The relationships between drought response and anatomical/physiological properties were assessed in two poplar clones belonging to the Aigeros section: Populusxeuramericana clone Dorskamp (drought-tolerant) and clone Luisa Avanzo (drought-sensitive). Cuttings of both clones were exposed for 12 h to 0 mM (control). 50 mM (osmotic potential -0.112 MPa), and 150 mM (-0.336 MPa) mannitol. In control, Dorskamp had smaller stomata than Luisa Avanzo, one or two layers of palisade cells, a spongy mesophyll, and high concentrations of antioxidative compounds (ascorbate, glutathione). After exposure to 50 or 150 mM mannitol, both clones closed their stomata: leaf conductance and opening of stomata decreased. When exposed to 50 mM mannitol, net photosynthetic rate (PN) and chlorophyll (Chl) and total solute contents remained stable; ribulose-1,5-bisphosphate carboxylase/-oxygenase activity, Chl synthesis and turn-over, ascorbate peroxidase and glutathione reductase activities were less affected in Dorskamp than in Luisa Avanzo. Following an exposure to 150 mM mannitol, Dorskamp exhibited higher PN and higher contents of antioxidants (ascorbate, glutathione) and antioxidative enzymes (ascorbate peroxidase, glutathione reductase) than Luisa Avanzo. Hence the drought-tolerant poplar was able to better avoid and tolerate osmotic stress. and M. Courtois, E. Boudouresque, G. Guerrier.
High CO2 concentrations (HC) in air induce partial deactivation of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO, EC 4.1.1.39). Under saturating irradiance, increase in [CO2] to 1 200 cm3 m-3 reduces the concentration of operating carboxylation centres by 20–30 %. At a further increase in [CO2], the activity remained on the same level. Under limiting irradiance, the lowest activity was reached at 600 cm3 (CO2) m-3. The presence of oxygen diminished deactivation, but O2 failed to stimulate reactivation under high CO2. Conditions that favour oxygenation of ribulose-1,5- bisphosphate (RuBP) facilitated reactivation. Even HC did not act as an inhibitor. HC induces deactivation of RuBPCO by increasing the concentration of free reaction centres devoid of the substrate, which are more vulnerable to inhibition than the centres filled with substrates or products. and J. Viil ... [et al.].
The specificity factor of Rubisco (S f) was estimated in intact leaves from the carboxylation of ribulose-1,5-bisphosphate (RuBP) at various CO2/O2 ratios. As oxygenation is calculated by the difference of the 14CO2 uptake by RuBP in the absence and presence of oxygen, it is important to choose the optimum CO2/O2 ratios. At high CO2 concentration (1,000 cm3 m-3 and higher) oxygenation consumes less than 50% RuBP but the difference of concentrations of CO2 at cell walls (Cw) and at the carboxylation centers (Cc) is 2-5% and the influence of mesophyll resistance
(rmd) is of minor importance. To accumulate large endogenous pool of RuBP, the leaves were preilluminated in the CO2- and
O2-free gas environments for 8 to 10 s. Thereafter the light was switched off and the leaves were flushed with the gas containing different concentrations of 14CO2 and O2. The specificity factor of Rubisco was calculated from the amount of the tracer taken up under different 14CO2/O2 ratios by the exhaustion of the RuBP pool. Application of 14CO2 allowed us to discriminate between the CO2 uptake and the concurrent respiratory CO2 release which proceeded at the expense of unlabelled intermediates., J. Viil, H. Ivanova, T. Pärnik., and Obsahuje bibliografii
Changes in the temperature dependence of the maximum carboxylation capacity (VCmax) of Rubisco during thermal acclimation of PN remain controversial. I tested for acclimation of the temperature dependence of VCmax in quinoa, wheat, and alfalfa. Plants were grown with day/night temperatures of 12/6, 20/14, and 28/22°C. Responses of PN to substomatal CO2 (Ci) and CO2 at Rubisco (Cc) were measured at leaf temperatures of 10-30°C. VCmax was determined from the initial slope of the PNvs. Ci or Cc curve. Slopes of linear regressions of 1/VCmaxvs. 1/T [K] provided estimates the activation energy. In wheat and alfalfa the increases in activation energy with growth temperature calculated using Ci did not always occur when using Cc, indicating the importance of mesophyll conductance when estimating the activation energy. However, in quinoa, the mean activation energy approximately doubled between the lowest and highest growth temperatures, whether based on Ci or Cc., J. A. Bunce., and Obsahuje bibliografii