Bot. Bull. Acad. Sin. (2003) 44: 291-296

Ge et al. Soil water effects on physioecological traits of two endangered species

Effect of soil water status on the physioecological traits and the ecological replacement of two endangered species, Changium smyrnioides and Chuanminshen violaceum

Y. Ge1, Jie Chang1,*, C.-X. Fu1, and G.Y.-S. Chan2

1The State Key Laboratory of Plant Physiology and Biochemistry, College of Life Science, Zhejiang University, 232 Wensan Road, Hangzhou 310012, P. R. China

2Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P. R. China

(Received April 21, 2003; Accepted June 9, 2003)

Abstract. Both Changium smyrnioides Wolff and Chuanminshen violaceum Shen et Shan are monotypic species of the family Umbellaceae. They have narrow distribution areas and can only be found in the Yangtze River basin in China. Changium is distributed from the east to middle subtropical zone of China, and Chuanminshen from the middle to the west; they almost meet in the middle of the Yangtze basin. Because the climate varies from humid to semi-humid, an experiment was carried out to find out the two species' demand for soil water and analyze their niche separation by determining changes in their physio-ecological and structural traits in response to different soil water statuses. Results showed that the two species have similar structural characters and functional intensity. Diurnal variations, daily mean values, and relationships of irradiation and temperature to PN, E and WUE among the three treatments indicate that Chuanminshen has more tolerance to drought than Changium. Changes in morphological traits also suggest that Changium adapts to medium to wet soil conditions while Chuanminshen adapts to drought soil conditions. The differences in the adaptation of physiological and morphological traits to the water environment should be part of the reason these two species have become geographically vicarious species. The results indicate that, at least, the mechanism and the approach to conservation for the two species are similar.

Keywords: Ecological replacement; Endangered species; Growth; Morphology; Photosynthesis; Subtropical zone; Transpiration; Water use efficiency.

Introduction

Both Changium smyrnioides Wolff and Chuanminshen violaceum Shen et Shan are monotypic species of the family Umbellaceae. There are some interesting viewpoints on the relationship between Changium and Chuanminshen, for they were classified as the same species before 1980. Shen et al. (1980) indicated that Chuanminshen violaceum should belong to different genera with a distant relation in systematic development with Changium smyrnioides.

Because their fleshy roots are prized in Chinese medicine, and because of land use, the natural populations of the two species have been seriously disturbed by human activity and have decreased in recent years to the point that they are now endangered species (Qiu et al., 2000). Chuanminshen has been cultivated as a medicinal material on a large scale, but Changium has not yet been cultivated.

The two species have narrow distribution areas and can be found only in the drainage basin of the Yangtze River in China. Changium is distributed in the area from the east

to the middle subtropical zone of China while Chuanminshen is from the middle to the west. The distribution areas of the two species meet in the middle of the Yangtze basin, but barely overlap (Figure 1). Thus, the distribution of the two species shows ecological replacement. They should be geographically vicarious from an ecological viewpoint. Because the climate from east to west of the subtropical zone of China varies from humid to semi-humid (Wu, 1983), we suppose each species should fit into different water environments with different morphological adaptations and physio-ecological requirements, especially different demands for water; i.e. there would be niche partitioning between the two species.

To test the hypothesis, we designed an experiment on the appearance and plasticity of the physioecological and structural traits of the two species in response to different soil water statuses (Rajendrudu et al., 1997), to find out their demands for soil water and analyze their niche separation.

Although there are studies of geographically vicarious species, much of the work is on their systematics, morphology, anatomy, molecular biology, genetics (Liu et al., 2002), or paleobiology (Prado and Gribs, 1993). Some work has been done on the distribution patterns and their relationship to climate and water conditions (Prado and

*Corresponding author. Tel & Fax: +86-571-8797-2193; E-mail: jchang@mail.hz.zj.cn


Botanical Bulletin of Academia Sinica, Vol. 44, 2003

species, and finally determining the distribution. In the period from late autumn to spring, the mean temperatures in the east and the west are similar while the west is significantly dryer with less precipitation (Figure 2).

Research was conducted at the plantation of Zhejiang University in the city of Hangzhou, eastern China (12010E, 3015N). Both species were cultivated in pots (height 14.5 cm and diameter of upper surface 17.5 cm) in January 2001, after the seeds germinated and the seedlings had reached 5 cm. Experiencing a growth season, Chuanminshen began dormancy in early June while Changium did so in mid-June. Chuanminshen began frondescing in late-September, and the treatments began in October 2001; Changium frondescence in mid-February 2002, and the treatments begin in early March 2002. The aim of the experiment was to provide the two species with the same treatments at the same life history stages. All treatments were carried out in a glass house with an open wall.

There were ten repetitions of three treatments, expressed by the relative soil water contents (RWC), which is the percentage of the soil water holding capacity (WHC) as follows: (1) Constant saturation (SS), replenished until 100% WHC whenever the RWC of samples dropped to 90% WHC due to evapotranspiration; (2) Alterative water (middle moist, AW50), plants were not watered until the RWC dropped to 50% WHC; and (3) Alterative water (drought, AW20), distilled water was added to saturation whenever RWC dropped to 20%. These treatments simulated field soil water conditions in which rainy and dry weather alternate: SS simulated waterside habitat, and AW50 and AW20 simulated soil moisture conditions with more or less intermittent precipitation, respectively.

Measurements of net photosynthetic rate (PN) and transpiration (E) were carried out from 06:30 to 18:30 on clear days at the beginning of April, every hour in triplicate. PN was measured using a CO2 analyzer (GHX305A, Beijing,

Figure 1. Distribution of Changium smyrnioides and Chuanminshen violaceum.

Gribs, 1993); however, little has been known about the physio-ecological reasons for the ecological replacement of the vicarious species. Our work can increase the understanding of the ecological mechanisms of vicarious distributions of the plant species and assist in the development of approaches to conserve the two endangered species.

Materials and Methods

Both species are winter species in the subtropical zone of the Yangtze basin of China, where the climate and soil environmental conditions are not too cool for plants. The main growth periods of the two species are winter and spring, and the seasonal climate should be the main factor limiting the physiological processes and growth of the

Figure 2. Combination of temperature (solid line) and precipitation (dot line) in east (e.g. Hangzhou) and west (e.g. Chengdu) subtropical zones of China, the distribution region of Changium smyrnioides and Chuanminshen violaceum. The drought period occurs when precipitation line falls below temperature line.


Ge et al. Soil water effects on physioecological traits of two endangered species

China-German cooperation) in the closed system. Diurnal mean photosynthetic rate (PNmean) was calculated by average the PN of every hour from morning to afternoon. E of whole shoots were measured by weighting the whole pots, the upper surfaces of which were covered with plastic film to prevent evaporation from the soil,

E = (Wt-Wt+1)/A

where Wt is the weight of pots the previous time, Wt+1 is the weight the current time, and A is the total area of the leaves in pots. Diurnal mean transpiration rate (Emean) was calculated by the average of the E of every hour. Water use efficiency (WUE) was calculated as PN/E. Photosynthetically active radiation (PAR) above the leaf, air temperature (Ta), leaf temperature (Tl), and relative humidity (RH) were measured simultaneously with PN. An admixture of acetones and ethanol (2:1) was used to distill chlorophyll, and then the absorbency of chlorophyll-a and chlorophyll-b were measured at 663 nm and 645 nm, respectively, with a 751 spectrophotometer (HP, Shanghai). Three individuals of every species were harvested from the three-replication pots. Before drying the samples in an oven at 80C for at least 72 h, leaf area was determined with a portable area meter. Leaf area per unit leaf mass (specific leaf area; SLA), leaf area per unit of total mass (leaf area ratio; LAR), and leaf mass per unit of total mass (leaf mass ratio; LMR) were determined (Hunt, 1978). The differences between the traits were statistically analyzed with ANOVA in SPSS (8.0).

Results and Discussion

Diurnal Pattern of Photosynthesis, Transpiration and Water Use Efficiency in Response to Soil Water Status

Daily changes of photosynthesis differed between the species in the same soil water status (Figure 3). In SS, the PN of Changium increased in the morning in response to PAR, peaked at 10:00, then did not respond to the increase of PAR and held steady until 14:00. In Chuanminshen the PN increased more slowly than Changium and peaked at 13:00, showing a single peak curve. In dry soil (AW20), the PN of Changium showed double peaks (at 9:00 and 14:00) while the PN of Chuanminshen fluctuated at midday (the peak was at 13:00). The diurnal curves of PN of Changium in AW50 were close to those of SS while that of Chuanminshen in AW50 approached AW20.

The diurnal variations of E of both species were similar. The values for Chuanminshen were lower than Changium in SS and AW50 and slightly higher than Changium in AW20.

The E of Changium increased much more slowly than PN in the morning in SS and AW50, causing an increase in WUE of Changium in the early morning, and the PN of Changium was not correlated with E (P>0.05). While the E increased simultaneously with PN for Chuanminshen (P<0.05), that made WUE of Changium higher than Chuanminshen in the morning.

Figure 3. Diurnal variations of the net photosynthetic rate (PN), transpiration (E), water use efficiency (WUE), photosynthetic photon flux density (PAR), leaf temperature (Tl) and air temperature (Ta) for Changium smyrnioides and Chuanminshen violaceum in different soil water statuses (SS, soil water near saturation constantly; AW50, soil water content altered from saturation to 50% soil water holding capacity; AW20, soil water content altered from saturation to 20% soil water holding capacity).

WUE of the two species had higher values and larger fluctuations in the morning than in the afternoon. The differences were more significant in the morning than in the afternoon among the treatments, especially for Changium.

The diurnal variation of PN, E and WUE in response to soil water status indicates that the physiological traits of Changium change little when soil water is kept at a wet or medium wet level, but change significantly when water levels varied from medium wet to drought level conditions. In contrast, the physiological traits of Chuanminshen change little when soil water varied from medium wet to drought level conditions; i.e., Chuanminshen can keep a high physiological vigor at drought level conditions.

The differences between PN and E among the three treatments in Changium outstrip those in Chuanminshen, indicating that Changium is more sensitive in response to soil water than Chuanminshen in photosynthesis and transpiration, while the two species did not show differences in WUE.

Daily Mean Photosynthesis, Transpiration, and Water Use Efficiency Responds to Soil Water Status

The PNmean of Changium showed a significant decrease when conditions changed from medium wet to drought level; in contrast, that of Chuanminshen showed an increase with a change from medium wet to drought level


Botanical Bulletin of Academia Sinica, Vol. 44, 2003

conditions. PNmean of Changium was similar in SS and AW50, but that of Chuanminshen was similar in AW50 and AW20 (Figure 4). The PNmean of Changium was higher than that of Chuanminshen in SS and AW50, but lower in AW20.

The Emean of two species was SS>AW50>AW20, entirely in response to soil water content. Like PNmean, the Emean of Changium was higher than that of Chuanminshen in wet to medium wet soil, but lower than Chuanminshen in drought.

The highest daily mean WUE of Changium was in AW50 while the highest in Chuanminshen was in AW20, with the smallest in AW50. The WUE of Changium was nearly two times that of Chuanminshen in medium wetness, but was lower than Chuanminshen in drought. This indicates that a medium soil water status suits Changium best and that it has a lower tolerance than Chuanminshen for drought.

The coefficient variation of PNmean and Emean of Changium (56%, 52%) was larger than that of Chuanminshen (30%, 35%) in three treatments, showing that the PN of Changium had a higher phenotypic plasticity in response to soil water content than Chuanminshen.

Diurnal Pattern of Photosynthesis, Transpiration and Water Use Efficiency in Response to Light and Temperature

An increase in PAR and Tl brought a linear increase in PN of Changium in wet to medium wet soil (SS and AW50), and the response was AW50>SS (Table 1). However, the PN of Changium did not respond to PAR or Tl in drought. Subsection analysis showed that the PN only responds to PAR (R2=0.725**, n=24) below light saturation (PAR<300

Figure 4. Daily mean net photosynthetic rate (PNmean), daily mean transpiration (Emean) and water use efficiency (WUE) of Changium smyrnioides and Chuanminshen violaceum in different soil water statuses (legends as in Figure 3).


Ge et al. Soil water effects on physioecological traits of two endangered species

the fourth euphylis on, i.e. they are almost the same in the early stages of individual development and differ later. We suggest that the systematic development of both Changium and Chuanminshen is very close.

Changium resides under the forest gap caused by defoliated trees in winter. Individuals finish vegetative growth after re-foliation of deciduous trees, so as to avoid shading and competition with other plants (Busso et al., 2001). Such a pattern of Changium could be seen as a competition-avoidance strategy through temporal niche partition. With higher SLA, LAR, and chloral content (Table 3), Changium uses irradiation fully in carbon accumulation in a short time in compensation for its short growth period. In humid environments, the plant often has a larger SLA and LAR than in drought (Fownes, 1999; Gurnier et al., 2001). The humid climate in Changium's distribution area, the east to middle subtropical zone of China, ensure it a larger SLA and LAR. Chuanminshen has a lower SLA and LAR, adapts to the dryer environment in the west, and has a longer growth period to meet the requirement of carbon accumulation.

Conclusions

The two species, Changium smyrnioides and Chuanminshen violaceum have similar structural charac

mol m-2s-1) and only to Tl (negative correlation, R2= 0.287**, n=28) above light saturation, meaning that temperature stress occurs at higher temperatures in drought conditions.

In contrast to Changium, PN of Chuanminshen responds to PAR and Tl in all three soil water statuses, and the degree of response was SS>AW20>AW50. It means that PN of Chuanminshen still functions normally in the dry soil in which the physiological process of Changium has changed.

The E of both species responds to PAR and T1 (positive) and RH (negative) significantly and the slope of correlation was SS>AW50>AW20. In wet to medium wet conditions, Changium responds to the above three factors more sensitively than Chuanminshen but does just the opposite in drought conditions (Table 2).

The diurnal pattern of the main physiological traits suggests that the PN of Chuanminshen has a higher tolerance to drought than that of Changium.

Morphology and Growth Traits of the Two Species

Our investigation concluded both species to be nearly identical in morphology from the first to the third euphylis of the seedling, and then the leaves become different from


Botanical Bulletin of Academia Sinica, Vol. 44, 2003

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ters and functional intensity, such as the net photosynthetic rate (PN), transpiration (E), water use efficiency (WUE), and the fact that each is endangered. Meanwhile, these two species showed different plasticity for the main physiological process and their morphological traits respond differently to different soil water statuses. Changium smyrnioides demands medium to wet soil conditions while Chuanminshen violaceum adapts readily to droughty soil. The differences in the adaptation of physiology and morphology to the water environment cause niche partitioning, part of the reason these two species are geographical replacements in distribution. The results indicate that, at least, the mechanism and the approach of conservation for the two species should be similar.

Acknowledgements. We are grateful for the funding provided by the National State Key Basic Research and Development Plan (973) (No. G2000046805).

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