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.].
Chlorophyll fluorescence kinetics was used to investigate the effect of 1,4-dithiothreitol (DTT) on the distribution of excitation energy between photosystem 1 (PS1) and photosystem 2 (PS2) in soybean leaves under high irradiance (HI). The maximum PS2 quantum yield (Fv/Fm) was hardly affected by the presence of DTT, however, photon-saturated photosynthesis was depressed distinctly. Photochemical efficiency of open PS2 reaction centres during irradiation (Fv'/Fm') was enhanced by about 30-40 % by DTT treatment, whereas photochemical quenching (qP) was depressed by about 40 % under HI. DTT treatment caused a 30 % decrease in allocation of excitation energy to PS1 under HI and a 20 % increase to PS2. An obvious shift in the balance of excitation energy distribution between photosystems was observed in DTT-treated leaves. Though high excitation pressure (1 - qP) resulted from DTT treatment, non-photochemical quenching (qN) was lower. DTT completely inhibited the formation of zeaxanthin and also distinctly depressed the state transition (qT). The shift in the balance of excitation distribution between the two photosystems induced by DTT was mainly due to the enhancement of excitation energy capture by PS2 antenna and the inhibition of state transition. It might be the shift in the balance between the two photosystems that mainly induced the depression of photosynthesis. Thus, to keep high utilization efficiency of absorbed photon energy, it is necessary to maintain the balance of excitation distribution between PS2 and PS1. 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 study the responses of winter wheat cultivars released in different years to short-term high O3 exposure, an old cultivar ('Nongda 311', released in 1960s) and a modern one ('Yannong 19', released in 1990s) were treated with an O3 exposure (145 ± 12 mm3 m-3, 4 h d-1 for 3 d) shortly after anthesis stage (> 50 % main stems blossomed). During the O3 exposure, light-saturated photosynthetic rate (PN) and stomatal conductance (gs) of both cultivars decreased considerably. Elevated O3 did not decrease dark-adapted maximum photochemical efficiency, but induced significant reduction in actual photochemical efficiency and thereby considerably increase in non-photochemical quenching. PN, gs of the modern cultivar 'Yannong 19' decreased more than the older one 'Nongda 311', indicating the former exhibited higher sensitivity to O3 than the latter. After O3 exposure, PN, gs and chlorophyll (Chl) content in flag leaf decreased more quickly than control, indicating induction of faster premature leaf senescence. As a result, the short-term O3 exposure caused substantial yield loss, with larger reduction in 'Yannong 19' (-19.2 %) than in 'Nongda 311' (-8.4 %). Our results indicated that high O3 exposure at grain filling stage would have greater negative impacts on the high yielding modern cultivar relative to the old one with lower yield. 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.].
The responses of gas exchange and chlorophyll fluorescence of field-growing Ulmus pumila seedlings to changes in simulated precipitation were studied in Hunshandak Sandland, China. Leaf water potential (Ψwp), net photosynthetic rate (PN), stomatal conductance (gs), and transpiration rate (E) were significantly increased with enhancement of precipitation from 0 to 20 mm (p<0.01), indicating stomatal limitation of U. pumila seedlings that could be avoided when soil water was abundant. However, PN changed slightly when precipitation exceeded 20 mm (p>0.05), indicating more precipitation than 20 mm had no significant effects on photosynthesis. Maximum photochemical efficiency of photosystem 2, PS 2 (Fv/Fm) increased from 0.53 to 0.78 when rainfall increased from 0 to 10 mm, and Fv/Fm maintained a steady state level when rainfall was more than 10 mm. Water use efficiency (WUE) decreased significantly (from 78-95 to 23-27 µmol mol-1) with enhancement of rainfalls. PN showed significant linear correlations with both gs and Ψwp (p<0.0001), which implied that leaf water status influenced gas exchange of U. pumila seedlings. The 20-mm precipitation (soil water content at about 15 %, v/v) might be enough for the growth of elm seedlings. When soil water content (SWC) reached 10 %, down regulation of PS2 photochemical efficiency could be avoided, but stomatal limitation to photosynthesis remained. When SWC exceeded 15 %, stomatal limitation to photosynthesis could be avoided, indicating elm seedlings might tolerate moderate drought. and Y.-G. Li ... [et al.].