Journal of Experimental Botany, Vol. 52, No. 362, pp. 1913-1923,
September 1, 2001
© 2001 Oxford University Press
Original Papers |
Interactive effects of soil temperature, atmospheric carbon dioxide and soil N on root development, biomass and nutrient uptake of winter wheat during vegetative growth
1 Department of Plant Research, Risø National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark
2 Department of Evolution, Ecology, and Organismal Biology, The Ohio State University, Columbus, Ohio, USA
Received 29 November 2000; Accepted 6 June 2001
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Abstract |
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Nutrient requirements for plant growth are expected to rise in response to the predicted changes in CO2 and temperature. In this context, little attention has been paid to the effects of soil temperature, which limits plant growth at early stages in temperate regions. A factorial growth-room experiment was conducted with winter wheat, varying soil temperature (10 °C and 15 °C), atmospheric CO2 concentration (360 and 700 ppm), and N supply (low and high). The hypothesis was that soil temperature would modify root development, biomass allocation and nutrient uptake during vegetative growth and that its effects would interact with atmospheric CO2 and N availability. Soil temperature effects were confirmed for most of the variables measured and 3-factor interactions were observed for root development, plant biomass components, N-use efficiency, and shoot P content. Importantly, the soil temperature effects were manifest in the absence of any change in air temperature. Changes in root development, nutrient uptake and nutrient-use efficiencies were interpreted as counterbalancing mechanisms for meeting nutrient requirements for plant growth in each situation. Most variables responded to an increase in resource availability in the order: N supply >soil temperature >CO2.
Key words: Elevated carbon dioxide, nitrogen, phosphorus, soil temperature, winter wheat.
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Introduction |
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Global climate change in the next 100 years is likely to affect plant C assimilation, growth, biomass allocation, and nutrient uptake, since two potentially limiting resources, atmospheric CO2 and temperature, will be modified in a range where large responses can be expected. Most C3 plants respond, at least in the short term, to an increase in CO2 availability since the activity of Rubisco (ribulose-1,5-biphosphate carboxylase/ oxygenase) is limited at the present CO2 concentration (Moore et al., 1999
). Experimental increases in atmospheric CO2 concentration have, therefore, often resulted in increased plant photosynthesis and growth (Curtis and Wang, 1998; Stitt and Krapp, 1999), and the amount of C allocated below-ground (Rogers et al., 1994). However, plants may acclimate to increased CO2 availability because nutrient limitation or insufficient sink strength may restrict the plant's ability to use the increased production of photosynthates and a reduction in photosynthesis may occur (Wardlaw, 1990; Farrar and Williams, 1991).
Mean global temperature is also predicted to rise by 1.5–4.5 °C, with local variations, within the next 100 years (Houghton et al., 1992). Low soil temperature has been surprisingly neglected in most studies although it is known that it can limit shoot and root growth, and nutrient and water uptake, especially below 10 °C which is a common temperature for early crop growth in temperate regions (Bowen, 1991). In this temperature range, specific absorption rates of nutrients can show marked temperature dependency, with large changes in plant growth and nutrient absorption after a relatively small increase in soil temperature (Moorby and Nye, 1984; Clarkson et al., 1992; Engels and Marschner, 1992). Temperatures below the plant's optimum range usually result in increased relative investment of biomass in roots, whether temperature for the entire plant or only root temperature is considered (Clarkson et al., 1988). Low soil temperatures reduce overall growth and tend to increase carbon allocation to roots because nutrient and water uptake are reduced (Lambers et al., 1995; Li et al., 1994). At low soil temperatures, shoot demand seems to control uptake and translocation of N, K and Ca, but not P (Engels et al., 1992). Phosphorus uptake is usually more depressed by low soil temperature than is the uptake of other nutrients (Bravo-F and Uribe, 1981; Engels et al., 1992). This may be mainly due to reduced root growth because at low temperatures phosphate uptake is dominated by root production rather than by chemical nutrient availability in soil or by physiological uptake capacity (Mackay and Barber, 1984). However, direct effects of low temperature on roots or on functioning of active transport systems cannot be ruled out (Engels et al., 1992; Engels and Marschner, 1992).
Three major growth-limiting factors of temperate regions are thus likely to be changed in the coming century: temperature, atmospheric CO2 concentration, and nutrient requirements. Moreover, their effects are known to be interactive and are therefore of particular interest in understanding and predicting climate change effects on agriculture. Experimental soil temperature studies are few since soil temperature is difficult to manipulate, yet it is much more stable than air temperature and changes slowly making its effects easier to model and predict than those of air temperature. Several authors have reported interactive effects of atmospheric CO2 and air temperature on wheat biomass (Mitchell et al., 1993, 1995; Kimball et al., 1995; Rawson, 1995; Batts et al., 1997), but the direction of the effects was not always the same or varied from year to year. However, most of these studies have been conducted in the field, with no or inadequate monitoring of soil temperature. Also, their emphasis was on plant development and crop yield measurements whereas much less attention was paid to the underlying physiological mechanisms.
An experiment was conducted here with winter wheat to explore the mechanisms for meeting the nutrient requirements for vegetative growth after varying soil temperature, atmospheric CO2 and N supply. Among the mechanisms hypothesized to counterbalance increased growth requirements at higher atmospheric CO2 concentrations and temperatures are changes in biomass allocation, root development, nutrient uptake and nutrient use efficiencies (BassiriRad et al., 1996). The authors were particularly interested in (1) how soil temperature would influence biomass allocation to roots, root development and P uptake, (2) the interactions among soil temperature, atmospheric CO2 and N supply in affecting photosynthetic rates, root length development, and nutrient uptake, and (3) whether root development, nutrient uptake, nutrient-use efficiency or biomass allocation would respond to changes in these environmental factors in order to meet altered plant growth requirements.
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Experimental design
The experiment was a complete randomized block factorial experiment with three factors and two levels in each factor: atmospheric carbon dioxide (ambient and elevated); soil temperature (10 °C and 15 °C ); soil N (high and low). There were six replicates for each treatment combination.
Soil and plant
The soil was collected from the arable layer of an organic cropping site in Denmark. This soil (49.9% sand, 31.8% silt, 16% clay, 1.36% organic matter) had 0.27 meq Mg l-1, 0.06 meq Na l-1, 10.98 meq Ca l-1, 0.27 meq K l-1, 11.58 meq CEC l-1, 0.14% total N, and 27 mg kg-1 NaHCO3-extractable P. Soil was air-dried, sieved through an 8 mm mesh and mixed with quartz sand 1:1, w:w. This resulted in a reduction of plant available P to 18 mg kg-1. The soil was then mixed with the following nutrients as powder (mg kg-1): KCl (100), MgSO4.7H2O (100), MnSO4.H2O (10), CuSO4.5H2O (5), ZnSO4.7H2O (5), CoSO4.7 H2O (1), Na MoO4.2H2O (0.5), and KH2PO4 (131.7). An initial application of 4 and 20 mg N kg-1 was also added as NH4NO3 to the low-N and high-N treatments, respectively. Pots were PVC cylinders, 20 cm diameter and 50 cm depth, sealed at the bottom. A butyrate-plastic mini-rhizotron tube, scribed with 1 cm2 quadrats, was placed diagonally in the pot and the pots were filled with 14 kg of the soil:sand mixture.
Winter wheat (Triticum aestivum L. cv. Terra) seeds were germinated, planted in trays, and maintained for 6 weeks in a growth room with low light and 2 °C constant temperature for vernalization. Five vernalized seedlings were planted in each pot. The planting density, 5 plants in 0.027 m2, was lower than that used in the field, but was considered suitable for root observations and prevented the plants from being pot bound.
Growing conditions
Pots were placed inside air-cooled, 12-pot capacity chambers where soil temperature could be controlled separately from room air temperature. Chambers were set to maintain soil temperatures of approximately 10 °C, the average for the corresponding growth period in Denmark at 25 cm depth (Jakobsen and Nielsen, 1983; B Jensen, personal communication) and 15 °C soil temperature to simulate soil warming. During the day, the actual soil temperature in the pots in the 10 °C treatment was 9 °C constant at the bottom of the pots, 10–12 °C at 20 cm depth, and 10–15 °C towards the soil surface. Soil temperature in the 15 °C treatment was 14 °C constant at the bottom of the pots, 15–17 °C at 20 cm depth, and 15–20 °C towards the soil surface. These fluctuations in the top 20 cm were observed during the day as a consequence of heating from lamps and changing air temperature. Soil temperature at night was more uniform: 9–10–11 °C and 14–15–16 °C (bottom–20 cm–surface) in the 10 °C and 15 °C treatments, respectively.
Two soil-cooling chambers, one for each soil temperature treatment, were placed in each of two growth rooms at Risø Experimental Risk Assessment Facility (RERAF, Risø National Laboratory, Denmark). The growth rooms were operated at ambient (measured experiment mean 368 ppm) or elevated (measured experiment mean 688 ppm) atmospheric CO2. Pots were not rotated within soil-cooling chambers, but each chamber was divided into six blocks to account for differences due to position. Each growth room was maintained at either ambient or elevated atmospheric CO2 for a 1 week period and then the CO2 concentration was switched. The soil-cooling chambers were moved to the appropriate room to maintain the CO2 treatments, and rotated within the room.
Plants were grown for 9 weeks with gradual increases in air temperature typical of field conditions. Air temperature was 8 °C constant during the night and gradually increased to 14 °C during the day for the first 3 weeks; for the last 6 weeks it was 10 °C constant during the night and gradually increased to 16 °C during the day. Daylength was also increased from 14 h during the first 3 weeks to 16 h during the rest of the experiment. Photosynthetic photon flux density gradually increased to, and gradually decreased from, a midday maximum of 750±50 µmol m-2 s-1, with a 6 h period at maximum light intensity during the first 3 weeks which was increased to 8 h during the rest of the experiment.
Pots were watered to field capacity for the first 2 weeks and then the water supply was increased by 1% of the gravimetric water content every subsequent week to ensure adequate moisture. Watering was done by weight and water use was registered along the experiment. Nutrient solution, containing 2 mM NH4NO3 for the high-N treatment or 0.4 mM NH4NO3 for the low-N treatment, was added once a week for the first 3 weeks and twice a week in the subsequent weeks to a final amount of 150 mg N kg-1 soil and 30 mg N kg-1 soil in the high-N and low-N treatments, respectively.
Sampling, measurements and harvest
Root images were taken every week using a mini-rhizotron camera (BTC-1.125, Bartz Technology Co., Santa Barbara, USA) and analysed using the computer program RooTracker (Duke University Phytotron, NC, USA). Measurements of root length and root diameter were used to calculate weekly root production, root loss, standing total root length (root production minus root loss), mean root diameter of standing total root length at each week, and the number of root segments initiated and their survival period. Root length production was defined as the appearance of new root segments or any increase in root length of previously measured segments. Root length loss was defined as the entire or partial disappearance of root segments. Disappearance was used as a criterion for root loss instead of changes in appearance because both dark and light root segments were observed in newly produced roots. Roots seen for the first time on weeks 2, 4 and 6 (therefore excluding growth of previously measured root segments) were considered discrete cohorts and were followed until they died or until the end of the experiment.
Photosynthetic rates were measured 6 weeks after planting using a randomly selected, second youngest, fully developed leaf from each pot. Measurements were made using a PP systems, CIRAS-1 with automatic leaf cuvette, at light saturation (1500 µmol m-2 s-1) and reproducing the conditions of the growth rooms (air temperature was 16 °C at that time), and the experimental air CO2 concentration of each treatment (Ca=360 or 700 ppm). A leaf disc of fixed area taken from the same leaf following the measurements was dried and weighed.
Plants were harvested 60 d after planting, 1 week after onset of flowering. Shoots were divided into stems, leaves, and inflorescences, dried and weighed, and then mixed, ground and wet oxidized for P and N analyses. Total P was measured by the molybdate-blue method (Murphy and Riley, 1962) and total N was measured by the Kjeldahl method (Bremner and Mulvaney, 1982) using a Technicon AutoAnalyser II. Roots were washed thoroughly, subsampled for root nutrient analysis, dried, weighed and analysed for P and N as described above.
Statistical analysis
Effects of soil temperature, atmospheric CO2, blocks, and soil N were tested by four-way ANOVA. A priori tests were carried out as linear contrasts regardless of significance level in ANOVA and treatment means were compared with a Tukey test. Repeated measures ANOVA, and ANOVAs for each measuring date were used to test the significance of main effects and factor interactions in root length variables that were measured weekly. Data sets not meeting assumptions for ANOVA were transformed as required, but the results are presented in their original scale of measurement. Differences were considered marginally significant at p<0.1 and significant at p<0.05.
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Results |
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Root development
Total root length increased steadily across all treatments until week 6 and remained unchanged afterwards (Fig. 1
). The effects of soil temperature and N supply on total root length were consistent, additive and highly significant (Fig. 1; Table 1). Total root length responded to atmospheric CO2 enrichment only in the first 2 weeks, with no significant effects of CO2 when considered across the entire 8 week period (Table 1). There were no treatment interactions on any given date, but 2-factor interactions and the 3-factor interaction were significant across all dates (Table 1). The 15 °C–high-N treatment had the highest, and the 10 °C–low-N treatment the lowest, root length at either CO2 level. There was a reduction in total root length in the elevated-10 °C–low-N treatment compared to the ambient-10 °C–low-N treatment (Fig. 1) but there were no other CO2 effects across the soil temperature-N supply combinations. Root lengths in the 10 °C–high N and the 15 °C–low N treatments were always similar, indicating that the increase in N supply at low soil temperature and the increase in soil temperature at low-N supply led to similar stimulatory effects.
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increase, 
decrease. +P<0.1, *P<0.05, **P<0.01, ns=not significant.Root length production increased in all treatments until week 3, remained relatively constant from weeks 3 to 6 and declined markedly after the onset of flowering in week 7 (Fig. 2
). Root production increased in response to elevated CO2 only in the first 2 weeks and the overall effect of CO2 was not significant (Table 1
). Increasing soil temperature and N supply consistently stimulated root production from weeks 3 to 7. No significant treatment interactions were observed.
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Root cohorts initiated on weeks 2, 4 and 6 showed, in general, similar treatment effects as observed for total root production (Table 2
). Root length at elevated compared to ambient CO2 was higher in the week 2 cohort, similar in the week 4 cohort and less in the week 6 cohort. More root length was produced at 15 °C than at 10 °C in the 2 and 6 week cohorts, with no differences in the week 4 cohort. The low-N and high-N supply treatments produced similar root lengths in the 2 and 4 week cohorts, and the high-N treatment produced more root length than the low-N treatment in the week 6 cohort. Cohort root diameters (Table 2) showed treatment effects more strongly than did the pooled age class root population (data not shown). In the week 2 cohort, roots produced under elevated CO2 had smaller diameter than those produced under ambient CO2, were of equal diameter in the week 4 cohort, and were significantly thicker in the week 6 cohort. Soil temperature had no effect on root diameter in the week 2 and 4 cohorts, but roots were significantly thinner at 15 °C than at 10 °C in the week 6 cohort. Roots were thicker at low-N than at high-N supply in the week 2 cohort, but this was reversed by the week 6 cohort. There were no treatment interactions for either root length or root diameter.
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Root loss began on week 3 and increased steadily through week 8 (Fig. 3
), reaching no more than 30% of total root production by the end of the experiment. Root loss was highly variable in all treatments and, since the experiment was terminated before crop senescence, these results should be interpreted cautiously. CO2 enrichment increased root loss across all weeks despite no significant differences in loss on any given week (Table 1). Soil temperature and N supply effects on root loss became evident from week 5 to the end of the experiment (Fig. 3). However, soil temperature and N supply interacted, with greater root loss at high-N than at low-N supply at 10 °C, but the reverse was apparent at 15 °C soil temperature. Survival rates of roots initiated in week 2 started to decline after week 6 but did not show any clear treatment effects (Fig. 3, inserts). Root turnover, or the proportion of total root loss to total root production, was only affected significantly by soil temperature, where turnover was 8% higher at 10 °C than at 15 °C soil temperature (p=0.002).
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Carbon uptake and mass allocation
Elevated CO2 significantly increased net CO2 assimilation rates (A) only at high-N supply (Tables 3, 4). There were no soil temperature or N supply effects on A at ambient CO2, or soil temperature effects at elevated CO2. The intercellular CO2 concentration differed significantly only between the CO2 treatments: ambient 125±49 and elevated 429±119 µmol m-2 s-1.
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Stem and leaf mass responded significantly to all treatments (Table 3
), increasing on average 9% with CO2 enrichment, 22% at higher soil temperature and 150% with high-N supply (Table 4). Specific leaf area was lower at elevated than at ambient CO2, at 15 °C than at 10 °C, and at low-N than at high-N supply (Table 4). Root mass was less responsive to CO2 (+6%, P<0.1) and soil temperature (+6%, ns) but nearly doubled with high-N supply (+98%, P<0.01). There were significant 3-factor interactions for treatment effects on root, shoot and total mass (Table 3) with the only consistent response to CO2 occurring with high-N supply and in 15 °C soil (Table 4).
Root/shoot ratio (R/S) also responded to CO2 enrichment only in this treatment combination (-9%, P<0.05). There were, however, significant increases in R/S at 10 °C compared to 15 °C soil temperature and at low compared to high-N supply. Furthermore, there was a significant interaction of soil temperature with N supply where R/S increased due to low-N supply to a higher extent at 10 °C (+33%) than at 15 °C soil temperature (+17%) (Table 4).
There were only minor CO2 effects on percentage allocation of biomass to different components, while the effects of soil temperature and soil N were highly significant (Table 3), although quantitatively modest. Significant temperaturexN supply interactions (Table 4) revealed larger effects of N supply on allocation to roots (P<0.05) and leaves (P<0.01), at 10 °C than at 15 °C soil temperature. Interestingly, increases in soil temperature and N supply reduced allocation to roots and leaves and increased allocation to stems (the same trend, though not significant, was observed with elevated CO2). The proportion of biomass allocated to reproduction showed the same general trends as did the other components but could not be interpreted further since flowering had only recently begun when plants were harvested. Although changes in shoot phenology were not followed thoroughly, no obvious differences among treatments (other than in size) were observed during the experiment. Time to flowering varied only a few days across all treatments. At harvest, however, there was a significant increase in the number of inflorescences and inflorescence dry mass (Table 4) due to increasing soil temperature. Number of tillers was significantly reduced at elevated CO2, at 15 °C and at low-N supply (Table 4).
Nutrient uptake
Root and shoot N concentrations were significantly reduced by CO2 enrichment, higher soil temperature and low-N supply (Fig. 4a). However, there were no differences in root N content due to CO2 or soil temperature, while low-N supply resulted in lower root N content compared to high-N supply (Fig. 4b). Shoot N and total N contents were higher at ambient than at elevated CO2 (+14%), at 15 °C than at 10 °C soil temperature (+9%), and at high than at low-N supply (+284%) (Fig. 4b). There also was a significant CO2xN supply interaction (Table 3) in which the reduction in total N content at elevated CO2 was more pronounced when plants were growing with high-N supply. N-use-efficiency (NUE) was generally higher with CO2 enrichment, higher soil temperature and low-N supply (Table 4). A significant 3-factor interaction indicated that N supply and soil temperature had stronger effects on NUE at elevated than at ambient CO2.
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Tissue P concentrations were reduced modestly by CO2 enrichment (–11%) and to a greater extent by low-N supply (–22%) but, contrary to expectations, were not affected significantly by soil temperature (Fig. 5a
). Root and shoot P contents were strongly affected by increased N supply (+290%), whereas effects of higher soil temperature (+11%) and CO2 enrichment (+5%) were much smaller (Fig. 5b). P-use-efficiency was higher with CO2 enrichment and low-N supply and was not affected by soil temperature (Table 4).
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Discussion |
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This study is one of the few highlighting interactive effects of climate change factors, including soil temperature, for a major crop species in temperate regions. Soil temperature effects were found on many growth and nutrient relations parameters as well as significant interactions with atmospheric CO2 and soil N supply. Importantly, soil temperature effects were manifest in the absence of any change in air temperature. Soil temperature is clearly an important variable to include in modelling exercises, and is more stable and hence easier to predict than air temperature. In addition, the adaptive mechanisms of plants for growth were examined under these altered environmental conditions.
The effects of increasing soil temperature, CO2 availability and N supply on plant biomass were, in general, positive and additive but not always independent. Shoot size responded to an increase in the availability of all resources in the following order: N supply >soil temperature >CO2. Interestingly, CO2 increased shoot growth at low N and 10 °C soil temperature, but not at low N and 15 °C soil temperature. Variable interactions between temperature and atmospheric CO2 have also been observed (Mitchell et al., 1993, 1995). In this study, in general, low soil temperature tended to reduce the stimulating effect of elevated CO2, but did not reverse it as reported previously (McKee and Woodward, 1994).
No soil temperature effect on C assimilation rate was observed. Although there is a sound theoretical background for the temperature dependency of photosynthesis, species like wheat and barley can give unexpectedly high responses to elevated CO2 at temperatures as low as 10 °C (Bunce, 1998). A highly significant interaction between CO2 and N supply was observed showing that without sufficient N supply plants could not use more carbon. These results agree with those of Delgado et al. for winter wheat (Delgado et al., 1994) and support the results of a considerable number of other studies (see review by Stitt and Krapp, 1999).
The response to CO2 enrichment is at the lower end of those reported in a survey of several species (Morison and Lawlor, 1999) and at the lower end of those reported for other winter wheat cultivars growing at similar temperatures (Mitchell et al., 1993, 1995; Batts et al., 1997; Kimball et al., 1995). Winter wheat growth stimulation by CO2 doubling falls well within the range reported by Rawson (Rawson, 1995) at 10 °C and 15 °C air temperatures and not, as discussed in the same paper, within the range predicted by Idso et al. (Idso et al., 1987). The latter study predicts no response to a 2-fold increase in atmospheric CO2 below 18 °C.
It was found that changes in nutrient uptake and nutrient-use efficiency accounted for most of the treatment effects on biomass production and root/shoot allocation. Nutrient uptake was stimulated by increasing N supply and increasing soil temperature, and was reduced by elevated CO2. Therefore, nutrient uptake appeared to be controlled by plant growth, which in turn was mainly restricted by N availability. Plants growing with low-N supply took up most (60–70%) of the N added whereas in the high supply treatment, a large amount of N (65–70%) remained unused.
Additionally, N concentration and uptake were reduced by elevated CO2 at both N supply levels. This could be related to lower transpiration at elevated CO2, since water use at elevated CO2 was approximately 20% less than at ambient CO2 during the experiment (data not shown). The reductions in N uptake did not affect plant biomass, however, because N-use efficiency increased. It is difficult to separate CO2 effects on plant growth from those on nutrient uptake kinetics. The response of N uptake kinetics to CO2 enrichment has been inconsistent and dependent on N-form, plant species and plant age. When differences in N uptake kinetics have been found (BassiriRad et al., 1999) counteracting mechanisms such as fine root production or nutrient-use efficiency have been observed.
Elevated CO2 had only a minor and inconsistent negative effect on P uptake, which was again overcome by increased P-use efficiency. The small effects of CO2 and soil temperature relative to the effect of N supply on most of the variables measured were not surprising since it is sink strength and metabolism, not carbohydrate supply, that controls growth at low temperatures (Gifford, 1992). The most pronounced effects of low temperature on shoot and root development, P uptake (Moorby and Nye, 1984; A
alsteinsson and Jensén, 1990), and N uptake (Clarkson et al., 1986, 1992) occur below 10 °C.
Root mass doubled from low to high-N supply, was less affected by soil temperature and unaffected by CO2. The relative investment in roots was higher, as expected, at 10 °C than at 15 °C soil temperature with a more pronounced increase in allocation to roots when N was severely limiting. Contrary to our expectation, however, relative biomass allocation to roots and other biomass components was not affected by CO2. Other studies have also reported minimal effects of atmospheric CO2 on percentage allocation of biomass of winter wheat (Franck and Bauer, 1996; Slafer and Rawson, 1997).
Root production and root loss were related to plant phenology, as reported for field-grown winter wheat (Gibbs and Reid, 1992) and other annual crops (Pritchard and Rogers, 2000; Gavito et al., 2001). The size of the winter wheat root system during vegetative growth was dominated by root length production, since root loss was low, and controlled mainly by nutrient availability and to a lower extent by soil temperature. Gibbs and Reid followed root development through the entire plant cycle and found that root mortality started before plant flowering and increased as flowering approached, but it took approximately 180 d to reach the flowering stage under field conditions (Gibbs and Reid, 1992). In the experiments reported here the plants were grown simulating a winter period before seedlings were planted in the experimental pots and this was followed by a simulated short spring. This considerably accelerates plant development. Gibbs and Reid calculated an average root longevity of 59±7 d for winter wheat roots produced after the winter (Gibbs and Reid, 1992). The experiment in this study was finished too early to make any firm conclusions about root longevity or turnover but the results suggested that treatment interactions were likely to occur. Roots of annual crops seem to live for 1 or 2 months, and to have a shorter life than roots from perennials, in the few studies conducted to date (Eissenstat and Yanai, 1997). Fitter et al. reported, however, almost no root death in spring wheat until 116 d after planting at plant maturity (Fitter et al., 1996).
In addition to changes in root distribution in the soil profile as a consequence of CO2 enrichment (observed by Van Vuuren et al., 1997), it was found that atmospheric CO2 and soil temperature, and to a lesser degree soil N supply, induced changes in root diameter. Changes in root diameter may have important consequences in root physiology and longevity, and plant nutrient absorption capacity. Also, it has been confirmed here for an important crop species that the response of root production and loss to soil temperature depends on soil fertility, as King et al. observed in Populus tremuloides (King et al., 1999).
Treatment effects on shoot size were evident during the experiment, but effects on shoot phenology were seen until the plants flowered and only in the case of soil temperature. Soil temperature affected mainly nutrient uptake and plant size during vegetative growth and its effects on root and shoot phenology became evident towards the end of vegetative growth. A marginally significant increase in root weight was found and a significant increase in N-use efficiency under elevated CO2. An increase in early root production was also observed. These results support the hypothesis of feedback mechanisms to optimize nutrient capture and use in the face of declining uptake. From these results, with a predicted increase of 0.3 °C and 35 ppm CO2 per decade in the next 100 years, progressively larger responses in winter wheat early biomass production may be expected as long as soil fertility is not limiting. With an increasing number of days with higher temperatures where plant growth and nutrient uptake are not severely restricted, more C could be assimilated and used for early growth. It has been reported, however, that warming may have negative effects on winter wheat yield (Mitchell et al., 1993, 1995; Batts et al., 1997). Therefore, further investigation of soil and air temperature effects is needed.
In conclusion, it was found, as predicted, significant soil temperature effects on most of the variables measured. Significant 3-factor interactions in root development, root, shoot and total plant biomass, N-use efficiency, and total shoot P supported this study's hypothesis. Changes in root development, nutrient uptake and nutrient-use efficiencies were found as counterbalancing mechanisms to meet plant growth in each situation, whereas changes in biomass allocation were modest. These results suggest a need for considering air and soil temperature increases together with soil fertility in studies of climate change effects on plant growth. In temperate regions where plants often experience low soil and air temperatures at early growth stages, model results could be misleading without an adequate accounting of both soil and air temperature.
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Acknowledgments |
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We wish to thank Anne Olsen and Anette Olsen for skilful technical assistance, and Henrik Jørgensen and Erling Johannsen for assistance on video and computing equipment. This work was funded by grant no. 9501743 of the Danish Agricultural and Veterinary Research Council. Partial funding was provided by the National Institute for Global Environmental Change through the US Department of Energy (Co-operative Agreement No. DE-FC03-90ER61010). Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the DOE.
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Notes |
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3 Present address: Department of Microbial Ecology, Lund University, Sweden.
4 To whom correspondence should be addressed: Fax +4546774282. E-mail: iver.jakobsen{at}risoe.dk
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References |
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