Effects of cadmium stress on the antioxidant system and chlorophyll fluorescence characteristics of two Taxodium clones.

KEY MESSAGE: The T.118 and T.406 seedlings showed strong adaptability under Cd concentrations ≤ 50 µM. The mechanisms of photoprotection in T.118 and T.406 differed in high-Cd concentrations. To explore the physiological response characteristics of Taxodium hybrids to cadmium (Cd) stress and provide basis for screening of Cd-tolerant species, the hydroponic cultivation of T.118 and T.406 seedlings was conducted to demonstrate the effects of Cd stress on seedling growth, antioxidant system, and chlorophyll fluorescence parameters. After 35 days of Cd stress at a concentration ≤ 50 µM, the dry weight biomass of the two clones did not significantly differ from that of the control. T.406 exhibited a significant increase in POD activity compared to T.118 and maintained high SOD activity after exposure to high concentrations of Cd, whereas MDA levels showed little changes. Under low-Cd stress, chlorophyll content and fluorescence parameters remained stable, especially for T.406. Under high-Cd concentration stress, the above parameters were lower than the control, with a more significant decrease in T.118 than in T.406. The non-photochemical quenching coefficient (NPQ) of both clones increased with increasing Cd concentration. T.118 showed a greater increase than T.406, particularly under high-Cd concentration stress. The T.118 and T.406 seedlings adapted to low-Cd concentration stress by enhancing their antioxidant enzyme activity to maintain the balance of reactive oxygen metabolism and reduce cellular damage. The photochemical activity of mesophyll cells remained high to maintain photosynthetic capacity and normal seedling growth. T.406 showed stronger resistance to Cd than T.118. T.406 prevented photodamage by promoting the photochemical utilization of the excitation energy and maintaining a strong antioxidant stress ability. Enhancement of heat dissipation capability may be the main photoprotection mechanism of T.118.

New evidence for primordial action site of Fluazifop-P-butyl on Acanthospermum hispidum seedlings: From the effects on chlorophyll fluorescence characteristics and histological observation.

Acanthospermum hispidum DC, an Asteraceae weed species, was very susceptible to fluazifop-P-butyl, but tolerant to other aryloxyphenoxypropionate herbicides, such as haloxyfop-P-methyl. However, other Asteraceae weeds including Bidens pilosa were all tolerant to fluazifop-P-butyl. Membrane lipid peroxidation by increasing the levels of reactive oxygen species (ROS) was proposed as an action mechanism of fluazifop-P-butyl in A. hispidum. To further clarify the primordial action site of fluazifop-P-butyl in this species, the effects on chlorophyll fluorescence characteristics and cytohistology of apical meristems were studied. Chlorophyll fluorescence characteristics (CFC) in sensitive A. hispidum seedlings were markedly affected by 10µM fluazifop-P-butyl, with the dark fluorescence yield (Fo), maximal fluorescence yield (Fm), maximal PS II quantum yield (Fv/Fm), effective photosystem II (PS II) quantum yield [Y(II)], and quantum yield of regulated energy dissipation [Y(NPQ)] declining, quantum yield of nonregulated energy dissipation [Y(NO)] rising, but these measures were not affected in Bidens pilosa. The effects of fluazifop-P-butyl on chlorophyll fluorescence properties were observed on the growing point before the mature leaves by about 4-6h. Haloxyfop-P-methyl, a control herbicide, had no effects on CFC of either A. hispidum or B. pilosa. In addition, damage to apical meristem cells of A. hispidum was observed at 6 HAT prior to changes in chlorophyll fluorescence parameters suggesting that the primary action site of fluazifop-P-butyl in this species is in the apical meristem and the effects on CFC may be the results of secondary action.

Chlorophyll fluorescence characteristics and H2O2 contents of Chinese tallow tree are dependent on population origin, nutrients and salinity.

Plants from invasive populations often have higher growth rates than conspecifics from native populations due to better environmental adaptability. However, the roles of improved chlorophyll fluorescence or antioxidant defenses in helping them to grow better under adverse situations are insufficient, even though this is a key physiological question for elucidating mechanisms of plant invasion. Here, we conducted experiments with eight native (China) and eight introduced (USA) populations of Chinese tallow tree (Triadica sebifera). We tested how salinity, nutrients (overall amount or N:P in two separate experiments) and their interaction affected T. sebifera aboveground biomass, leaf area, chlorophyll fluorescence and antioxidant defenses. Plants from introduced populations were larger than those from native populations, but salinity and nutrient shortage (low nutrients or high N:P) reduced this advantage, possibly reflecting differences in chlorophyll fluorescence based on their higher PSII maximum photochemical efficiency (F v/F m) and PSI maximum photo-oxidizable P700 in higher nutrient conditions. Native population plants had lower F v/F m with saline. Except in high nutrients/N:P with salinity, introduced population plants had lower electron transfer rate and photochemical quantum yield. There were no differences in antioxidant defenses between introduced and native populations except accumulation of hydrogen peroxide (H2O2), which was lower for introduced populations. Low nutrients and higher N:P or salinity increased total antioxidant capacity and H2O2. Our results indicate that nutrients and salinity induce differences in H2O2 contents and chlorophyll fluorescence characteristics between introduced and native populations of an invasive plant, illuminating adaptive mechanisms using photosynthetic physiological descriptors in order to predict invasions.

Effects of exogenous calcium on growth, chlorophyll fluorescence characteristics and antioxidant system of Fraxinus malacophylla seedlings.

Karst ecosystems are becoming increasingly problematic, and high calcium is one of the main characteristics of soils in rocky desertification areas. Chlorophyll fluorescence is one of the most important indicators of the extent to which plants are affected by their environment. There are few reports on the effects of changes in exogenous calcium levels on the chlorophyll fluorescence properties of Fraxinus malacophylla seedlings. In the present study, we investigated the growth, chlorophyll fluorescence properties and antioxidant system of Fraxinus malacophylla seedlings in response to exogenous calcium (as the concentrations of 0, 25, 50, 75 mmol L-1). The results showed that Ca2+ concentration (25-50 mmol L-1) treatment mainly promoted the growth, biomass accumulation, root activity, and chlorophyll synthesis and effect on chlorophyll fluorescence in Fraxinus malacophylla; the developed root system became a strong linking hub for calcium adaptation. In addition, the activities of the antioxidant enzymes peroxidase (POD) and catalase (CAT) are upregulated and play an important role in preventing excessive oxidative damage. OJIP test parameters changed significantly with the addition of exogenous calcium, and parameters related to each photosystem II (PSII) reaction centre, such as ABS/RC and DIo/RC, increased significantly in the OJIP test, with enhanced function of the PSII electron donor lateral oxygen evolution complex. In conclusion, the addition of exogenous calcium (25-50 mmol L-1) had an important protective effect on the photosynthetic mechanism of Fraxinus malacophylla, promoting photosynthesis, better growth and better adaptability.

High Potassium Application Rate Increased Grain Yield of Shading-Stressed Winter Wheat by Improving Photosynthesis and Photosynthate Translocation.

Wheat (Triticum aestivum L) production on the Huang-Huai Plain of China has substantially affected in the past 50 years as a result of the decreasing total solar radiation and sunshine hours. Potassium has a significant effect on improving leaf photosynthesis ability under stress conditions. Five potassium application rates (K), 0 (K0), 50 (K50), 100 (K100), 150 (K150), and 250 (K250) mg K2O kg-1 soil, combined with two shading levels, no shading (NS) and shading at early filling stage for 10 days (SE), were used to investigate the effects of K application on winter wheat growth under SE condition. Under NS condition, the parameters related to chlorophyll fluorescence characteristics, dry matter productivity and grain yields reached the maximum values at a middle K application rate (100 mg K2O kg-1 soil). Shading stress significantly reduced leaf SPAD value, showed negative effects on chlorophyll fluorescence characteristics and reduced grain yield of winter wheat. However, as the result of the interaction of K×S, compared to NS condition, higher K application rate (150 mg and 250 K2O kg-1 soil) was beneficial in terms of achieving a higher grain yield of winter wheat under SE by improving leaf SPAD value, alleviating the damage of SE on the winter wheat photosynthetic system, and increasing fructan content and dry matter translocation percentage.

Effects of cesium accumulation on chlorophyll content and fluorescence of Brassica juncea L.

The aim of our study was to investigate the toxicological mechanism of cesium on Indian mustard (Brassica juncea L.). The impact of cesium toxicity to plants was evaluated using phytophysiology and genetic methods. In this study, Brassica juncea was grown on Cs-contaminated Hoagland's nutrient solution, and chlorophyll content, chlorophyll fluorescence, and Cs bioaccumulation were measured. Transcriptome data was used to perform an in-depth analysis of the molecular mechanisms underlying the effects of Cs accumulation. The results showed that Cs accumulated up to 3586.70 mg kg-1 in B. juncea treated with 100 mg L-1 Cs. The chlorophyll content and several chlorophyll fluorescence parameters (Fv/F0, Fv/Fm, ΦPS II, qP, and NPQ) significantly decreased under Cs exposure. The starting process of PSII was also inhibited under higher Cs conditions. These results indicate that excessive Cs can damage PS II in leaves, decreasing photochemical activity and the energy conversion rate. Further analysis revealed that Cs interfered with the expression of chloroplastic metabolic genes (25 up and 36 down) and inhibited the expression of PsaB, psbC, PetF, LHCA1, and LHCB5. The results indicate that stable Cs leads to abnormal expression of genes related to photosynthesis pathway, blocking the electron transport process from plastoquinone-QA to plastoquinone-QB, resulting in abnormal photosynthesis, which leads to abnormal growth of B. juncea.

The Response of Photosynthetic Functions of F1 Cutting Seedlings From Physocarpus amurensis Maxim (♀) × Physocarpus opulifolius "Diabolo" (♂) and the Parental Seedlings to Salt Stress.

This paper selected clonal cutting seedlings from the F1 hybrid varieties of Physocarpus amurensis Maxim (♀) × P. opulifolius "Diabolo" (♂) as research material to study the response of the photosynthetic gas exchange parameters and chlorophyll fluorescence parameters of P. amurensis hybrids and their parental leaves to NaCl stress (with concentrations of 0, 50, 100, and 200 mmol⋅L-1). The results showed that under salt stress, the stomatal conductance (Gs), transpiration rate (Tr), and net photosynthetic rate (Pn) of the three kinds of P. amurensis all significantly decreased. When the NaCl concentration was below 100 mmol⋅L-1, the intercellular CO2 concentration (Ci) of leaves of the three samples declined with the increase of salt concentration; however, when the concentration increased to 200 mmol⋅L-1, Ci did not decrease significantly, especially when the Ci of P. opulifolius "Diabolo" presented a slight increase. This indicated that the decline of photosynthetic carbon assimilation capacity induced by salt stress was the consequence of interaction between stomatal factors and non-stomatal factors, and the stomatal factors played an important role when the salt concentration was below 200 mmol⋅L-1. Compared with P. amurensis, the photosynthetic gas exchange capability of P. opulifolius "Diabolo" leaves was more sensitive to salt stress, and the limitation of non-stomatal factors was relatively evident. However, the photosynthetic capacity of hybrid P. amurensis leaves with the desired purple color was improved compared with P. amurensis. Under salt stress, the PSII activity of the three kinds of P. amurensis leaves declined, the electron transfer was inhibited, and obvious signs of photoinhibition were present. The PSII activity of P. opulifolius "Diabolo" leaves was more sensitive to salt stress than that in P. amurensis. Under salt stress, the NPQ of P. opulifolius "Diabolo" leaves decreased greatly, while under high salt concentrations the degree of photoinhibition in P. amurensis and hybrid P. amurensis were reduced due to a relatively high NPQ. With the increase of salt concentration, the Vk of P. amurensis and hybrid P. amurensis leaves presented a decreasing trend. However, the Vk of P. opulifolius "Diabolo" leaves increased slightly. This suggested that the effects of salt stress on the oxygen-evolving complex (OEC) of the three P. amurensis sample types were relatively limited and only the OEC of P.s opulifolius "Diabolo" leaves were slightly sensitive to salt stress. The VJ of all leaves from the three P. amurensis types increased under salt stress, and the VJ increased significantly when the salt concentration increased to 200 mmol⋅L-1, indicating that salt stress obviously impeded the electron transfer chain from QA to QB on the PSII receptor side. Moreover, high salt concentrations caused thylakoid membrane dissociation. The electron transfer and degree of damage to the thylakoid membrane of P. opulifolius "Diabolo" leaves were obviously higher than that of P. amurensis. However, the electron transfer capacity on the PSII receptor side as well as the degree of damage of the thylakoid membrane of hybrid P. amurensis leaves was obviously lower than those of P. opulifolius "Diabolo." The salt tolerance of photosynthetic functions of hybrid P. amurensis (♀) × P. opulifolius "Diabolo" (♂) leaves was improved compared with that of parental P. opulifolius "Diabolo," and the hybrid shows obvious hybrid vigor for photosynthesis.