| The growth of the field plants often subjected to a number of different stresses simultaneously, such as salt stress, light irradiance, drought stress, heat stress and potassium (K) deficiency. However, the majority of the studies have focused on only single stress. Salt stress is one of the major abiotic stresses which inhibit plant growth and drastically decreases crop yields. The results of long-term cultivation often lead to nutrient element deficiency, such as nitrogen, phosphorus and potassium deficiency, in the soil. However, very little work has been done on the damages of maize caused by exposure to a combination stress of both salt stress and K deficiency. Exogenous potassium application will be possible to reduce the effects caused by K+deficiency, but might exacerbate soil salinization. Furthermore, China has only2.2%of the global K reserves that limit K application. On the hand other, China has rich rare earth resources. It has been proved that low concentrations of rare earths treatment could both improve the quality and increase crop yields. The question is raised whether trace amounts of rare earths can increase crop yield under the combined stresses of salt stress and K+deficiency in maize seedlings. Maize is one of the most important crops in China and occupies an important role in the national economy. In this study, the morphology, photosynthesis and oxidative damage were investigated by using the experimental techniques of microscopy, biochemistry and molecular biology in maize seedlings subjected to the combined stresses of salt stress and K+deficiency as well as the addition of cerium, one of the common rare earths used in agriculture. This study will not only provide insights into the biological effects of mineral elements, but also lay a solid ground for the efficient use of rare earth elements in agriculture. The results are below. 1. Salt stress, K+deficiency and the combined stresses of salt and K+deficiency decreased the growth of maize seedlings, affected the formation of the appendages of leaf epidermal cells. The maize seedlings exhibited thinner diameter of stem and root, and narrow leaves under K+deficiency, but the symptoms of K+deficiency were not apparent under a combination stress of salt and K+deficiency. Salt stress and the combined stresses reduced synthesis of middle lamella molecule as well as interaction between the molecules of cell wall and middle lamella. The reductions in the length of root hair and the number of fibrous roots caused by the combined stresses were greater than those of each individual stress, which led to the decreases of root absorption area and root activity.2. The reductions in plant growth, chlorophylls, carotenoids, K+, Mg2+, maximum quantum efficiency (Fv/Fm), quantum yield (Y(II)), non-photochemical quenching (such as NPQ and qN) and photochemical quenching (such as qP and qL) of photosystem II (PSII), photochemical efficiency (Y(I)) of photosystem I (PSI) and non-photochemical quantum yield of PSI accepted side (Y(NA)), electron transport rates of two photosystems, and the mRNA expression levels of Rubisco and light harvesting pigments protein (LHCII CABl) in maize seedlings caused by a combination stress of salt and K+deficiency were greater than those of each individual stress. Moreover, significant increases in Na+, Cl-and the parameters of quantum yield of energy dissipation of two photosystems, including Y(NPQ), Y(NO), and Y(ND) of maize leaves subjected to the combined stresses were observed. Further, the photochemical reaction of PSII of maize seedlings cultivated in the combinesd stresses was significantly inhibited than PSI. The results implied that a combination of salt stress and K+deficiency impaired the photochemical reaction of PSI, PSII and the carbon assimilation reaction in maize seedlings.3. The compatible solute accumulation, permeability of plasma membrane, malondialdehyde as a degradation product of lipid peroxidation, reactive oxygen species such as superoxide radicals, hydrogen peroxide of maize leaves cultivated in the combines stresses of salt stress and K+deficiency were higher than those of the individual potassium deficiency or salt-stress as expected. However, the antioxidative enzymes such as superoxide dismutase (SOD), glutathione reductase (GR), ascorbate peroxidase (APX), and antioxidants such as ascorbic acid and glutathione, and the ratios of AsA to DAsA and GSH to GSSG were lower than those of the individual stress. Taken together, salt stress impaired K nutrition of maize leaves, K deficiency at the cellular level might be a contributory factor to oxidative stress induced by salt stress and related to cell damages.4. Significant increases of the accumulation of active oxygen, the level of lipid peroxidation and the compatible solute accumulation in the maize leaves and roots subjected to the combined stresses were observed. Moreover, the combined stresses did not induce the generation of new isozymes. However, the generating rate of reactive oxygen species and the response of antioxidant defense systems in the maize leaves were different from roots under the combined stresses of salt stress and K+deficiency. For instance,(1) The generating rate of reactive oxygen species and oxidative damage in the leaves were higher than those in roots.(2) The activities of SOD, APX and GR and gene expressions of SOD, APX isozymes remain high in the roots, whereas, the enzymes activities and their gene expressions were inhibited, and CAT activity and its gene expression were increased in the leaves.(3) The activities of SOD and CAT in1g leaves were higher than roots, but the activities of APX, GR and POD in1g leaves were lower than roots. The isozyme ratios of SOD-A/SOD-5and CAT-1/CAT-2in the leaves were lower than those in the roots, and the isozyme ratio of acidic POD to basic POD in the leaves was higher than roots.(4) The content of total glutathione in leaves was lower than roots, and the content of total ascorbic acid in the leaves is higher than roots. The ratios of AsA to DAsA and GSH to GSSG in leaves were lower than roots, indicating a higher oxidative state in the leaves. It implies that the combined stresses of salt and K+deficiency caused ion imbalance which exacerbate the harm to plants, the accumulation of reactive oxygen species and the responses of antioxidant defense systems in maize leaves and roots to the combined stresses were also inconsistent.5. The combined stresses of salt stress and K+deficiency more significantly decreased nitrate uptake, plant growth, activities of nitrate reductase (NR), glutamate dehydrogenase (GDH), glutamate synthase(GS), urease, glutamic-pyruvic transaminase (GPT), glutamic-oxaloace transaminase (GOT), sucrose-phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPase), and the synthesis of free amino acids, chlorophyll and protein than those of each individual stress, respectively. However, the combined stresses significantly increase the accumulation of ammonium and carbohydrate products. The combined stresses also significantly decreased the ratio of free water to bound water. These results indicated that the combined stresses not only inhibited the light reaction of photosynthesis and carbon assimilation reaction, but also strongly inhibited the transport of photosynthetic products (sucrose accumulation), the conversion of photosynthetic carbon assimilation product into organic nitrogen-containing compounds, thus resulted in inhibition of protein and chlorophyll synthesis, and maize growth.6. Added cerium promoted the growth of leaves and roots, the synthesis of middle lamella molecule as well as the interaction between the molecules of cell wall and middle lamella, and increased the ratio of K+/Na+, photochemical reaction efficiency of PSI and PSII and the carbon assimilation, and the ratio of free water to bound water, and synthesis of organic molecules (chlorophyll, protein) under a combination stress of salt stress and K+deficiency. In addition, added cerium decreased active oxygen accumulation, and increase antioxidant enzymes activites and their gene expressions in the maize roots and leaves, which improved the growth of maize seedlings following exposure to a combination stress of salt stress and K+deficiency. |
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