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Keywords:

  • Allometric growth;
  • Bellis perennis;
  • Dactylis glomerata;
  • Poa annua;
  • root respiration;
  • temperature;
  • thermal time

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

1. Plants of Bellis perennis, Dactylis glomerata and Poa annua were grown from seed in controlled-environment cabinets at either 16 or 20 °C; at the higher temperature all three species had increased total dry mass and leaf area when assessed on the basis of chronological time. On the basis of thermal time (summation of degree-days above 0 °C; days °C) temperature decreased the dry mass in P. annua.

2. Partitioning was assessed as a change in the allometric coefficients relating shoot and root dry mass, leaf and plant mass, leaf area and plant mass, and leaf area and leaf mass. Of the 12 relationships examined only three were affected by temperature: there was increased partitioning towards the shoot relative to the root in D. glomerata and increased partitioning towards leaf area rather than leaf mass in D. glomerata and B.perennis.

3. Root respiration was unaffected by temperature of growth in D. glomerata and P.annua but was lower in B. perennis grown at elevated temperature.

4. Root respiration acclimated to temperature in P. annua and B. perennis (i.e. when measured at the same temperature, respiration was higher in plants grown at 16 °C).

5. Root soluble carbohydrate concentration was unaffected by temperature of growth in any of the species. Feeding sucrose to the roots for a short period had no effect on the rate of respiration of B. perennis or D. glomerata but increased root respiration of P. annua.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Global mean temperatures are predicted to increase by 4 °C by 2100 (Watson et al. 1990) and it has long been known that temperature has a profound effect on the growth of plants. Both dry mass and leaf area are generally increased by growth at higher temperatures (Brouwer 1962; Beevers & Cooper 1964; Cumbus & Nye 1982; Clarkson, Hopper & Jones 1986; Delucia, Heckathorn & Day 1992; Mitchell et al. 1993) although both may appear to be decreased owing to the increased rate of development at higher temperatures (Cumbus & Nye 1982; Mitchell et al. 1993). Increased developmental rate at higher temperatures has also been shown through the use of thermal time (summation of degree-days above a basal temperature) (Gregory 1983; Ong 1983a,b; Squireet al. 1984; Gregory 1986).

Much research has been carried out on the effect of temperature on the partitioning of dry mass or leaf area:dry mass by looking at changes in ratios (e.g. shoot:root ratio, leaf area ratio, leaf mass ratio, specific leaf area) (Eagles 1967; Woledge & Jewiss 1969; Jones 1971; Farrar 1989; Delucia et al. 1992; Rawson 1992). However, ratios give a misleading picture about partitioning because they may change with ontogeny (Jones 1971; Farrar & Gunn 1996; Gunn, Bailey & Farrar 1999). One way to overcome this problem is to assess the effect of treatment on the allometric coefficient which relates the two variables (Farrar, Minchin & Thorpe 1995; Gunn et al. 1999). Jones (1971) showed that temperature had no effect on the relationship between leaf and whole plant dry mass in dwarf French beans, whilst the allometric constant, k relating shoot and root rises with growth temperature in clover (recalculated from the data of Hatch & Macduff 1991) and sunflower (recalculated from the data of Szaniawski 1983) but decreases in Lolium perenne (Troughton 1960). A 4 °C increase in growth temperature increased the allometric coefficient relating shoot and root dry mass in Avena sativa but had no effect on k in Holcus lanatus (Gunn, Bailey & Farrar 1993). Because the optimum temperature for the growth of each of these species may be different, a single increase in temperature may represent a different treatment for each species, and so generalizations must be made with care.

Relative growth rates (RGR) and respiration are linearly related (Farrar & Williams 1991). Respiration is strongly affected by temperature changes in the short term (with a Q10 of 2; Lambers 1985) but the respiration of plants grown at different temperatures is generally similar (Lambers 1985; Farrar & Gunn 1996) because plants acclimate to a new temperature environment over a few days (Farrar 1989). The acclimation may be controlled by carbohydrate status, because there is a correlation between the rate of respiration and non-structural carbohydrates in long-term experiments (Williams & Farrar 1990; Pollock & Farrar 1996). The short- and long-term effects of temperature on respiration may be mediated through its effects on the rate of metabolism of the cytosolic sucrose pool (Farrar & Gunn 1996). In the short term an increase in temperature will increase the rate of metabolism of sucrose and hence growth, which will in turn change the concentration of the cytosolic sucrose pool. Carbohydrate-dependent changes in gene expression may then account for the acclimation of respiration seen in long-term exposure to elevated temperature (Farrar & Williams 1991; Farrar & Gunn 1996; Pollock & Farrar 1996). In the short term (minutes to hours) supplying sucrose exogenously generally has no effect on the rate of respiration unless roots have been depleted of sugars (Bingham & Farrar 1988; Farrar & Williams 1991).

The hypotheses we set out to investigate were that (1) dry mass and leaf area would be increased by a small (4 °C) increase in temperature but that (2) partitioning between dry mass and leaf area:dry mass would be unaffected. We also wished to investigate if there was a relationship between partitioning to roots, down-regulation of respiration, and carbohydrate content in roots of plants grown long-term at elevated temperature, and whether there was any growth parameter which could be used to predict the effects of the projected increase in the global temperature.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

PLANT GROWTH

Plants of Dactylis glomerata L. cv Sylvan (IGER, Aberystwyth, UK), Poa annua L. (Herbiseed, Wokingham, UK) and Bellis perennis L. (Chiltern Seeds, Cumbria, UK) were germinated on moist paper towelling in the dark at 20 °C for 8, 13 or 9 days, respectively. They were then transferred to nutrient solution (see below) and grown in controlled-environment cabinets (Sanyo Gallenkamp PG660, Leicester, UK) at ambient CO2 concentrations, with a 16 h day, at a photon flux density of 550 mmol m–2 s–1 at plant height produced by from HQI bulbs, an air temperature of 16 or 20 °C (± 1 °C) and a vapour pressure deficit of 0·46 or 0·7 kPa, respectively. Air was drawn into the cabinets from the roof of the building with a fan (Type 3MS11, Air Control Installations, Chard, UK) providing 5·5 complete changes of air per hour in each cabinet. Thirty plants were grown in 7 dm3 of solution aerated at a rate of 1 dm3 min–1. The temperature of the solution was not separately controlled but was measured at ± 1 °C of the air temperature. Dactylis glomerata and P. annua were grown in full strength modified Long Ashton solution whilst B.perennis was grown in a solution from Institut National de la Recherche Agronomique (Gunn et al. 1999). Solutions were changed every 3 or 4 days.

Plants of D. glomerata were harvested between 19 and 29 days old, P. annua between 25 and 38 days old and B. perennis between 19 and 56 days old. At each harvest five replicate plants of each species were taken at random. Dactylis glomerata and P. annua were divided into fully expanded leaf blades, rest of shoot (leaf sheaths and expanding leaves) and root, whilst B. perennis was divided into leaves (excluding petioles), rest of shoot (including petioles) and root. The leaf area was measured on a leaf area meter (Delta T, type TC2014/X video camera with electronic control unit, Cambridge, UK) and dry mass of all parts was determined after drying in a ventilated oven at 80 °C.

GROWTH ANALYSIS

The effect of temperature on total dry mass and leaf area was assessed on the basis of chronological age (days after sowing) and thermal time (summation of degree-days above 0 °C; days °C). Results for chronological time were assessed by analysis of variance of natural log-transformed data after a test for homogeneity of variance using Cochran's test. Significant interactions were compared using Tukey's honestly significant test (Zar 1996). No significance testing was carried out on the thermal time results.

A stepwise regression was used to determine if first- or second-order polynomials were the best fit for a natural log transformation of total dry mass plotted against time (days) for all plants (Hunt 1982). Student's t-tests were used to determine if the quadratic term differed significantly from zero (Zar 1996), using the computer package SPSS (version 7, SPSS, Chicago, IL, USA). A linear equation best described the data for D. glomerata and P. annua whilst a quadratic equation best described the data for B. perennis. The first differential was used to calculate RGRs; RGR at the time of the earliest harvest is given for B. perennis. Only if there are significant differences between the regression coefficients (b and c) of the equations describing the data for plants grown at ambient and elevated temperature will there be differences in RGR. Hence the regression coefficients were compared using t-tests (Zar 1996).

Mean net assimilation rate (also known as unit leaf rate) between the first and last harvests (NAR) was calculated using the equation

NAR = [(M2M1)/(t2t1)]×[(ln L2– ln L1)/(L2L1)],

where M1, M2 are the total plant dry mass, L1, L2 are the total leaf area (cm2) at times t1 (first harvest, days) and t2 (last harvest, days), respectively (Hunt 1982). Prior to performing the calculations plants from the two harvests were paired on the basis of total dry mass (Hunt 1978). NAR from the two treatments were compared using t-tests (Zar 1996).

ALLOMETRIC COEFFICIENTS

Allometric coefficients for geometric mean regressions (ν) were calculated for the relationships between shoot and root dry mass, leaf area and plant dry mass, leaf mass and plant mass, and leaf area and leaf mass by geometric mean regression (Gunn et al. 1999). In summary ν = k/r where k is the slope of the regression line of, e.g. ln shoot dry mass and ln root dry mass, and r is the correlation coefficient. The two variables for each regression are denoted by subscripts; thus νSR is the coefficient relating shoot and root dry mass. A comparison of the correlation coefficients (16 and 20 °C) within any one plot was carried out after a Fisher's z-transformation of r (Zar 1996). In only two cases, shoot vs root dry mass and leaf mass vs plant dry mass, both in D. glomerata, were there significant differences between the r-values of the two lines. No allowance for this was made in the final analysis. Allometric coefficients were compared using a t-test, and results are shown with 95% confidence limits.

ROOTS: RESPIRATION AND CARBOHYDRATE CONCENTRATION

Root respiration was measured in B. perennis and P.annua when 33 days old and in D. glomerata when 23 days old. O2 uptake by freshly detached roots was measured in a liquid phase Clarke-type electrode (Hansatech Instruments Ltd, Norfolk, UK) (Delieu & Walker 1972). Respiration was measured in the appropriate nutrient solution (minus Fe or Si) at both 16 or 20 °C and a fresh solution was used for each root. Handling of roots was kept to a minimum. Respiration was measured over a period of 15–20 min during which time the chart recorder trace was linear.

Effect of temperature

Separate roots from the same plant were used for each temperature. Prior to use solutions were continually aerated and kept at the appropriate temperature. Results were assessed in three ways. A t-test was used to compare the rates of respiration of plants grown and measured at 16 °C with those grown and measured at 20 °C. Acclimation of the rate of respiration was assessed, using a t-test, by comparing plants grown at 16 °C but measured at 20 °C with those grown and measured at 20 °C; down-regulation was defined as a lower rate of respiration in roots grown and measured at 20 °C than those grown at 16 °C but measured at 20 °C. A two-way factorial analysis of variance was used to assess the effect of growth and measurement temperature upon the rate of respiration.

Effect of feeding sucrose to the roots

Prior to the measurement of respiration the roots of whole plants were placed in nutrient solution (maintained at growth temperature) containing 20 mol m–3 sucrose for 30 min. At the end of this period roots were removed, gently washed in distilled water and then placed immediately into the electrode vessel in the appropriate nutrient solution minus sucrose. A two-way factorial analysis of variance was used to assess the effect of growth temperature and sucrose feeding upon the rate of respiration.

Carbohydrate concentration

Separate roots from the same plants used above (effect of feeding sucrose to the roots) were extracted twice in 10 cm3 of 95% ethanol at 80 °C. Roots were harvested from control plants (not root fed sucrose) and plants which had been root-fed sucrose immediately after feeding. These extracts were combined and made up to 25 cm3 with distilled water (ethanol soluble carbohydrates; these may include sucrose, hexoses and low degree of polymerization fructans). Total carbohydrate in each fraction was estimated by the phenol–sulphuric method of Dubois et al. (1956). A two-way factorial analysis of variance was used to assess the effect of growth temperature and sucrose feeding on the carbohydrate concentration for each species individually.

In a separate experiment 14C-sucrose ([U-14C]sucrose, Amersham Life Science, Buckinghamshire, UK) was used to ensure that sucrose was taken up by the roots. Roots were detached from plants grown at 20 °C when 13 days old and placed in the appropriate nutrient solution (minus Fe and Si) at 20 °C, with 20 mol m–3 sucrose and 1·1 MBq dm–314C-sucrose. The roots were left for 30 min with intermittent shaking, removed and washed three times (5 min per wash) in distilled water at 20 °C, blotted dry, weighed, and then placed individually in 10 cm3 Aquasol (New England Nuclear) prior to scintillation counting in a liquid scintillation counter (LKB-Wallac 1215 Rackbeta 11).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

GROWTH

Shoot, root and total dry mass and leaf area were higher for all three species grown at elevated compared to ambient temperature when assessed on the basis of chronological time (P < 0·001; temperature main effects averaged over all harvests, from ANOVA). Values from the final harvest are shown in Table 1. When dry mass and leaf area were assessed on the basis of thermal time (Fig. 1) the data for plants grown at 16 and 20 °C appeared to lie on the same line for all three species suggesting that, in general, the plants grown at elevated temperature were merely more advanced in their growth than those grown at ambient temperature. There was one exception: P. annua grown at elevated temperature had a lower dry mass than at ambient temperature (Fig. 1).

Table 1.  . The effects of temperature on growth of P. annua, D. glomerata and B. perennis in hydroponics. Dry mass and leaf area at the final harvest. Constants in the equations ln dry mass = a + b(time) or ln dry mass = a + b(time) + c (time)2 for shoot and root dry mass and maximum RGR (calculated at the earliest harvest for quadratic equations). Net assimilation rate (NAR). Standard errors are in brackets. NS, no significant difference; *P < 0·05, **P < 0·01 Thumbnail image of
image

Figure 1. . The effect of temperature on the dry mass (mg plant–1) and leaf area (cm2 plant–1) of B. perennis, P. annua and D.glomerata when measured on the basis of thermal time (summation of degree-days above 0 °C; days °C). Plants were grown in hydroponics at an air temperature of either 16 °C (▪) or 20 °C (▪). Values are the mean of four to five replicates.

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RGR of the shoot and root was not significantly affected by growth temperature in P. annua but was higher in B. perennis and D. glomerata (Table 1) grown at elevated temperature. In fact it must have been higher in all three species at some stage to account for the increased mass at final harvest. NAR was not significantly affected by temperature in D.glomerata or P. annua but was lower in plants of B.perennis grown at 20 °C than at 16 °C (Table 1).

ALLOMETRIC COEFFICIENTS

Shoot vs root dry mass SR)

The allometric coefficient νSR relating shoot and root dry mass was unaffected by temperature in B. perennis or P. annua but was higher in D. glomerata grown at 20 °C than at 16 °C; there was increased partitioning towards the shoot. The graphs of ln shoot dry mass vs ln root dry mass for the three species are shown in Fig. 2 as examples of allometric plots. The remaining coefficients are given as values only (Table 2).

image

Figure 2. . The effect of temperature on the allometric relationship between shoot and root dry mass of B. perennis, P. annua and D. glomerata grown in hydroponics. Plants were grown at either 16 °C (▪ solid line) or 20 °C (▪ dotted line).

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Table 2.  . The effect of temperature on the allometric coefficient relating shoot and root dry mass, leaf area and total dry mass, leaf mass and total dry mass, and leaf area and leaf mass of P. annua, D. glomerata and B. perennis. The allometric coefficient was calculated by plotting the natural log of Y against the natural log of X for all plants from all harvests and fitting a linear regression, where X is the root, leaf or total dry mass, Y is the shoot or leaf dry mass or leaf area, ln a the intercept and k the slope; r is the correlation coefficient. The allometric coefficient, ν, was then calculated as k/r. Values are shown with 95% confidence limits. NS, no significant difference; *P < 0·05 Thumbnail image of
Leaf area vs total dry mass AM) and leaf mass vs total dry mass LM)

Temperature had no effect on the allometric coefficient relating leaf area to total dry mass or on that relating leaf and total dry masss in any of the three species (Table 2).

Leaf area vs leaf mass AL)

The allometric coefficient relating leaf area and leaf dry mass was higher in plants of D. glomerata and B.perennis grown at elevated temperature (Table 2) with more leaf area being produced per unit of leaf dry mass. Temperature had no significant effect on νAL in P. annua (Table 2).

ROOTS: RESPIRATION AND CARBOHYDRATE CONCENTRATION

Effect of temperature

The rate of respiration of each species from each treatment was measured at both 16 °C and 20 °C. In D.glomerata and P. annua roots grown and measured at 20 °C had the same rate of respiration as those grown and measured at 16 °C (Table 3). However, roots of B. perennis grown and measured at 20 °C had a lower rate of respiration than those grown and measured at 16 °C (P < 0·01, Table 3).

Table 3.  . The effects of temperature and sucrose feeding on the ethanol-soluble carbohydrate concentration (μmol sucrose equivalents g–1 fresh mass) sucrose uptake by detached roots measured using 14C-sucrose (μmol sucrose equivalents g–1 fresh mass 0·5 h–1) and rate of root respiration (nmol O2 g–1 fresh mass s–1). Ethanol-soluble carbohydrates and respiration were measured in P. annua at 33 days old, D. glomerata 23 days old and B. perennis 33 days old whilst sucrose uptake was measured at 13 days old in all three species. Plants were grown at either 16 or 20 °C and root respiration was measured at both the growth temperatures for all plants, or with (20 mol m–3 sucrose 30 min prior to the measurement of respiration, plus sucrose) and without sucrose feeding (control). Values are the mean of four replicates, except D. glomerata which had three replicates (soluble carbohydrates) or five replicates (sucrose uptake and respiration). Standard errors are in brackets. ND, not determined Thumbnail image of

Short-term changes in root temperature had no significant effect on the rate of respiration in roots of D. glomerata (Table 3). However, respiration rate was higher when measured at 20 °C than at 16 °C in both B. perennis and P. annua (P < 0·01, Table 3). Consequently the rate of respiration was higher in roots grown at 16 °C and measured at 20 °C than those grown and measured at 20 °C in B. perennis (P < 0·001) and P. annua (P < 0·05, Table 3).

Effect of sucrose feeding

Detached roots from all species took up sucrose when this was measured using 14C-sucrose (Table 3). The rate of uptake was faster in P. annua than in the other two species (Table 3). A 30 min feed of 20 mol m–3 sucrose given to the roots of whole plants prior to measuring respiration had no effect on the rate of root respiration in B. perennis or D. glomerata but increased the rate of respiration in roots of P. annua when measured at the growth temperature (P < 0·05, Table 3).

Carbohydrate concentration

Growth temperature had no effect on the ethanol-soluble carbohydrate concentration of the roots of D.glomerata or P. annua when measured by the phenol-sulphuric method (Table 3). The ethanol-soluble carbohydrate concentration of the roots B. perennis was higher in plants grown at 20 °C than at 16 °C (P < 0·001; Table 3). Feeding 20 mol m–3 sucrose to the roots of whole plants for 30 min had no significant effect on the ethanol-soluble carbohydrate concentration (Table 3).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

GROWTH AND CARBON PARTITIONING

Both the total dry mass and leaf area were higher for B. perennis, D. glomerata and P. annua grown at 20 °C than at 16 °C when assessed on the basis of chronological age, and this is in agreement with similar work (Brouwer 1962; Beevers & Cooper 1964; Cumbus et al. 1982; Clarkson et al. 1986; Delucia et al. 1992; Mitchell et al. 1993). However, this was merely because growth was more advanced at elevated temperatures (shown by the relationships between dry mass or leaf area and thermal time) as has been found for other plants (Gregory 1983; Ong 1983a,b; Squireet al. 1984; Gregory 1986). There was one exception: the dry mass of P. annua was lower at 20 °C than at 16 °C at any given thermal time. The reason for this is unclear.

Thermal time is a method which can be used by modellers to assess the impact of climate change on plant growth although more work would have to be done to assess how universal is the phenomenon. Thermal time does not, however, tell us anything about the underlying mechanism(s) and it is these that need to be determined if accurate predictions, applicable to all plants, are to be made.

In much of the literature temperature is reported to have a profound effect on partitioning when assessed as ratios (Eagles 1967; Woledge & Jewiss 1969; Jones 1971; Farrar 1989; Delucia et al. 1992; Rawson 1992). Indeed in this work too, shoot:root ratio, leaf area ratio, leaf mass ratio and specific leaf area were different at the two temperatures (results not shown). By contrast, a 4 °C increase in growth temperature had relatively little effect on the partitioning of dry mass or leaf area:dry mass in P. annua, B. perennis, when these were assessed allometrically; in D. glomerata more shoot relative to root was produced at the higher temperature. Allometry is the preferred technique for demonstrating the effect of a treatment on net partitioning (Farrar & Gunn 1998).

ROOTS: RESPIRATION AND CARBOHYDRATE CONCENTRATION

The rate of root respiration was the same in plants of D. glomerata and P. annua grown at both 16 and 20 °C, similar to the finding for other species (Lambers 1985; Farrar & Gunn 1996), but was lower in B. perennis grown at 20 °C. However, increasing the temperature by 4 °C in the short term increased the rate of respiration (although not significantly in D.glomerata) by c. 30% (Table 4d) and this is in line with, although slightly less than, the 40% increase which would be predicted from a Q10 of 2 for respiration (Lambers 1985). There was no general correlation between respiration and RGR as has been found for other species (Table 4c).

Table 4.  . Percentage changes in whole plant RGR with temperature of growth; ethanol-soluble carbohydrate concentration in roots of whole plants ([CHO]): (a) with temperature of growth (from Table 3); (b) after feeding 20 mol m–3 sucrose for 30 min to roots of whole plants (estimated from 14C-sucrose uptake of detached roots, Table 3); respiration of (c) roots grown and measured at 20 °C compared to those grown and measured at 16 °C; (d) roots measured at 20 °C compared to those measured at 16 °C (regardless of growth temperature); (e) roots grown at 16 °C and measured at 20 °C compared with those grown and measured at 20 °C; (f) roots from whole plants fed 20 mol m–3 sucrose for 30 min compared to those not given a sucrose feed (regardless of growth temperature). NS, no significant difference; *P < 0·05, **P < 0·01, ***P < 0·001 Thumbnail image of

Respiration acclimated to temperature in B. perennis. We are persuaded of acclimation because the rate of respiration was higher in roots exposed for a short period of time to a higher temperature than in plants grown long term at the higher temperature (Table 4e). Thus there was down-regulation in the rate of respiration in plants grown for a long time at elevated temperature. In barley the rate of respiration also increased when roots were exposed to a higher temperature but within 5 days of the transfer to a higher temperature the rate of root respiration returned to pretreatment values (Abebe 1990). Although P. annua also apparently acclimated, its respiration responded to exogenous sucrose and so in this species acclimation and substrate-dependence of respiration are compounded. Note that direct evidence of acclimation is necessary; the fact that the rates at the two growth temperatures are different could otherwise be explained by the fact that specific respiration rate scales with plant size (Kidd, West & Briggs 1921).

In the short term, root respiration of B. perennis and D. glomerata was insensitive to the addition of exogenous sucrose, similar to results for barley (Bingham & Farrar 1988) although the rate of respiration was increased in P. annua. It has been suggested that differences in the absorption of exogenous sugars may account for species-specific differences in the effect of exogenous sugars on respiration (Lambers, der Warf & Konings 1991). In these experiments the increase in the concentration of ethanol-soluble sugars (which was calculated using the results from the feeding of 14C-sucrose to detached roots, because the phenol-sulphuric method was too insensitive to detect the increase in concentration in roots of whole plants) varied with species (Table 4b). The results from the two experiments are not directly comparable because 14C-sucrose was fed to detached instead of intact roots and the plants were younger. However, they indicate that the species-specific effect of sucrose feeding on the rate of respiration in these experiments may also be due to differences in uptake or to the sensitivity of respiration to increasing sucrose.

In the long term both the ethanol-soluble carbohydrate concentration and the rate of respiration was the same in plants grown at ambient and elevated temperature in P. annua and D. glomerata (Table 4a, c) as might be expected from the acclimation of respiration outlined in the Introduction. However, this was not true for B. perennis where respiration was lower, but the concentration of ethanol-soluble carbohydrates higher, in roots of plants grown at elevated temperature (Table 4a, c) suggesting that substrate was not limiting respiration in these plants.

PARTITIONING TO ROOTS

The differential response of shoot:root partitioning in these species gives an opportunity to ask if any of the measurements we have made on roots correlate with altered partitioning to them? Dactylis glomerata partitioned less net dry mass to the root at the higher temperature. It was the species showing the lowest down-regulation of root respiration at the higher temperature (although not significantly, Table 4e). When comparing results as percentages alone, the rate of respiration of D. glomerata was sensitive to short-term addition of exogenous sucrose (Table 4f) and the carbohydrate content of its root was slightly increased by temperature in the long term (Table 4a). This set of features was not shared by either of the other species (Table 4). However, there is no obvious reason why this unique set of features described above for D. glomerata may lead to the decrease in net partitioning to the roots found in these plants. When Engels & Marschner (1986) performed a very careful and comprehensive series of experiments on temperature-dependent partitioning between potato tubers, they too failed to find plausible causal explanations in the carbohydrate metabolism of the potato tubers. There does not appear to be a simple relationship between metabolism and the turgor gradients which determine partitioning (Farrar 1992; Minchin, Farrar & Thorpe 1994; Farrar et al. 1995). It is worth noting that in all three species, the soluble carbohydrate in the roots is sufficient to support respiration for about 10 h (calculated from data in Table 3); continued import is therefore necessary to sustain this pool.

GENERAL CONCLUSIONS

The results from these experiments are species-specific, suggesting that it will not be possible to use any one parameter to predict the effects of a small increase in growth temperature on C3 plants in general. Because it is impossible to measure the effect of temperature on all plant species the effects of climate change on a wide range of plants can only be predicted once the mechanisms involved in plant growth, and the effect of temperature on these mechanisms, are known.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We would like to thank NERC for support under the TIGER programme (section IV.1).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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