Dandelion (Taraxacum officinale Wigg.) seed reproduction indices such as the total number of seeds, the number of normally developed seeds and underdeveloped seeds per anthodium, and seed weight are suggested to assess the level of environmental pollution (bioindication). However, the non-monotonic dose-response dependences (hormesis and paradoxical effects) of these indices are insufficiently explored upon exposure to pollution. We studied the dependence of some T. officinale seed reproduction indices on intensity of motor traffic pollution in wide range of values over 2 years of observation. In 2010, the increase in traffic intensity induced a monotonic increase in the total seed number and the number of normally developed seeds. Besides, motor traffic pollution decreased the number of undeveloped seeds and seed weight in comparison with the control. In 2011, for all studied T. officinale indices except seed weight, complicated non-monotonic dependences on traffic intensity were found that could be attributed to paradoxical effects. It is hypothesised that the significant differences in the studied dependencies in 2010–2011 were caused by changes in weather conditions because traffic intensity did not differ significantly between the two observation years.
At the present time, evidence has accumulated in toxicology that, apart from classical monotonic dose-response dependences (S-shaped, exponential), non-monotonic responses, which include hormesis (Cedergreen et al. 2007; Calabrese and Blain 2009) and paradoxical effects (Schatz 1999; Batyan et al. 2009; Smith et al. 2012), are also found rather often. Hormesis is a biphasic dose-response phenomenon characterised by low-dose stimulation and high-dose inhibition (Calabrese 2008). It is known that the manifestation of paradoxical effects consists of the following: as the dose or concentration of the toxic agent is reduced, its toxicity increases, and vice versa, such that with an increase in the dose, the toxic effect is reduced (Schatz et al. 1964; Batyan et al. 2009). Therefore, it is necessary to study the reproductive capacity of different plant species under the action of anthropogenic stress in a wide range of values to determine the pattern of this dependence.
Dandelion (Taraxacum officinale Wigg.) seed reproduction indices are recommended by some authors for assessing of the level of environmental pollution (for bioindication). This species often grows in urbanised territories near roadsides with different intensities of traffic. At the same time, the pattern of the dependence of T. officinale seed reproduction on the contamination level has not been studied. Therefore, this species was selected for this study. The aim of this study was to investigate the dependence of some dandelion (T. officinale) seed reproduction indices on intensity of motor traffic pollution in a wide range of values over 2 years of observation.
MATERIALS AND METHODS
Study area and study plots
In the last decade of May in 2010–2011, T. officinale seeds were collected in 9–10 coenopopulations located at a distance of 1–4 m from the roads with different levels of pollution in the upland part of the city of Nizhni Novgorod, Russia (Table 1). Motor traffic is a major source of pollution in this part of the city. The coenopopulation locations were chosen so that traffic intensity varied by a wide range, with the minimum and maximum values differing by a factor of several tens. In 2010, 10 contaminated plots of T. officinale coenopopulations were studied. In 2011, there were only nine such plots because the majority of the coenopopulation was destroyed by the road-building on Gagarina Prospect (Surikova Street bus stop; Table 1). Control plot was located in Forest Park Shchelkovsky Farm in the upland part of the city. Control plot was located far from highways and other pollution sources. All plots were characterised by similar soil conditions (light grey forest soils with anthropogenically mixed upper horizons) and a normal moistening regime.
Traffic intensity in study plots of the city of Nizhni Novgorod where T. officinale grew.
No.
Study plots
Traffic intensity in 2010, vehicles per hour
Traffic intensity in 2011, vehicles per hour
1
Ashhabadskya Street
81
60
2
Nevzorovih Street
453
261
3
Pushkina Street
687
531
4
Nartova Street
795
822
5
Meditzinskay Street
909
942
6
Timiryazeva Street
933
1,068
7
Belinskogo Street
1,923
2,019
8
Rodionova Street
2,484
2,805
9
Gagarina Prospect (Surikova Street bus stop)
3,606
−
10
Gagarina Prospect (University bus stop)
4,524
4,731
Estimation of motor traffic pollution
Motor traffic pollution was estimated by the traffic intensity (vehicles per hour). The traffic intensity was a median of vehicles per hour, counted three times on a weekday: in the morning (from 8 until 10); in the afternoon (from 12 until 15); and in the evening (from 17 until 19) (Ruzskiy et al. 2008). We previously demonstrated that the traffic intensity was correlated with the content of the main pollutants (oxides of sulphur, nitrogen, carbon, benzine, kerosene, benzo[a]pyrene, and formaldehyde) in the air along highways in Nizhni Novgorod (r = 0.8−0.9; p < 0.05).
Estimation of seed reproduction parameters
We studied seed reproduction in middle-aged generative plants of T. officinale. All studied coenopopulations of T. officinale were located in unshaded areas and were not trampled. The density of plant location did not differ significantly in the studied coenopopulations. In each coeno-population, eight test plots (1 × 1 m) were studied. They were equally spaced within the coenopopulation. On each test plot, five anthodiums were collected from five different plants. Thus, 40 anthodiums were collected from 40 different plants in each coenopopulation (8 test plots × 5 anthodiums; n=40). We studied the following parameters of T. officinale seed reproduction: the total number of seeds per anthodium; the number of normally developed seeds (full seeds) and underdeveloped seeds (empty seeds) per anthodium; and the weight of 50 normally developed seeds (Savinov 1998). From anthodium of each plant in these coenopopulations, 50 normally developed seeds were randomly selected and weighed to within 1 mg to study the last parameter.
Statistical analysis
Statistical analyses were carried out using the programs Statistica 6.0. and Primer of Biostatistics 4.03. Regression analysis was used to evaluate the dependence of studied parameters on traffic intensity. One or two points of some dependences were outside the 95% confidence interval of values; therefore, they were excluded from regression analysis. Exclusion of such points from regression analysis is an accepted procedure in statistics (Glantz 2005).
Parametric criteria were used since the Shapiro-Wilk's criterion showed that the distribution in all samplings did not differ from the Gaussian distribution. One-way ANOVA and parametric Newman-Keuls test were used for multiple comparisons of studied parameters. Sampling means with standard errors were used for graphical data presentation.
RESULTS
Dependence of T. officinale seed reproduction indices on intensity of motor traffic pollution in 2010
Regression analysis showed that the studied T. officinale indices depended on traffic intensity in 2010 (p<0.05) (Figs. 1a–4a). Dose-response dependences of the total number of seeds and number of normally developed seeds were similar. An increase in traffic intensity induced an increase in these parameters in comparison with control (by 25.3% and 28.3%, respectively; Figs. 1a–2a). Normally developed seeds constituted an overwhelming majority per anthodium; therefore, their dose-response dependence was similar to that of the total seed number. This fact was confirmed by the positive correlation between these parameters (Table 2). At the same time, an increase in traffic caused a decrease in seed weight in comparison with control (not more than 25%; Fig. 4a). The number of underdeveloped seeds was also decreased under the action of traffic intensity, to not more than 75% in comparison with the control level (Fig. 3a). It should be emphasised that other authors also found an increase in the total number of T. officinale seeds and the number of normally developed seeds and a decrease in seed weight upon exposure to chemical pollution (Savinov 1998).
Spearman's correlation between the studied Taraxacum officinale parameters in 2010–2011.
Studied parameters
Total number of seeds per anthodium
Number of normally developed seeds per anthodium
Number of undeveloped seeds per anthodium
Number of normally developed seeds per anthodium
r=0.98; p=0.001r=0.98; p=0.001
−
−
Number of undeveloped seeds per anthodium
r=0.41; p=0.230r=0.72; p=0.048
r=0.30; p=0.390r=0.72; p=0.048
−
Weight of 50 seeds
r= −0.77; p=0.019r=0.18; p=0.610
r= −0.70; p=0.038r=0.08; p=0.810
r= −0.31; p=0.390r= −0.38; p=0.330
The data obtained in 2010 are presented in the numerator; the data obtained in 2011 are presented in the denominator. Statistically significant correlation coefficients (p<0.05) are shown in bold.
Dependence of total number of seeds per Taraxacum officinale anthodium on motor traffic intensity in 2010 (a) and in 2011 (b) (n=40). * indicates a significant difference in this parameter between plants growing in contaminated and control (0 vehicles per hour) plots at p<0.05. # indicates a significant difference between the minimum of the dependence (822 vehicles per hour) and other points at p<0.05.
Dependence of the number of normally developed seeds per Taraxacum officinale anthodium on motor traffic intensity in 2010 (a) and in 2011 (b) (n=40). * indicates a significant difference in this parameter between plants growing in contaminated and control (0 vehicles per hour) plots at p<0.05. # indicates a significant difference between the minimum of the dependence (822 vehicles per hour) and other points at p<0.05.
Dependence of the number of undeveloped seeds per Taraxacum officinale anthodium on motor traffic intensity in 2010 (a) and in 2011 (b) (n=40). * indicates a significant difference in this parameter between plants growing in contaminated and control (0 vehicles per hour) plots at p<0.05. # indicates a significant difference between the maximum (2019 vehicles per hour) of the dependence and other points at p<0.05.
Dependence of weight of 50 Taraxacum officinale seeds on motor traffic intensity in 2010 (a) and in 2011 (b) (n=40). * indicates a significant difference in this parameter between plants growing in contaminated and control (0 vehicles per hour) plots at p<0.05.
Dependence of T. officinale seed reproduction indices on intensity of motor traffic pollution in 2011
In 2011, dependences of all studied parameters of T. officinale on traffic intensity were identified (Figs. 1b–3b), except for the weight of 50 seeds (Fig. 4b). In 2011, similar dose-response dependences were identified for the total number of seeds and the number of normally developed seeds. This fact was confirmed by the positive correlation between these parameters (Table 2). However, the pattern of these dose-response dependences differed significantly in 2010 and in 2011. In 2011, an increase in traffic (to 531–942 vehicles per hour) induced a decrease in the total number of seeds and the number of normally developed seeds (by 21.0% and 23.2%, respectively). A further increase in traffic (to 2805 vehicles per hour), on the contrary, caused an increase in these indices in comparison with the control (by 13.5–14.5%) and with the minimum of the curve (by 44.3–47.8%). Finally, the highest traffic intensity (4731 vehicles per hour) led to a significant decrease in these parameters (by 44.3–47.8%) in comparison with the control level (Figs. 1b–2b).
The dose-response dependence of the number of underdeveloped seeds was biphasic. In the first phase, motor traffic load led to an increase in this parameter (to 180% of the control level); in the second phase, it decreased the parameter to the control level (Fig. 3b). The number of underdeveloped seeds had a positive correlation with the total number of seeds (Table 2).
DISCUSSION
Thus, dose-response dependences of the studied T. officinale parameters differed significantly in 2010 and 2011. In 2010, all identified dependences were monotonic but they had a non-monotonic pattern in 2011.
The dose-response dependences identified in 2011 can be attributed to paradoxical effects, because the low traffic intensity disturbed seed reproduction of T. officinale (it decreased the total number of seeds and number of normally developed seeds and increased the number of underdeveloped seeds). At the same time, higher traffic intensity induced more efficient T. officinale seed reproduction — the total seed number and number of normally developed seeds were increased and number of underdeveloped seeds was decreased in comparison with control (Figs. 1b–3b).
In 2011, a change of dose-response dependences apparently induced a change of correlations between some of the studied T. officinale parameters in comparison with the correlations obtained in 2010 (Table 2). For example, in 2011 the number of underdeveloped seeds had a positive correlation with the total number of seeds, because dose-response dependences of these parameters were similar. However, in 2010 dose-response dependences of these parameters differed significantly, so we did not find a correlation between them.
Why did dose-response dependences of the studied T. officinale parameters differ significantly in 2010 and 2011? Traffic intensity did not differ significantly in the plots in 2010 and 2011 (Table 1). This fact is confirmed by a functional positive correlation between traffic intensity in 2010 and 2011 (Spearman's correlation: r=1; p=0). Therefore, the observed difference of dose-response dependences was not induced by a change of traffic intensity in the two years of observation.
It has been shown that apart from chemical pollution, the weather conditions of the growing season have a significant effect on seed reproduction of T. officinale (Zhuikova et al. 2002). The combination of weather conditions and anthropogenic impact ultimately determines the state of T. officinale coenopopulations in urban ecosystems, including seed reproduction.
In May 2010, weather conditions were less favourable for T. officinale seed reproduction than those in the same month of 2011, since a lack of atmospheric precipitation (49% below the norm) was accompanied by a significantly increased average monthly air temperature (above the norm by 4.3 °C). In May 2011, the amount of precipitation was also below the norm, but the average monthly air temperature did not differ significantly from normal values.
In 2010, the total number of seeds and number of normally developed seeds were lower (p<0.05) in almost all studied coenopopulations of dandelion in comparison with the same parameters of coenopopulations in 2011, which also indicates more favourable weather conditions for dandelion seed reproduction in 2011.
Earlier, we found that a monotonic change of various physiological and biochemical parameters in seed plants was mainly observed upon exposure to sublethal doses of different pollutants (Erofeeva 2014). Similar results were obtained by other authors for animals and other biological objects (Calabrese 2008).
Apparently, in 2010 the combination of unfavorable weather conditions and chemical pollution created conditions close to the extreme values for the development of a dandelion. This induced a monotonic adaptive increase in seed production, which was accompanied by a decrease in seed quality (seed weight). It is believed that such redundancy of seed number is necessary to compensate for losses of descendants in subsequent stages of seed germination, seedling formation, etc. (Zhuikova et al. 2002). In 2011, the combination of more favourable weather conditions and motor traffic pollution induced paradoxical dose-response dependences for the seed number of dandelion, with no influence on seed weight.
At present, the causes of a non-monotonic response in living organisms are insufficiently explored. At the same time, it was shown that at different phases of non-monotonic dose-response dependence, an agent can act on different receptor types (subtypes) or different cell signalling pathways that induce the non-monotonic pattern of the dependence (Calabrese 2013). Previously, to explain this phenomenon, we proposed a hypothesis (Erofeeva et al. 2011) that was based on the concept of the gradual involvement of different adaptive mechanisms in the process of phenotypic adaptation to a factor and the existence of several regimes (levels) of functioning in living organisms (Veselova et al. 1993; Garkavi et al. 1998).
We can draw the following conclusions based on this study:
The studied T. officinale parameters had different types of dose-response dependences on exposure to motor traffic pollution in 2010 and 2011.
In 2010, an increase in traffic intensity induced a monotonic increase in total seed number and number of normally developed seeds in comparison with the control. At the same time, seed weight and number of undeveloped seeds decrease monotonically relative to the control level.
In 2011, an increase in traffic intensity induced a paradoxical effect for total seed number, number of normally developed seeds, and number of undeveloped seeds. Seed weight did not depend on traffic intensity.
Footnotes
ACKNOWLEDGMENTS
Three anonymous reviewers gave valuable comments on earlier versions of the manuscript.
