Diurnal cycle and photoinhibition of photosynthesis in palm Trachycarpus fortunei H. Wendl. under winter and summer conditions

A study was conducted to analyze the contribution of high irradiance and re sulting photoinhibition to the decline in net photosynthesis in the leaves of palm Trachycarpus fortunei during summer and winter as well as at normal growth and low temperatures in field and laboratory conditions, respective ly. Fluorescence induction measurements indicated that there was a 10% decr ease in the F-v/F-m ratio in field conditions at midday during both summer and winter, due to the relatively low intensity of incident light resulting from the partial leaf segment folding. Fluorescence parameters completely recovered by the evening hours. In summer the midday decay was due to the d ecrease of F-m which probably represents a rapidly reversible component of photoinhibition by the protective down-regulation of PSII mediated by the x anthophyll cycle. In winter, however, the initial F-v/F-m ratio was 40% les s than as measured in summer and its midday decline was associated with the decrease of F-v indicating the partial inactivation of PS II. The net CO2 assimilation rate followed the pattern of the F-v/F-m ratio but it could no t recover due to the stomatal closure after midday. Comparing the fluoresce nce and gas exchange measurements we have concluded that the photoinhibitio n of T. fortunei represented by the F-v/F-m ratio changes is a regulatory a djustment of PS II efficiency to limiting carbon utilization and to limitin g carbon availability imposed by stomatal closure. Leaves photoinhibited un der laboratory conditions at growth temperature showed a substantial decrea se of 50% in the F-v/F-m ratio due to the perpendicular exposure, but no ap parent changes in D-1 protein content could be detected. Phytotron grown pl ants exposed to cold stress (6 degrees C) and low irradiance (250 mu mol m( -2) s(-1)) under laboratory conditions showed a time related but much slowe r continuous decrease in F-v/F-m ratio. After high irradiance the recovery kinetics in the dark at normal growth temperature (28 degrees C) strongly d epended on the extent of the photoinhibition, while after low irradiance co mplete recovery occurred in 12 hours irrespective of the initial F-v/F-m va lue, independently from the time of cold treatment, indicating that at low light and cold treatments only reversible inactive PS IIs were formed.