The influences of a number of environmental factors on flowering time are well understood, yet the effects of rising atmospheric [CO2] have received less attention. A recent review indicated that elevated [CO2] can lead to delays and accelerations in flowering time among species, although less is known about intraspecific responses to elevated [CO2]. Previously, we showed that elevated [CO2] (700 ppm) can delay the flowering times of some Arabidopsis ecotypes, possibly due to signaling mechanisms from excess sugars. However, it is unknown if this occurs from a “dose-dependent” effect of elevated [CO2] (linear response with increasing exposure) or if there are thresholds that occur in flowering time with increased exposure time. This issue is key since atmospheric [CO2] is expected to reach 700 ppm within the next 50-100 years and may have a large impact on flowering time.
We subjected eight Arabidopsis ecotypes to varying lengths of time at elevated [CO2] (700 ppm). Plants of each ecotype were exposed to elevated [CO2] for 4, 8, 12, and 16 d. After the designated exposure time, plants were placed in modern conditions (380 ppm) until flowering occurred. Control plants spent the entirety of their life cycle in either modern or elevated [CO2].
Results/Conclusions
When combining all ecotypes, control plants that remained at either modern (380 ppm) or future (700 ppm) [CO2] flowered after 18 and 21 d, respectively. We found that ecotypes exposed to 700 ppm CO2 between 4 and 8 d did not significantly differ in their flowering times from plants that remained at 380 ppm. For exposure times between 8 and 12 d, we found that seven of the eight ecotypes experienced significant delays in flowering time that ranged from 1-4 d across these doses. This result suggests that once a threshold of elevated CO2 exposure is reached, a shift in flowering time can occur, and this appears as progressive delays with increasing exposure up to 12 d. Interestingly, however, only three ecotypes continued to display delayed flowering time between 12 and 16 d of exposure, whereas other ecotypes showed evidence of a return to accelerated flowering times. Thus, although there appears to be an initial “dosage” effect of elevated CO2 on flowering time, this may break down in some ecotypes following longer-term exposure. Moreover, this study suggests that the underlying mechanisms that control flowering times in response to elevated CO2 may exhibit high levels of intraspecific variation.