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Journal of Stress Physiology & Biochemistry, Vol. 8 No. 1 2012, pp. 232-240 ISSN 1997-0838 Original Text Copyright © 2012 by Ibn Maaouia-Houimli, Dridi-Mouhandes, ben Mansour-Gueddes and Denden
ORIGINAL ARTICLE
24-Epibrassinolide ameliorates the adverse effect of salt stress (NaCl) on pepper (Capsicum annuum L.)
Ibn Maaouia-Houimli Samira *, Bouthaina Dridi-Mouhandes, Samia ben Mansour-Gueddes, Mounir Denden
Department of Horticulture, High Institute of Agronomy, 4042 Chott-Mariem, Tunisia
Tel: +216 2159 2240 *E mail: h.samira@laposte.net
Received January 23, 2012
The present study investigates the role of 24-epibrassinolide (EBL) in inducing plant tolerance to salinity. Seedlings of pepper (Capsicum annuum L.) were grown in the presence of 70 mM NaCl and were sprayed with 10-6 M EBL at 7 days after transplantation and were sampled at 28 day. The plants exposed to NaCl exhibited a significant decline in relative growth rate, net CO2 assimilation, stomatal conductance, transpiration and water use efficiency. However, the follow up treatment with EBL significantly improved the above parameters. EBL treated plants had greater relative growth rate compared to untreated plants when exposed to salt stress. Application of EBL increased photosynthesis by increasing stomatal conductance in both control and salt stressed plants and may have contributed to the enhanced growth. The water use efficiency was improved because CO2 assimilation is more important than the transpiration.
Key words: 24-epibrassinolide /pepper /NaCl / relative growth rate /gas exchange.
ORIGINAL ARTICLE
24-Epibrassinolide ameliorates the adverse effect of salt stress (NaCl) on pepper (Capsicum annuum L.)
Ibn Maaouia-Houimli Samira *, Bouthaina Dridi-Mouhandes, Samia ben Mansour-Gueddes, Mounir Denden
Department of Horticulture, High Institute of Agronomy, 4042 Chott-Mariem, Tunisia
Tel: +216 2159 2240 *E mail: h.samira@laposte.net
Received January 23, 2012
The present study investigates the role of 24-epibrassinolide (EBL) in inducing plant tolerance to salinity. Seedlings of pepper (Capsicum annuum L.) were grown in the presence of 70 mM NaCl and were sprayed with 10-6 M EBL at 7 days after transplantation and were sampled at 28 day. The plants exposed to NaCl exhibited a significant decline in relative growth rate, net CO2 assimilation, stomatal conductance, transpiration and water use efficiency. However, the follow up treatment with EBL significantly improved the above parameters. EBL treated plants had greater relative growth rate compared to untreated plants when exposed to salt stress. Application of EBL increased photosynthesis by increasing stomatal conductance in both control and salt stressed plants and may have contributed to the enhanced growth. The water use efficiency was improved because CO2 assimilation is more important than the transpiration.
Key words: 24-epibrassinolide /pepper /NaCl / relative growth rate /gas exchange.
Salinity has been considered as one of the serious environmental stress on plant growth and development as well as crop production. About 20% of irrigated agricultural land is adversely affected by salinity (Pitman and Lauchli, 2002). Moreover about one-third of the irrigated land on the earth is affected by salinity (Taiz and Zeiger, 2006). Through its osmotic effect, salinity perturbs a multitude of physiological processes including photosynthesis, a process which is the main determinant of the dry matter accumulation and
productivity of the crops. The decline in
photosynthesis observed under salt stress is not only attributed to stomata closure leading to a reduction of intercellular CO2 concentration, but also to non-stomata factors (Qin et al., 2010). Non-stomatal restriction of net assimilation may originate from a reduced efficiency or regeneration capacity of ribulose-1,5-biphosphate (RuBP) carboxylase, a sensitivity of photosystem
II to NaCl, or a reduced leaf chlorophyll
concentration (Lycoskoufis et al., 2005). Salinity
reduces the ability of plants to utilize water and causes a reduction in growth rate, as well as changes in plant metabolic processes (Munns,
2002). All these events lead to poor plant growth and productivity.
Various agronomic and physiological practices are applied to minimize the adverse effects of salinity on the normal functioning of plants. Moreover, plant growth regulators, both natural and synthetic, are widely applied to agricultural crops as a means of crop improvement (Hayat et al., 2010).
Brassinosteroids (BRs) represent a group of naturally occurring steroidal lactones widely distributed in plant kingdom. Studies with BR biosynthesis mutants and BR insensitive mutants of Arabidopsis thaliana have also provided evidence that BRs are essential for plant growth (Clouse and Sasse, 1998). Since the identification of the first BRs, almost one hundred structurally related compounds were isolated from various parts of many plant species (Hola 2010). BRs promote plant growth through multiple mechanisms like cell division, cell elongation, vascular differentiation, germination, rhizogenesis, flowering, and modulation of gene expression (Sasse 2003). Exogenous application of BRs promote leaf photosynthesis by positively regulating synthesis and activation of a variety of photosynthetic enzymes including Rubisco (Xia et al., 2009), and this activity of BRs may contribute to increased crop yield after BRs application (Hayat et al., 2000; Yu et al., 2004). In addition to stimulating growth, BRs have an anti-stress effect on plants. Previous studies showed that salt tolerance of rice (Ozdemir et al., 2004), chickpea (Ali et al., 2007), Indian mustard (Ali et al., 2008) and mung bean (Hayat et al., 2010) could be markedly enhanced by the application of brassinosteroids.
Pepper (Capsicum annuum) is considered sensitive (Lycoskoufis et al., 2005) or moderately-sensitive to salt stress (Chartzoulakis and Klapaki, 2000). Salinity hampers pepper growth more during vegetative phase (Villa-Castorena et al.,
2003).Therefore, the present study was conducted with an objective to ameliorate pepper salt tolerance by using 24-epibrassinolide, a highly active and stable steroidal hormone (Khripach et al., 2000). For this purpose, pepper plants were sprayed with 24-epibrassinolide solution and the effects of salt exposure for 28 days on growth and photosynthesis parameters were studied. MATERIALS AND METHODS Plant material and experimental conditions
Experiment was carried out at ‘High Institute of Agronomy, Chott Meriam’ (Tunisia) with pepper plants (Capsicum annuum L. cv. Beldi) grown in glasshouse conditions: where the temperature for day/night was 28/20°C and the relative humidity was 60-80%. Uniforms pepper seedlings (true leaves) were transplanted at a rate of one plant per 17 cm plastic pot containing peat. A week after transplantation, the seedlings were irrigated with salt water (70 mM NaCl) and were sprayed with distilled water (control) or 10-6 M 24-epibrassinolide (EBL was dissolved in a minimal volume of ethanol and then made up to volume with distilled water as described in our previous study (Houimli et al. 2008). The plants were sprayed once on the leaves early in the morning. A Hoagland's solution was added weekly to the plants. Growth analysis
Plant growth was determined destructively over two harvests, the first at the time of EBL application (t0 and the second occurred 21 days after EBL application (t2), to examine plant responses to EBL treatment. The leaf area was
measured using a LI-3100 leaf meter (LI-COR. Inc., Lincoln, NE, USA). Plants materials (leaves, stem and roots) were dried in a 70 °C oven for 3 days to determine dry weights. Six individual plants were randomly selected per treatment at each harvest. The relative growth rate (RGR, in mg g-'day-1), the net assimilation rate (NAR, in mg.cm-2 j-') and the leaf area ratio (LAR, in cm2.g-1) of each plant was estimated using the equations:
RGR= (lnW2-lnW:)/ (t2-t:)
NAR= (W2-W1) [(lnA2-lnA1)/(A2-A1)(t2-t1)] LAR=0.5 [(LA1/W1) + (LA/W2)]
Where W1 and A1 are the plant dry weight and total leaf area, respectively, at the initial time (tO, and W2 and A2 are the plant dry weight and total leaf area at the final harvest (2) (Hunt, 1990). Chlorophyll fluorescence
Chlorophyll fluorescence measurements were made in attached leaves with a portable fluorometer F.I.M, 1500, ADC (Fluorescence Induction Monitor 1550, Analytical Development Company Limited). Leaves were dark-adapted for 30 min, then dark fluorescence Fo, maximal fluorescence Fm, and photochemical yield Fv/Fm (Fv = Fm- Fo) were recorded.
Fo is the initial fluorescence emission by antenna Chl a molecules. Fm is the maximum total fluorescence value; Fv = Fm-Fo is the variable fluorescence. The Fv/Fm ratio measures the efficiency of excitation energy captured by open PSII reaction centres representing the maximum capacity of light-dependent charge separation (Krause and Weis, 1991).
Gas exchange
Photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration (E) measurements were made on the upper fourth or fifth fully expanded leaf, between 9:00 h and 11:00h using a portable gas exchange system (CI- 301 CO2 GAS ANALYSER, CID, Inc.). The atmospheric conditions during the experiments were: PAR, 1100±50 ^mol m-2 s-1; Ci, 280±15^mol mol-1; atmospheric CO2, 355±5^mol mol-1; relative humidity, 65±5%; atmospheric temperature, 25±2 °C.
Statistical analysis
Data was subjected to one-way analysis of variance (ANOVA) and the mean differences were compared by Duncan Test. Each value was the mean of six replicates (n= 6) and comparisons with P-values <0.05 were considered significantly different.
Table 1: Evaluation of relative growth rate (RGR), net assimilation rate (NAR) and leaf area ratio (LAR) of pepper plants (var. Beldi) were subjected to foliar application of 24-epibrassinolide (EBL) under control or saline condition.
Treatments RGR (mg g-1j-1) NAR (mg cm-2 j-1) LAR (cm2g-1)
Control 56.2 b 193.2 b 22.21 b
EBL 70.3 a 217.5 a 24.39 ab
NaCl 49.9 c 178.5 c 21.02 b
NaCl +EBL 62.0 b 197.7 b 24.04 ab
Data were reported as means (n=6). Means were separated by Duncan's test, different letters in a single column show statistically significant differences for P<0.05.
Table 2. Chlorophyll fluorescence parameters (F0, Fm and Fv/Fm) of pepper plants (var. Beldi) were subjected to foliar application of 24-epibrassinolide (EBL) under control or saline condition.
Treatments Fo Fm Fv Fv/Fm
Control 289 a 1596 a 1307 a 0.819 a
EBL 288 a 1540 a 1252 a 0.813 a
NaCl 291 a 1582 a 1291 a 0.816 a
NaCl +EBL 281 a 1509 a 1228 a 0.814 a
Data were reported as means (n=6). Means were separated by Duncan's test, different letters in a single column show statistically significant differences for P<0.05.
Table 3. Effects of 24-epibrassinolide (10-6 M) and NaCl-stress (70mM) on the photosynthetic rate (Pn, ^mol CO2 m-2.s-1), stomatal conductance (Gs, mmol m-2.s-1), transpiration (E, mmol H2O m-s-1) and water use efficiency (WUE, ^mol CO2/mmol H2O) in pepper leaves.
Treatments Pn E Gs WUE
Control 14.2 b 3.6 a 301 b 4.0 ab
EBL 16.3 a 3.8 a 334 a 4.3 a
NaCl 9.6 d 2.9 b 229 d 3.3 bc
NaCl +EBL 12.3 c 3.3 ab 279 c 3.7 b
Data were reported as means (n=6). Means were separated by Duncan's test, different letters in a single column show statistically significant differences for P<0.05.
RESULTS Growth analysis
Application of 70 mM NaCl to pepper plants adversely influenced relative growth rate (RGR) as compared with control plants (Table 1). Analysis of RGR components shows that this inhibition is due to a significant reduction of net assimilation rate (NAR) and not to the leaf expansion (LAR). When plants grown in the presence or absence of sodium chloride are subjected to EBL treatment, RGR was significantly improved compared with control plants (Table 1). This increase is accompanied by significant increases in the NAR and unchanged for the LAR.
Chlorophyll fluorescence
The changes in PSII photochemistry were investigated in the dark-adapted leaves. Table 2 shows that neither NaCl nor EBL application affected the minimal Chl a fluorescence, Fo, the maximal Chl a fluorescence, Fm and therefore the
maximum quantum efficiency of PSII photochemistry (Fv/Fm).
Gas exchange
The net assimilation (Pn), transpiration (E) and stomatal conductance (Gs) were significantly (p<0.05) declined in the plants, exposed to NaCl-stress (Table 3). It appears from our results that the decline in net photosynthesis (Pn) is mainly attributed to the decrease in stomatal conductance, but not to the damage of the photosynthetic apparatus, since the activity of PSII (estimated by the ratio of Fv/Fm) was preserved in the dark adapted leaves under NaCl-stress (Table 2).
Treatment with EBR has a positive effect on the photosynthetic activity of control and NaCl-stressed plants, which is associated with a parallel positive effect on stomatal conductance. In relative terms, the impact of EBR on carbon fixation is greater than the impact on stomatal conductance (and thus the leaf
transpiration); the result is a positive effect of EBR on plant water use efficiency (Table 3). DISCUSSION
The growth analysis showed that 70 mM NaCl treatment induced a significant reduction in relative growth rate (RGR) of pepper plants as compared to the control (Table 1). This is consistent with previous reports that pepper plants are relatively sensitive to salt stress (Chartzoulakis and Klapaki, 2000, Lycoskoufis et al, 2005; Huez-Lopez et al., 2011). This reduction may be the result of slower leaf expansion (LAR) (Cramer et al., 1994) or the reduction of photosynthetic activity (NAR) (Bayuelo-Jimenez et al., 2003). Our results show that inhibition of biosynthetic activity (RGR) was estimated by a decline in photosynthesis (NAR) rather than a reduction in leaf area (LAR). This suggests that photosynthesis is the limiting factor for growth. Changes in CO2 assimilation may be attributable to either stomatal or non-stomatal factors or both (Bethke and Drew 1992). Our data show that the presence of salt in the root medium caused a reduction in stomatal conductance (Gs) of pepper plants (Table 2). Similar results were obtained by Martinez-Ballesta et al (2004) and Lycoskoufis et al (2005), where Gs was reduced by salt treatments. A decrease in Gs was observed for the NaCl treatments, probably caused by closure of the stomata or a decrease in water uptake through the roots (Martinez-Ballesta et al., 2004). The decrease in water flow by salinity may cause lowering in leaf water content that would result in stomatal closure in order to maintain their water status (Robinson et al., 1997). The stomtal closure is likely the first plant defense against desiccation and an important factor to control carbon fixation (Da Silva et al., 2008). Measurements of Chl a fluorescence parameters were attempted to evaluate
the direct effects of salt stress on PSII photochemistry. Results revealed that NaCl treatment have no effect on leaf photochemistry, and photosystem II (PSII) remained resistant to salt stress as reported by Belkhodja et al (1999) and Zribi et al (2009), they showed no significant change in the photosynthetic quantum yield (Fv/Fm) in response to NaCl treatments. It seems from the results that the decline in net photosynthesis (Pn) may result from a reduced stomatal conductance, but not to injuries to the photosynthetic apparatus, since PSII activity (estimated by the ratio Fv/Fm) of dark-adapted leaves were preserved under salt stress (Table 2). Parallel decreases in stomatal conductance and net photosynthesis due to NaCl salinity have been reported for pepper plants (Chartzoulakis and Klapaki, 2000; De Pascale et al., 2003; Martinez-Ballesta et al., 2004).
Exogenous application of EBL improved the RGR of pepper plants. These results are not surprising since brassinosteroids were implicated in cell elongation and differentiation (Hu et al., 2000). However, the increase in RGR was attributed to the increase in the physiological growth parameter (NAR) rather than the morphological growth parameter (LAR).Thus suggests that EBL-induced salt tolerance in pepper plant. Since salinity inhibits plant growth by adversely affecting various physiological and biochemical processes including photosynthesis, antioxidant capacity, and ion homeostasis (Ashraf, 2004), it is suggested that BRs enhanced growth of salt stressed plants might have been due to BRs-induced changes in these physiological or biochemical processes (Ozdemir et al., 2004; Ali et al., 2007; Ali et al., 2008; Hayat et al., 2010). For example, Ali et al (2008) reported that EBL increased growth in Indian mustard plants could be related to enhanced activity of antioxidative
enzymes and proline level that protect the plants from oxidative damage. Therefore, photosynthesis which is a major controlling factor for plant growth and yield might have been increased due to EBL application (Yu et al., 2004). In this study, foliar application of EBL enhanced CO2 assimilation rate in pepper plants. Therefore, the promotion of growth in pepper plants by EBL under NaCl-stress appears at least correlatively linked with enhanced photosynthesis. Increased growth and CO2 assimilation following the application of BRs under salt stress have been reported in case the mustard (Hayat et al., 2007; Ali et al., 2008), and mung bean plant (Hayat et al., 2010). EBL activated the rate of photosynthesis may be by increasing stomatal conductance (Table 3), facilitating the diffusion of carbon dioxide into the stomatal cavity (Fariduddin et al., 2006). However, EBL has no effect on quantum efficiency (Table 2). Similarly, no significant changes of Fv/Fm value in wheat, grown hydroponically in EBL-containing medium (Ali et al., 2008) or foliar sprayed (Shahbaz et al., 2008). Yu et al (2004) noted that EBL significantly increased the light saturated net CO2 assimilation rate from 3 h to7 d after spraying with 0.1 mg l-1 EBL in Cucumis sativus. Increased CO2 assimilation rate in EBL treated leaves was accompanied by a higher quantum yield of PSII electron transport, mainly due to a significant increase in the photochemical quenching and unchanged efficiency of energy capture by open PSII reaction centers (Yu et al., 2004). Moreover, BRs had a positive effect on the activation of Rubisco based on increased maximum Rubisco carboxylation rates (Vc,max), total Rubisco activity and, to a greater extent, initial Rubisco activity induced by an enhanced expression of genes encoding other Calvin cycle genes after BRs treatment might also play a positive role in
RuBP regeneration/(/max), thereby increasing maximum carboxylation rate of Rubisco (Vc>max). Thus, BRs promote photosynthesis by positively regulating synthesis and activation of a variety of photosynthetic enzymes including Rubisco (Xia et al, 2009).
In conclusion, the foliar spray of 24-epibrassinolide has a positive effect on growth and plant metabolism. Indeed, it reduces some disruption caused by NaCl-stress: attenuation of growth inhibition and stimulation of photosynthetic characteristics of pepper under control or saline conditions. These results suggest a potential use of 24-epibrassinolide which to be confirmed by other experiments and field trials.
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