^^^^^^^^^^^^ ЭКСПЕРИМЕНТАЛЬНЫЕ
СТАТЬИ
581.1
EFFECT OF EXOGENOUS POTASSIUM ON PHOTOSYNTHESIS AND ANTIOXIDANT ENZYMES OF RICE UNDER IRON TOXICITY1
© 2014 P. P. Gao*, G. H. Zheng*, Y. H. Wu**, P. Liu*
*Key Laboratory of Botany, Zhejiang Normal University, Jinhua, P.R. China **College of Life and Environmental Science, Hangzhou Normal University, Hangzhou, P.R. China
Received October 7, 2012
A solution culture experiment was conducted to determine the effects of different potassium concentrations on the chlorophyll fluorescence and oxidation resistance of the Fe2+-tolerant rice (Oryza sativa L.) genotype Xieyou 9308 and the Fe2+-sensitive genotype Ilyou 838 exposed to 250 mg/L of Fe2+. The minimal fluorescence, maximum fluorescence efficiency, maximum fluorescence, and non-photochemical quenching coefficient showed no significant changes. However, the photochemical quenching coefficient increased. Under 200 mg/L K+ concentration, the effects of Fe2+ stress decreased. Compared with the control group, chlorophyll content and peroxidase, superoxide dismutase, and catalase activities decreased, whereas MDA content increased under Fe2+ stress. Exogenous K+ alleviated Fe2+ toxicity in the test subjects compared with the control group. Overall, external K+ could alleviate the toxic effects of Fe2+ toxicity.
Keywords: Oryza sativa - iron stress - potassium - chlorophyll fluorescence - oxidation resistance
ФИЗИОЛОГИЯ РАСТЕНИЙ, 2014, том 61, № 1, с. 53-58
УДК
DOI: 10.7868/S0015330314010059
INTRODUCTION
Iron (Fe2+) is an essential trace element in plants and a component of many important oxidoreductases. Plant Fe2+ poisoning is a common physiological disease in tropical and subtropical regions. Such poisoning is caused by the accumulation of excess Fe2+ salts in soil solutions. Excess Fe2+ inhibits plant growth and yield significantly [1]. Ferrous poisoning is an important limiting factor to rice growth in gley soil [2]. Rice growth, biomass, and yield could be decreased significantly by 30 to 60% under serious Fe2+ toxicity [3]. As a major grain crop, rice occupies an important position in agricultural production. Thus, analyzing the physiological and biochemical responses of rice to Fe2+ toxicity is necessary for optimum productivity and yield [4].
ROS are overproduced in plants under various stress factors, such as temperature, drought, salinity, and aluminum toxicity. Such stress factors increase membrane oxidation and permeability [5]. Excess Fe2+ accumulation in plants causes ROS production and lipid peroxidation, which damage plant tissues
1 This text was submitted by the authors in English.
Abbreviations'. CAT — catalase; Chl — chlorophyll; POD — peroxidase; PS — photosystem; SOD — superoxide dismutase. Corresponding author. Peng Liu. Key Laboratory of Botany, Zhejiang Normal University, Jinhua, 321004 P.R. China. Fax. +86579-8228-2340; e-mail. pliu99@vip.sina.com
and induce membrane lipid peroxidation [6]. Various toxic free radicals induced by Fe2+ [7], especially OH, can usually be cleared by the protective enzyme systems of plants. Superoxide dismutase (SOD) directly decomposes ROS, and its increased activity plays an important role in plant tolerance to Fe2+ toxicity [8]. Peroxidase (POD) can remove hydrogen peroxide (H2O2) and accelerate cell-wall lignification, blocking the entry of Fe2+ [9, 10]. Catalase (CAT) also functions in plant tolerance to Fe2+ toxicity [4]. In general, plants with active POD and CAT can resist to ROS and eliminate them, inhibit membrane lipid peroxidation, and maintain membrane system stability [11, 12].
Chlorophyll, which exists in the chloroplasts of plant cells, is the main substance involved in photosynthesis. It is the ultimate chemical substance that can accept optical excitation. Chlorophyll also reflects the characteristics of crop growth and development. Chlorophyll fluorescence kinetics has been used to study the operation of plant photosynthetic organs as well as their response mechanisms to environmental stresses [13]. Reversible photosystem II (PSII) reaction center inactivation, its irreversible damage, or leaf thylakoid membrane injuries were presumed to increase the minimal fluorescence (F0) [14]. Maximum fluorescence (Fm) represents fluorescence when all PSII reaction centers are inactive, i.e., the Fm in the dark can reflect electron transfer through the PSII system [15]. Maximum fluorescence efficiency (Fv/Fm)
54
GAO h gp.
represents the maximum light energy conversion efficiency of PSII [15]. Photochemical quenching coefficient (qP) mainly reflects the capability to generate shock capacity in the excited state of the PSII reaction center for charge separation through the primary photochemical reactions [16]. A decrease in the non-photochemical quenching coefficient (qN) shows excess excitation energy dissipation through non-radiative energy dissipation to improve light usage efficiency [16]. Compared with the traditional gas exchange index, chlorophyll fluorescence parameters reflect plant photosynthetic capability and afford a quick, sensitive, and non-destructive method for analyzing the influence of adverse factors on photosynthesis [17].
Potassium (K+) can improve plant root oxidation status [18], promote Fe2+ rhizosphere oxidation and pH change, and prevent the rice roots from absorbing excess Fe2+ [19, 20]. It increases the oxidation of the rice root system, thus improving soil redox potential, decreasing the content of Fe2+, and reducing the quantity of aerobic microorganisms.
This study adopts solution cultures and uses the Fe2+-tolerant rice genotype Xieyou 9308 and Fe2+-sensitive genotype IIyou 838 as research materials to determine the effects of K+ different concentrations on rice growth and physiology under Fe2+ poisoning. This study aims to identify the optimum K+ concentration to alleviate Fe2+ toxicity. The results of this study may serve as a theoretical basis for the treatment of Fe2+ poisoning.
MATERIALS AND METHODS
Experimental design. The Fe2+-tolerant rice (Oryza sativa L.) genotype Xieyou 9308 and the Fe2+-sensitive genotype IIyou 838 were used as research materials. Nutrient solution was prepared following the method described by Jawardena et al. [8]. Seeds of uniform sizes were disinfected for 30 min with 0.1% H2O2 and then washed three times with tap and distilled water. After rinsing, the seeds were germinated at 30°C. A week later, the seedlings were transferred to a 48-L plastic tank and cultivated in 0.25 complete solution for a week, in 0.5 complete solution for another week, and then in complete solution for 2 weeks. The pH of the culture was adjusted daily to 5.0, and the nutrients were replaced every 3 days. Iron was supplied as EDTA-Fe2+ (EDTA chelation of FeSO4 x 7 H2O), and K+ was supplied as KCl.
Rice plants were cultivated in complete nutrient solutions: T0 (2 mg/L Fe2+ and 40 mg/L K+), T1 (250 mg/L Fe2+), T2 (250 mg/L Fe2+, 100 mg/L K+), T3 (250 mg/L Fe2+, 200 mg/L K+), and T4 (250 mg/L Fe2+, 400 mg/L K+). On day 14, the chlorophyll content, chlorophyll fluorescence, antioxidant enzyme activities, and MDA content were determined. In early experiments, Fe2+ and K+ concentrations were
screened through pre-tests. It was found that 250 mg/L Fe2+ causes significant plant stress.
Research methods. Chlorophyll fluorescence parameters, including F0, Fv/Fm, Fm, qN, and qP, were measured by a portable pulse modulated chlorophyll fluorometer (PAM-210, "Walz", Germany). The measuring methods used were similar to those used by Ralph et al. [21]. Chlorophyll content (Chl a, Chl b, and Chl (a + b)), MDA content, and POD, SOD, and CAT activities were determined according to the method of Li et al. [5].
Date processing. According to data from three independent experiments, we calculated the mean values and standard errors. Significant difference was evaluated through the least significance difference test using SPSS17.0 software.
RESULTS
Effects of exogenous K+ on the chlorophyll fluorescence characteristics of rice under Fe2+ stress
Under Fe2+ stress, the F0 and Fv/Fm of IIyou 838 showed no significant changes, whereas those of Xieyou 9308 increased and decreased, respectively. However, no significant difference was noted between the two genotypes. After the addition of exogenous K+, compared with the Fe2+ treatment group, the F0 value of IIyou 838 initially increased and then decreased, whereas its Fv/Fm decreased slightly. The F0 of Xieyou 9308 increased slightly, whereas its Fv/Fm declined.
The Fm values of IIyou 838 and Xieyou 9308 decreased and increased, respectively. The addition of KCl to nutrient solution increased the Fm value of both rice genotypes compared with the T1 treatment. A K+ concentration of 100 mg/L was favorable for improving Fm values. As shown in table 1, the Fm values of Xieyou 9308 and IIyou 838 initially increased and then decreased with increasing K+ concentration.
As shown in table 1, the qP was different in two rice genotypes: IIyou 838 was more sensitive to Fe2+ toxicity than Xieyou 9308. When the exogenous K+ concentration was 200 mg/L, the qP of both rice genotypes was significantly relieved.
The qN of both genotypes of rice increased under Fe2+ stress and then decreased after the addition of K+ to nutrient solution (table 1). The qN of IIyou 838 was inhibited to a greater extent than that of Xieyou 9308.
Effect of exogenous K+ on the chlorophyll content in rice under Fe2+ toxicity
As shown in table 2, compared with the control group, the Chl a, Chl b, and Chl (a + b) levels in Xieyou 9308 and IIyou 838 decreased significantly under Fe2+ treatment (T1 group). The Chl a, Chl b, and Chl (a + b) contents in IIyou 838 decreased by 10.4, 11.0, and
Table 1. Effects of exogenous K+ on the chlorophyll fluorescence characteristics of the rice cvs. Xieyou 9308 and Ilyou 838 under Fe2+ stress
Treatment Fo Fm Fv/Fm qP qN
Xieyou 9308 T0 0.240 ± 0.010d 1.150 ± 0.080b 0.790
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