This study investigated the main factors contributing to boron toxicity in plants. Growth was rapidly inhibited by internal B concentrations in the range 1-5 m M across a range of plant types that included monocot, dicot and algal species. In contrast, mature cells were able to withstand up to 60 m M B for several days. In wheat, rapid inhibition of root growth occurred if high B was applied to the root tip, but not if high B was applied to mature sections of the root. In leaves, there were gradations in B concentrations that correlated with visible symptoms of toxicity. However, there was no evidence to support the hypothesis that toxicity in leaves is due to osmotic stress induced by the accumulation of B. Analysis of the sensitivity to B of a range of metabolic processes including photosynthesis, respiration and protein synthesis leads to the conclusion that growth is not restricted by effects of B on energy supply and not directly by inhibition of protein synthesis. At higher B concentrations, many cellular activities were found to be partially inhibited and the toxicity to mature tissues was therefore considered not to arise from the disruption of a single process, but from the accumulated retardation of many cellular processes, exacerbated in light by photooxidative stress.

No abstract

POST, A., and VESK, M. 1992. Photosynthesis, pigments, and chloroplast ultrastructure of an Antarctic liverwort from sun-exposed and shaded sites. Can. J. Bot. 70: 2259-2264. Adaptations to severe environmental conditions were studied in the only recorded continental Antarctic hepatic, Cephaloziella exiliflora (Tayl.) Steph. During the summer growing season Antarctic plants melt out and many are exposed fully to the sun. C. exiliflora plants collected from sun-exposed and shaded sites were compared. Most plants were sun exposed, with dark purple leaves containing an anthocyanin-like pigment bound within the thick cell walls of the leaves. The purple plants grew in a dense turf, were larger with more closely spaced leaves, and had a higher carotenoid to chlorophyll ratio than the green plants. Shaded green plants contained more chlorophyll per unit weight. The chlorophyll a to chlorophyll b ratio did not vary. At low irradiances the green plants had higher photosynthetic oxygen evolution rates. Comparing leaves in similar positions along the stems showed that the chloroplasts in green leaves contained more appressed thylakoids than the chloroplasts in purple leaves. These differences are consistent with responses to varying light exposure. An understanding of these effects of changing irradiance on bryophytes is needed for monitoring the impact of increased ultraviolet light owing to the seasonal depletion of stratospheric ozone.

No abstract

Deuteriated analogues of ubiquinone 10 (Q10) have been dispersed with plasma membranes of Escherichia coli and with the inner membranes of beetroot mitochondria. Orientational order at various deuteriated sites was measured by solid-state deuterium nuclear magnetic resonance (2H NMR). Similar measurements were made, using the compounds dispersed in dimyristoylphosphatidylcholine (DMPC) and egg yolk lecithin and dispersions prepared from the lipid extracts of beetroot mitochondria. In all cases only a single unresolved 2H NMR spectrum (typically 1000-Hz full width at half-height) was observed at concentrations down to 0.02 mol % Q10 per membrane lipid. This result shows that most Q10 is in a mobile environment which is physically separate from the orientational constraints of the bilayer lipid chains. In contrast, a short-chain analogue of Q10, in which the 10 isoprene groups have been replaced by a perdeuteriated tridecyl chain, showed 2H NMR spectra with quadrupolar splittings typical of an ordered lipid that is intercalated into the bilayer. The NADH oxidase activity and O2 uptake in Escherichia coli and in mitochondria were independent of which analogue was incorporated into the membrane. Thus, despite the major difference in their physical association with membranes, or their lipid extracts, the electron transport function of the long- and short-chain ubiquinones is similar, suggesting that the bulk of the long-chain ubiquinone does not have a direct function in electron transporting activity. The physiologically active Q10 may only be a small fraction of the total ubiquinone, a fraction that is below the level of detection of the present NMR equipment.(ABSTRACT TRUNCATED AT 250 WORDS)