To clarify the photosynthetic physiological response of mulberry seedlings under mixed saline–alkali stress, different saline–alkali combinations were prepared to study its effect on leaf water content, chlorophyll content, and changes of chlorophyll fluorescence parameters of mulberry seedlings leaves. Different saline–alkali combinations were composed of three salt concentrations (50, 100, and 200 mmol L⁻¹) and four pH gradients (7.0, 8.0, 9.0, and 10.0) in different proportions, of which salt concentrations were prepared with two neutral salts (NaCl and Na₂SO₄) and two alkaline salts (Na₂CO₃ and NaHCO₃) in different proportions. The results showed that different pH levels had relatively limited influence on chlorophyll fluorescence parameters of mulberry seedling leaves when the salt concentration was 50 mmol L⁻¹ and 100 mmol L⁻¹, but the photochemical activity decreased slightly with the increase of the pH value in the 100 mmol L⁻¹ treatment. When salt concentrations increased to 200 mmol L⁻¹, the photochemical efficiency of PSII in mulberry seedling leaves decreased significantly. With the increase of the pH value, the reduction of the PSII reaction center activity in mulberry seedling leaves was accelerated. Mixed saline–alkali stress also resulted in a significant decline in the electron transport rate (ETR) of mulberry seedling leaves, which was mainly due to the decrease in activity of the oxygen-evolving complex (OEC) on the PSII donor side and the blockage of electron transfer from Q_A to Q_B on the PSII acceptor side. This electron transfer on the PSII acceptor side was the main target point of saline–alkali stress. At a salt concentration of 100 mmol L⁻¹, mulberry seedlings dissipated excess light energy by increasing non-photochemical quenching (NPQ). However, at the 200 mmol L⁻¹ salt concentration, the protective ability of NPQ decreased, and the reduction was more significant at a high pH value, which led to the accumulation of excess excitation energy (1 − q_P)/NPQ and the aggravation of photoinhibition. Saline–alkali mixed stress also significantly changed the energy distribution parameters of the PSII reaction center. A higher salt concentration and pH value led to inactivation or degradation of the PSII reaction center and reduced the ability of antenna pigment to capture light energy. Under the interaction of high salt concentration and high pH value, mulberry seedlings dissipated excess excitation energy mainly in the form of heat energy with the decrease of NPQ. In conclusion, the effects of salt and pH value on the PSII function in mulberry seedlings leaves were minimal under low salt concentrations, and there was no significant interaction between them. However, salt and pH showed significant interactions under the high salt concentration, and the higher pH value affected photoinhibition in PSII under saline–alkali stress. Therefore, the influence of total alkalinity and alkalinity should be considered when planting mulberry in high salinity areas.

中文翻译：

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盐碱混合胁迫下盐度和pH对桑叶PSII功能的影响

为了阐明在盐碱混合胁迫下桑树幼苗的光合生理响应，准备了不同的盐碱组合以研究其对桑树叶片含水量，叶绿素含量以及叶绿素荧光参数变化的影响。不同的盐碱组合由三种盐浓度（50、100和200 mmol L ^-1）和四个pH梯度（7.0、8.0、9.0和10.0）组成，比例不同，其中盐浓度由两种中性盐（NaCl和Na ₂ SO ₄）和两种碱性盐（Na ₂ CO ₃和NaHCO ₃）以不同的比例。结果表明，当盐浓度为50 mmol L ^-1和100 mmol L ^-1时，不同pH值对桑叶叶片叶绿素荧光参数的影响相对较小，但随着pH值的增加，光化学活性略有下降。 100 mmol L ^-1处理。当盐浓度增加到200 mmol L ^-1时，桑树幼苗叶片中PSII的光化学效率显着下降。随着pH值的增加，桑树幼苗叶片中PSII反应中心活性的下降加速。盐碱混合胁迫还导致桑树幼苗叶片的电子传递速率（ETR）显着下降，这主要是由于PSII供体侧放氧复合物（OEC）的活性降低和受阻在PSII受体一侧电子从Q _A传递到Q _B的过程。PSII受体一侧的电子转移是盐碱胁迫的主要目标。盐浓度为100 mmol L ^-1时桑树幼苗通过增加非光化学猝灭（NPQ）来耗散多余的光能。然而，在200 mmol L ^-1的盐浓度下，NPQ的保护能力降低，并且在高pH值下还原作用更为明显，这导致了多余的激发能（1-  q _P）/ NPQ和光抑制作用的加剧。盐碱混合应力也显着改变了PSII反应中心的能量分布参数。较高的盐浓度和pH值会导致PSII反应中心失活或降解，并降低天线色素捕获光能的能力。在高盐浓度和高pH值的相互作用下，桑树幼苗主要以热能的形式耗散过量的激发能，而NPQ降低。总之，在低盐浓度下，盐和pH值对桑树幼苗叶片PSII功能的影响很小，并且两者之间没有显着的相互作用。但是，在高盐浓度下，盐和pH值显示出显着的相互作用，较高的pH值会影响盐碱胁迫下PSII的光抑制。因此，在高盐度地区种植桑树时，应考虑总碱度和碱度的影响。