Responses of photosystem II of white elm to UV-B radiation monitored by OJIP fluorescence transients

Photosystem II (PSII) activities in both samara and leaf of white elm (Ulmus pumila L.) were significantly inhibited by enhanced UV-B radiation (UVBR). UVBR disturbed both the donor and acceptor sides of PSII. The plastoquinone (PQ) pool size on the acceptor side, the trapped excited energy for complete reduction of Q_A, and the proportion of closed PSII reaction centers (RCs) increased, with PSII RCs being transformed into dissipative sinks for excitation energy under UVBR. However, samara and leaf responded to UVBR in different ways. A decrease in the F ₀ for leaf induced by UV-B radiation suggests the formation of fluorescence-quenching centers. An increase in the V_I for leaf under UVBR might mean the accumulation of reduced Q_A and PQ. F ₀ and V_I for samara showed opposite change pattern. Leaf has the mechanism of regulation of the amount of light reaching the RC through decreasing the number of light-harvesting chlorophyll molecules under UVBR while samara may be unable to regulate the light-harvesting capacity. PSII in samara was more susceptible to UVBR than that in leaf, with PI_ABS for samara decreasing more rapidly by a factor of 6.4 than that for leaf. Samara can recover more easily from UVBR-induced damage to PSII than the leaf.