photoinhibition often occurs in winter for most evergreen plants, which is particularly true in sun leaves. Lots of photoprotective approaches could be induced to prevent from photodestruction for the plants. Photoinhibition and photoprotection mechanisms were studied much more on conifer tree species than broadleaves ones, and more in PSⅡthan PSⅠ. In this paper, two evergreen broadleaved plantstree species (Euonmymus japonicus and Buxus spervirens L.)under different light environments were studied during winter. The two woody species were introduced from subtropical areas to Beijing. Specificity of both photosystem Ⅱ (PSⅡ) and photosystemⅠ (PSⅠ) changes of the two woody species were studied using methods of synchro-monitoring chlorophyll fluorescence and P700 redox kinetics.
The results were shown that the low temperature above 0℃ didn't inhibit the effective PSⅡ yield Y(Ⅱ) for each species at the end of October, while Fo and OJIP values of the sun leaves of Buxus spervirens L. were lower than those of the shade leaves. Meanwhile P700 oxidization of the sun leaves of the two species kept at the same rate from 0ms to 20ms, but the oxidation rate of the shade leaves of the two species decreased after 2ms, particularly in Buxus spervirens L.. Under this case, Y(Ⅰ)/Y(Ⅱ) ratio of the sun leaves for both two species were 1, while the values of the shade leaves for both two species of were lower than 1.
During winter, PSⅡ photoinhibition occurred in almost all of the leaves for both species. The order of the Y(Ⅱ) value from the highest one to the lowest one in winter were as following: shade leaves of Buxus spervirens L., sun leaves of Buxus spervirens L., shade leaves of Euonmymus japonicus and sun leaves of Euonmymus japonicus. Whereas both the relative efficiency of PSⅠ and the ratio of Y(Ⅰ)/Y(Ⅱ) increased in winter. Low temperature led to disequilibrium between PSⅡ and PSⅠ of shade leaves of Euonmymus japonicus, while combined stresses of low temperature and high light led to the same disequilibrium in the sun-leaves of both two species. In the winter OJIP kinetics of sun leaves disappeared, meanwhile that of shade leaves was still active at some extent.
Lots of photoprotective channels could be induced in the overwintering leaves, which could not be consistent in different leaves. State transitions occurred in both of the two species. Nonlinear electron transport maybe more important to Euonmymus japonicus. The capacity of light harvesting of the sun leaves decreased to decrease the inhibition of photosystem center,. Photoinhibition of PSⅡ was one of the most protective mechanism for the electron transporters in downstream in both of the species in mid-winter. Photoinhibition of the sun leave was much more significant than that of shade leaves. Photoprotection capacity of Buxus spervirens L. was much higher than that of Euonmymus japonicus, which was probably due to the difference in ROS clearing ability under low temperature stress. In conclusion, the degree of photoinhibition and the of photoprotective channels for overwintering leaves in two evergreen broadleaved tree species were different, which leads to the specificity in the photosystems' function.