Photosynthetic function, photoprotection, and the response of related proteomics of mulberry (Morus alba L.) seedling leaves under NaCl and NaHCO3 stress with the same Na+ concentration (100 mmol•L-1) were studied by using photosynthetic gas exchange and chlorophyll fluorescence techniques combined with TMT proteomics. The results showed that NaCl stress had no significant effect on photosystem II (PSII) activity in mulberry seedling leaves, and the expressions of the related proteins, OEE3-1 and PPD4, of the PSII oxygen-evolving complex (OEC) and the antenna proteins, CP24 10A, CP26, and CP29, of LHCII in the leaves also increased to varying degrees. The photosystem I (PSI) activity in the leaves of mulberry seedling also increased, and the expressions of some proteins, PsaF, PsaG, PsaH, PsaL, PsaN, and Ycf4, in PSI increased significantly under NaCl stress. Under NaHCO3 stress, the activity of PSII and PSI and the expression of their protein complexes and the electron transfer-related proteins significantly decreased. NaCl stress had little effect on RuBP regeneration during dark reaction in the leaves and the expressions of glucose synthesis related proteins and net photosynthetic rate (Pn) did not decrease significantly. The leaves could adapt to NaCl stress by reducing stomatal conductance (Gs) to increase water use efficiency (WUE). Under NaHCO3 stress, the expression of dark reaction-related proteins was mostly down-regulated, and Gs was significantly reduced, which indicated that non-stomatal factors were important reasons for the significant inhibition of carbon assimilation. In the photoprotective mechanism under NaCl stress, the expression of cyclic electron flow (CEF) related proteins, ndhH, ndhI, ndhK, and ndhM, involved in NAD(P)H dehydrogenase (NDH) and the key enzyme of the xanthophyll cycle, violaxanthin de-epoxidase (VDE) were up-regulated. In addition, the ratio of xanthophyll cycle components (A+Z)/(V+A+Z) was increased. The expressions of proteins FTR and Fd-NiR, which are related to Fd-dependent ROS metabolism and nitrogen metabolism, were also significant up-regulated under NaCl stress, which can effectively reduce the electronic pressure on Fd. Under NaHCO3 stress, the expressions of CEF-related proteins, VDE, ZE, FTR, Fd-NiR, Fd-GOGAT, SGAT, and GGAT, were significant down-regulated, and the photoprotective mechanism, like the xanthophyll cycle, CEF, and photorespiration, might be damaged, resulting in the inhibition of PSII activity and carbon assimilation in leaves of mulberry seedling under NaHCO3 stress.

Five-day exposure of clary sage (Salvia sclarea) to 100 μM cadmium (Cd) in hydroponics was sufficient to increase Cd concentrations significantly in roots and aboveground parts and affect negatively whole plant levels of calcium (Ca) and magnesium (Mg), since Cd competes for Ca channels, while reduced Mg concentrations are associated with increased Cd tolerance. Total zinc (Zn), copper (Cu), and iron (Fe) uptake increased but their translocation to the aboveground parts decreased possible due to translocation barriers. Despite the substantial levels of Cd in leaves, without any observed defects on chloroplast ultrastructure, an enhanced photosystem II (PSII) efficiency was observed, with a higher fraction of absorbed light energy to be directed to photochemistry (ΦPSΙΙ). The concomitant increase in the photoprotective mechanism of non-photochemical quenching of photosynthesis (NPQ) resulted in an important decrease in the dissipated non-regulated energy (ΦNO), modifying the homeostasis of reactive oxygen species (ROS), through a decreased singlet oxygen (1O2) formation. Thus, when clary sage was exposed to Cd for a short period, tolerance mechanisms were triggered, with PSII photochemistry to be regulated by NPQ in such a way that PSII efficiency to be enhanced. However, exposure to a combination of Cd and high light or for longer duration (8 days) to Cd alone, resulted in an inhibition of PSII functionality pointing out towards Cd toxicity. Thus, the rapid activation of PSII functionality at short time exposures and the inhibition at longer duration suggests a hormetic response and describes these effects in terms of “adaptive response” and “toxicity”, respectively.