Effects of enhanced UV-B radiation on fitness of an alpine species Cerastium glomeratum Thuill

Abstract

Aims

Information about how species respond to extreme environments, such as high UV-B radiation, is very useful in estimating natural ecosystem structure and functions in alpine areas. Our aim is to examine the effect of enhanced UV-B radiation on the fitness of an alpine meadow annual species on Qinghai-Tibet Plateau.

Methods

Plants of Cerastium glomeratum Thuill. were exposed to ambient (control) or ambient plus supplemental UV-B radiation (enhanced), simulating a 9% ozone depletion over Gannan, China (102°53′E, 34°55′N, 2900 m in altitude), up to leaf senescence and fruit maturation. Plant height, flower phenology, biomass allocation and reproductive parameters of the species were measured.

Important findings

Plant height in C. glomeratum was reduced by enhanced UV-B radiation at early growth stages and compensated with ongoing development. Fruit biomass, aboveground biomass, total biomass and reproductive effort (fruit dry mass/aboveground biomass) were not affected by enhanced UV-B radiation, but a significant increase in root/shoot ratio was found. Enhanced UV-B radiation delayed onset of flowering by 1 day and shortened duration of flowering by 5 days in C. glomeratum. But because of the long period of flowering time (83–88 days), this did not make any significant effect on flower number, seed number, pollination success (number of seeds per fruit) or reproductive success (fruit to flower ratio) in C. glomeratum. Enhanced UV-B radiation had no effect on seed germination and seed mass either. And the high production and low germination rate of the seed might be the strategy of C. glomeratum to survive the extreme environments on alpine meadow. All these results showed that C. glomeratum was tolerant to enhanced UV-B radiation.

INTRODUCTION

Depletion of stratospheric ozone has increased solar ultraviolet-B (UV-B) radiation at high- and mid-latitudes in both northern and southern hemispheres (Madronich and de Gruijl 1994; Mckenzie et al. 1999; Seckmeyer et al. 1994), including Qinghai-Tibet Plateau in China (Zhou et al. 1995). Recent projections suggest that ozone depletion will reach its maximum in the coming years and forecasted to recover slowly over the next several decades (Gao et al. 2004; Koti et al. 2004). However, many factors, including feedbacks from rising concentrations of greenhouse gases, could delay this process (Montzka et al. 1999; Schindell et al. 1998).

Enhanced UV-B radiation can have many direct and indirect effects on plants including inhibition of photosynthesis, DNA damage, changes in morphology, phenology and biomass accumulation (Caldwell et al. 1995). There have been numerous studies examining physiological and growth responses by plants to enhanced UV-B (see reviews by Bornman and Teramura 1993; Caldwell et al. 1989; Liang et al. 2006; Runeckles and Krupa 1994; Teramura 1983; Tian et al. 2007; Xue and Yue 2004; Zheng et al. 1998). Enhanced UV-B radiation was generally characterized as harmful for plants and its effects were species specific (Teramura and Sullivan, 1993; Tevini and Teramura 1989).

Early studies were often misleading because they were conducted indoors under low intensities of photosynthetically active radiation, which reduced the capacity of plants to defend themselves against UV radiation, and so tended to overemphasize its damaging effects (Caldwell et al. 1995; Paul and Gwynn-Jones 2003). Unfortunately, only 15% of the reported studies have been conducted under field conditions (Caldwell et al. 1998). While laboratory and glasshouse studies provide information on mechanisms and processes of UV-B action, only field studies can provide realistic assessments of what will happen as the stratospheric ozone layer thins (Caldwell et al. 1995).

Many of the criteria used for assessing the sensitivity of a plant were mainly the UV-B radiation effects on growth and photosynthesis (Searles et al. 2001). However, the fitness of an organism, in addition to these parameters, depends on its ability to complete successfully its life cycle, giving offspring (Stephanou and Manetas 1998). Yet, studies on the effects of enhanced UV-B on reproduction were scarce, especially under field conditions (Bornman and Teramura 1993; Teramura and Sullivan 1993).

Although fruit and seed yield of a few crop species have been examined in various studies (see reviews of Gao et al. 2004; Kakani et al. 2003; Zu et al. 2004), little research has focused on the effects of UV-B radiation on the fitness of plants from natural populations (Conner and Neumeier 2002). In the few field studies on reproductive biology of wild species, more positive or none responses were found under enhanced UV-B radiation compared with ambient control (Conner and Neumeier 2002; Conner and Zangori 1997; Petropoulou et al. 2001a; Stephanou and Manetas 1998), which seems contrary to the viewpoint that enhanced UV-B radiation was harmful to plants and even ambient solar UV-B radiation may be adverse for growth (Phoenix et al. 2002; Rousseaux et al. 1998; Searles et al. 1995; Teramura and Sullivan, 1993; Tevini and Teramura 1989).

Based on the above, we conducted field experiments to investigate the effects of enhanced UV-B radiation on growth, flowering, biomass allocation and reproduction of an Alpine Meadow annual species (Cerastium glomeratum Thuill.) on Qinghai-Tibet Plateau. The purpose was to find out whether enhanced UV-B radiation was harmful, neutral or beneficial for the fitness of this species and the further influence might this result bring.

MATERIALS AND METHODS

Study site

The field experiment was conducted at Alpine Meadow Ecosystem Scientific Research Station of Lanzhou University at Gannan, Gansu province, China (altitude 2900 m, 102°53′E, 34°55′N). Gannan is situated in the east of Qinghai-Tibet Plateau. Average annual temperate is 2.0°C, the lowest daily temperature averages −8.9°C concentrated in December, January and February; the highest daily temperature averages 11. °C concentrated in June, July and August. The average precipitation of a year is 550 mm, concentrated in July, August and September. The vegetation there typifies alpine meadow (Wu 1980).

Plant material

Seeds of C. glomeratum Thuill. (Caryophyllaceae) were collected from east of the Qinghai-Tibet Plateau (Gannan, Gansu province), September 2003. C. glomeratum is a dicotylous annual forb widespread throughout most of China and the world. It occurs in hillside and grassland at an elevation of ∼3000 m. Flowering occurs in May to July. (Delectis Florae Reipublicae Popularis Sinicae Agendae Academiae Sinicae Edita, 1996; Wu 1983).

Methods

Field experiments were conducted during May to October, 2004, at Gannan, China, under two UV-B treatments (ambient or ambient plus supplemental UV-B radiation). Seeds of C. glomeratum were sown in 64 pots (28 cm in diameter, 25 cm in height) containing completely mixed local soil. The pots (three plants per pot) were randomly assigned to eight plots (four plots for each UV-B treatment) of 1.2 × 0.6 m each. Plants were treated with UV-B radiation from seedling emergence to leaf fall. During the experimental period, plants were watered regularly to maintain the soil water content at ∼35%.

Enhanced UV-B radiation was provided by three filtered 40-W fluorescence sun lamps (UV-B-313, Beijing Electric Light Sources Research Institute) in each plot. The lamps (spaced 30 cm apart) were suspended above and perpendicular to the canopy of plants and filtered with 0.13-mm thick cellulose diacetate (absorbs radiation <290 nm) for UV-B irradiance. The desired irradiation was obtained by changing the distance between the lamps and the top of plants. The spectral irradiance from the lamps was determined with an Optronics Model 742 (Optronics Labs, Orlando, FL, USA) spectroradiometer and weighted with the generalized plant action spectrum (Caldwell 1971) normalized to 300 nm to obtain the biologically effective UV-B radiation (UV-B_BE). The supplemental UV-B irradiation was 6.40 kJ m⁻² day⁻¹ that simulated 9% stratospheric ozone depletion on clear summer solstice (Gannan, 34°55′N, 2900 m), China, using the model of Green et al. (1980). UV-B radiation was supplemented for 6 h daily centered at solar noon (from 09:30 a.m. to 03:30 p.m.) except for cloudy or rainy days. Ambient UV-B radiation was provided by the emptied lamp hangers of the same size of that of the enhanced UV-B as control. The height and arrangement of the lamp hangers were the same as those of the enhanced treatment.

Measurements were conducted with randomly tagged seven plants per plot. Plant height was measured every 10 days beginning from 30 DAT (days after treatment) until no more increases in height was observed. During the flowering period, the ‘open’ inflorescences were visually counted and tagged daily, to study the phenological parameters of anthesis, and the B50 value (days after treatment when 50% of the plants were flowering) was calculated according to the method of Saile-Mark and Tevini (1997). Seeds of tagged plants were collected separately when matured every day until all seeds were matured and no seed loss occurred. Tagged plants were harvested separately after leaf fall and fruit maturation. After harvesting, plants were divided into fruits, stems and roots and their dry masses were measured after oven drying for 48 h at 80°C. We calculated aboveground biomass as fruit dry mass plus stem dry mass, total biomass as aboveground biomass plus root dry mass, root to shoot ratio as root dry mass/aboveground biomass and reproductive effort as fruit dry mass/aboveground biomass. In addition, fruits and seeds of each plant were counted; pollination success (number of seeds per fruit) and reproductive success (fruit to flower ratio) were calculated.

Seeds of untagged plants were harvested to determine seed germination. Before germination test, seeds were stored at 4°C to break dormancy. Three repeats of 100 seeds were counted and weighted and then evenly placed on three layers of filter papers in 9-cm Petri dishes. Germination tests were performed in a growth chamber in dark with temperature varied between 5°C (12 h) and 25°C (12 h). During the course of the test, the filter papers were humidified regularly with de-ionized water. A seed was considered to be germinated when radicle elongation was greater than the maximal seed diameter. Germinated seeds were counted every day until no more seed germinated.

Statistics

All data were statistically analyzed using one-way analysis of variance (ANOVA) in SPSS 11.0 to test significance (P < 0.05) of treatments. Plant height was also analyzed with two-way ANOVA (General Linear Model) to test for differences caused by UV-B, DAT and UV-B × DAT interactions. Data of reproductive effort, reproductive success and seed germination were transformed using the arcsin transformation before analysis.

RESULTS AND DISCUSSION

Plant height

The effects of enhanced UV-B radiation on plant height in C. glomeratum at different DAT were shown in Figure 1. A strong DAT main effect indicated that the plants were growing over time. Two-way ANOVA results showed significant UV-B main effects and UV-B × DAT interactions on plant height in C. glomeratum, indicating an effect of UV-B on plant height that varied over time.

Reduction in plant height has often been used as an index to assess the degree of UV-B radiation sensitivity (Li et al. 2000). In this study, plant height of C. glomeratum was significantly reduced by enhanced UV-B radiation at early growth stages (at 30–70 DAT) and compensated at last (at 80 DAT). This showed that sensitivity to UV-B radiation could be different at different growth stages. The compensation in growth with ongoing development was also found in several maize cultivars (Mark et al. 1996). Although the UV-B reduced plant height caught up at last, the decreases in plant height in C. glomeratum at early growth stages might keep it at a disadvantage at the very start in natural communities when it was competing with other species in which the plant height was not affected by enhanced UV-B radiation.

Flower phenology

Figure 2 showed flower phenology of C. glomeratum under two UV-B radiation conditions. Changes in flower phenology might affect pollination success in natural ecosystems because pollinators might not be available when the flowers were in their final reproductive phase (Mark et al. 1996). In our experiment, onset of flowering was delayed by 1 day under enhanced UV-B radiation (days to first flowering was at 65 DAT under control and at 66 DAT under enhanced UV-B radiation), termination of flowering under enhanced UV-B radiation was 4 days ahead of control (days to last flowering were at 152 DAT and 148 DAT under control and enhanced UV-B radiation, respectively) and duration of flowering was 5 days shorter under enhanced UV-B radiation than under control conditions (Figure 2a). But pollination success was not affected by enhanced UV-B radiation in C. glomeratum, nor was the number of seeds produced (Table 2).

From Figure 2b we can see, a prolonged 0.9 days in B₅₀ values (days after treatment when 50% of the plants were flowering) in C. glomeratum was found under enhanced UV-B radiation (87.8 DAT under control and 88.7 DAT under enhanced UV-B radiation), but this did not result in reductions in reproduction. While in the experiment of Rajendiran and Ramanujam (2004), UV-B stress delayed achievement of 50% flowering in Vigna radiata (L.) Wilczek by 1.6 days and both potential yield and seed number were reduced significantly by enhanced UV-B radiation. Similar results were found in experiments of Mark et al. (1996) and Saile-Mark and Tevini (1997). These suggested that C. glomeratum was tolerant to enhanced UV-B radiation and using only growth parameters to consider plant sensitivity to UV-B radiation could be misleading, the fitness of a plant consisted of not only growth parameters but more important the reproductive parameters (Stephanou and Manetas 1998).

Biomass allocation

Changes in biomass allocation as the result of UV-B treatment were also potentially of far reaching ecological importance (Björn et al. 1997). Although no significant effects of enhanced UV-B treatment were found on fruit biomass, aboveground biomass, total biomass and reproductive effort in this study, a significant increase in root/shoot ratio with percentage change of 36.5% under enhanced UV-B radiation was observed (Table 1). The increased root/shoot ratio may improve the leaf water relations and lead to higher growth rates (Petropoulou et al. 2001b). This may explain the compensation of plant height at later growth stages in C. glomeratum under enhanced UV-B radiation.

Reproduction

Although flowering was delayed by 1 day and the duration was 5 days shorter under enhanced UV-B radiation, total number of flowers and seeds were not significantly affected, so were pollination success (number of seeds per fruit) and reproductive success (fruit to flower ratio) in C. glomeratum (Table 2). These results indicated that changes in flower phenology might not have decisive effects on plant fitness (defined as the number of seeds produced) of this species. And seed yield might be due mainly to alterations in total number of flowers produced (Conner and Neumeier 2002; Feldheim and Conner 1996). In our case, changes in total number of flowers and seed yield were not consistent with changes in flower phenology. This may be due to three reasons. First, the duration of flowering was 5 days shorter in this species under enhanced UV-B radiation that was only ∼6% of the total flowering period (83–88 days) and onset of flowering was delayed by only 1 day, so this may not make significant changes in total flower number and seed yield. But in the research of Mark et al. (1996), the longest flower duration was only 14 days and a delay of 5 days in flowering caused a decreased yield. The second reason is that the 5 days shorter in duration of flowering under enhanced treatment compared with control was mainly due to the prolonged termination of flowering under control, and pollinators might not be available when the flowers were in their final reproductive phase (Mark et al. 1996). That is one of the reasons why pollination success was not affected too. Third, Feldheim and Conner (1996) proposed that timing of flowering could be much more important in species that flower early in the spring or that rely on specialist pollinators. Based on our observation, flowers of C. glomeratum occurred in summer and were visited by a variety of generalist pollinators. So timing of flowering was unlikely to affect pollination success and then seed yield of C. glomeratum. That was in correspondence with our results. Many factors might affect pollination success and seed yield, such as floral morphology, pollinators’ visitation, production of pollen, pollen germination and tube growth, nectar or flowers, all of which could be affected by UV-B radiation (Demchik and Day 1996; Feldheim and Conner 1996; Musil 1995; Petropoulou et al. 2001a; Stephanou et al. 2000). Our results of reproduction might be a combination of several of these factors. Further investigation was needed to explore the main factors that affected reproduction of the species under UV-B treatment.

Enhanced UV-B radiation had no significant effect on 100 seed mass and seed germination (Table 2). And seed production of C. glomeratum was high under both treatments and seed germination rate was very low. Seed germination patterns can be survival strategies in some species (Phillips et al. 2006). As previous work demonstrated that, for a number of species, not all viable seeds germinated under one set of conditions in the first year (Philippi 1993). The ungerminated seeds might be stored in seed banks to avoid bad conditions and allow species to survive episodes of disturbance and destruction (Thompson 2000). For instance, Song et al. (2005) reported that seeds of Suaeda physophora and Haloxylon ammodendron could remain ungerminated in a saline environment. So we speculated that the high seed production and low germination of C. glomeratum might be the strategy of it to survive extreme environments such as high UV-B radiation on alpine meadow. The above results showed that enhanced UV-B radiation was unlikely to have detrimental effects on fitness in C. glomeratum.

In all, our results showed that enhanced UV-B radiation was neutral for fitness in C. glomeratum. But considering the species-specific effects of UV-B radiation on growth and reproduction of plants (Caldwell et al. 1995; Petropoulou et al. 2001a, 2001b), the fitness of C. glomeratum under enhanced UV-B radiation might be changed in natural communities. Enhanced UV-B radiation might be harmful for C. glomeratum when it was competing with a species that was more resistant to UV-B radiation, and on the contrary, enhanced UV-B radiation might be beneficial for this species when it was competing with a UV-B-sensitive species. As UV-B is considered to be an important regulating factor at the ecosystem level (Caldwell et al. 1998; Rozema et al. 1997), knowledge about sensitivities of the Tibetan Plateau species to UV-B radiation will contribute to better estimates of ecosystem structure and functions.

FUNDING

Chinese key project for natural science (90202009); start-up fund of scientific research by QuFu Normal University (Bsqd2007012).

References