Photosynthetic performance and resistance to photoinhibition of Zea mays L. leaves grown at sub‐optimal temperature
The performance of the photosynthetic apparatus was examined in the third leaves of Zea mays L. seedlings grown at near-optimal (25 degrees C) or at sub-optimal (15 degrees C) temperature by measuring chlorophyll (Chl) a fluorescence parameters and oxygen evolution in different temperature and light conditions. In leaf tissue grown at 25 and 15 degrees C, the quantum yield of PSII electron transport phi PSII and the rate of O2 evolution decreased with decreasing temperature (from 25 to 4 degrees C) at a photon flux density of 125 micromole m-2 s-1. In leaves grown at 25 degrees C, the decrease of phi PSII correlated with a decrease of photochemical Chl fluorescence quenching (qP), whereas in leaves grown at 15 degrees C qP was largely insensitive to the temperature decrease. Compared with leaves grown at 25 degrees C, leaves grown at 15 degrees C were also able to maintain a higher fraction of oxidized to reduced QA (greater qP) at high photon flux densities (up to 2000 micromole m-2 s-1), particularly when the measurements were performed at high temperature (25 degrees C). With decreasing temperature and/or increasing light intensity, leaves grown at 15 degrees C exhibited a substantial quenching of the dark level of fluorescence F0 (q0), whereas this type of quenching was virtually absent in leaves grown at 25 degrees C. Furthermore, leaves grown at 15 degrees C were able to recover faster from photoinhibition of photosynthesis after a photoinhibitory treatment (1200 micromole m-2 s-1 at 25, 15 or 6 degrees C for 8 h) than leaves grown at 25 degrees C. The results suggest that, in spite of having a low photosynthetic capacity, Z. mays leaves grown at sub-optimal temperature possess efficient mechanisms of energy dissipation which enable them to cope better with photoinhibition than leaves grown at near-optimal temperature. It is suggested that the resistance of Z. mays leaves grown at 15 degrees C to photoinhibition is related to the higher content of carotenoids of the xanthophyll cycle (violaxanthin + antheraxanthin + zeaxanthin) measured in these leaves than in leaves grown at 25 degrees C.