- Top of page
- Materials and Methods
Global climate records show potentially important nonuniform changes in surface temperatures at a variety of spatial and temporal scales (Easterling et al., 1997; Alward et al., 1999; IPCC, 2001). For example, warming on land has been significantly more pronounced at night, increasing by approx. 0.2°C per decade between 1950 and 1993, or nearly twice the rate of increase in daytime maximum air temperature (IPCC, 2001). This strong diel pattern in the rate of warming means that photosynthesis and respiration are necessarily responding to fundamentally different temperature signals, and points to a clear need to investigate the direct and indirect impacts of nocturnal warming on plant carbon relations. We have previously shown that elevated night temperatures at ambient CO2 partial pressures can increase leaf photosynthesis during the following day (Turnbull et al., 2002). This impact of nocturnal warming on photosynthesis was significantly greater than the impact of daytime warming, and was a result of the alleviation of ‘feedback inhibition’ (Stitt et al., 1987) of photosynthesis by an increase in the rate of utilization or export of photosynthates (Turnbull et al., 2002).
In addition to temperature, global environmental change also involves a significant increase in CO2 partial pressure [p(CO2)a]. Therefore temperature impacts on the response of photosynthesis to elevated p(CO2)a are also likely to be important (Morison & Lawlor, 1999). Although there are sound theoretical reasons for expecting stimulation of photosynthesis by elevated p(CO2)a at higher operational (daytime) temperatures (Long, 1991; McMurtrie & Wang, 1993; Teskey, 1997; Ziska & Bunce, 1997), there is surprisingly little data supporting this unequivocally (Morison & Lawlor, 1999). Perhaps more importantly, very few studies have considered the indirect and interactive impact of nocturnal temperature and elevated p(CO2)a. Elevated nocturnal temperatures could well enhance the photosynthetic response of plants to elevated CO2 partial pressures through increased respiration, as the accumulation of leaf carbohydrates is one of the most ubiquitous plant responses to elevated CO2 (Körner & Miglietta, 1994; Wurth et al., 1998; Moore et al., 1999). Here we extend our previous study (Turnbull et al., 2002) by presenting the findings of an experiment conducted to determine the short-term response of leaf respiration and photosynthesis to increasing nocturnal temperature in cottonwood (Populus deltoides) trees growing at elevated p(CO2)a. We hypothesized that elevated night-time temperature should enhance the response of photosynthesis to elevated p(CO2)a through a reduction in carbohydrate feedback inhibition.
- Top of page
- Materials and Methods
Rates of night-time leaf dark respiration (Rd) were significantly influenced by night temperature (P < 0.0001). Rd increased over the 10°C range in nocturnal temperature at each growth p(CO2)a (from 1.16 mol m−2 s−1 at 15°C to 1.38 mol m−2 s−1 at 25°C at 42 Pa CO2; from 0.95 mol m−2 s−1 at 15°C to 1.31 mol m−2 s−1 at 25°C at 80 Pa CO2; and from 1.35 mol m−2 s−1 at 15°C to 1.55 mol m−2 s−1 at 25°C at 120 Pa CO2, Fig. 1a). These increases in Rd corresponded to an acclimated Q10 (15–25°C over the 9 d experiment) of 1.20 at 42 Pa, 1.38 at 80 Pa and 1.15 at 120 Pa CO2. Leaf carbohydrates (soluble sugars and starch) were significantly influenced by CO2 treatment at sunrise and sunset, but by nocturnal temperature treatment only at sunrise (Table 1). Soluble sugars and starch were generally greater in leaves from the elevated p(CO2)a treatments. Carbohydrates measured at sunrise, following the respiration measurements, were greatest under the 15°C temperature treatment and lower at 20 and 25°C. By sunset soluble sugar and starch had increased in all leaves, more substantially so in leaves previously subjected to the higher night-time temperature treatments. Starch content in trees experiencing the 25°C night temperature increased from sunrise to sunset by 320, 80 and 83% at 42, 80 and 120 Pa CO2, respectively. However, at 15°C starch content only increased by 33, 78 and 18% at 42, 80 and 120 Pa CO2, respectively. Percentage daily turnover of starch (but not sugars) increased significantly, but not consistently, in response to increased night temperature at 42 and 80 but not 120 Pa p(CO2)a (Table 1).
Figure 1. (a) Night respiration, Rd and (b) maximum assimilation rate at growth p(CO2)a, Amax in the subsequent light period (measured between 1000 and 1200 h) for leaves of Populus deltoides grown at three CO2 partial pressures and exposed to three night-time temperature regimes (15, 20 and 25°C; open, hatched and closed bars, respectively). Trees were exposed to each night temperature regime for 48 h before the day of measurement. Each point is the mean (± SEM) of 12 trees. For statistical comparisons see text.
Download figure to PowerPoint
Table 1. Carbohydrate contents (g m−2) at sunrise and sunset, and daily turnover (%), for leaves of Populus deltoides trees grown at three CO2 partial pressures and exposed to three night-time temperature regimes
|CO2 partial pressure (Pa)||Night temperature (°C)||Sunrise||Sunset||Turnover|
|Sugars (g m−2)||Starch (g m−2)||Sugars (g m−2)||Starch (g m−2)||Sugars (%)||Starch (%)|
|T|| ||< 0.05||< 0.001||ns||< 0.05||ns||<0.05|
|CO2|| ||< 0.0001||< 0.0001||< 0.001||< 0.0001||ns||ns|
Light-saturated photosynthesis at growth p(CO2)a (Amax) during the day (1000–1200 h) responded significantly to the night-time temperature treatment (P < 0.001). At 42 Pa, Amax was significantly greater in the 25°C night-time treatment than the 15°C treatment (P < 0.05; Fig. 1b), displaying an increase of 16%. At 80 Pa, Amax was significantly greater in the 25°C night-time treatment than the 15°C treatment (P < 0.05), displaying an increase of 12%. At 120 Pa the 4% increase was not significant. Photosynthetic parameters derived from A–Ci response curves showed that growth p(CO2)a had little impact other than on Rubisco-limited photosynthesis (Vcmax) (Table 2). Neither Jmax nor TPU was significantly affected by p(CO2)a. The responses of photosynthetic parameters to nocturnal temperature regime at the three growth CO2 partial pressures were consistent with that of Amax. Vcmax, Jmax and TPU increased significantly in response to the higher nocturnal temperature treatments, but only at 42 and 80 Pa (Table 2). The impact of the night-time temperature manipulation resulted in a positive correlation between Rd and night-time utilization of starch (starch turnover), and between night-time starch turnover and Amax (Fig. 2). The relationship between starch turnover and Amax did not hold in the 120 Pa treatment.
Table 2. Gas-exchange characteristics calculated from the response of assimilation, A, to internal CO2 partial pressure, Ci, for leaves of Populus deltoides trees grown at three CO2 partial pressures and exposed to three night-time temperature regimes
|CO2 partial pressure (Pa)||Night temperature (°C)||Vcmax (mol m−2 s−1)||Jmax (mol m−2 s−1)||TPU (mol m−2 s−1)|
|T|| ||< 0.01||< 0.0001||< 0.0001|
|CO2|| ||< 0.001||ns||ns|
|T × C|| ||ns||0.026||ns|
Figure 2. Relationships between (a) leaf dark respiration rate and night-time starch turnover; and (b) night-time starch turnover and maximum assimilation rate at growth p(CO2)a, Amax (determined during the subsequent light period from 1000 to 1200 h) in leaves of Populus deltoides grown at 42 Pa (open symbols) and 80 Pa (closed symbols) CO2 partial pressure and exposed to three night-time temperature regimes (15, 20 and 25°C: square, upwards triangle and downwards triangle, respectively). Figures are means (± SEM) for 12 replicate leaves on six different trees at each CO2 partial pressure and night-time temperature.
Download figure to PowerPoint
- Top of page
- Materials and Methods
We have previously shown that changes in night temperature may have a significantly greater impact on leaf photosynthesis than changes in day temperature at ambient p(CO2)a (Turnbull et al., 2002). Here we show that this short-term nocturnal temperature effect on photosynthesis may also be displayed at elevated CO2 concentrations predicted in global change scenarios (i.e. double current partial pressures by 2100). These findings are significant in extending our appreciation of the mechanistic links between respiration and photosynthesis that may underpin plant responses to global environmental change. They indicate that plants may be more photosynthetically responsive to elevated CO2 partial pressures than has been suggested to date, on the basis of CO2-only experimental manipulations (Leverenz et al., 1999; Bruhn et al., 2000). They also point to a need to develop our understanding of the interactive effects of nocturnal temperature and CO2 concentration on the balance between assimilate supply and sink activity in trees.
Increased nocturnal temperature had a pronounced effect on patterns of leaf respiration and carbohydrate concentration. We found that Rd on an area basis was lower at an elevated p(CO2)a of 80 than 42 Pa, but was greater at 120 Pa. There is some evidence that leaf respiration declines with growth at high p(CO2)a (Amthor, 1997), although this effect is likely to depend on the basis (mass or area) on which respiration is quoted, and may well be small or not occur (Gonzalez-Meler & Siedow, 1999; Amthor, 2000; Hamilton et al., 2001; Tissue et al., 2002). Regardless of this inconsistency in the response of Rd to p(CO2)a, the effect of increasing night temperature was clear at each p(CO2)a. Although leaf respiration rates increased significantly at higher night-time temperatures, the magnitude of the response of respiration to temperature was somewhat less than has been previously published. Values of Q10 in this whole-ecosystem temperature manipulation were 1.20, 1.40 and 1.15 at 42, 80 and 120 Pa CO2, respectively. While within the range previously found for plants (Azcon-Bieto, 1992), these values are lower than observed for a range of deciduous tree species (Bolstad et al., 1999; Amthor, 2000; Gunderson et al., 2000; Turnbull et al., 2001). This difference is primarily a result of the fact that we have measured the response of acclimated respiration after 3 d at each temperature, not an instantaneous response. Under these circumstances we would expect a lower Q10 (Gifford, 2003). The differences may also be the result of differences in growth temperature, the range of temperatures used to generate response curves (Tjoelker et al., 2001), and the timing of respiration measurements. We have previously found that Q10 in the same trees at the beginning of the dark period, when leaf carbohydrate concentrations were at a maximum, was 2.10 (Griffin et al., 2002b).
Higher night temperatures and increased Rd significantly reduced predawn leaf sugar and starch content, indicating increased sink demand at both leaf and whole-plant levels (Stitt & Quick, 1989; Harley & Sharkey, 1991; Stitt & Schulze, 1994). Soluble sugar and starch concentrations were significantly lower in leaves following the highest night temperature (25°C) in the ambient CO2 treatment (42 Pa), although in the elevated p(CO2)a treatments only starch concentration was significantly lower. Although the starch pool was relatively small in leaves experiencing the higher night temperatures, its accumulation during the light period, and subsequent utilization to support higher rates of respiration in the dark period, was greater. In addition to increases in leaf respiration, we have also measured significant increases in stem respiration in response to night warming in these trees (an increase from 2.5 ± 0.28 mol m−2 stem surface area s−1 at 11°C to 9.4 at 21°C; K.L.G., unpublished data). The increase in carbohydrate utilization in trees at higher night temperatures has important implications for photosynthetic capacity.
To date, the majority of studies investigating the likely responses of photosynthesis to temperature at elevated p(CO2)a have considered the temperature at which photosynthesis operates (day temperature: Long, 1991; McMurtrie & Wang, 1993; Teskey, 1997; Ziska & Bunce, 1997). It is reasonable to predict that stimulation of photosynthesis by high p(CO2)a is larger at higher daytime temperatures based on (1) the decreased ratio of photosynthesis to photorespiration; and (2) the decreased ratio of gross photosynthesis to dark respiration (Morison & Lawlor, 1999). Here we report an impact of night temperature on photosynthesis at elevated p(CO2)a. At low night temperature Vcmax was significantly lower at elevated p(CO2)a. As we increased the nocturnal temperature experienced by trees in this whole-ecosystem manipulation, photosynthesis at both ambient (42 Pa) and elevated (80 Pa) p(CO2)a increased (i.e. Vcmax increased). This nocturnal enhancement of photosynthesis was not evident at 120 Pa. These short-term responses indicate that, at future elevated night temperatures suggested by global climate monitoring and modelling (Easterling et al., 1997; Alward et al., 1999; IPCC, 2001), net photosynthesis at elevated p(CO2)a may be increased. Furthermore, there is some evidence that the response may saturate at CO2 partial pressures greater than double current ambient levels, although further research with a greater range of treatment p(CO2)a would be needed to support this assertion. Given the direct link between processes influencing carbon uptake (photosynthesis during the day) and loss (leaf and woody tissue respiration at night), the ‘decoupling’ of night and day temperatures in a global change world may be of significance.
We found that increased photosynthetic capacity (Amax) in cottonwood was associated with lower leaf carbohydrate concentration following warmer nights. This is consistent with previous findings (Azcon-Bieto & Osmond, 1983; Stitt & Quick, 1989; Harley & Sharkey, 1991; Azcon-Bieto, 1993; Stitt & Schulze, 1994). More specifically, Amax was positively correlated with leaf starch turnover. A number of direct and indirect feedback mechanisms have been proposed to regulate photosynthetic capacity (Krapp & Stitt, 1995). Indirect feedback mechanisms include downregulation of gene expression mediated by increases in carbohydrate concentration (Krapp et al., 1993), and reduced levels of photosynthetic raw materials such as RuBP and cytosolic Pi (Goldschmidt & Huber, 1992). We conclude that increased Amax was supported by an increase in Rubisco activity, as Vcmax increased at high night temperature and elevated p(CO2)a. This conclusion is supported by previous studies investigating temperature ×p(CO2)a effects (Warner et al., 1995).
The increase in Amax measured between the 15 and 25°C night temperatures (16 and 12% increase at 42 and 80 Pa, respectively) has the potential to significantly affect patterns of tree carbon gain in future climatic conditions (Easterling et al., 1997; Alward et al., 1999). We did find that the A : Rd ratio remained relatively constant at the three night temperature treatments (consistent with Gifford, 2003), but the increase in A must result in an increase in net photosynthate potentially available for growth (A − Rd increases). Although smaller in magnitude, the trend we observed here in response to nocturnal warming is consistent with our previous findings at ambient CO2 (Turnbull et al., 2002). The greater response in this 2002 study is likely to be a result of the differences in season, greater total sink capacity (trees were coppiced and 1 yr older so had larger root systems), and the addition of a daytime temperature increase (which increased Amax by 15% on top of the night warming treatment). The response of these trees to extremely elevated CO2 partial pressures is also noteworthy. At the leaf level there was a strong indication that the increase in photosynthetic capacity at higher night temperatures was much less pronounced at 120 Pa (only a 4% increase between 15 and 25°C). This may indicate a saturation of the photosynthetic response to increased night temperature at highly elevated p(CO2)a, although such high CO2 partial pressures are unlikely to occur this century (IPCC, 2001).
Having conducted two separate, but related, experiments over two different growing seasons (this experiment and those described by Turnbull et al., 2002), we are confident that the mechanism we propose regarding the impact of elevated nocturnal temperature on photosynthesis in this system is robust and repeatable. The large scale of this experiment did not allow for treatment replication and, although not strictly an independent study, the response to nocturnal warming has now been confirmed in newly coppiced growth in the same trees on a second occasion. We recognize that both experiments have been conducted over a short time scale (days). Assessment of the long-term implications of these findings for net carbon uptake in a global climate change environment must take account of long-term acclimatory responses to elevated night-time temperatures. In this experiment, trees were given a 48 h ‘acclimation’ period at each experimental temperature before making measurements. This was appropriate given the rapid acclimation of respiration to temperature (Atkin et al., 2000), and we found that this period was sufficient for leaves to return to their pre-experiment Amax when exposed to the middle (20°C) temperature treatment. However, we are mindful that further investigations are needed concerning impacts on larger-scale processes (e.g. growth) over longer time scales. Based on our present finding, that daytime photosynthetic capacity increases following warmer nights, we would predict that plant growth responses to elevated p(CO2)a may be enhanced at elevated night temperatures because of increased sink metabolism and depletion of assimilates (Gifford, 1992; Morison & Lawlor, 1999). However, the full significance of nocturnal warming can only be tested on the balance of night and day carbon exchange and, ultimately, plant growth rates (Myneni et al., 1997; Alward et al., 1999) in long-term studies.