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Acta Physiologiae Plantarum

, 37:1709 | Cite as

Salt stress induced sex-related spatial heterogeneity of gas exchange rates over the leaf surface in Populus cathayana Rehd.

  • Xiao Xu
  • Yunxiang Li
  • Bixia Wang
  • Jinyao Hu
  • Yongmei Liao
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Abstract

The sex-related spatial heterogeneity of gas exchange rates over the leaf surface under salt stress was investigated in the dioecious species, Populus cathayana Rehd. Cuttings were subjected to two salt regimes: 0 and 75 mM NaCl added to the Hoagland solution, the control and the treatment group, respectively. Measurements of gas exchange parameters were taken from over 40 sites on the surfaces of representative ‘non-stressed’ and ‘salt-treated’ leaves which had the same insertion point for two sexual cuttings. Compared to the control group, the treatment group showed a significant decrease in the mean values of the following: water use efficiency (WUE), Chlorophyll a (Chl a) concentration, chlorophyll b (Chl b) concentration, concentration of carotenoids (Caro), total chlorophyll concentration (TC) in two sexes, and net photosynthesis rate (P n), stomatal conductance (g s), and stomatal length/width ratio (SR) in females. Also, in the treatment group, females exhibited lower WUE, P n, g s, E, Chl a, Chl b, TC, and SR than males. Comparison of contour maps showed that the net photosynthesis rate decreased gradually from apical to basal zones over the leaf surface occurred in the two sexes under natural conditions, but under salt stress, the opposite trend was found in females only. The results suggest that the heterogeneity pattern of the gas exchange parameters in response to salt stress between the two sexes is quite different due to different strategies employed by males and females to maintain the photosynthesis rate under salt stress. This heterogeneity phenomenon under salt stress may mainly be attributed to the chlorophyll pigments in males and the stomatal apertures in females.

Keywords

Chlorophyll pigments Net photosynthesis rate Salt stress Stomatal characters Heterogeneity phenomenon 

Introduction

Since the finding that different stomata on a barley leaf were open to various degrees at the same time was reported in 1980 (Laisk et al. 1980), heterogeneity in gas exchange over the leaf has been acknowledged as an important phenomenon. At larger scales, the heterogeneity of gas exchange parameters is thought to partly depend on variations of stomatal number and size over the plant leaf (Weyers and Lawson 1997; Mott and Buckley 1998). This speculation was compatible with some previous studies that reported stomatal density and stomata length/width ratio (SR) varied from the tip to the base of the leaf, and was different between the edge and the center of the plant leaf (Tichá 1982; Croxdale et al. 1992; Pospíšilová and Šantrůček 1994; Weyers and Lawson 1997). At smaller scales, heterogeneity in the gas exchange parameters was observed to be related to non-uniform stomata behavior (Mott and Buckley 1998, 2000), especially patchy stomatal closure (Terashima 1992).

Conversely, some researchers have shown that the heterogeneity of the gas exchange parameters over the leaf surface can be induced by water stress (Gunasekera and Berkowitz 1992; Downton et al. 1988; Calatayud et al. 2006), air humidity (Loreto and Sharkey 1990; Mott et al. 1993, 1997; Nejad et al. 2006; Mott 2007), abscisic acid concentration (Terashima et al. 1988; Mott 1995), light intensity (Düring and Loveys 1996; Eckstein et al. 1996), and CO2 concentration (Poole et al. 2000; Kamakura and Furukawa 2008). It was believed that environmental factors can cause non-uniform gas exchange by changing stomatal size, stomatal density, and vein pattern (e.g. Smith et al. 1989; Pospíšilová and Šantrůček 1994; Poole et al. 1996, 2000; Mott and Buckley 1998), or by changing the chemical and/or hydraulic signals which trigger stomatal behaviors among adjacent regions (Terashima et al. 1988; Mott 1995, 2007; Mott and Buckley 1998; Mott et al. 1999; Buckley and Mott 2000; Beyschlag and Eckstein 2001; Nardini et al. 2008). However, whether such phenomenon can be induced by salt stress has not been investigated.

In addition to playing an important role in terrestrial ecosystems, dioecious plants were thought to be a consequence of different requirements for disseminating pollen and producing seeds and fruits (Darwin 1877). Some previous studies have explained the sexual differences in gas exchange under stress conditions. For example, water deficit reduced stomatal conductance more in females than in males (Dawson and Bliss 1989; Correia and Diaz Barradas 2000; Rowland 2001; Xu et al. 2008). Salt stress caused less negative effects on the photosynthetic ability in males and reduced the amount of leaf chlorophyll pigments more in females (Chen et al. 2010a). Short-day photoperiod shift significantly decreased gas exchange and chlorophyll pigments in both males and females, whereas males exhibited higher values of net photosynthesis rate, transpiration, stomatal conductance, chlorophyll pigments and carotenoid than females (Zhao et al. 2009). However, few studies have taken into account the sex-related heterogeneity in gas exchange parameters over the leaf in dioecious plants, especially under a stress condition.

Populus cathayana Rehd. is a dioecious, broad-leaved tree species. Because of its high survival rate and fast growth, it was chosen to be used as the model species in this study. To obtain a comprehensive understanding of sex-related spatial heterogeneity in gas exchange parameters induced by salt stress, we examined stomatal density, stomatal width, stomata ratio (length/width), photosynthesis rate and chlorophyll pigment concentration of the leaves between male and female grown under 75 mM NaCl treatment. Since previous studies have reported that photosynthetic capacity response to a stress environment is different between males and females due to sex-specific sensitivity in poplar cuttings (Xu et al. 2008; Chen et al. 2010b), we hypothesized that the heterogeneity pattern in gas exchange parameters over the leaf will be detected, and the pattern responses to salt stress between males and females will be different under a stress environment.

Materials and methods

Plant materials and experimental design

Cuttings of P. cathayana were collected from male and female trees in Nanchong City (106°04′E, 30°48′N), Sichuan province, China. The range of annual rainfall, temperature, and relative humidity in this area are 980–1150 mm, 15.8–17.8 °C and 76.0–86.0 %, respectively. In early spring, after sprouting and growing for about 2 months, healthy cuttings from 24 male and 24 female trees with uniform height were chosen and replanted in 12 L plastic pots (one cutting per pot) containing 10 kg of homogenized soil. The soil is composed of 15.53 % sand, 57.43 % silt, and 27.04 % clay and has 1.09 % of organic carbon. The replanted cuttings were grown under natural conditions.

The experimental layout was completely randomized with two main factors (sex and saline treatment). According to the revised method of Chen et al. (2010a), one liter of full-strength Hoagland solution was manually applied every week containing either 0 or 75 mM NaCl. The plants were watered to excess and were allowed excess solution drained into dishes placed under the pots. The dishes were kept at a constant water level by replacing water taken up by the plant on a daily basis. Four replications, with three cuttings in each replication, were used to minimize random errors. In addition, to quantify the level of stress imposed, soil pH and electric conductivity (EC) were measured with Clever-Chem 200 auto discrete analyzers (Langcheng Industry Co., Ltd., Shenzhen, China) under pre- and post-treatment conditions.

After 30 days of treatment, a fully expanded leaf from each plant was selected for measurement as described by Poole et al. (1996). The surface of the leaf was marked with a sampling grid (composed of 10 × 10 mm2 squares) 6 days before measurement by a neutral water pen, and gas exchange was measured on each square (10 × 10 mm2). Then, the leaf was cut into squares immediately along the sampling gridlines. Each square was divided into two pieces of equal area (5 × 10 mm2) to use for stomatal observation and chlorophyll pigment measurement, respectively.

Gas exchange measurement

Net photosynthesis rate (P n), transpiration rate (E), stomatal conductance (g s) and intercellular CO2 concentration (C i) were measured with a Li-Cor 6400 portable photosynthesis measurement system (Li-Cor Inc., Lincoln, NE, USA) on a cloudy day with relatively constant conditions: air temperature, 26 ± 2 °C; photosynthetic photon flux density (PPFD), 700 ± 100 µmol m−2 s−1; relative air humidity, 60 ± 5 %; and ambient CO2 concentration, 370 ± 10 µmol mol−1. The water use efficiency (WUE) was determined by the ratio of P n to E (Ashraf and Arfan 2005). During the measurement process, the Li-Cor 6400-15 Extended Reach 1 cm Chamber for leaves was employed. This chamber is designed with a 1.0 cm diameter aperture positioned 8.5 cm away from the main body of the infrared gas analyzer (IRGA). Both the top and bottom chambers were covered with clear Propafilm® windows which offer excellent light transmission with very little thermal trapping and minimal water sorption. The measurements were carried out between 8:30 and 10:30 a.m under the following conditions: leaf temperature, 26 ± 0.5 °C; photosynthetic photon flux density (PPFD), 700 ± 50 μmol m−2 s−1; and ambient CO2 concentration, 370 ± 10 μmol mol−1.

Stomatal feature measurement

Each piece (5 × 10 mm2) of leaf was dipped into cell dissociation solution (100 ml 10 % (v/v) chromic acid with 5 ml concentrated nitric acid) for 12 h to remove the mesophyll layer and then rinsed in distilled water. The leaf fragments were subsequently separated into abaxial and adaxial cuticles, and the remains of the mesophyll and vascular tissue were removed with forceps. The abaxial cuticle was stained in 1 % (v/v) methyl green solution for 5 h and mounted on slides with glycerol after the dye solution was rinsed off with deionized water. These slides were examined and photographed with an Olympus DP 71 microscope digital camera (Olympus America Inc., Melville, NY, USA).

To ensure the accuracy of the count of stomata, a systematic sampling strategy was adopted as described by Poole et al. (1996). The Motic Images pro-plus 6.2 software (Motic Instruments Inc. Richmond, BC, Canada) was used to measure number of stomatal cells, stomatal length (SL), and stomatal width (SW). Stomatal density (SD) and stomatal ratio (SR, the length divided by the width) were then calculated.

Chlorophyll pigment measurement

Chlorophyll was extracted using equal volumes of acetone and alcohol and was quantified by a UV/VIS spectrometer UV-574 (Shanghai APL Instrument Co. Ltd, Shanghai, China). Chlorophyll a (Chl a) absorbance was determined at 663 nm, chlorophyll b (Chl b) absorbance at 646 nm, and absorbance of carotenoids was determined at 470 nm. The absorbance values were converted to concentrations as described by Lichtenthaler (1987). The content of chlorophyll was expressed per unit leaf dry weight (g).

Mapping and statistical analysis

Two-dimensional contour maps illustrating net photosynthesis rate over the leaf surface were generated by using the Kriging interpolation option in the ArcView GIS program (ESRI, Redlands, CA, USA). The input data consisted of sets of x, y, and z values. The x and y coordinates represented the spatial location of the midpoint of the sampling square, and the value of z represented the mean net photosynthesis rate of four replications. All maps represent the view from above the leaf.

Data analyses were performed using SPSS Version 16.0 (SPSS Inc, Chicago, IL). An independent samples t test was used to determine the statistical significance of differences between the means, and a two-way analysis of variance (ANOVA) test was used to evaluate the interaction effect of sex and saline treatment. Pearson’s correlation coefficients were calculated to determine the relationships between the variables for different sexes under the saline treatment. Differences were considered significant at P < 0.05.

Results

Characterization of leaves and soil properties according to sex

Gas exchange, water use efficiency, chlorophyll pigments and stomatal characteristics of leaves were affected by the saline treatment in both male and female cuttings of P. cathayana (Table 1). Compared to the control, the saline treatment significantly decreased WUE, Chl a, Chl b, Caro and TC in two sexes, and decreased P n, g s and SR only in females. Females exhibited lower P n, g s, WUE, Chl a, Chl b, Caro, TC, SR, but higher SD than that of males under salt stress (Table 1). On the other hand, the saline treatment significantly increased soil pH and EC in two sex groups, and female group had higher soil pH and EC than that of male group (Table 1). In addition, P n, g s, E, Chl a, Chl b, TC, SR, pH, and EC were significantly affected by the interaction of sex and saline treatment (Table 1).
Table 1

Mean net photosynthesis rate (P n), stomatal conductance (g s), transpiration (E), intercellular CO2 concentration (C i), water use efficiency (WUE), chlorophyll a concentration (Chl a), chlorophyll b concentration (Chl b), carotenoid concentration (Caro), total chlorophyll concentration (TC), stomatal length/width ratio (SR), stomatal density (SD), soil pH, and soil electric conductivity (EC) in female and male groups of Populus cathayana as affected by salinity treatment (control or 75 mM NaCl)

Character

Female

Male

P > F S×T

Control

NaCl (75 mM)

P

Control

NaCl (75 mM)

P

P n (μmol m−2 s−1)

4.83 ± 0.39

2.73 ± 0.26

<0.001***

2.96 ± 0.17

5.22 ± 0.30

<0.001***

<0.001***

g s (mol m−2 s−1)

0.23 ± 0.04

0.14 ± 0.02

0.046*

0.11 ± 0.01

0.15 ± 0.02

0.106 ns

0.011*

E (mmol m−2 s−1)

3.50 ± 0.69

3.46 ± 0.47

0.962 ns

1.66 ± 0.23

4.06 ± 0.50

<0.001***

0.018*

C i (μmol mol−1)

303.5 ± 8.5

298.5 ± 6.0

0.631 ns

295.3 ± 7.8

279.5 ± 5.6

0.105 ns

0.448 ns

WUE (mmol mmol−1)

2.38 ± 0.35

1.11 ± 0.15

0.002**

2.29 ± 0.24

1.60 ± 0.15

0.019 *

0.223 ns

Chl a (mg g−1DW)

11.36 ± 0.60

4.42 ± 0.16

<0.001***

10.25 ± 0.30

6.73 ± 0.17

<0.001***

<0.001***

Chl b (mg g−1DW)

2.83 ± 0.04

1.29 ± 0.08

<0.001***

3.12 ± 0.15

2.16 ± 0.05

<0.001***

0.002**

Caro (mg g−1DW)

3.09 ± 0.27

1.19 ± 0.04

<0.001***

2.89 ± 0.22

1.64 ± 0.05

<0.001***

0.062 ns

TC (mg g−1DW)

14.19 ± 0.60

5.69 ± 0.25

<0.001***

13.37 ± 0.36

8.89 ± 0.21

<0.001***

<0.001***

SR (length/width)

1.65 ± 0.03

1.53 ± 0.02

0.001***

1.66 ± 0.02

1.74 ± 0.03

0.011*

<0.001***

SD (mm2)

168.0 ± 7.1

169.6 ± 4.0

0.849 ns

147.3 ± 2.6

139.4 ± 3.6

0.088 ns

0.314 ns

pH

8.10 ± 0.05

8.58 ± 0.03

<0.001***

8.11 ± 0.03

8.38 ± 0.06

0.004**

0.033*

EC (ms cm−1)

157.4 ± 9.99

545.0 ± 21.8

<0.001***

159.4 ± 6.21

412.5 ± 11.8

<0.001***

<0.001***

Values are mean ± SE (n = 4). Independent-samples T test was used to determine the significance of differences between two treatments for each sex, and two-way analysis of variance (ANOVA) was used to evaluate the interaction effect of sex and saline treatment (F S×T, sex × salinity effect). The significant values of analyses are denoted as: ns, not significant

* P < 0.05; ** P < 0.01; and *** P < 0.001

Contour maps of P n according to sex

Leaf size of P. cathayana cuttings was reduced by saline treatment, and the different spatial patterns of P n between the two sexes were detected (Fig. 1). Although the P n of males was lower than that of females in most regions in the control group, P n was observed to decrease gradually from apical to basal zones over the leaf surface in the two sexes (Fig. 1a and c). However, different patterns of P n were found in the saline treatment group. The P n of females was found to increase from apical to basal zones over the leaf surface, but this trend was not observed in males (Fig. 1b, d).
Fig. 1

Maps of net photosynthesis rate (P n) over the surface of male and female P. cathayana leaves subjected to 0 or 75 mM NaCl solution. Each value on the maps represents the mean P n of four replicate plants on the same site

Gas exchange in different regions of leaf according to sex

Saline treatment changed gas exchange parameters and induced differences on the leaf surface of both male and female cuttings (Fig. 2). For the control group, there were no significant sexual differences in the gas exchange parameters in all regions on the leaf (Fig. 2). However, under the saline treatment, males had higher P n than females in apical, middle, and left margin zones (P < 0.01 and P < 0.05, respectively). Moreover, no significant differences in g s, E, and C i between the two sexes in these regions were detected (Fig. 2).
Fig. 2

a Net photosynthesis rate (P n), b stomatal conductance (g s), c transpiration (E) and d intercellular CO2 concentration (C i) in females (open bars) and males (black bars) of P. cathayana cuttings subjected to 0 or 75 mM NaCl solution. Data are means + SE of n = 4 for five regions on the leaf (A apical zones; B middle zones; C basal zones; D left margin zones; E right margin zones). Asterisks above bars denote statistically significant differences between the sexes at the P < 0.05 according to the independent-samples t test. *P < 0.05; **P < 0.01; and ***P < 0.001

Chlorophyll pigments in different regions of the leaf according to sex

Compared with the control, the saline treatment induced sexual differences in carotenoid and chlorophyll pigment concentrations in most regions of the leaves (Fig. 3). Under the saline treatment, males had higher Chl b and TC in all five zones, and had a higher Caro in the apical, basal and left margin zones than females. Compared with males, females had higher Chl a/b in middle zones under the salt stress. However, there were no significant differences between the two sexes in these regions in the control group (Fig. 3).
Fig. 3

a Chlorophyll b concentration (Chl b), b carotenoid concentration (Caro) and c total chlorophyll concentration (TC), and d Chlorophyll a/b ratio in females (open bars) and males (black bars) of P. cathayana cuttings subjected to 0 or 75 mM NaCl solution. Data are means + SE of n = 4 for five regions on the leaf (A apical zones; B middle zones; C basal zones; D left margin zones; E right margin zones). Asterisks above bars denote statistically significant differences between the sexes at the P < 0.05 according to the independent samples t test. *P < 0.05; **P < 0.01; and ***P < 0.001

Stomatal characters in different regions of the leaf according to sex

In the control group, no significant differences in stomatal characters in each region for both males and females were detected on the leaves (Fig. 4). However, under the saline treatment, females had a higher SW over the leaves (except for apical margin zones) and a higher SD in the apical, and left margin zones, whereas males had a higher SR in apical, middle, and left margin zones than that of females (Fig. 4).
Fig. 4

a stomatal width (SW), b stomatal density (SD), and c stomatal length/width ratio (SR) in females (open bars) and males (black bars) of P. cathayana cuttings subjected to 0 or 75 mM NaCl solution. Data are means + SE of n = 4 for five regions on the leaf (A apical zones; B middle zones; C basal zones; D left margin zones; E right margin zones). Asterisks above bars denote statistically significant differences between the sexes at the P < 0.05 according to the independent samples t test. *P < 0.05; **P < 0.01; and ***P < 0.001

Relationships among gas exchange parameters, chlorophyll pigments, and stomatal characters

Under the saline treatment, there was a significant positive correlation between P n and SR, while a negative correlation between P n and C i in females was observed. Conversely, there were significant positive correlations among P n, g s, E and Chl a/b (chlorophyll a/b ratio) in males (Table 2).
Table 2

Correlation coefficients among gas exchange, chlorophyll pigments and stomatal characters of female (upper triangle) and male (lower triangle, italic numbers) P. cathayana cuttings grown under the saline treatment

Properties

P n

G s

E

C i

Chl a/b

Caro

TC

SR

SD

P n

 

−0.049

0.100

−0.457*

−0.356

−0.198

0.099

0.531*

0.005

g s

0.792***

 

0.943***

0.797***

0.556*

−0.110

−0.803***

0.039

−0.245

E

0.780***

0.994***

 

0.756***

0.448

−0.243

−0.806***

0.149

−0.298

C i

0.433

0.846***

0.853***

 

0.641**

−0.023

−0.781***

−0.239

−0.135

Chl a/b

0.603**

0.792***

0.774***

0.677***

 

0.452*

−0.574**

−0.393

0.217

Caro

0.109

0.378

0.380

0.395

0.733***

 

0.325

−0.794***

0.859***

TC

−0.110

0.094

0.098

0.148

0.435

0.881***

 

−0.179

0.323

SR

−0.422

−0.410

−0.455

−0.464

−0.132

0.242

0.301

 

−0.726***

SD

−0.253

0.213

0.260

0.388

0.180

0.357

0.281

0.024

 

P n net photosynthesis rate, g s stomatal conductance, E transpiration, C i intercellular CO2 concentration, Chl a/b chlorophyll a/b ratio, Caro carotenoid concentration, TC total chlorophyll concentration, SR stomatal length/width ratio, SD stomatal density

* 0.01 < P ≤ 0.05; ** 0.001 < P ≤ 0.01; *** P ≤ 0.001

Discussion

In the present study, saline treatment clearly decreased the gas exchange parameters, water use efficiency, chlorophyll pigments concentration, and stomatal length/width ratio in female P. cathayana cuttings (Table 1). Similar results that salinity induced limitation of photosynthetic capacity have been reported in some plants, such as Morus alba, Populus cathayana and Scytonema javanicum (e.g. Agastian et al. 2000; Tang et al. 2007; Yang et al. 2009). However, gas exchange was seen to be incremental in males under saline treatment in the study, which appears to be contrary to the finding of Chen et al. (2010a). We think these inconsistent results are likely to be due to the using of the full-strength Hoagland solution in our experiment. Compared with tap water used in the study by Chen et al. (2010a), the Hoagland solution provides more nutrition for plant growth in our study. Supplementary nutrient elements improves salt tolerance of plant have been reported in pepper, cucumber, strawberry, tomato and melon (Kaya et al. 2001, 2003a, b; Kaya et al. 2007), which was thought as application of fertilizer can reduce the Na+ absorption and accumulation in root and promote selective transport of K+ in leaf (Parida and Das 2005; Shabala and Cuin 2008). Thus, the salt stress experienced by plants in our experiment was actually lower than those plants watered with 75 mM NaCl solution. The phenomenon that lower salt stress will stimulate photosynthetic activity has been reported in some plants; e.g. in species Bruguiera parviflora (Parida et al. 2004), Populus bonatii (Chen et al. 2009b), and Jatropha curcas (Chen et al. 2009a). According to Parida and Das (2005), those differences in response to salt stress are linked to the abilities that facilitate retention and/or acquisition of water, protect chloroplast functions, and maintain ion homeostasis. In our study, net photosynthesis rate was maintained at a higher level and chlorophyll pigments were little affected by saline treatment in males, which indicated that males have greater ability than females to alleviate salt-induced damage on the gas exchange. This presumption had been corroborated by the proteome analysis results of Chen et al. (2011), they found that male P. cathayana cuttings showed a higher abundance and lower degradation of proteins involved in electron transfer, photosystem stabilization and redox homeostasis than females under NaCl treatment, which resulted in more efficient stress responses to alleviate salt stress than females (Chen et al. 2011).

On the other hand, in the control group, the trend of the photosynthesis rate decreased gradually from apical to basal zones over the leaf surface in both males and females (Fig. 1a, c). This result is consistent with the findings of Nardidi et al. (2008) in which Nicotiana tabacum leaf areas near the apex had higher gas exchange rates than near the basal areas because of better supplied water. Except for unevenness of vein density, heterogeneity in gas exchange parameters was reported to be the result of non-homogeneous stomatal distribution (Laisk et al. 1980; Smith et al. 1989; Beyschlag and Pfanz 1990), non-uniform stomata behavior (Downton et al. 1988; Siebke and Weis 1995; Lawson et al. 1998; Mott and Buckley 2000; Prytz et al. 2003; Nejad et al. 2006), and the patch of metabolic limitation (Meyer and Genty 1999; Calatayud et al. 2006). In our experiment, no significant differences in stomatal density, stomata length/width ratio, and chlorophyll pigments concentration were observed along the length of either female or male P. cathayana leaves in the control group, as supported by the data in Figs. 3 and 4. Since Nardini et al. (2008) have reported that the apical zone had higher vein density and hydraulic conductance than basal zone on the leaf, the heterogeneities seen in our study in which photosynthesis decreases gradually from apical to basal zones may be attributed to the different hydraulic mechanisms caused by uneven vein density.

Furthermore, comparison of contour maps showed that spatial patterns of photosynthesis were quite different between males and females under salt stress. Net photosynthesis rate in females was found to be higher from apical to basal zones over the surface; however, this trend was not observed in males (Fig. 1b, d). Salt stress causing a decrease in the photosynthetic CO2 fixation rate has been reported in plants Gossypium hirsutum, Phaseolus vulgaris, Plantago maritima, Populus cathayana, Vitis vinifera (Flanagan and Jeffries 1989; Downton et al. 1990; Parida and Das 2005; Yang et al. 2009); and non-uniform photosynthesis in response to salinity was attributed to the patchy closure of stomata (Downton et al. 1990; Terashima 1992). On the other hand, some studies have reported that males and females exhibited dissimilar photosynthetic capacity, with males having higher photosynthesis rate than females under stress environments, but there were no significant differences between the two sexes under natural conditions (Gehring and Monson 1994; Correia and Diaz Barradas 2000; Li et al. 2004; Xu et al. 2008; Schultz 2009). Hence, we speculated that salt stress may induce females and males to employ different strategies to maintain photosynthesis processes. More, we observed that males had higher chlorophyll pigments, carotenoid concentration, and stomata length/width ratio but lower stomatal density in apical and some margin zones than females; whereas, there were no significant differences between the two sexes in these regions in the control group (Figs. 3, 4). Then, we inferred that the sex-related spatial heterogeneity in gas exchange parameters response to salinity may be related to the differences in the distribution of photosynthetic pigments, stomatal density, and the stomatal length/width ratio over the leaf between males and female, and that this heterogeneous phenomenon under salt stress may be mainly related to the chlorophyll pigments in males, but with the stomatal apertures in females. As expected, a significant positive correlation between P n and Chl a/b in males, while between P n and SR in females was observed in our study (Table 2).

In addition, although sexual differences in spatial heterogeneity of gas exchange parameters over the leaf surface were investigated in our study, the mechanism is still very complex. Except for inherent sexual difference responses to salt stress, this phenomenon could be also motivated by other factors, such as light intensity, leaf temperature, atmospheric CO2 concentration, and even by temporal oscillations of stomatal characteristics (Poole et al. 1996; Prytz et al. 2003; Kamakura and Furukawa 2008). Thus, an advanced experimental approach will be required to determine the reasons for the above findings, and the results presented here lay the foundation for further investigations.

In conclusion, our study showed that males had higher chlorophyll pigments, carotenoid concentration, and stomata length/width ratio, and had a lower stomatal density in apical and left margin zones than females under saline treatment, yet there were no significant differences between the two sexes in the control group. Moreover, a similar trend in that net photosynthesis rate decreased gradually from apical to basal zones over the leaf surface in the two sexes for the control group, but the opposite trend was found only in females under salt stress. Therefore, the heterogeneity pattern of gas exchange in response to salt stress between males and females is quite different, which is a result of different strategies employed by males and females to maintain photosynthesis rate under salt stress.

Author contribution

Xu, X. designed the experiments and wrote the manuscript. Li, Y. and Wang, B. performed the experiments. Hu, J. provided the plant materials. Whole exploratory analysis was conducted by Liao, Y. All authors discussed the results and implications and commented on the manuscript at all stages.

Notes

Acknowledgments

The research was supported by the National Natural Science Foundation of China (No. 31170389 and 30771721) and the Dr Start-up Foundation of China West Normal University (No. 08B074).

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Copyright information

© Franciszek Górski Institute of Plant Physiology, Polish Academy of Sciences, Kraków 2014

Authors and Affiliations

  • Xiao Xu
    • 1
    • 2
  • Yunxiang Li
    • 1
  • Bixia Wang
    • 1
  • Jinyao Hu
    • 3
  • Yongmei Liao
    • 1
  1. 1.Key Laboratory of Southwest China Wildlife Resources ConservationMinistry of EducationNanchongChina
  2. 2.Institute of Rare Animals and PlantsChina West Normal UniversityNanchongChina
  3. 3.College of Life Science and BiotechnologyMianyang Normal UniversityMianyangChina

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