Submitted to: Tree Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/22/2009
Publication Date: 1/1/2010
Citation: Hozain, M.I., Salvucci, M.E., Holaday, A.S., 2010. The differential response of photosynthesis to high temperature for a boreal and temperate Populus species relates to differences in Rubisco activation and Rubisco activase properties. Tree Physiology, 30:32-44. Interpretive Summary: In the process of photosynthesis, plants convert light into chemical energy. The energy produced by photosynthesis is then used to synthesize sugars and other foodstuffs. Heat stress inhibits photosynthesis, reducing the overall yield of the plant. Previous research identified an enzyme called Rubisco activase as the component of photosynthesis that is most sensitive to inhibition by heat. Rubisco activase is a regulatory enzyme that controls the activity of the major carbon dioxide-fixing enzyme in plants. In this manuscript, we collaborated with colleagues at Texas Tech University on a proof-of-concept study to determine the factors that constrain photosynthesis under moderate heat stress. The study examined photosynthesis in two Populus species adapted to contrasting thermal environments. The data verified our hypothesis that differences in the temperature response of photosynthesis can be ascribed to differences in the response of Rubisco activation and ultimately to the thermal properties of Rubisco activase. The results provide new insights into our understanding of how photosynthesis responds to heat stress by showing that the close correlation between the thermal sensitivity of photosynthesis and the thermal properties of Rubisco activase is a general phenomenon in plants. The information from the study, particularly the sequence differences between the two poplar species, can be used to make changes that improve the stability of Rubisco activase in order to improve the efficiency of photosynthesis under heat stress.
Technical Abstract: Significant inhibition of photosynthesis occurs at temperatures only a few degrees (less than or equal to 10 degrees Celsius) above the optimum, resulting in considerable loss of potential productivity. Most studies of heat stress have focused on crop or weedy annual plants, whereas similar studies with trees have been limited in number. Since temperature is a major factor limiting the geographic ranges of most plants, the aim of this study was to use two Populus species adapted to contrasting thermal environments to determine the factors that constrain photosynthetic assimilation (A) under moderate heat stress in tree species. Consistent with its native range in temperate regions, P. deltoides exhibited a significantly higher temperature optimum for A than did P. balsamifera, a boreal species. The higher A exhibited by P. deltoides at 33 to 40 degrees Celsius compared with P. balsamifera was associated with a higher carboxylation efficiency and a higher activation state of Rubisco and correlated with higher ATPase activity of Rubisco activase. The temperature response of minimal chlorophyll a fluorescence for darkened leaves was similar for both species and was not consistent with a thylakoid lipid phase change contributing to the decline in A in the range of 30 to 40 degrees Celsius. Taken together, these data support the idea that differences in the temperature response of A for the two Populus species could be attributed to differences in the response of Rubisco activation and ultimately to the thermal properties of Rubisco activase. That the primary sequence of Rubisco activase differed between the species, especially in regions associated with ATPase activity and Rubisco recognition, indicates that genotypic differences in Rubisco activase might underlie differences in the heat sensitivity of Rubisco activase and photosynthesis at moderately high temperatures.