The effect of a heat wave on plant performance
An immediate and significant decline in Anet was observed in seedlings that were subjected to a +12°C heat wave, but not in seedlings subjected to a +6°C heat wave. Our first hypothesis, that heat waves would be more detrimental to Anet than a constant elevation of air temperature, was confirmed, but it depended on the severity of the heat wave. Our results were similar to results of studies that imposed short term temperature increases on other plant species, for example, Phaseolus vulgaris (Hüve et al., 2011), Quercus pubescens (Haldimann & Feller, 2004), Pisum sativum (Haldimann & Feller, 2005) and Vitis amurensis (Luo et al., 2011). These studies applied the temperature treatment for a few hours, while we imposed the temperature treatments for a week. After the third day of the +12°C heat wave, Anet values stabilized at positive values and did not show signs of further reduction, indicating that the photosynthetic apparatus did not accrue additional stress or damage as the heat wave continued. During the heat-wave period, we observed a slow decline in Anet of the Tamb and Tamb + 3 seedlings of Q. rubra at ambient [CO2]. As the ambient temperature was higher during this period when compared with the period before the heat wave (Table 1), air temperature might have been supra-optimal for photosynthesis (Gunderson et al., 2010).
Decreases in photosynthetic performance in response to heat stress have generally been attributed to stomatal limitations, increased respiration, photorespiration and/or heat damage to different parts of the photosynthetic apparatus (Saxe et al., 2001). As gs was not reduced during the heat wave in either P. taeda or Q. rubra, reductions in Anet cannot be explained by stomatal limitation. We found that gs was negatively related to leaf temperature in P. taeda but not in Q. rubra. Reports of the response of gs to heat stress have been highly variable. For example, a decrease in gs with rising temperature was reported in two oak species (Quercus macrocarpa and Quercus muehlenbergii) (Hamerlynck & Knapp, 1996), an increase was reported in gs in wheat (Triticum aestivum) and barley (Hordeum vulgare) (Bunce, 2000) and soybean (Glycine max) (Wilson & Bunce, 1997), and no change in gs was reported in two eucalyptus species (Eucalyptus saligna and Eucalyptus sideroxylon)(Ghannoum et al., 2010). We also found no significant relationships between VPD and gs, perhaps because of the high temperatures and VPDs in which the experiment was conducted. Similar to findings in P. taeda (Bongarten & Teskey, 1986; Teskey et al., 1986), gs exhibited diminished responses to changes in VPD at high values of VPD. It was noted by Salvucci & Crafts-Brandner (2004a) that plants under heat stress, in the presence of adequate water supply, keep their stomata open to evaporatively reduce their leaf temperature. We observed no stomatal closure and high transpiration rates during the heat wave, suggesting that seedlings of P. taeda and Q. rubra employed transpirational cooling to cope with heat stress. This result raises potential concerns about depletion of soil water if heat waves persist in time or increase in severity (Heath, 1998). In the absence of an adequate water supply it can be expected that leaf temperature will rise substantially, leading to more severe reductions in photosynthetic performance, and possibly increased tree mortality (Allen et al., 2010; Albert et al., 2011).
As the +12°C heat wave caused no apparent stomatal limitations to the diffusion of [CO2], inhibition of the photosynthetic apparatus must have contributed to the reduction in Anet. It has been shown that PSII is a thermolabile component of the photosynthetic apparatus. For example, thermal deactivation of oxygen evolution in PSII was observed in spinach (Spinacia oleracea)(Pueyo et al., 2002). In addition to reduction in oxygen evolution, heat stress has been shown to cause increased proton permeability of the thylakoid membranes (Bukhov et al., 1999), limitation of electron transport (Wise et al., 2004), cellular lesions (Hüve et al., 2011) and reversible deactivation of Rubisco activase (Haldimann & Feller, 2004; Salvucci & Crafts-Brandner, 2004b; Sharkey, 2005; Allen et al., 2010), all of which result in decreased photosynthetic activity.
InPAHW12 and PEHW12 we found a decrease in F0′ in response to the heat wave (data not shown). This result is consistent with the findings of Haldimann & Feller (2004), and suggests that the thylakoid membrane was not significantly damaged and was stable during heat stress. In contrast to P. taeda, an increase in F0′ was found inQAHW12, which indicates increased thylakoid proton permeability. This finding is in contrast to Ghouil et al. (2003), who concluded that through acclimatization, the thermal stability of the thylakoid membrane was enhanced in Q. suber seedlings exposed to temperatures up to 50°C. A similar acclimatization process may have taken place with repeated exposure to heat waves in P. taeda, but not in Q. rubra. Our analysis showed a similar relationship between Anet and ΦPSII values in QAHW12 and QEHW12. This relationship indicates that electron transport was reduced by heat stress in Q. rubra, resulting in a reduction in the production of ATP and NADPH, which would eventually cause reduced Anet. Values of ΦPSII remained constant for PAHW12 and PEHW12, which indicates maintenance of electron transport during the heat wave. As electron transport remained constant but a decrease in Anet was observed in both PAHW12 and PEHW12, electrons must have been transferred to alternative sinks such as photorespiration. Because increased leaf temperature results in reduced CO2 solubility relative to O2, it is likely that photorespiration increased during the heat wave (Wang et al., 2012). Photorespiration provides a mechanism that prevents over-reduction of the electron transport chain, thus protecting the photosynthetic apparatus from heat stress (Wingler et al., 2000; Leakey et al., 2003). In addition, a significant reduction in Fv′/Fm′ was found for PAHW12 and PEHW12, indicating a reduction in the efficiency of excitation energy capture and an increase in the thermal dissipation of excitation energy (Maxwell & Johnson, 2000; Rohacek, 2002).
We observed recovery of Anet during the 3 d after the heat wave in PAHW12 but not QAHW12, which indicates that the photosynthetic apparatus sustained damage, and lends support to our third hypothesis that a deciduous broadleaf tree species would be more susceptible to heat waves than an evergreen needle-leaf tree species. In a previous study of Q. rubra at the same site in which ambient temperature was raised by a constant 3°C or 6°C, no evidence of damage to photosynthetic apparatus was observed (Wertin et al. 2011). This result, combined with the lack of reduction to Anet in the +6°C heat wave in this study, suggests that the daily maximum temperatures in the +12°C heat wave, which exceeded 45°C, may have been the cause of the damage. These results also indicate that a mean increase in air temperature of +3°C caused by a series of heat waves may have a more negative effect on photosynthesis than a constant elevation of temperature. However, we observed partial recovery of Anet indicating that inhibition of photosynthesis during the heat wave was to some extent reversible. As damage to PSII is only slowly, if at all, reversible (Sinsawat et al., 2004), recovery of PSII cannot explain the reversible inhibition of Anet. However, a reversible reduction in Rubisco activation state through thermal deactivation of Rubisco activase has been reported (Haldimann & Feller, 2004, 2005; Sharkey, 2005). These studies showed that after alleviation of heat stress, Anet quickly returned to pre-stress values. In addition, leaf temperature decreased after the heat wave, thus reducing photorespiration (Sage & Kubien, 2007),suggesting another explanation for the partial recovery of Anet.
No evidence of thermal acclimatization of Anet to higher temperatures
Through thermal acclimatization of photosynthesis, plants can increase their optimum temperature for Anet (Hikosaka et al., 2006). This phenomenon has been reported for some conifer and deciduous tree species (Medlyn et al., 2002; Hikosaka et al., 2006; Ghannoum et al., 2010; Gunderson et al., 2010), but most studies have shown no acclimatization of photosynthesis to temperature (e.g. Ow et al., 2008a,b). Our observations indicated that a constant temperature increase of 3°C did not have a significant effect on Anet in P. taeda or Q. rubra, compared with Anet at ambient temperature. Wertin et al. (2010) also found that a temperature increase of 2.3°C did not result in thermal acclimatization of Anet in P. taeda. They attributed the lack of response to the broad temperature optimum for Anet in P. taeda (Teskey et al., 1987). In addition, seedlings exposed repeatedly to the +6°C heat-wave treatment did not acclimatize, although there was some inconclusive evidence of possible acclimatization in P. taeda.
Less heat stress under elevated [CO2]
The higher Anet we observed under elevated [CO2] was consistent with previous results in P. taeda (Teskey, 1997; Tissue et al., 1997) and Q. rubra (Kubiske & Pregitzer, 1996; Anderson & Tomlinson, 1998; Cavender-Bares et al., 2000), as well as many other species. Our data also revealed that elevated [CO2] compensated for the negative effect of heat stress on Anet, which is consistent with other studies (Teskey et al., 1987; Faria et al., 1996, 1999; Taub et al., 2000; Hamilton et al., 2008; Wang et al., 2008)and supported our second hypothesis that elevated [CO2] would mitigate heat stress induced by heat waves. This result suggests that under predicted future atmospheric [CO2] conditions, well-watered seedlings will be better able to cope with heat waves than in current [CO2] conditions. However, this finding must be interpreted with caution. As photosynthesis can acclimatize by downregulation to higher [CO2] (Wang et al., 1996; Long et al., 2004; Ainsworth & Rogers, 2007), the potential for future long-term mitigating effects of elevated [CO2] on heat stress might be less than the effect seen in our study. If this is the case, the future impact of heat waves on Anet may be similar to that at the present time. It is also important to consider that drought stress may strongly exacerbate heat stress regardless of [CO2].
Our results showed that Anet was significantly reduced in seedlings that were subjected to a +12°C heat wave, but that Anet remained positive throughout most or all of the heat wave and the overall level of stress was less than expected, especially under elevated [CO2]. The decline in Anet during the heat wave was similar in P. taeda and Q. rubra, but the underlying mechanism was different; in P. taeda, reduced Anet was linked to lower Fv′/Fm′, while in Q. rubra it was linked to lower ΦPSII.
We conclude that even under well-watered conditions, a future increase in the frequency or severity of heat waves will have detrimental effects on photosynthesis of P. taeda and Q. rubra. We observed a mitigating effect of elevated [CO2] on the thermal stress imposed by a severe heat wave. However, photosynthetic downregulation caused by long-term exposure to elevated [CO2] could negate this mitigating effect and lower available soil water could exacerbate the stress caused by heat waves.