Bot. Bull. Acad. Sin. (2004) 45: 229-236

Guan et al. — Plasticity in Mosla chinensis and M. scabra

Phenotypic plasticity of Mosla chinensis and M. scabra (Labiatae) response to soil water status

Bao-Hua GUAN, Ying GE, and Jie CHANG*

Zhejiang University, College of Life Science, Xixi Campus, 232 Wensan Road, Hangzhou 310012, P. R. China

(Received October 8, 2003; Accepted April 1, 2004)

Abstract. The growth and architectural plasticity of Mosla chinensis Maxim. in response to soil water status were compared with the congeneric plant, Mosla scabra (Thunb.) C. Y. Wu et H. W. Li. Two-week-old seedlings were exposed to five levels of soil water for a 6-week period. The results indicated that: an individual's total mass, root mass, apical height, basal diameter, accumulative branch length and branch fresh weight / dry weight ratio (FWB / DWB) of both species had high plasticity in response to soil water content (P < 0.05), and the plasticity of these traits in M. scabra is mostly higher than in M. chinensis. The leaf mass ratio (LMR), specific leaf area (SLA), root mass ratio (RMR), and root / shoot ratio (R / S) of both species had low plasticity. Furthermore, leaf mass, branch mass, branch mass ratio (BMR), and branch length ratio (BLR) had high plasticity (P < 0.05) in M. chinensis but not in M. scabra (P > 0.05) while branch number exhibited contrary trends. In response to soil water, M. scabra adjusted the traits of total mass and size, in terms of a bigger PI, more than M. chinensis while M. chinensis only adjusted partial branch and root traits, such as BMR, BLR, FWB / DWB, RMR and R / S, more than M. scabra. The optimum water niches (OWN) of both M. chinensis and M. scabra are from 40% soil water holding capacity (WHC) to constant saturation, but M. chinensis is only found in relatively dry environments while M. scabra is distributed from dry to wet environments in the field, so the actual water niche (AWN) was separated from the OWN in M. chinensis, but not in M. scabra. Mosla chinensis grew slower and remained smaller than M. scabra and other neighbor species in the field, and it therefore had no competitive superiority in the community. Mosla scabra was very competitive because of its higher yield and taller growth.

Keywords: Interspecific difference; Mosla chinensis; Mosla scabra; Phenotypic plasticity; Relative soil water content; Water niche.


Plants of the same genotype can produce different phenotypes in different environments, a property called phenotypic plasticity (Sultan, 2001). Adaptive phenotypic plasticity is the predicted evolutionary response to environmental factors, such as soil moisture in plant habitats (Sultan and Bazzaz, 1993). Differences among species in plasticity patterns may contribute to their differences in ecological breadth with respect to soil factors and climatic conditions (Cook et al., 1980). Bell and Sultan (1999) investigated how two Polygonum species altered their root growth and deployment over time in response to different soil moisture conditions, and they found that species differences in plastic response to specific moisture conditions would correspond to differences in their field distribution. Ryser and Eek (2000) reported two congeneric grass species with contrasting shade tolerance responded to low resource availability, and they indicated that interspecific differences in phenotypic plasticity were crucial factors for survival and distribution.

It is well known that the distribution and abundance of most plant species greatly depend on water supply

(Schulze et al., 1987). The influence of water availability on plant performance in natural and managed ecosystems has been of great concern in plant physiological ecology and related disciplines (Schulze et al., 1987). Plant growth may decrease in dry soils due to tissue dehydration as well as reduced mineral availability (Fitter and Hay, 1993).Conversely, flooding also reduces plant growth by decreasing the availability of oxygen to roots (Etherington, 1984; Ernst, 1990). There have been a number of studies on the phenotypic plasticity of plants in response to the water supply (Stevens et al., 1997; Šrùtek, 1997; Bell and Sultan, 1999; Clifton-Brown and Lewandowski, 2000; Kotowski et al., 2001), including a lot of studies elaborating the effect of plasticity on local traits for individuals, e.g., leaf and root traits (Bell and Sultan, 1999; Clifton-Brown and Lewandowski, 2000; Ryser and Eek, 2000).

Mosla chinensis Maxim., distributed in east, south, middle and southwest China (Fang et al., 1986), is a medicinal herb which contains many volatile oils and has a long history of use in China for rheum and heliosis (Zhang, 1989; Fang et al., 1986). Many studies about the medicinal components and physiology of M. chinensis have appeared (Zhang and Xu, 1988; Zheng et al., 1996; Zhou et al., 1996, 1998; Pan et al., 1997), but ecological studies of it are rare (Ge et al., 1999; Ge and Chang, 2001; Guan et al., 2003). Though its distribution in the field is wide, M. chinensis has only a few individuals in each population,

*Corresponding author. Tel & Fax: +86 571 8797 2193; E-mail:

Botanical Bulletin of Academia Sinica, Vol. 45, 2004

so it could only be a concomitant, not a dominant species, in the community. Guan et al. (2003) found that M. chinensis normally lives in droughty environments with thin soil and more detritus such as at roadsides or around the rocks with plenty sunlight.

Mosla scabra (Thunb.) C. Y. Wu et H. W. Li occurs as a common weed, distributed widely in China, Japan and Vietnam (Fang et al., 1986). It is also a medicinal herb (Fang et al., 1986). In the field, M. scabra is distributed in dry to moist habitats. Few studies on M. scabra have been done either (Zhang and Xu, 1988).

Both Mosla species suffer from similar water stress in slightly dry environments, where M. scabra can become weedy and dominant (outnumbering any other species in the community) (Guan et al., 2003) while M. chinensis is a concomitant species. What factors result in the different abundance between these two Mosla species? Here we present a comparative study to probe the adaptive mechanisms of these two Mosla species using the phenotypic plasticity patterns in response to different soil water conditions, aiming to understand the relationship between the adaptive mechanism and field abundance.

Materials and Methods


Research was conducted at the plantation of Zhejiang University, Hangzhou (120°10'E, 30°15'N), eastern China. Mosla scabra and M. chinensis germinated at the end of April and early May, respectively. After seedlings had grown for two or three weeks, we transplanted 120 seedlings of each species into pots at May 2000. The two hundred and forty plants were transplanted into pots 17 cm in height and 15 cm in diameter. Pots with the same soil were placed in a greenhouse, which had only a roof of colorless plastic but no wall, in an attempt to replicate the temperature and irradiation conditions of the outside. The soil was a mixture of 30% sand soil, taken from a field in which Mosla grows and 70% loamy, fine garden soil, which included 10% humus. Each species used forty pots, with three plants in each pot. All treatments began from saturated soil water content on May 31, 2000.

Experimental Design

There were five treatments, each of which had twenty-four plants for repetition, interpreted in terms of relative soil water content (RWC) and measured in terms of soil water holding capacity (WHC) (Gituru et al., 2002; Misra

and Tyler, 1999). The testing in dry soil base was 50.39%. For the 1st treatment, soil water was maintained as constant saturation, occasionally dropped to 90% WHC. For the 2nd treatment, plants were not watered unless the RWC dropped to 80% WHC. Similarly, distilled water was added to saturation whenever WHC dropped to 60%, 40%, and 20% in the 3rd, 4th and 5th treatments, respectively. The five treatments were defined as constant saturation (CS), W80, W60, W40 and W20, respectively (Table 1). Our goal in the above treatments was to simulate field soil water conditions in which rainy and fine days alternate. To prevent water leakage from the bottom of the pots, plants were watered slowly so that water would be fully absorbed by the soil. Watering was performed around 6:00 p.m. everyday.

Individuals were harvested after 6 weeks of growth under the treatments. Apical height, basal diameter, branch number, and accumulative branch length of the individuals were measured before harvest. Plant samples were categorized into roots, branches, and leaves. Fresh weights and leaf area were measured immediately after harvest. Leaf areas of samples were determined using a leaf area meter (Li-cor-3000, Lincoln, NE, USA). The mass of every component was determined after oven-drying at 80°C for at least 72 h.

The following parameters of the two Mosla species were determined according to Hunt (1978), Bell and Sultan (1999), and Ryser and Eek (2000): leaf mass ratio (LMR, leaf mass / total mass), specific leaf area (SLA, leaf area / leaf mass), branch mass ratio (BMR, branch mass / total mass), branch length ratio (BLR, branch length / total mass), branch fresh weight to dry weight ratio (FWB /DWB), root mass ratio (RMR, root mass / total mass), and root: shoot ratio (R/S, root mass / shoot mass). Biomass allocated parameters such as (LMR, BMR, RMR and R / S) can reflect the functional plasticity for plant response to soil water (Sultan, 2001). Generally, a bigger SLA means more water rising per leaf area per leaf mass, and BLR can reflect the extension of branch competition for light in response to soil water (Ryser and Eek, 2000). FWB / DWB is an index of water deposition in the plant branch (Schulze et al., 1987). A plasticity index (PI) of phenotypic ranging from zero to one was calculated for each variable and species as the difference between the minimum and the maximum mean values among the five water treatments divided by the maximum mean value (Valladares et al., 2000). Mean phenotypic plasticity was calculated for each species by averaging the indices of plasticity obtained for each of the variables.

Guan et al. — Plasticity in Mosla chinensis and M. scabra

Statistical Analysis

Statistical analysis was conducted using Microsoft Excel 2000 and SPSS 8.0 for Windows. The means and standard errors (SE) of every trait were calculated. Interspecific differences and influence of treatments were tested with a nested ANOVA using the General Linear Model (GLM) with species or (and) treatments. In order to clearly show the differences among treatments, we used histograms instead of lines to present data in Figures 1 and 2.


Overall Patterns of Plasticity

Both M. chinensis and M. scabra had the highest biomass at W60 and the lowest at W20 (Figure 1), and significant growth limitation was observed in both species at W20 (P < 0.05; Table 2). However, the mass of M. chinensis at W20 was 30% of the mass at W60, while the mass of M. scabra at W20 was only 21% of W60. Even so, M. scabra always had higher biomass than M. chinensis at all water

Figure 1. Individual total mass of Mosla chinensis and M. scabra (Mean ± SE) at five soil water statuses. W20: 20% water holding capacity (WHC); W40: 40% WHC; W60: 60% WHC; W80: 80% WHC; CS: constant saturation.

Figure 2. Morphological traits of Mosla chinensis and M. scabra (Mean ± SE) at five soil water statuses. W20: 20% water holding capacity (WHC); W40: 40% WHC; W60: 60% WHC; W80: 80% WHC; CS: constant saturation.

Botanical Bulletin of Academia Sinica, Vol. 45, 2004

statuses. There were no significant differences (P > 0.05) in mass among the W40, W60, W80 or CS in either species. The PI of total biomass of M. chinensis was lower than that of M. scabra (Table 2).

Almost all architectural traits—including apical height, basal diameter and branch number of M. scabra—increased with increasing RWC, the only exception being accumulative branch length, which increased from W20 to W80 and then decreased at CS (Figure 3). All architectural traits of M. chinensis were the lowest at W20, and the api

cal height, basal diameter, and branch number had the highest values at W80, but the accumulative branch length was highest at W60. Mosla scabra always had higher apical height, more branch number, and less accumulative branch length than M. chinensis, and the branch length of M. scabra degressed from nether to top to form a compact crown like a tower while M. chinensis formed an incompact crown like a sphere.

The PI of all architectural parameters of M. scabra was higher than that of M. chinensis (Table 2), indicating that

Figure 3. Growth parameters of Mosla chinensis and M. scabra (Mean ± SE) at five soil water statuses. LMR: leaf mass ratio; SLA: specific leaf area; BLR: branch length ratio; FWB/DWB: branch fresh weight / branch dry weight; RMR: root mass ratio; R / S: root shoot ratio.

Guan et al. — Plasticity in Mosla chinensis and M. scabra

M. scabra can better regulate its architecture than M. chinensis in response to soil water status.

Mass Allocation

The LMR of the two species had the same trends with increasing RWC: it decreased firstly from W20 to W60, then ascended from W60 to CS, the highest value appeared at W20. However the SLA of the two species followed different trends: that of M. scabra decreased from W20 to W60, and then increased from W60 to CS while that of M. chinensis increased from W20 to W80, then decreased from W80 to CS. With increasing RWC, the BLR of M. chinensis increased, but M. scabra decreased, indicating that with increasing soil water availability and at the same level of biomass yield, M. chinensis produced longer branches than M. scabra. At the lower RWC (W20 and W40), M. scabra had higher FMB / DMB than M. chinensis. Mosla scabra had low RMR at W20 and CS, and similar values in the other three treatments; M. chinensis had a similar RMR from W20 to W80, the lowest at CS. With increasing RWC, the R/S of M. chinensis decreased, but M. scabra increased.

The PIs of BMR, BLR, FMB/DMB, RMR, and R/S of M. chinensis were all larger than that of M. scabra (Table 2), indicating that M. chinensis could better adjust branch and root traits than M. scabra in response to soil water status.

Interspecies Differences

In all treatments, the total biomass, apical height, basal diameter, and branch number of M. chinensis were lower than that of M. scabra, indicating that the adaptation of

M. scabra is better than M. chinensis. M. chinensis always had higher accumulative branch length, LMR and BLR, lower RMR, SLA, and R/S than M. scabra in all treatments (Table 3). Furthermore, leaf mass, apical height, basal diameter, LMR, SLA, and RMR showed significant differences between two species (P<0.05).

Leaf mass, branch mass, BMR, and BLR had high plasticity (P<0.05; Table 2) in M. chinensis but not in M. scabra (P>0.05) in response to soil water while branch number showed contrary trends.


General Phenotypic Plasticity

Water is an important factor restricting plant growth (Schulze et al., 1987). Analyzing plant growth and architecture traits is an approach to understand how plants adapt to soil water content changes (Stevens et al., 1997). In the present study, M. chinensis and M. scabra each displayed traits of considerable phenotypic plasticity in response to soil water variety, such as total biomass accumulation, root mass, branch fresh weight / branch dry weight (FWB / DWB) and architectural traits. Plants can adapt to various water environments with these adjustments (Bell and Sultan, 1999).

The phenotypic plasticity index of M. chinensis was lower than that of M. scabra. Mosla chinensis mainly adjusts plasticity of branch and root traits in response to soil water while M. scabra mainly adjusts plasticity of total biomass and architectural traits, such as apical height, basal diameter, branch number, and accumulative branch length.

Botanical Bulletin of Academia Sinica, Vol. 45, 2004

light is available at the lower layer when all the neighbors grow higher than it after late spring. Evasive strategies like this may be the main reason M. chinensis becomes rare and is a concomitant species.

At a low water status, M. scabra can transform its water supply into biomass more efficiently than M. chinensis. It produced more biomass, higher apical height, bigger basal diameter, much higher branch number, less accumulative branch length, and less branch length per branch mass (BLR), forming a much more compact tower crown than M. chinensis most neighbor species in the community. That is a superior adaptation feature and allows a plant growing higher than its neighbors in a dry environment to acquire more sunlight (Ryser and Eek, 2000; Sultan, 2001), but it cannot grow higher than some grasses in a wet environment, and may be a reason M. scabra can be the dominant species in a dry but not in a wet environment.

Acknowledgement. We thank the NSFC for its financial support (No. 39970058).

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Optimum and Actual Water Niche

Both M. chinensis and M. scabra grew well from 40% water holding capacity to constant saturation, for they appeared to have higher biomass and bigger plant sizes at these water statuses. In the field, M. chinensis is mainly found in quite arid environments with the relative soil water content around 20% of water holding capacity (Guan et al., 2003), meaning that the optimum and actual water niches are separated in M. chinensis. Meanwhile, M. scabra is distributed from arid environments (like those of M. chinensis) to moist environments (beside water) in the field (Fang et al., 1986), suggesting that the actual water niche can meet the optimum one in M. scabra.

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Botanical Bulletin of Academia Sinica, Vol. 45, 2004