S. M. Glidewell’s research while affiliated with Royal Botanic Gardens, Kew and other places

The detection by electron paramagnetic resonance (EPR) spectroscopy of free radical centres trapped in cellulosic materials has been proposed as a potential method for the identification of irradiated foodstuffs of plant origin. However, the published spectra of irradiated cellulose show distinct differences from those obtained from unpurified cellulosic materials. In an attempt to explain these differences we have carried out a detailed investigation of the EPR spectra of gamma-irradiated celluloses and of the products from a lignocellulosic material (oat straw) that had been subjected to various chemical treatments. We conclude that chemical treatments of cellulosic material involving strong alkali (15% in the present measurements) during purification result in subtle structural alterations which produce a different chemical environment for the unpaired electron that is trapped on irradiation.

Optimization of the EPR signal from the ‘cellulosic’ radical of irradiated plant materials can be achieved with an incident microwave power of approximately 250 μW. This is lower than previously recommended (0.5–2.0 mW) and results in signal intensities of up to 200% greater amplitude. the use of this recommended power in any future EPR-based detection system will result in fewer false negatives, greater sensitivity, and is more likely to enforce compliance with existing labelling requirements.

NMR imaging was used for the non-invasive acquisition of three-dimensional images of developing barley grains from anthesis to maturity, 40 days after anthesis. Quantitative T2 maps were generated, and chemical shift imaging detected changes in the tissue distribution of water, soluble carbohydrate and lipids. Complete 3D data are accessible on a website.

Fertile complexes (individual reproductive units of ovulate cones) of three Prumnopitys species and Afrocarpus falcatus (Podocarpaceae) were subjected to histological examination and non-destructive NMR imaging. The latter technique allowed the display, frame-by-frame analysis and electronic ‘dissection’ of internal structures such as the number and courses of vascular traces and resin canals and their morphology. Characters of these internal structures distinguished all three Prumnopitys species from each other and thus were shown to be taxonomically diagnostic. Fertile complexes of Prumnopitys andina and P. taxifolia were most similar, possessing simple vascular traces and few unbranched resin canals. Those of P. ferruginea were very different and possessed an interconnected network of resin ducts within the sarcotesta. These findings are congruent with relationships inferred from molecular phylogenetic studies, in which two subclades were recovered within Prumnopitys. The anatomy of the female fertile complexes of Afrocarpus falcatus was very distinct from all Prumnopitys species analysed. Its most distinctive feature was the existence of a complex network of radial vascular strands originating from within the outer layers of the sarcotesta and penetrating the inner layers of the fertile complex. The surface texture and morphology of the sclerotesta of the seed was also unique to each species. © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society, 2004, 145, 295–316.

For recalcitrant seeds, mortality curves of germination versus water content typically imply a wide range of desiccation sensitivities within a seed population. However, seed to seed differences in water content, during desiccation, may confound our interpretation of these mortality plots. Here, we illustrate this problem for two batches of Vitellaria paradoxa (Sapotaceae) seeds collected in 1996 and 2002. Whole seeds were desiccated to various target water contents (TWCs) using silica gel. During desiccation, smaller seeds in the population dried most rapidly. Consequently, there was a significant linear relationship between whole-seed water content and seed mass during the drying process. In addition, following desiccation to low TWCs, only the largest seeds in the population retained viability. Taken together, this suggests that the larger seeds survived, not as a consequence of great relative desiccation tolerance, but as a result of taking longer to desiccate. Subsequently, the critical water content (CWC) for viability loss was calculated, based on the assumptions that in the seed population whole-seed water content during desiccation was normally distributed and the smallest, and hence driest, seeds were killed first. Using this approach, the driest seeds in the population that were killed, at each TWC, were always below a single CWC (c. 20% and 26% in 1996 and 2002, respectively). In subsequent experiments the effect of seed size variation on the response to desiccation was confirmed by conducting desiccation screens on seeds sorted into two discrete size classes, i.e. the seed-lot heterogeneity in mass was reduced. Using this approach, the mortality curves had a steeper slope. Furthermore, data for 24 tropical tree species from the Database of Tropical Tree Seed Research (DABATTS) revealed that seed lots with less variability in mass had steeper mortality curves. Thus, taken together, the data suggest that, at least for whole seeds, the wide range of desiccation sensitivities typically inferred is an artefact of seed to seed variation in mass, and hence water contents, during drying.

Electron paramagnetic resonance (EPR) spectroscopy has been used to study free radicals formed in a range of plant tissues as a result of physical damage, with the objective of gaining some insight into the free radical chemistry that is initiated when uncooked vegetable products (eg salads) are eaten. Chemical spin traps were used to aid the detection of unstable free radicals; more stable radicals were detected directly. Commonly observed ‘stable’ species were the monodehydroascorbate radical, which has a characteristic doublet spectrum, and a single‐peak resonance, which is presumed to come from free radical centres stabilised in macromolecules. In mushrooms ( Agaricus spp), spin‐trapping experiments using either α‐(4‐pyridyl‐1‐oxide)‐ N ‐ t ‐butyl‐nitrone (4‐POBN) or phenyl‐ t ‐butyl‐nitrone (PBN) showed the formation of large quantities of adducts of the radical from 4‐hydroxymethylbenzene diazonium salts. Pleurotus species, in contrast, gave signals consistent with the formation of unidentified C‐centred radicals. With other foodstuffs, reaction with 4‐POBN was complex and signals from 4 − POBN· and ${\bf CO_{2}^{\cdot -}}$ adducts were observed along with the t ‐butylhydronitroxide radical (an adduct breakdown product). Investigation of carrot hypocotyl rootstock in the presence of 5‐(diethoxyphosphoryl),5‐methyl‐1‐pyrroline‐ N ‐oxide (DEPMPO) revealed adducts of ·OH and unidentified C‐centred radicals. Free radical interactions between food components were demonstrated by the suppression of the signal from the 4‐POBN adducts of lettuce by onion, garlic, satsuma or vinaigrette, but not by olive oil. In addition, an appreciable decrease in spectral intensity of the 4‐POBN adduct from lettuce was observed in the presence of saliva, which suggests that saliva contains free radical scavengers which are able to compete successfully with the spin trap. © 2002 Society of Chemical Industry

The spring malting barley mutant TL43 and its parent cultivar, Triumph, were grown in field trials at Dundee, Scotland and Lleida, Spain in 1997 and 1999, to study water uptake and distribution after steeping and the grain properties that influence them. In 1997, TL43 absorbed more water than Triumph at both sites despite having higher protein and hordein contents. However, it had a lower proportion of B hordein. In 1999 a different steeping regime was employed, incorporating longer air rests, and water uptake of both TL43 and Triumph from Spain was extremely high. At Dundee, Triumph showed greater water uptake than TL43, but poorer distribution of water through the endosperm. This was attributed to the high B-hordein content in the sub-aleurone region of Triumph, which created an effective barrier to initial hydration of the endosperm. Subsequent hydration was, as demonstrated by the Spanish-grown samples, more effective in Triumph compared to TL43, due to the less compacted structure of the inner endosperm. NMR imaging suggested that hydration in TL43 spread throughout the sub-aleurone layer before moving to the inner endosperm, but, in Triumph, proceeded on a broader front from the scutellum towards the distal parts of the grain. A general conclusion was that water uptake in malting, although influenced both by genotype and environment, was more dependent, here, on the latter. B-hordein quantity and distribution were significant factors, but soluble β-glucan content, previously implicated in water uptake differences between Spain and Scotland, appeared to have little effect on the germplasm in this study.