Taraxacum officinale ( Asteraceae ) in the urban environment : seasonal fluctuations of plant traits and their relationship with meteorological factors

Plants can be used as effective bioindicators of the quality of the urban habitat. In this study, physiological traits were examined in plants growing outdoors, in the proximity of a road. All material was collected from robust, well-grown plants. All measurements were performed during the generative phase. Here, the evaluations of some gravimetric parameters (fresh weight, turgid weight, dry weight, water content, ash content) and physiological parameters (initial water content, mineral content, organic content, organic content/mineral content ratio, succulence, mineral deposition in tissues, tissues density, leaf relative water content, specific leaf area) were calculated for Taraxacum officinale from urban unmanaged areas, across the different seasons (winter, spring, summer, fall). One-way analysis of variance (ANOVA) was used to compare these traits in leaves, scapes, and roots. Initial water content and relative water content are good indicators of water status in T. officinale. With regard to succulence, its level was generally considerably lower in roots than in aboveground organs. The mineral content differed significantly among plant organs. For all the parameters analyzed, the most considerable seasonal differences were found in leaves. The Spearman correlations were calculated for the relations between plant traits. Mineral deposition in tissues is the most sensitive parameter, regardless of the organ. The data were subjected to Spearman correlation and linear regression analysis for each physiological parameter and some meteorological factors. The strongest association was generally found with the wind. Variations in the investigated parameters in roots and scapes are more associated with meteorological factors. Taraxacum officinale plants are able to tolerate urban conditions in proximity to a road. Plastic responses to environmental cues make the plant useful in biomonitoring the quality of the urban habitat.


Introduction
The advantages of street vegetation for the quality of the urban environment include the mitigation of the heat island effect, climate regulation [1][2][3], air pollution removal via absorption of gaseous pollutants [4][5][6], storm water management [7], indirect impact on human health [8], beauty and aesthetic value [9].Vegetation cover can also serve as an indicator of the degree of surface shading [10,11].The biomass of aboveground vegetation can serve as an indicator of the conversion of solar energy into plants [12].Street vegetation is subjected to high stress from environmental influences due to its location and poor maintenance.Anthropogenic stressors (such as the number of houses on a street or the level of traffic passing through a neighborhood) reduce the lifespan of many plants in the urban environment [13][14][15].The strength, duration, and rate of progression of stress also have an impact on the physiological responses of a plant, as well as on its developmental stage and sensitivity to different environmental components [16][17][18].
The phenology of the species is dependent on weather patterns that differ in particular climate zones across geographical areas [19].Changes in climatic conditions have the strongest impact upon the annual seasonal dynamics of perennial plants [20,21].It was stated that Taraxacum officinale F. H. Wigg.(common dandelion) may be used as a phenological indicator [19,22].It can grow in a wide range of soil types, wide range of soil pH, it resists drought, and adapts to a wide range of light and shade intensity [23].Taraxacum officinale has been used in a number of regional-scale studies as a biomonitor of environmental pollution in Bulgaria [24], Poland [25], Hungary [26], Italy [27], the Czech Republic [28], and Switzerland [29].
It is known that phenotypic plasticity in T. officinale increases its ability to colonize a wide range of habitats.On the other hand, the vitality of plants is a complex phenomenon resulting from different factors and physiological parameters assessed through various diagnostic methods [30].The gravimetric method, which has been described as optimal with respect to reliability and simplicity, remains widely used [31][32][33][34][35][36].In this study, we used such a method to investigate the hypothesis that some physiological traits of T. officinale differ in their flowering phenophase and express phenotypic plasticity.
Seasonal fluctuations are known for more compounds in T. officinale, but seasonal variations in physiological traits in the conditions of urban stress are not known.Those traits are easily and rapidly screened, and the screening procedures are relatively inexpensive.Such physiological parameters can be used to characterize the habitat.The objective of this study was to investigate variations in physiological traits in plants with increased vitality when T. officinale are grown under inconstant environmental conditions in the urban habitat of the city of Timisoara (Romania).

Study location
The study was performed in 2013 in the city of Timisoara, western Romania.The characteristic vegetation of Timisoara is anthropogenic forest steppe which is typical of the Banat Plain [37].The climate is moderate temperate-continental [38].The main pollutants tend to be airborne particles, sulfur dioxide, nitrogen dioxide, carbon monoxide, ozone, and volatile organic compounds.The principal sources of pollution are industry, transportation, and households.The public transportation system consisting of bus, trolleybus, and tramlines and the complex system of regional transportation, providing road, air, and rail connections to major cities in the country and Europe are the principal sources of pollution in Timisoara [39,40].
February was the driest and cloudiest month of 2013 (with 54% of days being more cloudy than clear).The longest warm spell was from April 11 to May 12, constituting 32 consecutive days with warmer than average temperatures.July had the highest average cloud ceiling, with an average cloud ceiling of 1508 m.The longest dry spell was from July 16 to July 30, constituting 15 consecutive days with no precipitation.September had the largest fraction of cooler than average days, with 70% days with lower than average temperatures.November was characterized with 77% of days with higher than average temperatures.

Plant material and sampling
Taraxacum officinale (Asteraceae) appear to be a good material, simple to identify, easy and inexpensive to collect.To reach full maturity, they require 85 to 95 days.The stems are acaulescent with very short internodes at or below the soil surface [41].
It forms a leaf rosette close to the ground and a taproot.The runcinate-pinnatifid leaves have glabrous to sparsely pubescent lower surfaces.The basal rosette gives rise to numerous glabrous and cylindrical scapes.Each scape bears a single terminal inflorescence of 2-5 cm diameter, composed of up to 250 ligulate yellow florets [23].The development of the inflorescence is quick.Dandelions flower throughout the entire vegetative season, with the peak in spring (April) and fall (September).The taproot can be up to 2-3 cm in diameter and grow up to 1-2 m in length.The lateral roots are distributed relatively regularly along its length.The roots are highly regenerative, capable of producing shoots and roots from very small fragments.The plant may regrow from cut roots due to the presence of callus buds [42].In winter, the root is shorter and comes down into the soil, where it is better protected from unfavorable conditions.The roots remain viable during the winter and are a source of nutrients which facilitate the resumption of growth in early spring.
Plants were collected during the flowering stage.The samples were taken from plants in full light.The soil had low aeration, porosity, and drainage due to compaction [43].It was contaminated with pollutants associated with demolition of buildings and vehicle emissions [44].As consistently as possible, plants were of the same size and had leaves of the same form.All plant material was collected during winter (late February), spring (late April), summer (mid-July), and fall (mid-September and early November).We collected the samples during the late morning.Immediately after cutting, plants were sealed within plastic bags and quickly transferred to the laboratory.The plants were dug from the earth as a whole, so as not to damage the root system.We did not wash the leaves before the analysis.We selected juvenile leaves as well as fully expanded and hardened leaves from adult plants.Leaf age classes were defined based on preliminary field observations: ■ young (juvenile) -usually from 0-2 weeks old, before attaining full expansion; ■ mature -from 2-4 weeks old; a fully grown and structurally developed leaf; ■ senescent -a leaf in the process of dying and just prior to abscission; characterized by a basal position, the loss of chlorophyll, or the onset of decomposition.

Physiological measurements
All mass measurements were made using an analytical balance, with precision of 0.0001 g.For each organ, the fresh weight was quickly measured (FW).Turgid weight (TW) was obtained after soaking leaves in distilled water in Petri dishes for 2 hours at room temperature and under the low light conditions of the laboratory.After soaking, leaves were quickly and carefully blotted dry with tissue paper for analysis.Finally, all the leaves (and all the organs) were dried in an oven at 80°C for 24 hours and weighed (DW).Dried samples were burned in ash in a furnace at 500°C for 2 h.After cooling, the mass of ash content (AC) was determined.Ash content is the inorganic residue left after burning and it is the reflection of the mineral content of biomass.Organic matter was calculated by subtracting crude ash weight from dry matter weight.Several physiological parameters were calculated: mineral deposition in tissues (TDM = AC/DW × 1000 -in g/kg dry weight), tissue density (TD = DW/FW × 1000 -in g/kg).Mineral content (MC; on the basis of the fresh weight) and organic content (OC; on the basis of the fresh weight) are expressed as mass percentage of fresh biomass and organic content / mineral content ratio.Succulence (S) was defined as total water mass divided by the dry mass of organic material (total dry mass minus ash content) [45].Leaf relative water content (LRWC) was calculated according to the following equation: LRWC% = (FW − DW) / (TW − DW) × 100.LRWC is the ratio of the amount of water in leaf tissue at sampling to that present when fully turgid (TW).Because excess water may be absorbed and held in the apoplast, it is likely that the RWC of an artificially hydrated organ is outside of the natural range for plants in natural conditions.This is an argument for simply measuring initial water content as plants are collected, without further rehydration (for scapes and roots) [46].Initial water content (IWC) is expressed as the mass percentage of fresh biomass.This parameter can be compared across all plant organs.Specific leaf area (SLA) is defined as the amount of leaf area per unit leaf weight.Fifteen randomly selected leaf discs (with calculated areas) were sampled and dried in an oven at 80°C for 24 hours.Dry weight (DW) of the samples was measured and used for the calculation of a specific leaf area, where S is the area of leaf discs: SLA (cm −2 / g −1 ) = S (cm 2 ) / DW (g) [47,48].

Statistical analysis
Statistical differences between measurements were analyzed following the analysis of variance (ANOVA), using SPSS software (IBM SPSS Statistics for Windows, version 22.0.Armonk, NY: IBM Corp.).A value of p < 0.05 was considered significant.The Spearman correlations were calculated for the relations between plant traits.A linear regression analysis was performed between physiological parameters and seasonal sampling.A t-test analysis was carried out on mature and juvenile leaves.A linear regression analysis was performed in order to determine how much of total variance in physiological parameters can be explained by meteorological variables.

Results
The descriptive statistics of gravimetric and physiological parameters calculated for each organ are shown in Tab. 1, Tab. 2, and Fig. 1-Fig.3.

Leaf-level traits
Mature leaves were found to weigh above 0.2000 g.The leaves of plants blooming in February were the lowest in fresh weight.These organs showed significant differences from one season to another for all the parameters analyzed.LIWC was higher in February.In November, they had the lowest values of LOC and LOC/LMC ratio, but the highest values of LMC and LTMD.The leaves in November had the highest average of succulence and the highest average of LRWC.Simple regression analysis showed that there was a linear relationship between seasonal samplings and average values of LAC (y = 0.0009x + 0.0036; R 2 = 0.4092), LMC (y = 0.2228x + 1.5131; R 2 = 0.8889), LTDM (y = 17.352x + 110.8;R 2 = 0.6447).LOC/LMC ratio showed a negative trend during 2013 (y = −0.5889x+ 7.3737; R 2 = 0.6186).The values of fresh weight, turgid weight, dry weight, ash content and water content were significantly different between mature leaves and juvenile leaves, for all seasons (Tab.2).LRWC indicated a greater ability to allow mature leaf tissue to soak water in winter and spring.In winter, significant differences to the following parameters were found: leaf organic content, LOC/LMC ratio, and mineral deposition in tissues.The values of LRWC, LOC, and LTMD were higher in mature leaves than in young leaves.For other parameters, no significant differences were observed.In our study, SLA of old leaves was significantly lower than SLA of young leaves (p = 0.035533).The average value of SLA was 454.8213 cm −2 /g −1 .SLA was higher in spring (April) and lower in July (summer).In our study, seasonal changes of SLA in leaves did not show a linear decrease or increase over the growing season.

Correlations between physiological parameters
The Spearman's rho coefficient indicated the same correlations (positive or negative) between physiological parameters in all organs studied.Organic and mineral content was negatively correlated to the amount of moisture in samples.A correlation coefficient of −1 indicated a perfect linear relationship.Therefore, the density of tissues was perfectly inversely correlated with the initial water content, while tissues mineral deposition was perfectly inversely correlated with the organic content / mineral content ratio.The most sensitive parameter for all organs analyzed was mineral deposition in tissues (LTDM, STDM, RTDM).
Correlations and regression analysis between physiological parameters and meteorological factors (Tab.3-Tab.5) Increasing average wind speeds correlated significantly with RS and RIWC.This increase is correlated with decreased RTD and ROC.The average wind speed showed no significant correlations with leaf and scape parameters.In terms of maximum wind speed, we identified several physiological parameters that have no correlation with it: LTMD, LOC/LMC ratio, STMD, SOC/SMC ratio, SMC.On the other hand, this parameter correlated significantly and positively with succulence of organs (roots and scapes), but significantly negatively with STD.Temperature showed no significant correlation with physiological parameters analyzed, with one exception: an increase in average and minimum temperature was correlated significantly with a decrease in succulence in roots.Average humidity demonstrated significant positive correlations with LRWC and RS.Atmospheric pressure had no significant correlations with any of the calculated parameters.
The regression analysis between temperature and initial water content showed relatively high values of coefficients between these variables (for scapes and roots).Minimum temperature explained variance of roots (38.44%) and scapes (49.49%).Leaf variance was explained in the proportion of 29.26% by the maximum temperature.Initial water content for roots and scapes was more associated with wind and humidity.The variations in the initial water content in leaves were better explained by humidity and atmospheric pressure.However, in the case of LRWC, the importance of minimum temperature (39.62%), average wind speed (66.68%), and average humidity (60.34%) increased.The most important meteorological factor which explained changes in mineral content in all plant organs, but especially in leaves, was air pressure (38.02%).Variations in the organic content in roots and scapes were explained by the minimum temperature in proportion of 29.52% and 32.76%, respectively.In roots and scapes, the following factors had the biggest influence on the organic content: average wind speed, maximum wind speed, and average humidity.Linear regression revealed that organic content variations in leaves were mostly explained by average humidity.The organic/mineral ratio and mineral deposition in tissues are not significantly influenced by meteorological factors.The same factors explained variance in tissue density of roots and scapes: minimum temperature, average wind speed, maximum wind speed, and average humidity.Temperature, humidity, and atmospheric pressure affected the density of leaf tissue.

Discussion
Plants from a heterogeneous environment exhibit a greater plasticity in terms of photosynthetic performance and flowering time in response to water shortage than plants from a less variable environment [49].Taraxacum officinale has wide ecological valence in relation to anthropogenic influences [50].In proximity to a high-traffic road, this species is an indicator of heavy metal and nutrient concentrations resulting from atmospheric deposition.It is hypothesized that significant differences in seed reproduction indices of Taraxacum were caused by changes in weather conditions, but not by traffic intensity [51].For example, achenes from urban plants showed higher germination and flowering success, producing a significantly higher number of flowers [52].Other studies have shown that these plants exhibit a variability of leaf blade shapes depending on the habitat conditions [53].Fig. 3 The mean values of specific leaf area (SLA).
In this study, the highest content of dry matter was found in roots and decreased in the following order: root > scape > leaf.Fresh or dry biomass is a common measure of growth for herbaceous plants [54].Dry matter content has been proposed as an indicator of plant resource use [55].This trait is related to leaf lifespan and it is involved in the fundamental trade-off between rapid production of biomass and efficient conservation of nutrients [56,57].Herbaceous perennials maintain the capacity to develop new leaves and continue to grow throughout their life.The carbohydrates stored in vegetative organs are important in terms of balancing the source-sink relationship in perennial plants.Great amounts of carbohydrate reserves stored in roots support the emergence of shoots when temperature and soil moisture are favorable [58].Dandelions are able to delay the onset of water stress by both controlling transpiration and by increasing the water uptake through having a large and deep root volume Tab. 3 Correlations and regression analysis between physiological parameters of scapes and meteorological factors.Correlation is significant at the 0.05 level.[59].Plants of T. officinale lose a part or all of their aboveground tissues before the onset of the unfavorable season, which suggests that they invest resources to protect the regenerating tissues located below the ground and/or to accumulate reserves in belowground tissues instead of cold hardening the aboveground organs [60].Taraxacum officinale has a medium drought tolerance, but generally grows with a minimum rooting depth of 15 cm and most root activity at a depth of 10-15 cm [61].Roots, on average, contained approximately 12% less water than floriferous scapes and leaves.Therefore, roots had the highest density of tissues among the analyzed organs.The levels of density of tissues showed considerable variations in samples throughout the study period.The organic content in the collected roots (24.97%) was higher than in scapes and leaves.The differences between these organs were significant.The organic content / mineral content ratio in roots was generally higher than that in leaves and

Scapes
Tab. 4 Correlations and regression analysis between physiological parameters of roots and meteorological factors.scapes.As far as mineral deposition in tissues is concerned, leaves showed the highest value (157.4590g/kg dry weight), followed by scapes (113.2690g/kg dry weight), and roots (67.1375 g/kg dry weight).The analysis of variance showed significant differences for this parameter.

Roots
In the present study, ash values give an idea of the amount of mineral content in plants.Leaves usually had a higher mineral content (2.17%).We observed an increase in ash content related to leaf maturity.Mineral nutrients can be measured in organs to assess vitality [62].Ash refers to the sum of all other oxidized elements in addition to carbon, hydrogen, oxygen, and nitrogen.Most of them are calcium, phosphorus, and potassium oxides, which reflects the total amount of minerals [63].These minerals increase the abrasiveness of leaf material, and reduce herbivore growth rates and digestion efficiency [64].Our results indicate that the variation of minerals at the growing stage is more complex than the variation of organic matter [65].The analysis of the accumulation of minerals in plants has been proposed as an inexpensive and simple way to predict yield and genotypic adaptation to drought in some C 3 cereals [66].
Plant water status can be indicated by the features of such tissues as root, scape, and leaf.Leaf initial water content (LIWC), scape initial water content (SIWC), and root initial water content (RIWC) give an overall estimate of the amount of water in these organs of a plant at the time of sampling.The highest initial water content was found in scapes and decreased in the following order: scape > leaf > root.Slow water uptake has been shown to be associated with growth, and is greater in young tissue than in mature tissue.Water storage in stems can serve as a buffer against transitional insufficient supply of water from soil [67].In another study, leaf structures were shown to reflect the effects of water stress more clearly than stem and/or root structures [59].
LRWC is considered as an alternative measure of plant water status, reflecting the metabolic activity in plant tissues [68].In our study, we observed only in July that young leaves had higher values of LRWC, but there were no significant differences in comparison to mature leaves.The rate of transpiration is the fastest when air temperature is between 20°C to 30°C.High temperatures in April may explain the decrease in value to 80.4% for LRWC in juvenile leaves.LRWC correlates closely with a plant's physiological activities and soil water status, and is a reliable trait, e.g., for screening for drought tolerance of different genotypes [69].It is a sensitive variable, which quickly responds to environmental conditions such as temperature, light, humidity, and water supply.Some researchers believe that LRWC is higher in the initial stages of leaf development and declines as dry matter accumulates and the leaf matures [70].
In the present study, the highest succulence was found in scapes and decreased in the following order: scape > leaf > root.This parameter is interpreted as providing a measure of water stored per gram carbon expenditure on the part of the plant [46].In larger-scale comparative studies, succulence is treated as a discrete characteristic.However, it has clear ecophysiological implications [71,72].
In this study, SLA of old leaves is generally lower than SLA of young leaves.Lower SLA contributes to long leaf survival, nutrient retention, and protection from desiccation.In fall, lower SLA is associated with higher leaf dry-matter content.Lower SLA is a consequence of an increase in the density of foliar tissue [73].In spring, higher SLA is associated with lower concentrations of phenolic compounds, including lignin [74].The decrease is at least partly caused by the accumulation of secondary compounds like lignin or other phenolics.A decrease in SLA may also occur in response to drought in herbaceous leaves as a result of an increased investment in structural tissues, allowing increased resistance to adverse environmental conditions [75].SLA declines along the gradients of decreasing moisture and/or nutrient availability [76].Water status during leaf development has been identified as an important factor influencing SLA of mature leaves [77].SLA is frequently used in growth analysis as it is often positively related to the relative growth rate [78].SLA is a measure of density and, therefore, the health of individual leaves is often used for assessing growth [79,80].Taraxacum officinale plants have a greater aboveground surface area coupled with a greater capacity to extract water from the soil [81].
Leaf is considered the most sensitive organ and can easily change its morphology and anatomy to adapt to the existing conditions.Nevertheless, in our study, variations in the investigated parameters of roots and scapes are more associated with meteorological factors.Plants react simultaneously to a combination of climate factors.The same meteorological factor may have different influences depending on the season.In our analysis, the values of minimum temperature were generally more significant than values of maximum and mean temperature.The strongest association was generally found with the wind.It seems that in natural conditions, profiles of temperature and humidity are much less important for photosynthesis and transpiration than profiles of wind [82].It is possible that other meteorological factors which were not measured in the present study may have some influence on physiological parameters.Likewise, non-climatic parameters such as soil type or nutrient supply can also affect phenological appearance in the urban habitat [83].Moreover, a plant's phenology is affected by its genotype and age, as well as by the presence of other plant species [84].On the other hand, it has been suggested that plants in closely mowed areas devote a lot of their energy to seed production (r-strategist), and those located in less frequently mowed areas devote more energy to vegetative reproduction (k-strategist) [85].These plants allocate 24% of their net production to seed production and only 13% percent to vegetative reproduction.Furthermore, in the urban environment, establishment and performance of herbaceous vegetation depend on the cutting height and frequency [86].
Several studies on T. officinale have examined the general rhythm of flowering, seasonal variation in flowering, environmental factors which control the opening and closing of the inflorescence and seed reproduction [87].It is known that inflorescences of T. officinale demonstrate thermonasty and photonasty [35].A decrease in temperature is usually connected with rainfall and can even stop blooming.The capitulum of T. officinale is reopened up to four times.Generally, an increase in relative humidity and decrease in temperature result in inflorescence and/or flower closing [88].Long-term studies have proven that it is mostly temperature that influences the process of flower opening.However, nutrients can also modulate flowering time [89].During anthesis, the production of flowers is a likely major metabolic sink for photosynthates.Expressions of plant traits are constrained by other traits, according to the patterns of resource allocation among the organs of an individual plant.Flowering time in natural populations might primarily be determined by environmentally induced phenotypic plasticity.
Our results show that T. officinale plants bloom throughout the year in different seasonal conditions.Plants tolerate a wide range of temperatures.In urban areas in proximity to a high-traffic road, the plants are resistant and show phenotypic plasticity.Short sampling durations provide only a snapshot of physiological parameters.Obviously, it is necessary to investigate multiannual dynamic, which can conclusively illustrate correlations with environmental factors.All in all, the present study reveals important differences in T. officinale properties throughout the various seasons in the urban habitat.These plants can serve as indicator of how well the urban environment is doing.

Conclusions
Taraxacum officinale is a highly successful weed which grows profusely in urban landscapes and colonizes diverse urban habitats.These plants exhibit exhibits seasonal variations in physiological traits.Several of these differences (the highest initial water content was found in scapes; leaves had a higher mineral content, and tissues -a higher mineral deposition; roots had the highest density of tissues) result from adaptive responses to seasonal changes.In our study, variations in the investigated parameters of roots and scapes are more associated with meteorological factors.It could be speculated that a particular pattern of variation in meteorological factors in the urban habitat, with decreasing and increasing trends during the seasons, determines phenotypic responses to physiological traits.Taraxacum officinale can serve as an important indicator of the quality of the urban habitat.
Roots showed significant differences for the following parameters: RMC, ROC/RMC ratio, and RTDM.The average values of RIWC and RS were relatively constant.In July, roots had the highest values of ROC.Fresh weights of roots were lowest in November.Blooming plants had the strongest roots in July.Two physiological parameters have increasing average values: RMC (0.1977x + y = 1.0617;R² = 0.9391) and RTDM (8.2382x + y = 39.248;R² = 0.8293).RTDM and RMC values increased gradually.Two parameters indicated that average values were decreasing during the investigated seasons: ROC/RMC ratio (y = −2.5645x+ 24.346; R² = 0.8169) and ROC (−0.747x + y = 27.344;R² = 0.4554).

Fig. 1
Fig.1The mean values of gravimetric parameters for all organs.

Fig. 2 a
Fig. 2 a-c The mean values of physiological parameters calculated for leaves (a), scapes (b), and roots (c).
Descriptive statistics and results of one-way ANOVA for gravimetric and physiological parameters in 2013 (with significant p-values indicated in bold).
Tab. 1 Tab. 2 Analysis of differences (with significant p-values indicated in bold; ns -not significant) between mature leaves (ML) and juvenile leaves (JL).
Correlations and regression analysis between physiological parameters of leaves and meteorological factors.