑叶的光合碳同化能力降低，这是气孔和非气孔因素共同作用的结果。主要的非口腔因素是光系统II（PSII）和光系统I（PSI）活性和羧化效率（CE）降低。PSII反应中心的供体侧的损伤大于受体侧。用100μmolL ^-1处理后在Pb和Cd中，桑叶通过开始非光化学猝灭（NPQ）继续能够消散多余的激发能，但是当Pb和Cd浓度增加到200μmolL ^-1时，依赖于NPQ的保护机制就会受到损害。 。叶绿体的过量激发能促进了ROS的大量增加，例如超氧阴离子（O ₂ ^•-）和H ₂ O ₂。此外，在高Pb和Cd胁迫下，超氧化物歧化酶（SOD）和抗坏血酸过氧化物酶（APX）也受到一定程度的抑制，并且过量的ROS也导致了更高程度的氧化损伤。与Cd相比，相同浓度水平下Pb胁迫的影响对Chl含量，光合碳同化和气孔导度的影响要低得多。同时，Pb应力主要破坏了位于PSII供体侧的析氧复合物（OEC）的活性，但降低了PSII受体侧和PSI的电子压力。此外，在铅胁迫下，NPQ，SOD和APX活性均显着高于镉胁迫下。因此，在铅的压力下，