ФИЗИОЛОГИЯ РАСТЕНИЙ, 2015, том 62, № 5, с. 654-659
ЭКСПЕРИМЕНТАЛЬНЫЕ СТАТЬИ
УДК 581.1
THE IMPACT OF SALICYLIC ACID AND SILICON ON CHLOROPHYLL a FLUORESCENCE IN MUNG BEAN UNDER SALT STRESS1
© 2015 K. Ghassemi-Golezani, R. Lotfi
Department of Plant Eco-physiology, Faculty of Agriculture, University of Tabriz, Tabriz, Iran
Received September 2, 2014
The ameliorative effect of SA and Si on PSII activity of mung bean plant under salt stress (control, 3, 6, and 9 dS/m) was studied by chlorophyll a fluorescence measurement. Results showed that Na+ accumulation decreased, but K+ accumulation increased in mung bean (Vigna radiata (L.) Wilczek) leaves by application of Si and especially SA, leading to improvement in PSII activity under salinity. Initial fluorescence (F0) and energy necessary for the closure of all reaction centres (Sm) were increased with increasing salt levels. Exogenous foliar application of SA and Si decreased initial fluorescence (F0) and increased photosynthesis relative vitality (PI). Maximum fluorescence (Fm), variable fluorescence (Fv), the activity of the water-splitting complex on the donor side of the PSII (proportional to Fv/F0) and the average redox state of QA in the time span from 0 to Tfm (Sm/Tfm) were also enhanced by foliar application of SA. Exogenous application of SA improved maximum quantum efficiency of PSII (Fv/Fm) and performance index (PI) under both saline and non-saline conditions. Increasing in the size of the plastoquinone pool (Area) was much greater for SA than that for Si treated plants. It was concluded that foliar application of Si and SA in particular could play a key role in salt stress tolerance of mung bean plants.
Keywords: Vigna radiata — fluorescence — photosystem II — salicylic acid — silicon — salinity
DOI: 10.7868/S0015330315040089
INTRODUCTION
Analysis of the kinetics of chlorophyll a fluorescence has been a widespread non-invasive technique used extensively for the study of oxygenic photosyn-thetic organisms. It has provided both qualitative as well as quantitative information on a large variety of photosynthetic events [1]. For higher plants, chlorophyll a fluorescence induction curve measured under continuous light has a fast (within a second) increasing phase, and a slow (within a few minutes) decreasing phase. PSII is responsible for chlorophyll a variable fluorescence, and the rate ofPSII photochemical conversion is limited by the electron acceptor side (i.e., the side that is reduced by PSII). The fluorescence yield was suggested to be controlled by a PSII acceptor quencher (called Q), later identified as the bound QA in its oxidized state [2].
1 This text was submitted by the authors in English.
Abbreviations'. Area — the area above the fluorescence induction curve between F0 and Fm; F0 — initial fluorescence; Fm — maximum fluorescence; Fv — variable fluorescence; PI — performance index; PSII — photosystem II; SA — salicylic acid; Si — silicon;
— energy necessary for the closure of all reaction centres; Tfm — the time span from 0 to Fm.
Corresponding author. Kazem Ghassemi-Golezani. Department of Plant Eco-physiology, Faculty of Agriculture, University of Tabriz, Tabriz, Iran; fax. +98-41-33356003; e-mail. golezani@gmail.com
Pulse amplitude modulation chlorophyll fluorescence of PSII was primarily developed to assess primary photosynthetic reactions and quenching mechanisms in plant physiology studies of higher plants [3]. The usefulness of chlorophyll fluorescence as an indicator of photosynthesis requires demonstrating its relationship with the quantum yield of gas evolution (O2 or CO2). Quantum yields of photosynthesis are usually defined as the quantum yield for oxygen production or carbon fixation. Chlorophyll a fluorescence, though corresponding to a very small fraction of the dissipated energy from the photosynthetic apparatus, is widely accepted to provide an access to the understanding of its structure and function [4].
Chlorophyll a fluorescence allows us to study the different functional levels of photosynthesis indirectly (processes at the pigment level, primary light reactions, thylakoid electron transport reactions, dark enzymatic stroma reactions and slow regulatory processes). It is useful to investigate the effects of environmental stresses on plants since photosynthesis is often reduced in plants experiencing adverse conditions. Therefore, analysis of chlorophyll a fluorescence parameters is considered as an important approach for evaluating the health or integrity of the internal apparatus during photosynthetic process within a leaf and
provides a rapid and accurate technique for detecting and quantifying the tolerance of plants to stress [5].
Salt stress involves both osmotic stress and ionic stress. NaCl (0.5 M) was found to inactivate both PSII and PSI [6]. Salt adapted cells can maintain a high conversion efficiency of excitation energy through the down regulation of PSII RCs [7]. Plant responses to salinity depend on PSII response to this stress [8]. One viable strategy of overcoming the salt-induced injurious effect on plant growth is the exogenous application of osmoprotectants and inorganic nutrients. Survival under these stressful conditions depends on the plant's ability to perceive the stimulus, generate and transmit signals and instigate biochemical changes that adjust the metabolism accordingly [9]. SA plays an important role in the defense response to pathogen attack and stresses in plant species. Several studies also supported a major role of SA in modulating the plant response to several abiotic stresses including salt and water stress [10]. Although, Si is not generally listed in the list of essential elements, it is considered as one of the important beneficial nutrient for plant growth and tolerance to stress [11].
Mung bean (Vigna radiata) is a crop of special importance and provides an inexpensive source of vegetable dietary protein. It is popular for its nutritive value and digestibility, containing higher protein contents (28%), oil (1.3%), carbohydrates (60.4%), and reasonable amount of vitamins and essential micronutri-ents [12]. The production of mung bean in developing countries is low, which is mainly due to its cultivation on marginal lands mostly affected by salt stress. Its cultivation on such soils could only be made profitable by applying chemicals or plant growth regulators exoge-nously. Thus, for the first time this research was aimed to investigate the target site of salt stress on PSII of mung bean plant in response to exogenous foliar application of salicylic acid and silicon.
MATERIALS AND METHODS
Plant materials and growth conditions. A pot experiment with a factorial arrangement on the bases of randomized complete block (RCB) with three replications was conducted in 2013 (Tabriz, Iran) to investigate the changes of chlorophyll a fluorescence of mung bean (Vigna radiata (L.) Wilczek) under salt stress and exogenous foliar application of salicylic acid (SA) and silicon (Si).
In each plastic pot (30 x 30 cm) containing 1.0 kg of perlite and coco peat (4 : 1) 15 seeds of mung bean were sown at a depth of 3 cm and then tap water (0.8 dS/m) and saline solutions (3, 6, and 9 dS/m) were added to achieve 100% field capacity. All pots kept inside a glass greenhouse under natural light. Minimum and maximum temperatures in greenhouse were 25 and 30°C, respectively. After germination, plants were thinned to 10 plants per pot. During the growth period, the pots
were weighed and the losses were made up with Hoag-land solution (electrical conductivity = 1.3 dS/m, pH 6.5—7.0). Perlite substrate within the pots was washed every 20 days, and non-saline and salinity treatments were reapplied in order to prevent further increase in electrical conductivity, due to adding the Hoagland solution. Two levels of SA (0 and 1 mM) and Si (0 and 2 mM) as silicic acid (H2SiO3) were sprayed on plants at growing and flowering stages.
Measurement of K+ and Na+ in leaves. Leaves of two plants from each sample were dried in 60°C for 48 h. Then 1 g of leaves was powdered and burned at 560°C and the ashes digested in 10 mL of 1 N HCl. The concentration of Na+ and K+ in the digested samples was determined, using a flame photometer (410 Corning Flame Photometer, "Sherwood Sci. Ltd.", UK).
Chlorophyll a fluorescence measurements. Induction of chlorophyll a fluorescence was monitored with a handy-PEA portable fluorometer ("Hansatech", UK) at flowering stage. Fluorescence emission was monitored from the upper surface of the leaves. Dark-adapted leaves (30 min) were initially exposed to the weak modulate measuring beam, followed by exposure to saturated white light to estimate the initial (F0) and maximum (Fm) fluorescence values, respectively. This device had a software for calculation, numerical presentation, and memorization of chlorophyll a fluorescence parameters. The Performance Index (PI) parameter (photosynthesis relative vitality), Tfm (ms; time taken to reach Fm, an indicator of QA reduction rate of the PSII acceptor, i.e., the rate of PSII electron transport) and Area (the area above the fluorescence induction curve between F0 and Fm. The size of the plastoquinone pool in PSII) were also monitored.
For calculating the parameters of chlorophyll fluorescence the following formulas were applied:
Fv = Fm — F0 (variable chlorophyll fluorescence);
Fv/F0 = (Fm — F0)/F0 (a value that is proportional to the activity of the water-splitting complex on the donor side of the PSII);
Fv/Fm = (Fm — F0)/Fm (a value that is related to the maximum quantum yield of PSII);
Sm = Area/(Fm — F0) (representing energy necessary for the closure of all reaction centers);
Sm/Tfm = Area/(Fm — F0) x Tfm (the ratio representing the average redox state of QA in the time span from 0 to Tfm and, concomitantly, the average fraction of open reaction centres during the time needed to complete their closure).
Statistical analysi
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