Two rice chlorophyll (Chl) b-less mutants (VG28-1, VG30-5) and the respective wild type (WT) plant (cv. Zhonghua No. 11) were analyzed for the changes in Chl fluorescence parameters, xanthophyll cycle pool, and its de-epoxidation state under exposure to strong irradiance, SI (1 700 µmol m-2 s-1). We also examined alterations in the chloroplast ultrastructure of the mutants induced by methyl viologen (MV) photooxidation. During HI (0-3.5 h), the photoinactivation of photosystem 2 (PS2) appeared earlier and more severely in Chl b-less mutants than in the WT. The decreases in maximal photochemical efficiency of PS2 in the dark (Fv/Fm), quantum efficiency of PS2 electron transport (ΦPS2), photochemical quenching (qP), as well as rate of photochemistry (Prate), and the increases in de-epoxidation state (DES) and rate of thermal dissipation of excitation energy (Drate) were significantly greater in Chl b-mutants compared with the WT plant. A relatively larger xanthophyll pool and 78-83 % conversion of violaxanthin into antheraxanthin and zeaxanthin in the mutants after 3.5 h of HI was accompanied with a high ratio of inactive/total PS2 (0.55-0.73) and high 1-qP (0.57-0.68) which showed that the activities of the xanthophyll cycle were probably insufficient to protect the photosynthetic apparatus against photoinhibition. No apparent difference of chloroplast ultrastructure was found between Chl b-less mutants and WT plants grown under low, LI (180 µmol m-2 s-1) and high, HI (700 µmol m-2 s-1) irradiance. However, swollen chloroplasts and slight dilation of thylakoids occurred in both mutants and the WT grown under LI followed by MV treatment. These typical symptoms of photooxidative damage were aggravated as plants were exposed to HI. Distorted and loose scattered thylakoids were observed in particular in the Chl b-less mutants. A greater extent of photoinhibition and photooxidation in these mutants indicated that the susceptibility to HI and oxidative stresses was enhanced in the photosynthetic apparatus without Chl b most likely as a consequence of a smaller antenna size. and Z.-F. Lin, G.-Z. Lin, C.-L. Peng.
The relationships between dark respiration rate (RD) and net photosynthetic rate (PN) in Quercus ilex L. shrubs growing at the Botanical Garden in Rome were analysed. Correlation analysis of the data sets collected in the year 2006 confirmed the dependence among the considered leaf traits, in particular, RD was significantly (p<0.05) correlated with PN (r = 0.40). RD and PN increased from March to May [1.40±0.10 and 10.1±1.8 µmol(CO2) m-2 s-1 mean values of the period, respectively], when air temperature was in the range 14.8-25.2 °C, underlining the highest metabolic activity in the period of the maximum vegetative activity that favoured biomass accumulation. On the contrary, the highest RD [1.60±0.02 µmol(CO2) m-2 s-1], associated to the lowest PN rates (44 % of the maximum) and carbon use efficiency (CUE) in July underlined the mobilization of stored material during drought stress by a higher air temperature (32.7 °C). and L. Gratani, L. Varone, A. Bonito.
Activities of crucial enzymes involved in the Calvin cycle, glycolysis, and oxidative pentose phosphate cycle (PPC) were investigated in green calli of sugar beet {Beta vulgaris L.) during the transitíon from photoheterotrophic to photoautotrophic growth. The actívities of the Calvin cycle enzymes were increased by lowering the sucrose concentration of the medium, whereas the actívities of dissimilatíon related enzymes were either decreased or not inftuenced. The photoautotrophic culture cultívated on sucrose-free medium and 2 % CO2 grew at a rate six tímes slower than the photoheterotrophic one. Its growth continued on polyurethane pads soaked with agar-free medium and was fully inhibited by 0.5 pM DCMU.
In order to evaluate the effect of leaf age on enzymes of carbon and sulphur metabolism, chlorophyll (Chl) and protein contents and the activities of RuBP- carboxylase (RuBPC), ATP-sulphurylase (ATPS), and 0-acetylserinesulphydrylase (OASS) were determined in leaves of different ages from luceme (Medicago sativa L. cv. Lodi and Sewa) plants grown outdoors. The enzymes of sulphur and carbon metabolism were differently regulated throughout development of luceme leaves, as ATPS reached the highest activity at very early stages of leaf growth, when RuBPC activity was negligible. OASS activity inereased 2-fold during leaf expansion, and deereased with leaf senescence. The two cultivars showed rather similar trends of development in Chl, protein content and enzyme activities. The cultivars differed also in the absolute level of enzyme activities and other biochemical parameters.
The exciton equilibration in the light-harvesting complex of photosystem 2 at room temperature was studied with pump-probe experiments under quasi-stationary conditions. The measurements were performed with two dye lasers, which had a pulse diuation of 10 ns and a spectral bandwidth of less than 0.01 nm. The relative probe beam transmission ffom 640-690 nm was recorded at different excitation wavelengths, varied from 645 to 685 nm. Two maxima were observed at 650 nm (Chl b absorption region) and at 680 nm (Chl a absorption region). The relative probe beam transmission in the Chl a region showed a course nearly independent of the pump beam wavelengťh in contrast to a weak dependence in the Chl b region. Measurements of the relative probe beam transmission as a function of pump beam intensity revealed a dominating exciton-exciton annihilation in the Chl a absorption region. In the Chl b region exciton-exciton annihilation was only marginal.