Influence of Elevated Air Temperature on the Relationship between Photochemical Reflectance Index (PRI) and Photosynthetic Light Use Efficiency of Poplar Leaves

Abstract

To clarify the application of the PRI to evaluation of photosynthetic light use efficiency under the varied air temperature, we investigated experimentally the PRI and photosynthesis in the leaves of 4-month-old poplar clones (Populus x Canadensis clone I45/51) which were exposed to 5 levels of air temperature (20, 25, 30, 35 and 40 ºC) and 4 levels of light intensities (250, 600, 1100 and 1700 µmol m-2 s-1 PPFD). Irrespective of air temperature treatment, increasing light intensity reduced the PRI, epoxidation state in the xanthophyll cycle (EPS) and quantum yield of photosystem II (ΔF/Fm’). The elevating air temperature significantly increased the PRI without clear changes of EPS, indicating that temperature-induced increase of the PRI could not be explained by only the de-epoxidation of xanthophyll cycle pigments. Consequently, the PRI positively correlated with EPS and ΔF/Fm’, but the regression lines differed among temperature treatments. The high air temperature above 30 ºC reduced light use efficiency of net photosynthetic rate (LUEN) at the light intensities of 250 and 1700 µmol m-2 s-1 PPFD, accompanied with lowered stomatal conductance and elevated respiration rate. Therefore, the PRI did not correlate to LUEN of poplar leaves in two higher temperature treatments. On the other hand, in all temperature treatments, the PRI positively correlated to the light use efficiency of gross photosynthetic rate (LUEG) which took into account of CO2 loss caused by increasing respiration. Although foliar PRI is increased by the elevating temperature, we conclude that the PRI can qualitatively trace the photosynthetic light use efficiencies both in photosystem II and gross photosynthetic rate in even heat-stressed poplar leaves.