expanded to cooler places (Long 1983; Leegood and Edwards 1996; Naidu et al. 2003; Wang et al. 2008). This conclusion is supported by the findings that PPDK overexpression transformants of maize maintain greater photosynthetic rates than control lines at low temperature (Ohta et al. 2006).
As with C3 plants (Yamori et al. 2010b), the limiting step of C4 photosynthesis may differ depending on the plant species (e.g., cold tolerant vs cold sensitive) and growth condition (low temperature vs high temperature), leading to different conclusions regarding the limiting step of photosynthesis at low temperature. Now, various antisense constructs in F. bidentis are available: (1) Rubsico (Furbank et al.1996); (2) Rubisco activase (von Caemmerer et al. 2005); (3) Carbonic anhydrase (von Caemmerer et al.2004); (4) NADP-ME (Pengelly et al. 2012); (5) PPDK (Furbank et al. 1997); and (6) NADP-MDH (Furbank et al. 1997). This would therefore be an excellent time to elucidate what limits C4 photosynthesis at low temperature, as well as high temperature, using F. bidentis as a model case, since the temperature response of C4 photosynthesis in these transgenic plants has not been closely examined.
Different temperature response of photosynthesis at day and night in CAM plants
The difference in temperature responses of photosynthetic reactions during the day and night have not been examined in CAM species. We may expect differential temperature responses of the different phases of CAM photosynthesis in desert CAM plants, since these species often experience a drastic alteration in day and night temperatures during a 24-h period. Thus, we analyzed the temperature responses of nocturnal CO2fixation rates (phase I), as well as chloroplast electron transport rates in the day (phase III) in two CAM species (Kalanchoe daigremontiana and K. pinnata), grown at day/night air temperatures of either 20/10 °C or 30/20 °C. More detailed information on plant growth conditions and photosynthetic measurements is described in Fig. S3.
The temperature response of CO2 fixation rates at night differed depending on the growth temperature in both species (Fig. 3). CO2 fixation rates at low temperatures were greater in 20 °C grown plants than in 30 °C grown plants, whereas CO2 fixation rates at high temperatures were greater in 30 °C grown plants than in 20 °C grown plants. The average optimum temperature for nocturnal CO2 fixation rates were increased by elevated growth temperatures (K. daigremontiana: 16.1 and 21.1 °C and K. pinnata: 15.3 and 19.6 °C, in 20 and 30 °C grown plants, respectively). The 30 °C grown plants showed higher optimum temperatures for electron transport rate in the day than 20 °C grown plants in both species, although a clear optimum temperature for daytime electron transport rate could not be obtained. Thus, both CO2 fixation rates at nighttime and electron transport rates in the daytime acclimated to shifts in growth temperatures.