ALBERT

Notes: The photosynthetic response of Larrea tridentata Cav., an evergreen Mojave Desert shrub, to elevated atmospheric CO2 and drought was examined to assist in the understanding of how plants from water-limited ecosystems will respond to rising CO2. We hypothesized that photosynthetic down-regulation would disappear during periods of water limitation, and would, therefore, likely be a seasonally transient event. To test this we measured photosynthetic, water relations and fluorescence responses during periods of increased and decreased water availability in two different treatment implementations: (1) from seedlings exposed to 360, 550, and 700 μmol mol–1 CO2 in a glasshouse; and (2) from intact adults exposed to 360 and 550 μmol mol–1 CO2 at the Nevada Desert FACE (Free Air CO2 Enrichment) Facility. FACE and glasshouse well-watered Larrea significantly down-regulated photosynthesis at elevated CO2, reducing maximum photosynthetic rate (Amax), carboxylation efficiency (CE), and Rubisco catalytic sites, whereas droughted Larrea showed a differing response depending on treatment technique. Amax and CE were lower in droughted Larrea compared with well-watered plants, and CO2 had no effect on these reduced photosynthetic parameters. However, Rubisco catalytic sites decreased in droughted Larrea at elevated CO2. Operating Ci increased at elevated CO2 in droughted plants, resulting in greater photosynthetic rates at elevated CO2 as compared with ambient CO2. In well-watered plants, the changes in operating Ci, CE and Amax resulted in similar photosynthetic rates across CO2 treatments. Our results suggest that drought can diminish photosynthetic down-regulation to elevated CO2 in Larrea, resulting in seasonally transient patterns of enhanced carbon gain. These results suggest that water status may ultimately control the photosynthetic response of desert systems to rising CO2.

Notes: Abstract. Stomatal conductance and needle water potential of P. radiata clones were measured after 2, 5 and 8 months on plants grown in controlled environment rooms with markedly different water vapour saturation deficits (D). Conductance was significantly lower at high D, but water potential differences between treatments were not significant. When trees were moved between treatments most of the changes in conductances occurred within 2 h, with residual changes after 24 h. Water potentials were not different 24 h after the trees were moved. The effects were completely reversible.Transpiration rates of individual trees were highest in the high D treatment and lowest in the low D treatment. They were not linearly related to D because of decreasing conductance with increasing D.Height growth, diameter growth and foliage areas were not significantly different between treatments. Tracheid lumen diameters tended to be larger in trees grown at higher D although treatment differences were not significant.There were significant clonal differences in shoot conductance and tracheid dimensions.

Notes: The growth rates of woody plants depend on both the rate of photosynthetic carbon gain and the availability of essential nutrients. Instantaneous carbon gain is known to increase in response to increasing atmospheric CO2 concentration, but it is uncertain whether this will translate into increased growth in the longer term under nutrient-limited conditions. An analytical model to address this question was developed by Comins & McMurtrie (1993, Ecological Applications 3, 666–681). Their model was further tested and analysed. Manipulation of various assumptions in the model revealed its key assumptions and allowed a more confident prediction of expected growth responses to CO2 enrichment under nutrient-limited conditions.The analysis indicated that conclusions about the CO2 sensitivity of production were strongly influenced by assumptions about the relationship between foliar and heartwood nitrogen concentrations. With heartwood nitrogen concentration proportional to foliar nitrogen concentration, the model predicted a strong response of plant productivity to increasing CO2 concentration, whereas with heartwood nitrogen concentration set constant, the model predicted only a very slight growth response to changing CO2 concentration. On the other hand, predictions were only slightly affected by: (1) assumptions about the extent of nitrogen retranslocation out of senescing roots and foliage or wood during heartwood formation; (2) the effects of nitrogen status on specific leaf area or (3) leaf longevity; (4) carbon allocation between different plant parts; or (5) changes in the N:C ratio of organic matter sequestered in the passive pool of soil organic matter. Modification of the effect of foliar nitrogen concentration on the light utilization coefficient had only a small effect on the CO2 sensitivity for pines. However, this conclusion was strongly dependent on the chosen relationship between single-leaf photosynthesis and leaf nitrogen concentration. Overall, the analysis suggested that trees growing under nitrogen-limited conditions can respond to increasing atmospheric CO2 concentration with considerable increases in growth.

Notes: The effects of CO2 enrichment on photosynthesis and ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco) in current year and 1-year-old needles on the same branch were studied on Pinus radiata D. Don. trees growing for 4 years in large, open-top chambers at ambient (36 Pa) and elevated (65 Pa) CO2 partial pressures. At this age trees were 3·5–4 m tall. Measurements made late in the growing cycle (March) showed that photosynthetic rates at the growth CO2 concentration [(pCO2)a] were lower in 1-year-old needles of trees grown at elevated CO2 concentrations than in those of trees grown at ambient (pCO2)a. At elevated CO2 concentrations Vcmax (maximum carboxylation rate) was reduced by 13% and Jmax (RuBP regeneration capacity mediated by maximum electron transport rate) by 17%. This corresponded with photosynthetic rates at the growth (pCO2)a of 4·68 ± 0·41 μmol m–2 s–1 and 6·15 ± 0·46 μmol m–2 s–1 at 36 and 65 Pa, respectively (an enhancement of 31%). In current year needles photosynthetic rates at the growth (pCO2)a were 6·2 ± 0·72 μmol m–2 s–1 at 36 Pa and 10·15 ± 0·64 μmol m–2 s–1 at 65 Pa (an enhancement of 63%). The smaller enhancement of photosynthesis in 1-year-old needles at 65 Pa was accompanied by a reduction in Rubisco activity (39%) and content (40%) compared with that at 36 Pa. Starch and sugar concentrations in 1-year-old needles were not significantly different in the CO2 treatments. There was no evidence in biochemical parameters for down-regulation at elevated (pCO2)a in fully fexpanded needles of the current year cohort. These data show that enhancement of photosynthesis continues to occur in needles after 4 years’ exposure to elevated CO2 concentrations. Photosynthetic acclimation reduces the degree of this enhancement, but only in needles after 1 year of growth. Thus, responses to elevated CO2 concentration change during the lifetime of needles, and acclimation may not be apparent in current year needles. This transitory effect is most probably attributable to the effects of developmental stage and proximity to actively growing shoots on sink strength for carbohydrates. The implications of such age-dependent responses are that older trees, in which the contribution of older needles to the photosynthetic biomass is greater than in younger trees, may become progressively more acclimated to elevated CO2 concentration.

Notes: The influence of vesicular–arbuscular mycorrhizal (M) colonization on biomass production and photosynthesis of Trifolium repens L. was investigated in two experiments in which the foliar nitrogen and phosphorus contents of non-mycorrhizal (NM) plants were manipulated to be no lower than that of M plants. Throughout both experiments there was a stimulation in the rate of CO2 assimilation of the youngest, fully expanded leaf of M compared with NM plants. In addition, M plants exhibited a higher specific leaf area compared with NM plants, a response that maximized the area available for CO2 assimilation per unit of carbon (C) invested. Despite the increased rate of photosynthesis in M plants there was no evidence that the additional C gained was converted to biomass production of M plants. It is suggested that this additional C gained by colonized plants was allocated to the mycorrhizal fungus and that it is the fungus, by acting as a sink for assimilates, that facilitated the stimulation in the rate of photosynthesis of the plant partner.

Notes: SUMMARY. The growth and recession of macrophytes on a shaded section of the R. Lambourn were documented by a mapping procedure. With the exception of Ranunculus spp., the changes in total cover did not indicate directly the pattern of growth and recession of the macrophyles. Analysis of gross changes, expressed as gains and losses on cover, indicated that colonization of gravel and silt by the dominant macrophyte, Berula erecta, did not vary seasonally. Colonization was at a constant rate of about 8% of the site each month throughout the year and this accounted for 50% of the total number of gains by Berula. Gains of Berula from Ranunculus showed an annual cycle with a maximum during the summer when Ranunculus was in recession. Gains of Berula from Callitriche spp. also varied annually but the maximum was during the autumn. Total losses of Berula were at a constant rate throughout the year but were to gravel and silt during the winter, to Ranunculus in spring and early summer and to Callitriche in late summer and autumn. Analysis of loss of Berula with time indicated that the position of the Berula carpet was constantly changing. The growth and recession of Berula could not be linked in a meaningful way to environmental variables. Callitriche and Ranunculus both showed an annual pattern of growth and recession. There was temporal separation of the two macrophytes with Ranunculus growing mainly in spring and early summer and Callitriche showing maximum growth in late summer and autumn, and some evidence of spatial separation. The observed differences between years in the growth of Callitriche could not be attributed to any of the environmental variables measured.Discharge was thought to be an important variable controlling the growth of Ranunculus since increase of Ranunculus in the spring was positively correlated with the mean discharge at that time. In years when discharge was low, the growth of Rununculus appeared to be restricted by shading from epiphytic algae which accumulated on the plant surfaces under these conditions.

Notes: A repetitive element (IS986), previously isolated from Mycobacterium tuberculosis and shown to detect multiple restriction fragment-length polymorphisms (RFLPs), has been sequenced. It consists of a potential insertion sequence of 1358bp, with 30-bp inverted repeat ends. IS986 has four potentially significant open reading frames (ORFs): ORFa1, ORFa2 and ORFb on one strand and ORFc on the complementary strand. The sequences of the potential translated products identify IS986 as a member of the IS3 family, with an apparent frameshift between ORFa1 and ORFa2. IS986 has potential as a highly specific probe for detection and typing of M. tuberculosis, as well as for transposon mutagenesis of mycobacteria. The sequence of IS986 is virtually identical to that of another recently described element, IS6110 (Thierry et al., 1990).

Notes: Polygalacturonase (Peh) and other pectolytic enzymes play a crucial role in the maceration of vegetables by soft rot Erwinia spp. We have sequenced the peh gene of Erwinia carotovora subsp. carotovora, and identified its product as a precursor of molecular weight 42639, and a mature protein of molecular weight 42200. A putative KdgR-binding site was identified in the region 5’to the peh gene. The Peh protein showed significant homology with Peh from tomato. In addition, we have found homologies between pectin methylesterase and pectate lyase from Erwinia and their counterparts in tomato. These homologies are described, and their significance discussed.

Notes: Erwinia Stewartii contains a large cluster of wts genes that are required by this bacterium for pathogenicity on com plants. Three complementation groups within the right half of this cluster, wtsA, wtsC, and ivtsB, were previously identified. In this study, WtsA was found to be a positive activator of ivfsB::lacZ expression. The WtsA locus was sequenced and a single open reading frame is present within the wtsA locus, which has the capacity to encode a 323 amino acid polypeptide. A corresponding 38kDa protein was observed in Escherichia coli minicells containing the cloned wtsA gene. The predicted WtsA polypeptide has significant similarity to HrpS from Pseudomonas syringae pv. phaseolicola, as well as other members of the NtrC class of prokaryotic regulatory proteins. Similar to other genes activated by NtrC regulators, wtsB::lacZ expression in E. coli was dependent upon rpoN.

Notes: Coxiella burnetii undergoes a poorly defined developmental cycle within phagolysosomes of eukaryotic host cells. Two distinct developmental forms are part of this cycle: a small-cell variant (SCV) and large-cell variant (LCV). Ultrastructurally, the SCV is distinguished from the LCV by its smaller size and condensed chromatin. At a molecular level, little is known about morphogenesis in C. burnetii, and no proteins specific to the SCV have been identified. Preparative isoelectric focusing was conducted to purify basic proteins possibly involved in SCV chromatin structure. A predominant protein of low Mr was present in the most basic fraction, eluting with a pH of approx. 11. Degenerate deoxyoligonucleotides corresponding to the N-terminal sequence of this protein were used to recover a cosmid clone from a C. burnetii genomic library. Nucleotide sequencing of insert DNA revealed an open reading frame designated scvA (small-cell-variant protein A) with coding potential for a 30 amino acid protein (ScvA) with a predicted Mr of 3610. ScvA is 46% arginine plus 46% glutamine with a predicted pi of 12.6. SDS-PAGE and silver staining of lysates of SCV and LCV purified by caesium chloride-equilibrium density centrifugation revealed a number of proteins unique to each cell type. Immunoblot analysis with ScvA antiserum demonstrated the presence of ScvA only in the SCV. By immunoelectron microscopy, ScvA antiserum labelled only the SCV, with the label concentrated on the condensed nucleoid. In addition, ScvA bound double-stranded DNA in gel mobility-shift assays. A 66% reduction in the mean number of gold particles per Coxiella cell was observed at 12 h post-infection when compared with the starting inoculum. Collectively, these data suggest that synthesis of ScvA is developmentally regulated, and that the protein may serve a structural or functional role as an integral component of the SCV chromatin. Moreover, degradation of this protein may be a necessary prerequisite for morphogenesis from SCV to LCV.