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Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau



Gaolin Wu, Guozhen Du, Min Chen and Dashuai Sun
 
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ABSTRACT

Six herbaceous plants native to alpine meadows of the eastern Qinghai-Tibetan Plateau were examined for their plasticity in root morphology and root biomass allocation in response to several light and nutrient levels in a field experiment. Root length, root dry weight, root length to total seedling height ratio (R/Th) and root dry weight to total seedling dry weight (R/Tw) were examined. The root morphology and biomass allocation have significant response to variation of light and nutrient availability. The seedlings root length, root weight; R/Th and R/Tw of these six species were all significantly affected by the light and nutrient. There are significant interaction effects on root length and R/Th between light and nutrient. The nutrient availability significantly increased the root length and biomass, the maximum value generally occurred in moderate nutrient availability. The shade reduced the biomass allocation to root. A higher nutrient availability improved the R/Th and R/Tw in shade conditions. But, the poor nutrient availability could increase the R/Th and R/Tw in a strong light condition. There are significant maternal effects of species on the root morphological growth and root biomass allocation. These indicated that these seedlings have different root morphology and biomass allocation strategies in different circumstance heterogeneities.

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Gaolin Wu, Guozhen Du, Min Chen and Dashuai Sun, 2006. Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau. International Journal of Botany, 2: 395-401.

DOI: 10.3923/ijb.2006.395.401

URL: https://scialert.net/abstract/?doi=ijb.2006.395.401

INTRODUCTION

Plasticity in morphology usually is understood as phenotypic accommodation-a change in a plant’s life-history strategy in response to the resources availability of environment. The success seedling recruitment also is affected by high growth rates and high allocation to vegetative spread via rhizomes (Rees et al., 2001). Biomass allocation between roots and shoots is one of the most important processes regulating plant growth. The consequences of below-ground allocation for water uptake and nutrient acquisition cannot be over-emphasized (Norby et al., 2001). Even after seed germination, the primary sources of light and nutrient availability are major determinant of seedling survival. Shallow root development in the first year of growth for some species can induce high seedling mortality, either directly by drought or indirectly by competition with grasses and herbs for available water (Casper and Jackson, 1997). The germination and survival of seedlings depend on light, nutrient, temperature and soil water status (Bugmann et al., 1997). Nutrient effects on seedling biology are likely to be important in some cases. Nutrients at the soil surface can potentially induce changes in seedling regeneration (Tyler, 1995).

At the seedling stage, it is difficult to make predictions of plant establishment and how root morphogenesis responds to light and nutrient availability. While roots vary widely in morphology and physiology, little is known about what selection pressures govern this variation or how this variation may be related to plant function. Ecologists have developed theories on plant growth strategies to explain variation in plant traits such as tissue morphology, biomass allocation (Grime 1977; Chapin, 1980; Falster and Westoby, 2003). The allocation of biologically expensive resources to tissue structure and function is governed by biotic and abiotic selection pressures. Broad patterns in tissue structure and function correlating with potential growth ratio have been found in leaves (Reich et al., 1997), but broad studies of root traits have been limited.

Changes in the allocation of biomass to root may also be a critical component of a plant's response to the different microsites, changing the distribution of biomass to the shoot and the root under a given environment condition (Andalo et al., 1996). In natural situations, the plant may lead to greater competition between root systems by increasing the overlap of root foraging zones between individuals (Bazzaz, 1990). The overall morphology of root architecture is changed by the aerial environment (Bernston and Woodward, 1992). The morphology and allocation response of root growth are important component of the response to different irradiance and nutrition conditions. They also mediate an evolutionary response. Therefore these characteristics have been studied to assess the potential selective effect of special climate condition. Studies should be given priority to the biomass allocation above-and below-ground response to a changing habitat. In considering how light and/or nutrient availability influence seedling biology, it is important to conduct a manipulative experiments in field. At the seedling stage, how root morphogenesis response to different light and nutrition conditions. There have been few investigations into the effects of light and nutrition conditions at the stages of root growth in alpine meadow.

Given that allocation of biomass to below-ground organs is important, it is important to understand how seedling root cope with low light and nutrient availability. The present research studied the effect of different irradiance and nutrient conditions on the seedlings root growth of six herbaceous species in alpine meadow in east of the Qinghai-Tibetan Plateau with special attention to the morphology of root length and biomass. Specially, the questions we addressed were: (1) is the irradiance and nutrition variation acting directly on the six seedling root morphological variation? ; (2) is there a maternal effect of species on the root biomass allocation?; (3) what root biomass allocation strategies would take for these species in circumstance heterogeneity?

MATERIALS AND METHODS

Material studied: Seeds of six alpine meadow herbaceous species (Vicia sepium L., Astragalus polycladus, Cremanthodium lineare Maxim., Trollius farreri Stapf, Artemisia desertorum Spreng and Artemisia hedinii Ostenf.) were obtained from sites in the alpine meadow grassland of the eastern Qinghai-Tibetan Plateau, China (Table 1). All six species are perennial and widespread species in the east of Qinghai-Tibetan Plateau. The six species were widely distributed species and are able to establish and persist in many habitat conditions in the alpine meadow. The six species were chosen for study for two reasons. First, they represent an important component of the seedling and/or adult herbage community of a primary alpine meadow site.

Table 1: Species and mean seed weight (SW) (±SE) per hundred seeds studied
Image for - Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau

Second, the six seedling species used in this study all had a relatively extensive habitats in alpine meadow. Because of the importance of these seedling species in alpine meadow and their extensive adaptability, they were considered to be among the most important species to examine for seedling root growth responses to microenvironments.

Study site: The field experiment was carried out in a alpine meadow belt at Gannan, Gansu Province, China (altitude 2900 m, 102.53-E, 34.55-N), which is situated in the east of Qinghai-Tibetan Plateau. Average annual temperature is 2.0°C the lowest daily temperature averages 8.9°C concentrated in December, January and February; the highest daily temperature averages 11.5°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).

Methods: Seeds of the six species were defleshed, surface-sterilized (10% sodium hypochlorite for 10 min) and germinated in plastic pots (40x40x50 cm) containing washed sand (no nutrition) in field at the alpine meadow condition, Gannan, Gansu Province, China. The 300 seeds/species from the seed collections used in this study were weighed to determine average seed mass (Table 1). The seeds of six species were sown at the same time. In each plastic pot, 3 seeds were sown uniformly, which avoided the competition among seedlings and every species in each treatment were repeated 10 plots. Each species were sown in one of twelve treatments.

The light intensity conditions were controlled with different-density black shade cloth over a frame of the same dimensions. Light treatments include four levels: L100, 100% of full sunlight; L50, 50% of full sunlight; L25, 25% of full sunlight; L12, 12% of full sunlight. The light intensity within each shade cloth of the experimental treatments was measured using a Decagon Model SF80 Sunfleck Ceptometer (Decagon Devices, Inc. Pullman, Washington, USA) on a cloudless day. Thirty light measurements (photon flux density, μmol m-2 s-1) using the single sensor setting were taken from each light regime. Three nutrient treatments were: N0, without nutrition; N50, 50% of full-strength Hoagland’s nutrient solution; N100, 100% of full-strength Hoagland’s nutrient solution.

Table 2: Summary of micro habitats characteristics of the twelve treatments plots in the experiment
Image for - Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau
We selected an extensive field area to carry out the experiment in alpine meadow. Shown for each variable is the mean at the initiation of the experiment in April 2004. These symbols (in Table 2) express five light regimes and two nutrient regimes: 100% full sunlight (L100), 50% full sunlight (L50), 25% full sunlight (L25) and 12% full sunlight (L12) and three nutrient regimes, no addition of nutrient solution (N0), 10% of full-strength Hoagland’s nutrient solution (N10) and 100% of full-strength Hoagland’s nutrient solution (N100)

This experiment controlled principally light and nutrient treatments and divided into twelve treatment conditions (Table 2): L100N0, L100N50, L100N100, L50N0, L50N50, L50N100, L25N0, L25N50, L25N100, L12N0, L12N50, L12N100; others were in the nature field environments. Apart from the normal rain, the seedlings were well watered during the all growth seasons; all treatments were meted out the same water. The nutrient solutions were added one time a day, N0 treatments were added the same water. From sowing, these treatments were being at all times.

Seedlings were grown for a constant time period after seed reserve depletion because seed reserves can influence the stage at which plants become dependent upon external supplies of nutrients (Atkinson, 1973). The time of independence was determined by noting when seeds/cotyledons were no longer attached to the seedling. In all plant species, seedling growth continued throughout the study period and no roots emerged from the bottom of the pots indicating that seedlings had not become pot-bound. At 60 days after seedling emergences for each treatment of each species indicated that the four species were at similar developmental stages at the time of seedling harvest, suggesting that harvest at a constant time following seed reserve depletion was appropriate. Fifteen seedlings per treatment each species which were identical growing state were harvested intact and the following data collected. Root length and seedling total height were measured. Seedlings were carefully washed and measured its total height and root length, then seedlings were oven dried (70°C) for 48 h and dry biomass of root and total seedling was obtained by electronic balance (the precision is 0.0001 g). Root-to-total seedling ratios were calculated as root dry weight/total seedling dry weight (R/Th) and root length/total seedling height (R/Tw) compared among treatments.

Statistical analyses: The data were analyzed using Two-way ANOVA statistically with Spss 12.0 to test significance (p<0.05) of treatments. Correlations were performed considering the all nutrition and light intensity treatments for length and dry weight (p<0.05).

RESULTS

Effect of light, nutrient and seed mass on root growth: The seedlings root length, root weight, root length/total seedling height (R/Th) and root weight/total seedling weight (R/Tw) of these six species were all significantly affected by the light and nutrient (Table 3). There are significant interaction effects on root length (ANOVA: F = 5.695, p<0.001) and R/Th (ANOVA: F = 9.529, p<0.001) between light and nutrient, but no significant interaction effect on root weight and R/Tw (Table 3). The responses of seedling root to light and nutrient availability were larger in morphology than in biomass allocation.

The different seed-mass species were significantly difference in root length (ANOVA: F = 439.321, p<0.001), root weight (ANOVA: F = 32.880, p<0.001), R/Th (ANOVA: F = 185.430, p<0.001) and R/Tw (ANOVA: F = 62.673, p<0.001) (Table 3). So, there are significant maternal effects of species on the root morphological growth and root biomass allocation.

There are significant positive relationship between root length and total seedling weight (p<0.01) and between root weight and total seedling weight (p<0.01) (Table 4). The root length and root weight are important for seedling growth. There is a significant negative relationship between R/Th and total seedling weight (p<0.01) and a slight negative relationship between R/Tw and total seedling weight (Table 4). These indicated that the larger seedling has shorter root length in these species.

Root morphology and biomass allocation: The root morphology and biomass allocation have significant response to variation of light and nutrient availability. The seedling had a shorter and lighter root in shade conditions. The nutrient availability significantly increased the root length and biomass, the maximum value generally occurred in moderate nutrient availability (Fig. 1 and 2). A higher nutrient availability improved the R/Th and R/Tw in shade conditions. But, the poor nutrient availability could increase the R/Th and R/Tw in a strong light condition (Fig. 3 and 4). The shade reduced the biomass allocation to root.

These indicated that these seedlings have different root morphology and biomass allocation strategies in different circumstance heterogeneities.

Table 3: Three-ways (light, nutrition and species) ANOVA summary of all effects on the growth of root length, root weight and R/Tw, R/Th at 40 days
Image for - Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau

Table 4: Pearson’s coefficients of correlation (r) for relationships (with significant p<0.05 coefficients in bold ) between all pairings of six species seedlings
Image for - Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau
Correlations were performed considering the all nutrition and light intensity treatments,*Correlation is significant at the 0.05 level (2-tailed).; ** Correlation is significant at the 0.01 level (2-tailed). R/Th is the ratio of root length to the total seedling height; R/Sw is the ratio of root dry weight to the total seedling dry weight

Image for - Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau
Fig. 1:
The mean seedling root length of six species in different light and nutrient availability. All seedling root dry weight of seedlings differed significantly (p<0.05) between light and nutrient levels

Image for - Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau
Fig. 2:
The mean seedling root dry weight of six species in different light and nutrient availability. All seedling root dry weight of seedlings differed significantly (p<0.05) between light and nutrient levels

Image for - Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau
Fig. 3:
The mean ratio of seedling root length to seedling total height of six species in different light and nutrient availability. All seedling root dry weight of seedlings differed significantly (p<0.05) between light and nutrient levels

Image for - Response of Seedling Root of Six Herbaceous Species to Light and Nutrient in Alpine Meadow of Qinghai-Tibetan Plateau
Fig. 4:
The mean ratio of seedling root dry weight to seedling total dry weight of six species in different light and nutrient availability. All seedling root dry weight of seedlings differed significantly (p<0.05) between light and nutrient levels

DISCUSSION

Research indicated potentially important interactions among light, nutrient and species that could influence regeneration dynamics (Walters and Reich, 2000). The seedling root growth and allocation were affected significantly by the light and nutrient availability directly and in interaction with the maternal effects of different seed-mass species. Abiotic and biotic factors may restrict seedling recruitment processes. Resource addition has a more positive effect on the seedling survival of species (Milberg et al., 1999). Some finding observed that many of the same environmental constraints and organismal tradeoffs that shape the evolution of plant morphologies, life histories and physiologies also influence the dynamics of interspecific interactions and the mechanisms of coexistence that control community and ecosystem functioning (Tilman and Pacala, 1993; Rees et al., 2001).

The variation in seed size may be an important adaptive factor in the future changing environment (Westoby et al., 1992). The seedling root growth response may be considered by the genetic variability in seed volume (Andalo et al., 1998). Krannitz et al (1991) have shown that, independent of root growth, seedlings from genotypes of Arabidopsis thaliana with larger seeds survived longer than seedlings from genotypes with smaller seeds when nutrient supply was deficient. We considered that the root biomass variety of seedling from the smaller-seed species was influenced by the nutrient and light availability, but the seedling from the larger-seed species was influenced slightly.

Pigliucci (2005) suggests that adaptive phenotypic plasticity occurs in natural populations and average differences among environments across genotypes. The recognition that plasticity can be adaptive has stimulated a wealth of studies on how plasticity alters interactions between individual organisms and their environments (Sultan, 2000). Plasticity in morphology usually was understood as phenotypic accommodation-a change in a plant’s life-history strategy in response to the resources availability of environment. It is now clear that a wide diversity of organisms express phenotypic plasticity in response to biotic and abiotic aspects of their environments (Miner et al., 2005). The success seedling recruitment also was affected by high growth rates and high allocation to vegetative spread via rhizomes (Rees et al., 2001).

Biomass allocation was one of the central concepts in modern ecology, providing the basis for different strategies; a plant had a given amount of resources at any point in time and it allocated these resources to different structures (Weiner, 2004). Plasticity in allocation usually was understood as a change in a plant’s life-history strategy in response to the resource availability of environment. These plastic responses in our study include change in morphology, growth, life history. Comas and Eissenstat (2004) have concluded that many factors and root morphology and architecture influence root growth rates, such as carbohydrate supply and environmental conditions. Plants alter the growth and structure of roots in response to different concentrations of nutrients, which maximizes nutrient foraging in patchy soils. This response increases the capture of essential nutrients and affects competitive interactions among plant species (Hodge, 2004). Seedling biomass and root-to-total biomass ratio responded positively to increased irradiance (Robakowski et al., 2003). The vertical foliage distribution also strongly influenced physiological processes such as photosynthesis and affected individual plant growth (Xiao and Ceulemans, 2004). C-S-R strategy theory (Grime, 1979) predicts that chronic shortages of soil nutrients selectively favor conservative use of resources, including low turnover rates of tissues, low reproductive effort, low relative growth rate and extreme longevity (the stress-tolerator strategy). We concluded that the seedlings will alter the root biomass allocation to improve their fitness and establish successfully in different environment heterogeneities, which is also a life-history strategy. The seedlings preferentially allocated biomass to the part obtaining the resource that is limiting growth and survival. So the root morphology and biomass allocation strategies are seemed to very important, in many circumstance where resource is limiting, especially in the inclement climate environment of the Qinghai-Tibetan Plateau which the circumstances are not in favor of seedling establishment.

In conclusion, the seedlings will have different root morphological variation and root biomass allocation strategies and the different seed-mass species have different root growth strategies. These showed that seedling growth strategy evolved in the face of selective pressure.

ACKNOWLEDGMENTS

This research was funded by the Key Project of Natural Science Foundation of China (90202009) and Project of Natural Science Foundation of China (30470307).

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