Conclusions and future perspective
The earth’s climate is predicted to be warm by an average of 1.1–6.4 °C during the next century as a result of the increased greenhouse gases in the atmosphere. High leaf temperatures can reduce plant growth and limit crop yields, with estimates of up to a 17 % decrease in yield per 1.0 °C increase in average growing season temperature (Lobell and Asner 2003). It has been argued that a new “green revolution” is needed in world agriculture to increase crop yields for food demands (Fischer and Edmeades 2010), and enhancing photosynthesis is a promising approach for increasing crop yield. However, to reach this goal, we must understand what process limits photosynthesis under a range of growth conditions, and how well photosynthesis can acclimate to predicted changes in temperature. We found clear differences in the ability to acclimate photosynthesis to increases in growth temperature between species from differing photosynthetic pathways. C4 species had higher optimum temperatures of photosynthesis, but a reduced ability to acclimate the temperature optimum of photosynthesis to growth temperature, than C3 species, while C3 species tended to maintain the same photosynthetic rate at their growth condition across a range of growth temperatures (e.g., had better homeostasis) than C4 species. We also found that, within C3 species, evergreen woody plants and perennial herbaceous plants showed greater temperature homeostasis of photosynthesis than deciduous woody plants and annual herbaceous plants. In addition, we found that in CAM plants, the temperature response of CO2 fixation at night was much different from that of chloroplast electron transport in the day, and that both CO2 fixation rates and electron transport rates acclimated to shifts in growth temperatures. This could be considered to be an adaptive response since CAM plants from desert environments can experience a drastic alteration in day and night temperatures during a 24-h period.
Advances in plant transformation technology now make it possible to manipulate photosynthesis by overexpressing particular genes for alleviating bottleneck steps of photosynthesis. Thus, understanding the mechanisms of temperature acclimation of photosynthesis via comparisons of species differences and/or changes in growth temperature is of immense importance for identifying a biomolecular target for enhancing leaf photosynthesis. What would be a useful biomolecular target for enhancing leaf photosynthesis? There is no single answer, since the limiting step of photosynthesis differs depending on plants species, and also differs depending on growth and measurement temperatures even in a single plant species (Yamori et al.2010b). Therefore, the impact on the control of carbon fixation by manipulation of one enzyme would differ depending on the plant species and growth conditions. More attention should be paid to studying differences in the photosynthetic limiting step depending on species and growth conditions, as this might provide opportunities for achieving faster improvements in crop production.