Susceptibility to photoinhibition of three deciduous broadleaf tree species with different successional traits raised under various light regimes

The susceptibility to photoinhibition of tree species from three different successional stages were examined using chlorophyll fluorescence and gas exchange techniques. The three deciduous broadleaf tree species were Betula platyphylla var. japonica, pioneer and early successional, Quercus mongolica, intermediate shade-tolerant and mid-successional, and Acer mono, shade-tolerant and late successional. Tree seedlings were raised under three light regimes: full sunlight (open), 10% full sun, and 5% full sun. Susceptibility to photoinhibition was assessed on the basis of the recovery kinetics of the ratio of vaviable to maximum fluorescence (F-v/F-m) of detached leaf discs exposed to about 2000 mu mol m(-1) s(-1) photon flux density (PFD) for 2 h under controlled conditions (25 to 28 degrees C, fully hydrated). Differences in susceptibility to photodamage among species were not significant in the open and 10% full sun treatments. But in 5% full sun, B. platyphylla sustained a significantly greater photodamage than other species, probably associated with having the lowest photosynthetic capacity indicated by light-saturated photosynthetic rate (B. platyphylla, 9.87, 5.85 and 2.82; Q. mongolica, 8.05, 6.28 and 4.41; A. mono, 7.93, 6.11 and 5.08 mu mol CO2 m(-1) s(-1) for open, 10% and 5% full sun, respectively). To simulate a gap formation and assess its complex effects including high temperature and water stress in addition to strong light on the susceptibility to photoinhibition, we examined photoinhibition in the field by means of monitoring Delta F/F'(m) on the first day of transfer to natural daylight. Compared with Delta F/F'(m) in AM, the lower Delta F/F'(m) in PM responding to lower PFD following high PFD around noon indicated that photoinhibition occurred in plants grown in 10 and 5% full sun. The diurnal changes of Delta F/F'(m) showed that Q. mongolica grown in 5% full sun was less susceptible to photoinhibition than A. mono although they showed little differences both in photosynthetic capacity in intact leaves and susceptibility to photoinhibition based on leaf disc measurements. These results suggest that shade-grown Q. mongolica had a higher tolerance for additional stresses such as high temperature and water stress in the field, possibly due to their lower plasticity in leaf anatomy to low light environment.