Impact of elevated CO₂ concentration under three soil water levels on growth of Cinnamomum camphora

Impact of elevated CO 2 concentration under three soil water levels on growth of Cinnamomum camphora first-author †E-mail:zhaoxingzheng@zju.edu.cnwanggx@zju.edu.cn †E-mail:zhaoxingzheng@zju.edu.cnwanggx@zju.edu.cn zhaoxingzheng@zju.edu.cn corresp ‡Corresponding Author ‡Corresponding Author †E-mail:zhaoxingzheng@zju.edu.cnwanggx@zju.edu.cn †E-mail:zhaoxingzheng@zju.edu.cnwanggx@zju.edu.cn zhaoxingzheng@zju.edu.cn Agro-Ecology Institute, School of Life Sciences, Zhejiang University, Hangzhou 310029, China Agro-Ecology Institute, School of Life Sciences, Zhejiang University, Hangzhou 310029, China Forest plays very important roles in global system with about 35% land area producing about 70% of total land net production. It is important to consider both elevated CO 2 concentrations and different soil moisture when the possible effects of elevated CO 2 concentration on trees are assessed. In this study, we grew Cinnamomum camphora seedlings under two CO 2 concentrations (350 μmol/mol and 500 μmol/mol) and three soil moisture levels [80%, 60% and 40% FWC (field water capacity)] to focus on the effects of exposure of trees to elevated CO 2 on underground and aboveground plant growth, and its dependence on soil moisture. The results indicated that high CO 2 concentration has no significant effects on shoot height but significantly impacts shoot weight and ratio of shoot weight to height under three soil moisture levels. The response of root growth to CO 2 enrichment is just reversed, there are obvious effects on root length growth, but no effects on root weight growth and ratio of root weight to length. The CO 2 enrichment decreased 20.42%, 32.78%, 20.59% of weight ratio of root to shoot under 40%, 60% and 80% FWC soil water conditions, respectively. And elevated CO 2 concentration significantly increased the water content in aboveground and underground parts. Then we concluded that high CO 2 concentration favours more tree aboveground biomass growth than underground biomass growth under favorable soil water conditions. And CO 2 enrichment enhanced lateral growth of shoot and vertical growth of root. The responses of plants to elevated CO 2 depend on soil water availability, and plants may benefit more from CO 2 enrichment with sufficient water supply. Cinnamomum camphora CO 2 concentration Soil moisture Plant growth Root to shoot ratio INTRODUCTION 1 Current atmospheric carbon dioxide (CO 2 ) concentration has increased by about 100 μmol/mol since the industrial revolution and is predicted to continue rising approximately 1~2 μmol/mol each year (Keeling et al., 1995 1995 ). During this century CO 2 levels could be doubled or tripled compared to pre-industrial revolution levels (IPCC, 2001 2001 ). And there is about 35% of land area covered with forest ecosystems producing about 70% of total land net production (Kramer, 1981 1981 ; Melillo et al., 1993 1993 ; Meyer and Turner, 1992 1992 ). Forest plays very important roles in the global system than we have always thought. So it is important to consider both elevated CO 2 concentrations and the differences in soil moisture when the possible effects of elevated CO 2 concentration on trees are assessed. Numerous experiments showed that high atmospheric CO 2 concentration leads to increases in photosynthetic rate and whole-plant growth in many C 3 species, while in C 4 species the increasing effects were much lower (Bowes, 1993 1993 ; Finzi et al., 2001 2001 ; Ghannoum et al., 1997 1997 ; 2000 2000 ; Gifford, 1992 1992 ; Griffin et al., 2000 2000 ; Gunderson et al., 2000 2000 ; Idso and Idso, 1994 1994 ; Hymus et al., 2001a 2001a ; 2001b 2001b ; Jach and Ceulemans, 2000 2000 ; Watling et al., 2000 2000 ). The effect of CO 2 enrichment on plants was limited by soil fertility levels (Coruzzi and Zhou, 2001 2001 ; Cotrufo et al., 1998 1998 ; Deng and Woodward, 1998 1998 ; Loladze, 2002 2002 ; Poorter, 1998 1998 ; LaDeau and Clark, 2001 2001 ; Oren et al., 2001 2001 ; Walch-Liu et al., 2001 2001 ) and varies under different soil moisture regimes (Wu et al., 2002 2002 ; 2004 2004 ). Most studies were carried out under favorable water conditions. However, data on the interactive effects of CO 2 and soil moisture on plants are scarce and often contradictory. Some authors claim that the percentage increase in plant growth due to elevated CO 2 concentration is generally not reduced by water stress (Idso and Idso, 1994 1994 ; Kang et al., 2002 2002 ) whereas the results of many other theoretical projections and field or greenhouse experiments suggest that the relative effects of CO 2 enrichment on plants are constrained by less than optimal levels of soil moisture (Poorter, 1993 1993 ; 1998 1998 ; Wu and Wang, 2000 2000 ). Experiments on broad bean ( Vicia faba ), spring wheat under elevated CO 2 concentration of different soil water contents had been conducted formerly by our group (Wu and Wang, 2000 2000 ; Lin and Wang, 2002 2002 ; Wu et al., 2002 2002 ; 2004 2004 ). Our hypothesis is that plant morphology of shoot or root would vary to adapt to environment changes, and that the responses to elevated CO 2 concentration may be controlled by soil water availability and experiments with growing seedlings of Cinnamomum camphora under two CO 2 concentrations (350 μmol/mol and 500 μmol/mol) and three soil moisture levels [80%, 60% and 40% field water capacity (FWC)] were conducted to observe the effects of exposure of tree seedlings to elevated CO 2 concentration on the morphology and biomass of underground and aboveground plant parts, and their dependence on soil moisture. MATERIALS AND METHODS 1 Plant materials and growth conditions 2 Cinnamomum camphora is a dense broadleaved evergreen that can grow to 15~46 m tall and 5 m in diameter. The shiny foliage is made up of alternate 2~10 cm oval leaves dangling from long petioles with each leaf having three distinct yellowish veins and with the area of whole adult leaf being about 3000~6000 mm 2 . The flowers come out in the spring on branching, followed by large crops of fruit comprised of round pea sized berries. It comes from China, Japan, Korea and adjacent parts of East Asia, where it grows in mesic forests at well-drained sites. An experiment was conducted at Huajiachi campus, Zhejiang University, Hangzhou, China. Plants were grown in two identical controlled greenhouses (Conviron, Controlled Environments Ltd., Canada), one supplied with ambient CO 2 concentration ((350±30) μmol/mol), and another with elevated CO 2 concentration ((500±30) μmol/mol). There were three water level treatments [80%, 60% and 40% field water capacity (FWC)] with ten replicate pots per water level in each greenhouse. The environmental variables including CO 2 concentration, temperature and light intensity inside the two greenhouses were continuously monitored. Temperature and light intensity were the same in both greenhouses. Only CO 2 concentration was varied in the two greenhouses, one with ambient CO 2 , the other with elevated CO 2 . The environmental sensors and controlling systems of the two greenhouses were carefully calibrated before start of the experiment, and the environmental factors in the greenhouses were periodically monitored during the entire course of experiment in order to minimize the variance induced by the station in the greenhouses and between greenhouses heterogeneity of environmental conditions. Air-conditionings inside the greenhouses facilitated the circulation and thorough mixing of air. The temperature inside the greenhouses was controlled at 25~30 °C during daytimes, and to that of the atmosphere during nighttimes. Average relative humidity inside the greenhouses was about 40% during the growth seasons and was measured but not controlled. The environmental variables such as CO 2 concentration, and daytime temperature inside the greenhouses were continuously monitored and controlled by a computer. Before sowing, the soil was irrigated to 80% FWC. Then, soil samples were taken and analyzed at the laboratory. The results of analysis revealed that soil properties were: pH 7.0, organic matter 1.61%, available N 85.38 mg/kg, available P 31.01 mg/kg, available K 46.58 mg/kg, and FWC 35.6%. Three soil water levels, 40%, 60% and 80% FWC, were applied to each greenhouse (ten pots per treatment), and kept constant throughout the entire experiment period by simply weighing each pot every 2 d and adding the water lost accordingly (Wu et al., 2004 2004 ). At the late growth phase, when total biomass of plant accounted for more than 0.5% of the total pot weight (plastic pot+soil+soil water+biomass), that fraction of biomass was taken into account. Growth measurements 2 Shoot height and root length were measured at first, then six seedlings were randomly selected to determine the wet and dry weight of shoot and root before transplanting, and all the plants were harvested at the end of the experiment. All component dry weights were measured following oven-drying to constant weight at 85 °C. And the water content of shoot and root was calculated by (wet weight−dry weight)/(dry weight). Plants were finally harvested on 20 July, 3 months (92 d) after transplanting. Experimental design and statistical design 2 Our experiment consisted of two CO 2 levels (350 μmol/mol and 500 μmol/mol) and three soil water levels (40%, 60% and 80% FWC). A factorial design was used with a total of six treatments, which were designated as HC, HD, MC, MD, LC and LD, respectively, where H, M and L represented high (80% FWC), medium (60% FWC) and low soil moisture (40% FWC), C and D represented current (350 μmol/mol) and elevated CO 2 concentration (500 μmol/mol), respectively. Each treatment had ten replicate pots in the greenhouses. Since the environment was the same in the two greenhouses throughout the plant growth period, pot replication was adequate. Thirty pots were placed in each greenhouse and controlled to three soil moisture levels. H, M and L pots were placed alternately in the greenhouses and randomly changed every 2 d after weighing for soil moisture control, and the greenhouses were changed every week to minimize the variance induced by the station in the greenhouses and by the between-greenhouses heterogeneity of environmental conditions. Data were analyzed using SPSS 11.5 software for two-way ANOVA and standard deviation. Two-way ANOVA was carried out on shoot height/root length, shoot/root weight, water content of shoot/root, as well as length/weight ratio of root to shoot, and ratio of shoot/root weight to height/length to determine the effects of CO 2 level, soil moisture level and their interactions. Because ANOVA and most other statistical tests of significance do not work very well with ratio in very high or very low numbers, the data on ratio of shoot weight to height (WH) and ratio of root weight to length (WL) whose values were less than 0.3 were arcsine transformed with the equation of y =arcsin x (where y is the data for ANOVA analysis, and x is the original data) before the analyses. Mean values and error bars are calculated on the ten replicate pots of each treatment. And the standard errors are shown with error bars in the figures, respectively. RESULTS 1 Impacts on plant shoot growth of higher CO 2 concentration under three soil water levels 2 Although there were no significant differences between the CO 2 concentrations, higher CO 2 concentration increased shoot height by 6.39%, 6.92% and 1.72%, and shoot weight by 1.45%, 27.04%, 36.25% under 40%, 60% and 80% field water capacity (FWC) soil moisture, respectively (Table 1 1 ). The positive effect of high CO 2 concentration on shoot biomass growth of Cinnamomum camphora was greater under high soil moisture conditions. As a result, the difference in shoot weight among the three soil moisture levels was greater under elevated CO 2 . Elevated CO 2 concentration strongly affected shoot water content (SWC) (Table 2 2 ). SWC was increased greatly under 40% and 60% FWC soil moisture, and decreased by 7.38% under 80% FWC soil moisture. Plants grown under elevated CO 2 concentration had larger ratio of shoot weight to height (WH), while plant height was no different between the two CO 2 concentrations (Table 1 1 ). The WH exposed to the higher CO 2 concentration increased by 39.58% and 20.45% under 80% and 60% FWC soil moisture, respectively. However, under 40% FWC soil water level, the ratio decreased by 3.37% (Fig. 1 1 , P < 0.05). On the other hand, water deficit significantly decreased plant WH under both ambient and elevated CO 2 concentration (Fig. 1 1 , P < 0.01). Significance between 350 μmol/mol and 500 μmol/mol CO 2 concentration (NS: P > 0.05, ** : P < 0.01, n =10); for each element, values in the same list followed by different letters are significantly different ( P < 0.05, n =10), and the data are shown with mean value± SE 350 μmol/mol 500 μmol/mol P 3 SH (cm) 40% FWC 30.83±2.24 a 32.80±1.57 a NS 60% FWC 33.11±1.46 ab 35.40±1.69 a NS 80% FWC 37.66±3.22 b 38.31±2.30 a NS 3 SW (g) 40% FWC 4.15±0.64 a 4.21±0.57 a NS 60% FWC 4.59±0.67 a 5.83±0.59 a NS 80% FWC 5.90±1.02 a 8.04±0.97 b NS 3 RL (cm) 40% FWC 37.18±1.65 a 39.25±2.25 a NS 60% FWC 34.22±3.91 a 43.93±1.39 ab ** 80% FWC 46.24±1.91 b 47.81±0.80 b NS 3 RW (g) 40% FWC 4.48±1.00 a 3.69±0.63 a NS 60% FWC 5.11±0.73 a 4.94±0.67 a NS 80% FWC 4.48±0.71 a 5.44±0.84 a NS Significance between 350 mmol/mol and 500 mmol/mol CO 2 concentration (NS: P > 0.05, * ： P < 0.05, ** ： P < 0.01, n =10); for each element, values in the same list followed by different letters are significantly different ( P < 0.05, n =10), and the data are shown with mean value± SE 350 μmol/mol 500 μmol/mol P 3 SWC 40% FWC 1.89±0.04 a 2.06±0.08 a * 60% FWC 1.96±0.03 a 2.52±0.04 b ** 80% FWC 2.14±0.02 b 1.99±0.05 a * 3 RWC 40% FWC 1.46±0.06 a 1.69±0.20 a NS 60% FWC 1.22±0.03 a 1.93±0.03 a ** 80% FWC 1.81±0.03 b 1.93±0.07 a NS Effects of elevated CO 2 concentration on plant root growth under different soil moisture 2 Root length was increased by 5.57%, 28.37% and 3.40% by the higher CO 2 concentration under 40%, 60% and 80% FWC soil moisture, and there was significant difference between the CO 2 concentrations ( P < 0.01, Table 1 1 ). While high CO 2 concentration decreased root weight by 21.66% under low soil moisture (40% FWC), and increased by 21.26% under high moisture (80% FWC), but there was no significant difference between them (Table 1 1 ). The positive effect of high CO 2 concentration on C . camphora root growth was only shown under high soil moisture conditions. Root water content (RWC) was obviously increased by high CO 2 concentration under favourable soil water condition (60% FWC, Table 2 2 ), while under 40% and 80% FWC soil moisture, there were no significant responses of RWC to CO 2 content variability. The tendency of ratio of root weight to length (WL) was similar to that of root weight, and decreased under low water levels but increased under high soil moisture conditions. However, there were no differences between the two CO 2 concentrations and three water levels of plant W/L ratio (Fig. 2 2 ). Responses of ratio of root to shoot to elevated CO 2 concentration and different water supply levels 2 The positive effect of high CO 2 concentration on length ratio of root to shoot of C . camphora was only shown under 60% FWC soil moisture, while under 40% and 80% FWC soil water levels CO 2 enrichment resulted in 5.03% and 3.53% decrease, respectively (Fig. 3 3 ). ANOVA analysis indicated that the interaction between elevated CO 2 concentration, soil water levels, and CO 2 concentration×water on plant growth was not significant. The effects of CO 2 concentration enrichment and different soil water conditions on weight ratio of root to shoot of C . camphora are shown in Fig. 4 4 indicating that high CO 2 concentration decreases the ratio. And high soil water content (80% FWC) decreases the ratio by 42.4% and 29.4% compared with 60% FWC under current and elevated CO 2 concentration, respectively. CO 2 concentration enrichment decreased the ratio by 20.42%, 32.78%, 20.59% under 40%, 60% and 80% FWC soil water conditions, respectively. There were significant differences between different CO 2 concentration ( P < 0.01) and soil moisture ( P < 0.01), while the interaction between CO 2 enrichment and soil water levels was not significant. DISCUSSION 1 Effects of CO 2 enrichment on plant morphology of Cinnamomum camphora 2 In the present experiments, CO 2 enrichment significantly increased shoot weight as reported previously in many other studies (Curtis and Wang, 1998 1998 ; DeLucia et al., 1999 1999 ; Eichelmann et al., 2004 2004 ; Niklaus et al., 2001 2001 ; Norby et al., 1999 1999 ; Smith et al., 2000 2000 ; Tissue et al., 2001 2001 ; Usami et al., 2001 2001 ; Woodward, 2002 2002 ) but without obvious impacts on its height, the ratio of shoot weight to height (WH) was bigger under elevated CO 2 concentration ( P < 0.05, Fig. 1 1 ). While the effects of CO 2 concentration enrichment on root growth was significant on length rather than weight, especially under favorable conditions (60% FWC soil moisture), the ratio of root weight to length (WL) in higher CO 2 concentration was half of that in current concentration (Fig. 2 2 ). The positive effects of high CO 2 concentration on length ratio of root to shoot of C . camphora was not significant, while there was significant differences between different CO 2 concentration ( P < 0.01) on weight ratio. CO 2 and soil water levels had significant effects on plant water content (Table 2 2 , P < 0.01). Then we concluded that CO 2 enrichment should favour plant water conservation which accords with reported positive effects of elevated CO 2 concentration on plant water use efficiency (Allen, 1990 1990 ; Ellsworth, 1999 1999 ; Gavazzi et al., 2000 2000 ; Hui et al., 2001 2001 ; Liao and Wang, 2002 2002 ; Wu and Wang, 2000 2000 ; Wu et al., 2002 2002 ; 2004 2004 ). Then we suggest that plant morphology could be altered under future high CO 2 concentration conditions. High CO 2 enhances plants shoot lateral growth more than vertical growth, whereas there was little effect on root growth. Interactive effect of CO 2 concentration and soil moisture on plant growth 2 Observation results indicated that CO 2 concentration and soil moisture had significant interactive effects on plant growth. High CO 2 could alleviate the negative effects of water deficit on plants on the one hand, and the positive effects of high CO 2 concentration on plant growth were constrained by less favorable soil moisture conditions on the other hand. This accords with most previous reports (Conroy and Hocking, 1993 1993 ; Poorter, 1998 1998 ; Catovsky and Bazzaz, 1999 1999 ; Ward et al., 1999 1999 ; Wu and Wang, 2000 2000 ). Moreover, still other reports on similar experiments suggested that growth induced by high CO 2 was greater under drought stress than under high soil moisture (Gifford, 1992 1992 ). This may be partly attributed to the different method of water control. In their experiments, dry treatment was realized by periodically supplying a preset amount of water (very little) or giving no water. The quantity of water added to maintain the soil moisture gradient did not give out the actual soil moisture. This may lead to actually better soil water conditions in high CO 2 treatment than in ambient treatment since plants use water more economically under high CO 2 concentration conditions. Additionally, use of different factors, such as temperature and light intensity, may alter the interaction between CO 2 concentrations and soil moisture. Thus, based on the results of ours and those from the literature, it can be concluded that the positive effects of CO 2 enrichment on plants are greater under more suitable conditions. Depending on the life history and evolutionary traits of species, different species of wild plants and their cultivated relatives or even different cultivars of the same domesticated species may respond differently to an environmental gradient as realized by the researchers. For instance, Catovsky and Bazzaz ( 1999 1999 ) found that under elevated atmospheric CO 2 levels, the seedling growth of paper birch often found on more xeric, well-drained soils, was enhanced more by low soil moisture treatment than by high soil moisture treatment, while yellow birch usually associated with more mesic sites, showed more improved growth under high soil moisture treatment (Catovsky and Bazzaz, 1999 1999 ). 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Effects of elevated CO 2 concentration on growth, water use, yield and grain quality of wheat under two soil water levels . Agriculture, Ecosystems and Environment , 2004 . 104 ( 3 ): 493 - 507 . DOI: 10.1016/j.agee.2004.01.018 http://doi.org/10.1016/j.agee.2004.01.018 . Effects of elevated CO2 concentration on growth, water use, yield and grain quality of wheat under two soil water levels 10.1631/jzus.2006.B0283 Project supported by the National Natural Science Foundation of China (Nos. 30170161 and 90102015) and the Doctoral Disciplines Programs Foundation of Ministry of Education of China (No. 20030335043) 2005-08-10 2005-10-19 2006-03-13 2022-01-05 Copyright © 2006, Journal of Zhejiang University Science 2006 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 04 07