Under a moderate water stress (pod water potential, Ψ№, - 1.5 MPa), induced by stopping irrigation for 3 d, the net photosynthetic rate (PN) decreased to 50 %, transpiration rate (£) to 85 %, stomatal conductance to 65 % and chlorophyll (Chi) content to 82 %, while the activities of photosystems (PS) and of some enzymes of the photosynthetic carbon reduction cycle (NAD- and NADP-glyceraldehyde-3-P dehydrogenases, aldolase) were almost without changes. The exceptions were ribulose-5-P kinase and 3-phosphoglycerate kinase, the activities of which were reduced to 55 and 79 %, respectively. Under a severe water stress (5 d without irrigation, VPW -2.1 MPa) all the above characteristics were strongly reduced (PN to 0.5 %, E to 57 %, PS1 to 62 %, PS2 to 37 %, enzyme activities to 48-68 %), but after rehydration the initial activities were restored. The reduction of PN in pods at a moderate water stress is probably related to the decline in activities of 3-phosphoglycerate kinase and ribulose-5-P kinase, while under a severe stress, when PN is lowered almost to zero, the decline in gs and electron transport activities is very important.
Most organisms inhabiting earth feed directly or indirectly on the products synthesized by the reaction of photosynthesis, which at the current atmospheric CO2 levels operates only at two thirds of its peak efficiency. Restricting the photorespiratory loss of carbon and thereby improving the efficiency of photosynthesis is seen by many as a good option to enhance productivity of food crops. Research during last half a century has shown that several plant species developed CO2-concentrating mechanism (CCM) to restrict photorespiration under lower concentration of available CO2. CCMs are now known to be operative in several terrestrial and aquatic plants, ranging from most advanced higher plants to algae, cyanobacteria and diatoms. Plants with C4 pathway of photosynthesis (where four-carbon compound is the first product of photosynthesis) or crassulacean acid metabolism (CAM) may consistently operate CCM. Some plants however can undergo a shift in photosynthetic metabolism only with change in environmental variables. More recently, a shift in plant photosynthetic metabolism is reported at high altitude where improved efficiency of CO2 uptake is related to the recapture of photorespiratory loss of carbon. Of the divergent CO2 assimilation strategies operative in different oraganisms, the capacity to recapture photorespiratory CO2 could be an important approach to develop plants with efficient photosynthetic capacity. and S. K. Vats, S. Kumar, P. S. Ahuja
Activity of carbonic anhydrase (CA) was studied in wild type (WT) Chlamydomonas reinhardtii and five mutants distinguished only by retaining photosystem 1 (PS 1), photosystem 2 (PS 2), light-harvesting complexes (LHC) of PS 1 or PS 2 and both photosystems without LHC. Two forms of CA, soluble CA (sCA) and membrane- bound (mbCA), were found in the cells. The highest activity of mbCA was obseiwed in mutants which retained both photosystems, oř only PS 1 and PS 2. The mutants deprived of photosystems but retaining LHC almost lacked mbCA. Thus, there was a correlation between the retention of mbCA and mutant abilities to perform the photosynthesis. Photosynthetic characteristics of the high CO2 requiring mutant of C. reinhardtii CIA-3 were compared with those of WT grown at 2 % CO2 and after various times of exposure to limiting CO2 concentration (0.03 %). CIA-3 cells háve a lower photosynthetic affmity for inorganic carbon (Cj) as compared to WT when grown at high and low CO2 concentrations. Only in the WT cells the photosynthetic affmity for Cj was increased when the gas phase was changed to air. In CIA-3 the photosynthesis was inhibited by a high concentration of Cj.
Grain sorghum [Sorghum bicolor (L.) Moench. cvs. TX430 and KS82] was grown in a Haynie very fine sandy loam (coarse-silty, mixed, superactive, calcareous, mesic Mollic Udifluvents) under constant 47 % shade or full irradiance in a greenhouse under two watering regimes to see the combined and individual effects of low irradiance (LI) and low water (LW) on the sorghum genotypes. Under the high-irradiance (HI) and high-water (HW) treatment (control) and the LI-HW treatment, TX430 grew taller than KS82. Both LI and LW reduced several times the fresh and dry masses. Under the control conditions, TX430 reached its maximum net photosynthetic rate (PNmax) of 28.93 μmol m-2 s-1 at a photosynthetic photon flux density (PPFD) of 1 707 μmol m-2 s-1, and KS82 reached its PNmax of 28.32 μmol m-2 s-1 at a PPFD of 2 973 μmol m-2 s-1. The fact that TX430 had PNmax under a lower PPFD than KS82 may relate to its taller growth under LI conditions. Hence genotypes of sorghum might be selected for low irradiance using curves relating PN to PPFD. and Qingzhang Xu, M. B. Kirkham.
Tomato (Lycopersicon esculentum Mill. cv. Pearson) plants were grown in growth chambers for 25 days with cadmium (Cd) and then exposed briefly to ozone (O3). Gas exchange, chlorophyll a fluorescence, and pigment composition were analysed in leaves at the end of the treatment to assess the effects of a single pollutant and their combination on photosynthesis. The CO2 assimilation rate was dramatically reduced in plants subjected to the combined treatment, while the single effect of Cd appeared less severe than that of O3. The decline of CO2 photoassimilation found in all
O3-exposed plants was attributed to both stomatal and nonstomatal limitations. Tomato plants seemed to detoxify Cd to a great extent, but this resulted in growth suppression. In response to O3 exposure, the plants protected their photosystems by heat dissipation of excess energy via the xanthophyll cycle. Cd combined with O3 affected adversely this cycle resulting in an increase in photosynthetic performance under the same experimental light conditions., E. Degl’Innocenti, A. Castagna, A. Ranieri, L. Guidi., and Obsahuje bibliografii
Drought stress is one of the main environmental factors limiting plant growth and productivity of many crops. Elevated carbon dioxide concentration (eCO2) can ameliorate, mitigate, or compensate for the negative impact of drought on plant growth and enable plants to remain turgid and functional for a longer period. In order to investigate the combined effects of eCO2 and drought stress on photosynthetic performance and leaf structures, we analyzed photosynthetic characteristics and structure and ultrastructure of cucumber leaves. The decline in net photosynthetic rate under moderate drought stress occurred due to stomatal limitation alone, while under severe drought stress, it was the result of stomatal and nonstomatal limitations. Conversely, eCO2 improved photosynthetic performance under moderate drought stress, increased the lengths of the palisade cells and the number of chloroplasts per palisade cell under severe drought stress, and significantly increased the grana thickness under moderate drought stress. Additionally, eCO2 significantly decreased stomatal density, stomatal widths and stomatal aperture on the abaxial surface of leaves under moderate drought stress. In conclusion, eCO2 can alleviate the negative effects of drought stress by improving the drought resistance of cucumber seedlings through stomatal modifications and leaf structure., B. B. Liu, M. Li, Q. M. Li, Q. Q. Cui, W. D. Zhang, X. Z. Ai, H. G. Bi., and Obsahuje bibliografii
The combined effects of UV-B irradiation and foliar treatment with selenium on two buckwheat species, common (Fagopyrum esculentum Moench) and tartary [Fagopyrum tataricum (L.) Gaertn.] buckwheat, that underwent different intensity of breeding, were examined. Plants grown outdoors under three levels of UV-B radiation were studied for 9 weeks, from sowing to ripening. At week 7 they were sprayed with solution containing 1 g(Se) m-3 that presumably mitigates UV-B stress. Morphological, physiological, and biochemical parameters of the plants were monitored. Elevated UV-B radiation, corresponding to a 17 % reduction of the ozone layer, induced synthesis of UV absorbing compounds. In both buckwheat species it also caused a reduction in amounts of chlorophyll a during the time of intensive growth, an effect, which was increased in tartary buckwheat in the presence of selenium. The respiratory potential, measured as terminal electron transport system activity, was lower in plants subjected to enhanced UV-B radiation during the time of intensive growth. The effective quantum yield of photosystem 2 was also reduced due to UV-B radiation in both buckwheat species and was mitigated by the addition of Se. Se treatment also mitigated the stunting effect of UV-B radiation and the lowering of biomass in common buckwheat. and B. Breznik ... [et al.].
The state of some parameters of photosynthetic activity in Norway spruce (Picea abies [L.] Karst.) seedlings during the first autumn temperature transition to frost was monitored in October 1991. The trees were grown under field conditions of the Beskydy Mts. (North Moravia, The Czech Republic). Simultaneous measurements of Chi a fluorescence and C02 gas exchange revealed two phases in the functional transition of the assimilatory apparatus. Immediately upon the temperature transition to frost a distinct decrease in the radiant energy saturated rate of C02 uptake was observed and radiationless dissipation was indicated by higher values of the nonphotochemical quenching coefficient. The second period of the transition, a period with prolonged influence of frost together with a higher level of irradiance, was connected with a decrease of photosynthetic efficiency. The overwhelming capacity for protective non-photochemical energy dissipation and the complete reduction of acceptor QA occurred especially at medium and high incident irradiance documenting photoinhibitory damage to the photosynthetic apparatus.