Barbara Rank

Kinetic Studies on the Xanthophyll Cycle in Barley Leaves (Influence of Antenna Size and Relations to Nonphotochemical Chlorophyll Fluorescence Quenching)

Xanthophyll-cycle kinetics as well as the relationship between the xanthophyll de-epoxidation state and Stern-Volmer type nonphotochemical chlorophyll (Chl) fluorescence quenching (qN) were investigated in barley (Hordeum vulgare L.) leaves comprising a stepwise reduced antenna system. For this purpose plants of the wild type (WT) and the Chl b-less mutant chlorina 3613 were cultivated under either continuous (CL) or intermittent light (IML). Violaxanthin (V) availability varied from about 70% in the WT up to 97 to 98% in the mutant and IML-grown plants. In CL-grown mutant leaves, de-epoxidation rates were strongly accelerated compared to the WT. This is ascribed to a different accessibility of V to the de-epoxidase due to the existence of two V pools: one bound to light-harvesting Chl a/b-binding complexes (LHC) and the other one not bound. Epoxidation rates (k) were decreased with reduction in LHC protein contents: kWT > kmutant >> kIML plants. This supports the idea that the epoxidase activity resides on certain LHC proteins. Irrespective of huge zeaxanthin and antheraxanthin accumulation, the capacity to develop qN was reduced stepwise with antenna size. The qN level obtained in dithiothreitol-treated CL- and IML-grown plants was almost identical with that in untreated IML-grown plants. The findings provide evidence that structural changes within the LHC proteins, mediated by xanthophyll-cycle operation, render the basis for the development of a major proportion of qN.

The α-tocopherol content of leaves of pedunculate oak (Quercus roburL.) - variation over the growing season and along the vertical light gradient in the canopy

This study was performed in order to investigate whether the actual requirement for defence against photo-oxidative stress is reflected by the alpha-tocopherol (alpha-Toco) content in leaves of pedunculate oak (Quercus robur L.). Antioxidants and pigments were quantified in leaves that were collected on six days between May and September 2000 in a mixed pine/oak forest at canopy positions differing in light environment. Pools of hydrophilic antioxidants and photo-protective xanthophyll cycle pigments (V + A + Z) reflected the anti-oxidative demand, as these pools increased with the average light intensity to which the leaves were acclimated. The photo-protective demand was not the determinant of the alpha-Toco content of oak leaves, as (1) foliage of a young oak, exposed to low light levels in the understorey, contained higher amounts of this lipophilic antioxidant than leaves sampled from semimature oaks at canopy positions with a similar light environment, and (2) a strong increase in the alpha-Toco content over the growing season was detected at each investigated crown position, whereas the V + A + Z pool did not show a concomitant accumulation during leaf ageing. The rate of alpha-Toco accumulation differed distinctly between samples taken at different canopy positions.

Free radical formation in chloroplasts. Methyl viologen action

Abstract The generation of free radicals under various conditions in the presence of methyl viologen (MV 2+ ) was investigated in chloroplasts of wheat and lettuce by means of electron spin resonance (ESR) spectroscopy including spin trapping using 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). An increase in the formation of different radical species, depending on the concentration of MV 2+ , the light intensity and the amount of chloroplasts, was observed. The monocation radical of MV 2+ (MV ) accumulated in the light under conditions of exhaustion of oxygen, and vanished in the dark. The disappearance of the ESR signal was strongly dependent on the amount of chloroplasts both for wheat and lettuce, indicating the transfer of electrons from MV to acceptors within the chloroplasts. NADPH stimulated and NADP + decreased the formation of reactive oxygen species. The addition of the NADPH-specific enzyme glutathione reductase diminished the DMPOOH signal both in the absence and in the presence of MV 2+ . This is explained by the restoration of the NADP + -pool by the action of the enzyme.

Photosynthesis and photoprotection inNicotiana tabacumL.in vitro-grown plantlets

Nicotiana tabacum L. plantlets were cultured in vitro photoautotrophically (0% sucrose) and photomixotrophically (3% or 5% sucrose) at two irradiances (80 or 380 mumol m-2 s-1) with the aim of investigating the effect of these culture conditions on photosynthetic parameters and on protective systems against excess excitation energy. In plantlets grown photoautotrophically under higher irradiance photoinhibition was demonstrated. These plantlets had a decreased chlorophyll (Chl) a + b content and Chl a/b ratio, an increased content of xanthrophyll cycle pigments and a higher deepoxidation state, a decreased maximum photochemical efficiency of photosystem II (PS II) and actual photochemical efficiency of PS II, and an increased non-photochemical quenching. In the photoautotrophically grown plantlets and those photomixotrophically grown with 3% sucrose, the increase of growth irradiance from 80 to 380 mumol m-2 s-1 stimulated the activities of ascorbate-glutathione cycle enzymes with the exception of ascorbate peroxidase. Ascorbate peroxidase activity was not affected by the increase in growth irradiance but a significant decrease with increasing sucrose concentration was evident. The higher concentration of sucrose in the medium (5%) in combination with the higher irradiance inhibited photosynthesis (decrease in Chl a + b content and net photosynthetic rate) but no significant changes in activities of ascorbate-glutathione cycle enzymes were found. These results suggest that exogenous sucrose added to the medium improved high irradiance and oxidative stress resistance of the plantlets but the effect of sucrose is concentration dependent.

Is the Lower Shade Tolerance of Scots Pine, Relative to Pedunculate Oak, Related to the Composition of Photosynthetic Pigments?

Foliage of Scots pine (Pinus sylvestris L.) and pedunculate oak (Quercus robur L.) was collected in a mixed pine/oak forest at canopy positions differing in radiation environment. In both species, chlorophyll (Chl) a/b ratios were higher in foliage of canopy positions exposed to higher irradiance as compared to more shaded crown layers. Throughout the growing season, pine needles exhibited significantly lower Chl a/b ratios than oak leaves acclimated to a similar photon availability. Hence, pine needles showed shade-type pigment characteristics relative to foliage of oak. At a given radiation environment, pine needles tended to contain more neoxanthin and lutein per unit of Chl than oak leaves. The differences in pigment composition between foliage of pine and oak can be explained by a higher ratio of outer antennae Chl to core complex Chl in needles of P. sylvestris which enhances the efficiency of photon capture under limiting irradiance. The shade-type pigment composition of pine relative to oak foliage could have been due to a reduced mesophyll internal photon exposure of chloroplasts in needles of Scots pine, resulting from their xeromorphic anatomy. Hence, the higher drought tolerance of pine needles could be achieved at the expense of shade tolerance.

Photoinduced damage in leaf segments of wheat (Triticum aestivum L.) and lettuce (Lactuca sativa L.) treated with 5-aminolevulinic acid. II. Characterization of photodynamic damage by means of delayed chlorophyll fluorescence and P700 photooxidation

Summary In a preceding paper (Hartel et al., this issue, pp. 230–236), light-induced damage of structural components of the photosynthetic apparatus has been demonstrated after pretreatment of green leaf segments of wheat and lettuce with 5-aminolevulinic acid (ALA) in darkness. In the present study, the time course of this process has been investigated by means of chlorophyll fluorescence and measurement of the lightinduced electron spin resonance signal I of P700 + . After exposure of leaves pretreated with ALA in darkness to continuous light, a decrease in delayed chlorophyll fluorescence intensity coincided with an increase in P700 + formation. This was observed in both plant species during the first two hours, indicating disturbance of the electron transfer between the two photosystems. Later a complete inhibition of photosystem-II-mediated electron transfer was found in lettuce. P700+ measurements reflected a severe structural disintegration of thylakoid membranes which increased with duration of light exposure and light intensity, and which appeared to be accelerated when photosystem II activity was lost. In contrast, photodynamic damage in wheat leaf segments did not increase after 2 h of illumination. The electron transfer inhibition was incomplete, and P700 + measurements revealed no indication of strong structural injury.