Melatonin is important in the protection of plants suffering various forms of abiotic stress. The molecular mechanisms underlying the melatonin-mediated protection of their photosynthetic machinery are not completely resolved. This study investigates the effects of exogenous melatonin applications on salt-induced damage to the light reaction components of the photosynthetic machinery of tomato seedlings. The results showed that melatonin pretreatments can help maintain growth and net photosynthetic rate (PN) under salt stress conditions. Pretreatment with melatonin increased the effective quantum yield of photosystem II (ΦPSII), the photochemical quenching coefficient (qP) and the proportion of PSII centers that are “open” (qL) under saline conditions. In this way, damage to the photosynthetic electron transport chain (PET) in photosystem II (PSII) was mitigated. In addition, melatonin pretreatment facilitated the repair of PSII by maintaining the availability of D1 protein that was otherwise reduced by salinity. The ROS levels and the gene expressions of the chloroplast TRXs and PRXs were also investigated. Salt stress resulted in increased levels of reactive oxygen species (ROS), which were mitigated by melatonin. In tomato leaves under salt stress, the expressions of PRXs and TRXf declined but the expressions of TRXm1/4 and TRXm2 increased. Melatonin pretreatment promoted the expression of TRXf and the abundances of TRXf and TRXm gene products but had no effects on the expressions of PRXs. In summary, melatonin improves the photosynthetic activities of tomato seedlings under salt stress. The mechanism could be that: (1) Melatonin controls ROS levels and prevents damaging elevations of ROS caused by salt stress. (2) Melatonin facilitates the recovery of PET and D1 protein synthesis, thus enhancing the tolerance of photosynthetic activities to salinity. (3) Melatonin induces the expression of TRXf and regulates the abundance of TRXf and TRXm gene products, which may facilitate repair of the light reaction parts of the photosynthetic machinery.

To obtain new insights into the mechanisms of the positive effects of exogenous melatonin applications to cucumber seedlings during chilling, we investigated its role in regulating photosynthesis, the transcription level of csZat12 and the metabolism of polyamines (PAs) and of abscisic acid (ABA). The negative effects of chilling were clearly alleviated in cucumber seedlings by irrigation with 200 μM melatonin solution. This was evidenced by alleviation of the decline in net photosynthesis rate and also in electrolyte leakage in chilled plants. The reasons for this can be explained as follows. First, melatonin up-regulates CsZat12, an important stress-related gene. Second, melatonin increases the content of putrescine (Put) and spermidine (Spd) and stabilized spermine (Spm) by altering the activity of the PA metabolic enzymes. And, third, ABA is also involved in these effects as melatonin modulated the expression of the key ABA biosynthesis genes (CsNCED1 and CsNCED2) and also the key ABA catabolism genes (CsCYP707A1 and CsCYP707A2). This study provides new evidence suggesting melatonin mitigates chilling stress in cucumber by up-regulating the expression of CsZat12 and by modulating the metabolism of PAs and ABA.

Silicon can improve drought tolerance of plants, but the mechanism still remains unclear. Previous studies have mainly concentrated on silicon-accumulating plants, whereas less work has been conducted in silicon-excluding plants, such as tomato (Solanum lycopersicum L.). In this study, we investigated the effects of exogenous silicon (2.5 mmol L-1) on the chlorophyll fluorescence and expression of photosynthesis-related genes in tomato seedlings (Zhongza 9) under water stress induced by 10% (w/v) polyethylene glycol (PEG-6000). The results showed that under water stress, the growth of shoot and root was inhibited, and the chlorophyll and carotenoid concentrations were decreased, while silicon addition improved the plant growth and increased the concentrations of chlorophyll and carotenoid. Under water sterss, chlorophyll fluorescence parameters such as PSII maximum photochemical efficiency (F v /F m), effective quantum efficiency, actual photochemical quantum efficiency (Ф PSII), photosynthetic electron transport rate (ETR), and photochemical quenching coefficient (q P) were decreased; while these changes were reversed in the presence of added silicon. The expressions of some photosynthesis-related genes including PetE, PetF, PsbP, PsbQ, PsbW, and Psb28 were down-regulated under water stress, and exogenous Si could partially up-regulate their expressions. These results suggest that silicon plays a role in the alleviation of water stress by modulating some photosynthesis-related genes and regulating the photochemical process, and thus promoting photosynthesis.

The aim of the study was to monitor the effects of exogenous melatonin on cucumber (Cucumis sativus L.) chloroplasts and explore the mechanisms through which it mitigates chilling stress. Under chilling stress, chloroplast structure was seriously damaged as a result of over-accumulation of reactive oxygen species (ROS), as evidenced by the high levels of superoxide anion (O2−) and hydrogen peroxide (H2O2). However, pretreatment with 200 μM melatonin effectively mitigated this by suppressing the levels of ROS in chloroplasts. On the one hand, melatonin enhanced the scavenging ability of ROS by stimulating the ascorbate–glutathione (AsA–GSH) cycle in chloroplasts. The application of melatonin led to high levels of AsA and GSH, and increased the activity of total superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) dehydroascorbate reductase (DHAR, EC 1.5.5.1), glutathione reductase (GR, EC1.6.4.2) in the AsA–GSH cycle. On the other hand, melatonin lessened the production of ROS in chloroplasts by balancing the distribution of photosynthetic electron flux. Melatonin helped maintain a high level of electron flux in the PCR cycle [Je(PCR)] and in the PCO cycle [Je(PCO)], and suppressed the O2-dependent alternative electron flux Ja(O2-dependent) which is one important ROS source. Results indicate that melatonin increased the chilling tolerance of chloroplast in cucumber seedlings by accelerating the AsA–GSH cycle to enhance ROS scavenging ability and by balancing the distribution of photosynthetic electron flux so as to suppress ROS production.