Some plants have the ability to maintain similar respiratory rates (measured at the growth temperature), even when grown at different temperatures, a phenomenon referred to as respiratory homeostasis. The underlying mechanisms and ecological importance of this respiratory homeostasis are not understood. In order to understand this, root respiration and plant growth were investigated in two wheat cultivars (Triticum aestivum L. cv. Stiletto and cv. Patterson) with a high degree of homeostasis, and in one wheat cultivar (T. aestivum L. cv. Brookton) and one rice cultivar (Oryza sativa L. cv. Amaroo) with a low degree of homeostasis. The degree of homeostasis (H) is defined as a quantitative value, which occurs between 0 (no acclimation) and 1 (full acclimation). These plants were grown hydroponically at constant 15 or 25 °C. A good correlation was observed between the rate of root respiration and the relative growth rates (RGR) of whole plant, shoot or root. The plants with high H showed a tendency to maintain their RGR, irrespective of growth temperature, whereas the plants with low H grown at 15 °C showed lower RGR than those grown at 25 °C. Among several parameters of growth analysis, variation in net assimilation rate per shoot mass (NARm) appeared to be responsible for the variation in RGR and rates of root respiration in the four cultivars. The plants with high H maintained their NARm at low growth temperature, but the plants with low H grown at 15 °C showed lower NARm than those grown at 25 °C. It is concluded that respiratory homeostasis in roots would help to maintain growth rate at low temperature due to a smaller decrease in net carbon gain at low temperature. Alternatively, growth rate per se may control the demand of respiratory ATP, root respiration rates and sink demands of photosynthesis. The contribution of nitrogen uptake to total respiratory costs was also estimated, and the effects of a nitrogen leak out of the roots and the efficiency of respiration on those costs are discussed.
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Temperature is one of the important factors affecting respiratory rates of plants. Althogh respiration rates are usually slower when measured at lower temperatures, some plants maintain similar respiration rates when grown at different temperatures (as long as respiration rates are measured at those growth temperatures). This is called homeostasis of respiration (Lambers, Chapin & Pons 1998; Atkin & Tjoelker 2003). This homeostasis has been observed in roots of Plantago lanceolata (Smakman & Hofstra 1982), Festuca ovina, Juncus squarrosus, Nardus stricta (Fitter et al. 1998), Poa annua, Bellis perennis (Gunn & Farrar 1999) and Silene uniflora (Loveys et al. 2002). On the other hand, there are many species that do not show respiratory homeostasis, such as Picea glauca (Weger & Guy 1991), Dactylis glomerata (Gunn & Farrar 1999), Poa costiniana and Acacia aneura (Loveys et al. 2002). Tjoelker, Oleksyn & Reich (1999) observed that broad-leaved species had a lower degree of respiratory homeostasis than conifers in a comparison of five boreal tree species. Atkin, Edward & Loveys (2000a) theoretically estimated the effects of global warming on annual CO2 release in plants with or without homeostasis. Their results suggest that the amount of CO2 released per year by plants displaying homeostasis are only half of that by plants that do not display homeostasis when temperature increases, and that the effect of homeostasis depends on the Q10 of the respiratory system; a higher Q10 causes greater reduction of annual CO2 release. Therefore, in terms of carbon economy, respiratory homeostasis may be important for plant growth (Atkin & Tjoelker 2003). However, the underlying mechanisms and biological meaning of this respiratory homeostasis have not been elucidated.
Respiration provides energy (ATP and reducing equivalents) and carbon skeletons that are required in various biological processes, and changes in respiration rate relate to changes in plant growth (Amthor 1989). Respiratory rates are generally regulated by carbohydrate supply and/or ATP demands of cellular processes (Noguchi & Terashima 1997). Poorter, Remkes & Lambers (1990) reported that the rate of root respiration correlated positively with relative growth rate (RGR) among 24 herbaceous species. Tjoelker et al. (1999) indicated that root respiration rates in five boreal tree species correlated with RGR, irrespective of growth temperature. Hansen et al. (2002) suggested that plant growth rate is equal to a function of respiration rate and efficiency, using their model and simultaneous measurements of calorimetry and respiration. This raises the question of whether respiratory homeostasis is correlated with RGR. That is, is whole plant RGR constant irrespective of growth temperatures in plants that show respiratory homeostasis?
In roots, respiratory ATP is used for uptake of anions such as nitrate, and for their growth and maintenance. Poorter et al. (1991) reported that, in the root of 24 herbaceous species, 50–70% of the respiratory energy is consumed for nutrient uptake. Since much nitrogen is needed for the synthesis of photosynthetic enzymes (Poorter et al. 1990), the rate of nutrient uptake would be an important determinant of photosynthetic capacity, and thereby of the growth rate of whole plants. This possibility should be addressed to clarify causes and consequences of respiratory homeostasis. This is most easily done with roots, as their respiratory rates are easily and accurately measured, whereas this is complicated during the day in shoots (Atkin et al. 2000b), because of their photosynthetic oxygen production.
In order to investigate underlying mechanisms and ecological meaning of respiratory homeostasis in roots, we measured rates of root respiration and RGR of different plants with high and low degrees of homeostasis, and examined the relationship between respiratory homeostasis and various parameters of growth analysis. Moreover, we measured the nitrogen concentrations of tissues, estimated the net rate of nitrogen uptake (NNUR), and analyse the cost for nitrate uptake to determine whether this contributes to homeostasis of root respiration.
MATERIALS AND METHODS
Plant material and growth conditions
Seeds of three cultivars of wheat (Triticum aestivum L. cv. Brookton, cv. Patterson, cv. Stiletto) were obtained from Dr T. L. Setter (Agriculture Western Australia, Australia) and Dr M. Schortemeyer (University of Western Australia, Australia). Seeds of rice (Oryza sativa L. cv. Amaroo) were obtained from Dr A. H. Millar (University of Western Australia, Australia). All the seeds were germinated on moistened paper, and then the wheat cultivars were transferred into tubs containing 20 L hydroponic nutrient solutions 3 d after germination. Rice seeds were grown in moistened sand for 7 d starting immediately after germination, and then transferred into hydroponic solutions. A total of 12–48 seedlings were placed in each tub. The tubs were placed in a walk-in growth chamber [15 or 25 °C constant day and night temperature, 14 h photoperiod, photosynthetic photon flux density 400 µmol m−2 s−1]. The nutrient solution (1/8 strength for the first 3 d after transfer and full-strength thereafter) contained (full strength): 4 mm KNO3, 4 mm Ca(NO3)2, 1.5 mm MgSO4, 1.33 mm KH2PO4, 0.05 mm EDTA-Fe, 0.01 mm MnSO4, 1 µm ZnSO4, 1 µm CuSO4, 0.05 mm H3BO3, 0.5 µm Na2MoO4, 0.1 mm NaCl, 0.2 µm CoSO4. The pH of the nutrient solutions was adjusted to 6.0 with NaOH, and the solutions were replaced weekly. At 13–15 d and at 20–23 d after germination, four replicates of excised roots were taken for measurements of root respiration for T. aestivum and O. sativa, respectively.
Measurement of root respiration
Respiratory rates of whole detached roots were measured polarographically with a Clark-type oxygen electrode (Rank Brothers, Cambridge, UK) at 15, 25 and 35 °C in 5 mL of the air-saturated nutrient solution supplemented with 10 mm 2-morpholinoethanesulfonic acid (MES) (pH 6.0). A piece of nylon mesh was used to keep the roots above the stirrer bar and electrode surface. The oxygen concentration in air-saturated buffer was assumed to be 314, 258 and 222 µm at 15, 25 and 35 °C, respectively. We calculated the degree of homeostasis (H) using the method described by Atkin, Bruhn & Tjoelker (2004). The long-term acclimation ratio (LTR10), which was defined by Larigauderie & Körner (1995), has been used widely to express homeostasis of respiration. However, the degree of homeostasis depends not only on the above LTR10 but also on Q10. By comparing LTR10 and Q10 values, the degree of acclimation in respiration can be compared (Atkin et al. 2004). The degree of homeostasis (H) is calculated as
where Rn(m) denotes a respiratory rate of roots of n°C-grown plants, which were measured at m°C. This value occurs between 0 (no acclimation) and 1 (full acclimation). H in this study is the same value as AcclimLTR10 in Atkin et al. (2004).
For the wheat cultivars, from 8 to 23 d after germination, three to four samples were harvested at least every 3 d for measurements of RGR, leaf area and dry weight. For 25 °C-grown rice, from 13 to 27 d after germination, three to four samples were harvested at least every 3 d, and for 15 °C-grown rice, from 15 to 27 d after germination, three to four samples were harvested at least every 3 d. Samples were separated into shoots and roots; leaf area was measured with a scanner and an imaging software package (NIH Image, written by W. Rasband at the National Institute of Health and available from NTIS, 5285 Port Royal Road, Springfield, VA, USA). All of the samples were dried at 70 °C for at least 3 d, and then weighed. RGR was calculated as the slope of a linear regression of the natural logarithm of plant mass as a function of time. Growth analysis was performed using the following equation (Hunt 1978; Lambers & Poorter 1992)
where W is whole plant mass and SM is shoot mass. NARm was divided into LA/SM and NARa by using the following equation
where LA is total leaf area of plants. Averages and standard deviations of all the parameters of growth analysis were calculated for all of the data throughout the harvesting period.
Nitrogen concentration and net nitrogen uptake rate
The samples for analysis of nitrogen concentration were ground with a mortar and pestle, and the powdered sample analysed with a CN analyser (Vario EL; Elementar Analysensysteme GmbH, Hanau, Germany). Net nitrogen-uptake rate (NNUR) was obtained from the following equation (Garnier 1991),
where RMR is root mass ratio, and Nc is nitrogen concentration of whole plants.
where H+/Ij is the number of protons required for a membrane symport of nitrate (2 mol H+ mol−1 NO3; Crawford & Glass 1998), Mj is the number of membranes to be crossed actively (= 1; the plasma membrane), H+/P is an amount of protons pumped over the plasma membrane by the H+-ATPase per hydrolysis of one ATP to ADP (= 1 mol H+ mol−1 ATP; Crawford & Glass, 1998), and P/O2 is the efficiency of the oxidative phosphorylation (29/6 when only the cytochrome pathway is engaged, and 11/6 when only the alternative pathway is engaged; Amthor 1994). Therefore, the respiratory rate needed for the nitrogen uptake (VNNUR) is calculated as;
where I and E denote the influx of nitrate into the cytoplasm and efflux (or leak) of nitrate out of the cytoplasm, respectively (Scheurwater et al. 1999). Since VNNUR of Stiletto and Patterson gradually increased, the average values from 13 to 15 d after germination are shown in the figures.
All the data were analysed with StatView (ver. 5.0 J, SAS, Cary, NC, USA).
Respiration rates and relative growth rates at different temperatures
Respiratory rates of excised roots were measured at 15, 25 and 35 °C for all of the plants, and showed the expected increase with temperature (Fig. 1). The respiratory rates of plants grown at 15 °C were significantly faster than those grown at 25 °C, except for cv. Patterson, which showed a significant interaction between growth and measurement temperatures (Table 1). The degree of respiratory homeostasis (H) was calculated as described above (Eqn 1), and is also shown in Fig. 1. The values of H for Stiletto and Patterson were 0.65 and 0.69, respectively, which is higher than those for Brookton and Amaroo: 0.36 and 0.28, respectively.
Table 1. anova summary for the effect of growth temperature (15 and 25 °C) and measurement temperature (15, 25 and 35 °C) on root respiration rate
In all four cultivars, plants grown at 15 °C (15 °C-plants) showed slower whole plant RGR (RGRw) than those grown at 25 °C (25 °C-plants; Table 2). However, the differences in RGRw between 15- and 25 °C-plants in the cultivars with high H, 10% for Stiletto and 18% for Patterson, were smaller than those for cultivars with low H, 28% for Brookton and 55% for Amaroo, respectively (Table 2). Stiletto-15 °C-plants showed slower shoot RGR (RGRs) and root RGR (RGRr) than 25 °C-plants (7 and 16%, respectively). In Patterson, RGRs and RGRr of the 15 °C-plants were 17 and 18% lower than those of the 25 °C-plants, respectively. In Brookton, on the other hand, RGRs and RGRr of 15 °C-plants were 29 and 27% lower, respectively, than those of 25 °C-plants. Amaroo-15 °C-plants showed markedly lower RGRs and RGRr than 25 °C-plants (50 and 68%, respectively).
Table 2. Rates of root respiration at the growth temperature, degrees of homeostasis (H), and relative growth rate of whole plant (RGRw), shoot (RGRs), and root (RGRr)
Growth temperature (°C)
Respiratory rate (nmol g−1 DW s−1)
RGRw (g g−1 d−1)
RGRs (g g−1 d−1)
RGRr (g g−1 d−1)
Ratio, denotes ratio of respiratory rate at 15 and at 25 °C.
In Stiletto, NARm, SMR and LAR in 15 °C-plants were slightly lower than those in 25 °C-plants, whereas NARa and LA/SM of these plants were similar (Table 3). In Patterson, all of these parameters were lower in 15 °C-plants than in 25 °C-plants (Table 3). However, NARa and LA/SM in Patterson-15 °C-plants were only 5% lower than those in 25 °C-plants. Brookton-15 °C-plants showed 24% lower NARm and 20% lower NARa than 25 °C-plants. SMR, LAR and LA/SM in Brookton-15 °C-plants were slightly lower than those in 25 °C-plants (Table 3). Amaroo plants showed much more dramatic responses. In 15 °C-plants NARa was 18% greater, but NARm, LAR and SMR decreased by 52, 60 and 7%, respectively, compared with those for 25 °C-plants (Table 3).
Table 3. Parameters of growth analysis in each cultivar grown at 15 and 25 °C
Growth temperature (°C)
NARm (g g−1 d−1)
NARa (g m−2 d−1)
LA/SM (m2 kg−1)
LAR (m2 kg−1)
NNUR (mg g−1 d−1)
Ratio, denotes ratio of respiratory rate at 15 to that at 25 °C.
Figure 2 shows the relationships between RGR and root respiration rate. All of the correlation coefficients were high: 0.92 for RGRw and RGRr, and 0.91 for RGRs. In the cultivars with high H (Stiletto and Patterson), 15 °C-plants showed slightly lower RGR and root respiration rates than 25 °C-plants, whereas in the cultivars with low H, 15 °C-plants exhibited markedly lower values for RGR and root respiration rate than 25 °C-plants.
RGRw, RGRs and RGRr showed a significant dependence on NARm (Fig. 3a–c). In the cultivars with high H, 15 °C-plants showed slightly lower NARm than 25 °C-plants, whereas 15 °C-plants of cultivars with low H showed markedly lower NARm. These trends were similar to the relationship between the root respiration rate and RGR (Fig. 2). Leaf area per plant mass (LAR), partitioning of dry mass to shoot (SMR) and root (RMR) did not show significant correlations with RGR (data not shown).
In order to investigate the variation in NARm (Fig. 3a–c), we examined which component, NARa or LA/SM (see Eqn 3 in Materials and Methods), varied. For the high-H cultivars, Stiletto-15 °C-plants and 25 °C-plants showed similar NARa and LA/SM (Table 3) which resulted in similar NARm. Patterson-15 °C-plants showed slightly lower NARa and LA/SM than 25 °C-plants which resulted in slightly lower NARm. In the low-H cultivars, NARm was much lower at 15 °C. The causes for this differed between Brookton and Amaroo. In Brookton, 15 °C-plants had 20 and 5% lower NARa and LA/SM, respectively, than 25 °C-plants which caused NARm to decrease by 24% (Table 3). In Amaroo-15 °C-plants, LA/SM was 57% lower than in 25 °C-plants, which was entirely responsible for the 52% lower NARm (Table 3). Figure 4a shows the relationship between the root respiration rate and NARm. Significant dependency of root respiration on NARm was observed (R2 = 0.80, P < 0.01).
Respiration and plant nitrogen content
Plants with high H had a higher nitrogen concentration in whole plants (Nw), shoots (Ns) and roots (Nr), than the plants with low H (Table 3). The dependence of respiration rate on nitrogen concentration was not significant (Fig. 4b). The relationship between root respiration and Nw or Ns was similar (data not shown). Net nitrogen-uptake rates (NNUR) are shown in Table 3. The 15 °C-plants of Stiletto, Patterson, Brookton and Amaroo showed 24, 40, 18 and 60% lower NNUR, respectively, than the 25 °C-plants of each cultivar. Both RGR and root respiration significantly depended on NNUR (Figs 3d–f & 4c).
We estimated respiratory costs for nitrogen uptake assuming three different scenarios.
A Only the cytochrome pathway (CP) is active in the mitochondria, and there is no leak of nitrate across plasma membranes in root cells, namely P/O2 = 29/6 and E/I = 0 (Fig. 5a).
B Only the CP is active, and half of the nitrate absorbed into the cytosol leaks back across the plasma membrane, namely P/O2 = 29/6 and E/I = 0.5 (Fig. 5b).
C CP and alternative pathways (AP) contribute equally to mitochondrial electron transport, and half of the nitrate absorbed into the cytosol leaks back across the plasma membrane, namely P/O2 = 20/6 and E/I = 0.5 (Fig. 5c).
In scenario A, the contribution of nitrate uptake to the total respiratory cost was 20–26% in the plants with high H (Fig. 5a). Stiletto-15 °C-plants showed a similar ratio to 25 °C-plants, whereas Patterson-15 °C-plants showed a slightly lower ratio than 25 °C-plants. In plants with low H, the contribution of nitrate uptake to the total respiratory cost was less than in the plants with high H (10–18% of total respiratory costs). Brookton-15 °C-plants showed a slightly higher contribution than 25 °C-plants, and Amaroo-15 °C-plants showed a smaller contribution than 25 °C-plants (Fig. 5a). In scenario B, the costs for nitrate uptake were twice as high as those in scenario A (Fig. 5b and 40–52% in high-H plants, 21–36% in plants with low H). In scenario C, the costs for nitrate uptake were higher again, because the AP contributes less ATP than the CP. In this case, plants with high H consumed more than half of their total respiratory energy in nitrate uptake (Fig. 5c and 63 and 67% in Stiletto and 58 and 76% in Patterson). For the low-H cultivars, values were 52 and 38% for Brookton, and 30 and 48% for Amaroo.
Effects of growth temperature on RGR and parameters of growth analysis
In the plants we tested, cultivars with high H showed less variation in RGR with temperature than those with low H (Table 2), showing that acclimation in respiration coincides with acclimation in growth rate. These results agree with those obtained in previous studies. For example, Gunn & Farrar (1999) compared growth between plants grown at 16 °C and those grown at 20 °C. Poa annua, a species displaying homeostasis in root respiration, maintained its RGR, irrespective of growth temperatures, whereas in Bellis perrennis, which also displays homeostasis, plants grown at low temperature had only a 10% lower RGR than those at the higher temperature. In Dactylis glomerata, a plant not showing homeostasis, RGRs and RGRr were 31 and 25% less, respectively, at the lower temperature. Smakman & Hofstra (1982) found much larger changes in the RGR of Plantago lanceolata grown at 13 and 21 °C. Loveys et al. (2002) measured root respiration rates of six species grown at 18, 23 and 28 °C. Among these, Silene uniflora showed homeostasis, irrespective of growth temperature, and two other species, Acacia melanoxylon and Poa trivialis, showed partial homeostasis. In all cases, plants with similar root respiration rates maintained similar values for RGR at each temperature. However, the reverse is not necessarily true, as some plants that maintained RGR displayed variation in root respiration rates. Despite this, our results, together with those in the literature, suggest that plants showing homeostasis in root respiration have a greater ability to maintain RGR at lower growth temperatures.
When the various parameters contributing to RGR were analysed, we found that changes in NARm strongly correlated with the changes in RGR (Fig. 3a–c), whereas changes in other parameters were not correlated. A strong correlation between NARm and maximum RGR has been observed also in 28 species of monocots and 22 species of dicots grown with free access to nutrients, at a moderate temperature and adequate irradiance (Garnier 1991). Therefore, changes in NARm seem to be correlated with variation in RGR across species as well as within species.
How is RGR linked to root respiration rates?
There was a strong relationship between root respiration and RGR (Fig. 2), largely due to the relationship between respiratory rates and NARm (Fig. 4a). Since NAR and estimated rates of photosynthesis are tightly correlated (Funayama & Terashima 1999), it is tempting to speculate that respiratory homeostasis was simply due to maintenance of photosynthate supply to the roots of the high-H plants. For example, while high-H P. annua plants had similar concentrations of soluble carbohydrates at 16 and 20 °C, in high-H B. perennis the 20 °C-plants had higher concentrations of soluble carbohydrates than the 16 °C-plants (Gunn & Farrar 1999). In the same study, D. glomerata, which does not display respiratory homeostasis, maintained similar concentrations of soluble carbohydrates at two growth temperatures. In Picea glauca, which also does not show respiratory homeostasis, soluble carbohydrate level increased when grown at 4 °C (Weger & Guy 1991). However, in this case respiration would have been severely suppressed at such a low temperature (Farrar 1988), and the increase in carbohydrates might have been caused by a large decrease in the respiratory demand in the roots of these plants. Inhibition of phloem loading at very low temperature might also have contributed to elevated shoot carbohydrate concentrations. These different responses of carbohydrate levels to low growth temperatures suggest that the level of soluble carbohydrates is not the only factor responsible for homeostasis of root respiratory rates. Cellular ATP/ADP ratios may also affect root respiration rates, and it is interesting to note that exogenous sugars enhanced root respiration less than addition of an uncoupler in roots of Plantago lanceolata grown under a moderate temperature and transferred to low temperature, except for 6 °C of measurement temperature (Covey-Crump, Attwood & Atkin 2002). However, Dewar, Medlyn & McMurtrie (1999) showed that the stable ratio of respiration to photosynthesis in long-term experiments was effectively demonstrated by a simple substrate-based model, in which respiration is limited by the supply of carbohydrates fixed through photosynthesis. Obviously, further study is required to understand the relationships between temperature, NAR, carbohydrate availability and root respiratory rates.
Relationship between respiratory homeostasis and nitrogen uptake
Although we observed a significant correlation between NNUR and root respiratory rates (Fig. 4c), NNUR was not correlated with respiratory homeostasis. Thus, in the low-H cv. Brookton NNUR was only 18% lower at 15 °C than at 25 °C, whereas in the high-H cv. Stiletto and Patterson NNUR decreased by 24 and 40%, respectively (Table 3). That is, respiratory homeostasis was not per se influenced by a lower NNUR at lower growth temperature. NNUR is a net rate – the difference between influx and efflux of nitrogen. Since nitrate uptake is an active process, whereas efflux is not, the costs for nitrate uptake depend on influx rates, irrespective of NNUR. The ratio of efflux to influx was reported to vary between 0.2 and 0.52 in Spinacia oleracea (Ter Steege et al. 1999), and between 0.3 and 0.6 in four grass species (Scheurwater et al. 1999). Oilseed rape grown at 7 °C showed a higher ratio of efflux to influx than those grown at 17 °C (Macduff, Jarvis & Cockburn 1994). Thus, it is possible that the ratio of efflux to influx varied with growth temperature and cultivar also in our study.
As shown in Fig. 5, the costs for nitrate uptake would be at least 10–20% of total respiration, and would consume more than half the total respiratory ATP, if nitrate efflux were considerable or if the AP were engaged. Differences in root respiration between the high- and low-H cultivars used in this study may therefore have been due to differences in the ratio of nitrate influx and efflux. For example, it is possible that the high-H plants had a more rapid N efflux at the lower temperature, but maintained influx rates. Consequently, energy demand and respiratory rates did not change much as growth temperature decreased. In the low-H plants, on the other hand, both influx and efflux rates may have decreased in concert, keeping NNUR more or less constant, but decreasing respiration rates when growth temperature decreased.
When the AP contributes to respiration, more substrates are consumed to produce the same amount of ATP, and the energy costs associated with nitrate uptake would be a greater proportion of the energy budget. Figure 5c illustrates such a situation: under conditions where 50% of respiration occurred via AP, and there was 50% efflux of nitrate, the costs of nitrate uptake would be 30–76% of total respiration. Could therefore changes in AP activity contribute to differences in root respiratory rates in the high- and low-H plants during temperature changes? Increases in the amount of alternative oxidase (AOX) protein have been observed when maize (Stewart et al. 1990) and mung bean (Gonzàlez-Meler et al. 1999) were grown at low temperatures, but this did not necessarily lead to greater AOX activity in vivo (Gonzàlez-Meler et al. 1999). In the cultivars used here, we observed that 15 °C-plants with high H had a lower amount of AOX and higher amount of COX protein than 25 °C-plants, but this was not observed in the plants with low-H (data not shown). This suggests that the respiration of plants showing respiratory homeostasis uses more efficient pathways at low growth temperature. In general, monocotyledonous species with higher RGR show higher root respiration rates and in vivo activity of AOX than those with lower RGR (Millenaar et al. 2001). Further experiments measuring in vivo activity of AOX under the different growth temperatures are necessary to confirm this.
Our study suggests that homeostasis of root respiration could be advantageous for plant growth at low temperature, as long as these temperature are not extreme. NARm was mainly responsible for the differences in RGR and the rates of root respiration between plants with high H and low H. In plants with high H, the smaller decrease in NARm upon lowering the growth temperature may have led to a smaller decrease in carbohydrate supply to roots. However, it is also possible that variation in the efflux of nitrate and/or engagement of AOX influenced root respiration rates, and these processes could also play a role in respiratory homeostasis. Further experiments are required to assess the relative importance of these parameters.
We would like to thank the Drs M. Schortemeyer and T. L. Setter for providing the seeds of wheat. We also acknowledge Greg Cawthray and Robert Creasy for their kind help, and Drs O. K. Atkin and A. H. Millar for their technical and useful advice for this study.