Research articleEffects of molybdenum on water utilization, antioxidative defense system and osmotic-adjustment ability in winter wheat (Triticum aestivum) under drought stress
Introduction
Mo is an essential trace element for plants, however, it is biologically inactive until it becomes complexed to form molybdenum cofactor. More than 50 enzymes are known to be Mo-dependent in all organisms, but only five of them have been found in plants (Mendel, 2013). These Mo-enzymes are involved in nitrogen, sulphur, purine and phytohormone metabolisms in plants (Mendel, 2013). Mo also has effects on the ascorbate–glutathione cycle in Chinese cabbage and secondary metabolic processes of glycyrrhizic acid in Glycyrrhiza uralensis Fisch (Nie et al., 2007, Wang et al., 2013). In addition, Mo plays important roles in resisting various environmental stresses, such as drought, cold and salt stress (Al-Issawi et al., 2013, El-Samad et al., 2005, Ghafarian et al., 2013, Sun et al., 2009, Zhang et al., 2012). Previous study indicated that Mo deficiency inhibits the biosynthesis of chlorophyll and leads to a decrease of chlorophyll of winter wheat cultivars under cold stress (Yu and Wang, 2006) and there is a positive effect on photosynthetic rate, decreased transpiration rate and stomatal conductance in winter wheat by Mo application under low temperature stress (Sun et al., 2005). However, Mo excess decreased the biomass, seed yield and decreased the quality of produce in chickpea (Nautiyal and Chatterjee, 2004).
Drought stress is well known as a major environmental stress that restricts the growth and yield of crop plants worldwide, and can trigger a series of physiological and biochemical responses in plants (Yue et al., 2012). The main physiological and biochemical characteristics are not only related to stomata closure, inhibition of photosynthesis, decrease in chlorophyll content, and reduction in transpiration, but are also involved in antioxidant capacity and osmotic adjustment ability (Nezhadahmadi et al., 2013), which are important ways to increase plant resistance to drought. Many studies have shown that drought stress can result in an increased production of active oxygen species such as superoxides, hydroxyl radicals, and superoxide radical (Farooq et al., 2009), which can cause membrane damage and MDA accumulation and cell death, and MDA has been considered an indicator for the degree of oxidative stress (Gill and Tuteja, 2010, Li et al., 2013). Plants scavenge active oxygen species by enhancing the activities of antioxidant enzymes such as superoxide dismutase (SOD; EC 1.15.1.1), peroxidase (POD; EC 1.11.1.7), catalase (CAT; EC 1.11.1.6) and ascorbate peroxidase (APX; EC 1.11.1.11), or by increasing the levels of non-enzymatic antioxidants such as ascorbic acid (AsA), glutathione (GSH) and carotenoid (CAR) (Jiang and Zhang, 2002, Yordanov et al., 2000). Moreover, osmotic adjustment is also an important way to resist drought stress in plants. The osmoregulation substances such as proline, soluble sugar and soluble proteins play vital roles in maintaining osmotic equilibrium and the integrity of membranes under water deficit (Mahajan and Tuteja, 2005).
Winter wheat (Triticum aestivum), as a main food crop in China and also being a Mo-sensitive crop, often suffering drought stress. Its productivity and quality are significantly limited by drought stress. There are approximately 446 million hectares of molybdenum deficient arable land in China, and Mo deficiency is a limiting factor for crop production in many provinces of China (Sun et al., 2009). Recently, some papers have indicated that Mo might improve the drought resistance of wheat by increasing accumulation of K+ ions, improving agronomic traits and photosynthetic rate (El-Samad et al., 2005, Ghafarian et al., 2013, Zakhurul et al., 2000). It is also reported that overexpression of Arabidopsis molybdenum cofactor sulfurase gene confers drought tolerance in soybean, maize, cotton and tobacco (Li et al., 2013, Lu et al., 2013, Yue et al., 2011, Yue et al., 2012). In order to obtain a better understanding of the physiological mechanisms for Mo mediated drought tolerance, here we use Mo-efficient and Mo-inefficient cultivars of winter wheat to investigate the effects of Mo on water utilization, antioxidative defense and osmotic-adjustment systems under drought stress.
Section snippets
Experimental materials and treatments
A hydroponic trial was conducted in a controlled environment chamber. The conditions in the controlled environment chamber were: 24/18 °C; 14/10 h day/night, 400 μmol m−2 s−1 irradiance, 60% relative humidity. Two cultivars of winter wheat Mo-efficient ‘97003’ and Mo-inefficient ‘97014’, which are different in Mo uptake and distribution (Yu et al., 2002), were cultivated in a modified Hoagland solution. The modified Hoagland solution contained: Ca(NO3)2·4H2O 4 mM; KNO3 6 mM; NH4H2PO4 1 mM; MgSO4
Effects of Mo application on wheat growth
Compared with the Mo-deficiency treatment (−Mo), Mo application (+Mo) significantly increased the dry matter at 12 d of PEG-6000 stress and rewatering for 2 d, however, no significant differences were observed at 0, 2 and 6 d of PEG-6000 stress in either cultivar of winter wheat (Fig. 1). Mo application significantly increased chlorophyll a and chlorophyll b at 6 d of PEG-6000 stress in the Mo-efficient cultivar ‘97003’ (Fig. 2), while there was no significant difference at 0, 2, 12 d and
Discussion
Drought stress severely decreased chlorophyll a and chlorophyll b contents and Pn (Asrar and Elhindi, 2011, Miyashita et al., 2005), which resulted in the decrease of dry matter in plants (Asrar and Elhindi, 2011). However, in this study, the contents of chlorophyll a and chlorophyll b have not been decreased (Fig. 2). The decreased or unchanged chlorophyll content depended on the duration and severity of drought stress (Anjum et al., 2011). Drought stress depressed the growth of plant which
Conclusion
Application of Mo significantly enhanced Pn, chlorophyll a, chlorophyll b, dry matter, grain yield, biomass and mature spike number of wheat under drought stress. Mo application also significantly enhanced the RWC and WUE and decreased Gs, Tr and water loss in wheat under drought stress. The activities of antioxidant enzyme and non-enzymatic antioxidants were markedly increased and MDA content was decreased by Mo application under drought stress in wheat. The osmotic-adjustment products such as
Acknowledgements
The authors acknowledge Professor Ron McLaren who comes from Lincoln University, New Zealand for critical review and revision of the manuscript. This work was supported by the National Natural Science Foundation of China (Program No. 41171240) and the Fundamental Research Funds for the Central Universities (Program No. 2010QC037, 2010PY025, 2011PY150).
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