The influence of habitat structure on arthropod diversity in Argentine semi-arid Chaco forest

Abstract. The Argentine Chaco is a mosaic of grassland and open forest habitats maintained by natural disturbance activities such as fire. Since the introduction of domestic livestock and other human activities, the balance of this mosaic has been significantly altered, both in plant species and structural composition. This study focuses on the impact of such changes on the diversity of ground-dwelling arthropods within semi-arid Chaco forest. Quantitative measures of habitat structure and arthropod diversity were taken in forest areas previously subjected to grazing, logging and ploughing. Results indicated that arthropod diversity was smaller on sites with reduced structural complexity, with marked changes in arthropod family composition. The habitat components relating to plant architectural and vertical diversity were particularly influential on arthropod diversity. The guild size ratio of predatory to non-predatory arthropods also differed significantly between habitats suggesting a change in the resource base available to some groups. The latter suggests a shift in the functional organisation of the forest ecosystem which could have important repercussions for the diversity of other trophic levels.     

Fertile plant regeneration from protoplasts of a seed-propagated cultivar of Lilium × formolongi by utilizing meristematic nodular cell clumps

Plant regeneration from protoplasts of Lilium × formolongi cv. Azusa was achieved by utilizing suspension cultures of meristematic nodular cell clumps with a high plant regeneration ability. Creamy-white calli with embryogenic potential were initially induced from the seeds on Murashige and Skoog (MS) medium containing 4.1 μM picloram. The calli were then transferred into a liquid medium of the same composition, in which they turned into compact cell clumps which consisted of meristematic nodules. Protoplasts were readily isolated from these meristematic nodular cell clumps. Colonies were successfully formed from the protoplasts by embedding in 0.1% gellan gum-solidified MS medium containing 4.1 μM picloram and 0.5 M glucose. They regenerated shoots and roots on MS medium containing 2.2 μM benzylaminopurine (BAP). The plants thus obtained produced flowers with normal fertile pollen 8 months after successful transfer into soil. These plants had normal chromosome numbers (2n = 24) but had shorter leaves than original plants. They set seeds after as well as cross pollination.     

Genistein as an endogenous natural substrate of acidic peroxidases in lupin hypocotyls

Crude steam distillate from Ocimum gratissimum sprayed onto infection courts on detached cocoa pods moments after inoculation with Phytophthora palmivora completely inhibited the pathogen and blackpod lesion development on 75% of the infection courts. Disease suppression obtained with the extract was comparable to that obtained with a 2% Kocide 101 suspension. In the field, the O. gratissimum extract also suppressed lesion development although to a significantly lower (P = 0.05) extent in comparison with Kocide 101. Blackpod lesion expansion rates of 3.80, 3.56, 2.71 and 0.78 cm/day, respectively, were associated with pods treated in the field with C. citratus extract, tap water, O. gratissimum extract and 2% Kocide 101. The extract from Cymbopogon citratus was also ineffective on detached pods. Sporangia of P. palmivora from sporu-lating blackpod lesions on both detached and non-detached pods lost their infectivity within 1 h of treatment with the O. gratissimum extract. This effect was superior to that obtained with Kocide 101. Fungitoxicity of the extract on pods, however, was lost within 3 h of application. Thus, despite its in vivo effectiveness as an eradicant, the O. gratissimum extract, in its present form, has limited utility as a protectant fungicide.     

Localization of proteasomes in plant cells

Summary Proteasomes, also known as multicatalytic proteinase complexes, were localized in suspension cells of potato (Solanum tuberosum) by direct immunofluorescence using polyclonal antibodies labelled with fluorescein isothiocyanate. The method used allows an estimate of relative amounts of proteasomal antigens in different cell components. Proteasomes are present in the nuclei and the cytoplasm. The nucleoplasm contains small areas of weak fluorescence. The peripheral cytoplasm and possibly elements of the cytoskeleton show higher fluorescence than other parts of the cytoplasm. This indicates a localization of proteasomes similar to that known from animal cells.Abbreviations DMSO dimethylsulfoxide - EGTA ethyleneglycol-bis-(-aminoethylether)-N,N,N,N-tetra acetic acid - FITC fluorescein isothiocyanate - PBS phosphate buffered saline - PIPES piperazine-1,4-bis-(2-ethanesulfonic acid)     

Dynamic approaches to the mechanism of photosynthesis

An account of the author's life and scientific research is presented. Two main lines of research have been pursued: (1) Studies on the physiological aspect of photosynthesis started from experiments with crops under field conditions and then extended to the study of photosynthesis in nature; and (2) studies on the mechanism of photophosphorylation and related problems which began with the measurement of quantum requirement of photophosphorylation. This work led to the discovery of the high energy state of phosphorylation and many other interesting findings. In recent years, efforts have been made to study the operation and regulation of photosynthetic apparatus with a view to link the above-mentioned lines of research together.Written at the invitation of Govindjee.     

High fidelity extrachromosomal recombination and gene targeting in plants

The precision of extrachromosomal homologous recombination and gene targeting in plant cells was investigated. Recombination was directed to introns of selectable marker genes where potential changes could persist without affecting the function and therefore the selectability of the genes. Approximately 9 kb of crossover regions was rescued and sequenced. Changes were detected at a frequency below one point mutation per 1000 bp, indicating that extrachromosomal recombination and gene targeting both appear to occur with high fidelity.     

Indirect versus direct effects of grasses on growth of a cactus (Opuntia fragilis): insect herbivory versus competition

Variation in plant performance between microhabitats is usually attributed to direct mechanisms, such as plant physiological tolerances or competitive interactions. However, indirect mechanisms, such as differences in herbivore pressure mediated by microhabitat differences, could create the same pattern of variation. In this study, we investigated the effect of insect herbivore pressure on the growth of the grassland cactus Opuntia fragilis under different regimes of grassland canopy cover. Our purpose was to establish the extent to which canopy cover plays a direct, competitive role versus an indirect, mediatory role in cactus growth. We manipulated aboveground microhabitat, specifically the cover of adjacent grasses. The three treatments were: (1) open canopy, with grass pinned down away from the cactus; (2) shaded canopy, with a partial mesh cage staked over the cactus; and (3) ambient grass canopy. We measured seasonal plant growth and recorded changes in insect herbivore occurrence and damage in relation to cover. Cactus growth, defined as the change in number of live cladodes, was higher in the open than under either treatment where the plant was more shaded (P<0.05). However, allocation to new growth, measured as the proportion of new segments (cladodes) in a patch, did not differ among cover treatments. Thus, the hypothesis that physiological constraints, or competition for light, limited cactus performance in grass is rejected. Instead, we found that both cladode mortality, caused by the larvae of a cactus moth borer (Melitara dentata), and occurrence of the moth were lower in the open microhabitat than in either shaded microhabitat. Thus, higher net growth in the open, unshaded treatment, rather than representing a release from competition for light with grasses, was better explained as an indirect effect of grass cover on the activity and impact of the cactus moth. These results show that indirect effects can lead to a misinterpretation of experimental data on direct effects. These data also contribute to an improved understanding of mixed results in the biological control of weedy cacti. Clearly, future evaluations of the relative importance of physiology, competition, and insect herbivory in plant performance must be environmentally explicit.     

Drought response of a native and introduced Hawaiian grass

The alien grass, Pennisetum setaceum, dominates many of the lowland arid regions that once supported native Heteropogon contortus grassland on the island of Hawaii. Response to drought in a glasshouse was compared between these C₄ grasses to test if success as an invader is related to drought tolerance or plasticity for traits that confer drought tolerance. Pennisetum produced 51% more total biomass, allocated 49% more biomass to leaves, and had higher net photosynthetic rates (P _n) on a leaf area basis than Heteropogon. Plants of both species under drought produced less total biomass and increased their allocation to roots compared to well-watered plants, but there was no difference between the two species in the magnitude of these responses. The decline in P _n with decreasing leaf water potential (₁) was greater for Pennisetum compared to Heteropogon. Plasticity in the response of P _n to ₁, osmotic potentials, and the water potentials at turgor loss in response to drought were not different between the two species. Stomata were more responsive to w in Heteropogon than in Pennisetum and for well-watered plants compared to droughted plants. Plasticity for the stomatal response to w, however, was not different between the species. There was no evidence that the alien, Pennisetum, had greater plasticity for traits related to drought tolerance compared to the native, Heteropogon. Higher P _n and greater biomass allocation to leaves resulted in greater growth for Pennisetum compared to Heteropogon and may explain the success of Pennisetum as an invader of lowland arid zones on Hawaii.     

Results and recommendations

In the first phase of a collaborative study by the International Programme on Chemical Safety (PRCS), four coded chemicals, i.e. azidoglycerol (AG, 3-azido-1,2-propanediol), methyl nitrosurea (MNU), sodium azide (NaN₃) and maleic hydrazide (MH), and ethyl methanesulfonate (EMS) as a positive control were tested in four plant bioassays, namely the Arabidopsis embryo and chlorophyll mutation assay, the Tradescantia stamen hair assay (Trad-SH assay), the Tradescantia micronucleus assay (Trade-MCN), and the Vicia faba root tip assay. Seventeen laboratories from diverse regions of the world participated with four to six laboratories each using one plant assay. For the Arabidopsis assay, laboratories were in agreement with MNU and AG giving positive responses and NaN₃ giving a negative response. With the exception of one laboratory which reported MH as weakly mutagenic, no mutagenic response was reported for MH by the other laboratories. For the Vicia faba assay, all laboratories reported a positive response for MNU, AG, and MH, whereas two of the six laboratories reported a negative response for NaN₃. For the Trad-SH assay, MH was reported as giving a positive response and a positive response was also observed for MNU with the exception of one laboratory. NaN₃, which exhibited a relatively high degree of toxicity, elicited a positive response in three of the five laboratories. AG was found positive in only one of the two laboratories which tested this chemical. For the Trad-MCN assay, MNU and MH were reported as positive by all laboratories, while four out of five laboratories reported NaN₃ to be positive. Only one of three laboratories reported AG to be positive. The major sources of variability were identified and considered to be in the same range as found in similar studies on other test systems. Recommendations were made for minor changes in methodology and for initiating the second phase of this study.     

Non-random needle orientation in 1-year-old Scots pine (Pinus sylvestris L.) seedlings when adjacent to non-shading vegetation

Summary Two experiments were performed to test needle architecture of Scots pine (Pinus sylvestris L.) seedlings when affected by non-shading neighbouring vegetation. Both experiments showed that seedlings reacted to the presence of such adjacent vegetation by changes in needle distribution around the main stem: more needles were found on the opposite side of the main stem to where the vegetation was located. However, this reaction was confined to needles from the main stem. Responses of seedlings to dead neighbours differed from the response to live neighbours and also from the response to a black curtain substituted for plant neighbours. It is suggested that reactions are mediated by the red/far red ratio of incident light.