We examined photosynthetic activities and thermostability of photosystem 2 (PS2) in leaves of elm (Ulmus pumila) seedlings from initiation to full expansion. During leaf development, net photosynthetic rate (PN) increased gradually and reached the maximum when leaves were fully developed. In parallel with the increase of PN, chlorophyll (Chl) content was significantly elevated. Chl a fluorescence measurements showed that the maximum quantum yield of PS2 (ϕPS2), the efficiency a trapped exciton, moved an electron into the electron transport chain further than QA- (Ψo), and the quantum yield of electron transport beyond QA (ϕEo) increased gradually. These results were independently confirmed by our low irradiance experiments. When subjected to progressive heat stress, the young leaves exhibited considerably lower ϕPS2 and higher minimal fluorescence (F0) than the mature leaves, revealing the highly sensitive nature of PS2 under heat in the newly initiating leaves. Further analysis showed that PS2 structure in the newly initiating leaves was strongly altered under heat, as evidenced by the increased fluorescence signals at the position of the K step. We therefore demonstrated an inhibition in the oxygen-evolving complex (OEC) in the young leaves. This resulted in decrease in amount of the functional PS2 reaction centres and relative increase in the PS2 reaction centres with inhibited electron transport at the acceptor side under heat. We suggest that the enhanced thermostability of PS2 during leaf development is associated with improved OEC stability. and C.-D. Jiang ... [et al.].
Winter wheat (Triticum aestivum L. cv. Jingdong 8) was exposed to short-term high ozone treatment after anthesis and then was either well irrigated with soil water content (SWC) of 80-85 % (O3+W) or drought treated (SWC 35-40 %, O3+D). Short-term ozone exposure significantly decreased irradiance-saturated net photosynthetic rate (PN) of winter wheat. Under good SWC, PN of the O3-treated plant was similar to that of control on 2 d after O3-exposure (6 DAA), but decreased significantly after 13 DAA, indicating that O3 exposure accelerated leaf senescence. Meanwhile, green flag leaf area was reduced faster than that of control. As a result, grain yield of O3+W was significantly decreased. PN of O3+D was further notably decreased and green flag leaf area was reduced more than that in O3+W. Consequently, substantial yield loss of O3+D was observed compared to that of O3+W. Although PN was significantly positively correlated with stomatal conductance, it also had notable positive correlation with the maximum photochemical efficiency in the dark adapted leaves (Fv/Fm), electron transport rate (ETR), photochemical quenching (qP), as well as content of chlorophyll, suggesting that the depression of PN was mainly caused by non-stomatal limitation. Hence optimal soil water condition should be considered in order to reduce the yield loss caused by O3 pollution. and H. Xu ... [et al.].
In order to investigate the effect of chromosome doubling on ozone tolerance, we compared the physiological responses of a diploid honeysuckle (Lonicera japonica Thunb.) and its autotetraploid cultivar to elevated ozone (O3) exposure (70 ng g-1, 7 h d-1 for 31 d). Net photosynthetic rate (PN) of both cultivars were drastically (P<0.01) impaired by O3. Although there were significantly positive correlation between PN and stomatal conductance (gs) in both cultivars under each treatment, the decreased gs in O3 might be the result rather than the cause of decreased P N as indicated by stable or increasing the ratio of intercellular to ambient CO2 concentration(Ci/Ca). PN under saturating CO2 concentration
(PNsat) and carboxylation efficiency (CE) significantly decreased under O3 fumigation, which indicated the Calvin cycle was impaired. O3 also inhibited the maximum efficiency of photosystem II (PSII) photochemistry in the dark-adapted state (Fv/Fm), actual quantum yield of PSII photochemistry (ΦPSII), electron transport rate (ETR), photochemical quenching coefficient (qP), non-photochemical quenching (NPQ), the maximum in vivo rate of Rubisco carboxylation (Vcmax) and the maximal photosynthetic electron transport rate (Jmax) which demonstrated that the decrease in PN of the honeysuckle exposed to elevated O3 was probably not only due to impairment of Calvin cycle but also with respect to the light-harvesting and electron transport processes. Compared to the diploid, the tetraploid had higher relative loss in transpiration rate (E), (gs), (PNsat), Vcmax and Jmax. This result indicated that the Calvin cycle and electron transport in tetraploid was damaged more seriously than in diploid. A barely nonsignificant (P=0.086) interaction between O3 and cultivar on PN suggested a higher photosynthetic sensitivity of the tetraploid cultivar. and L. Zhang ... [et al.].