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Understanding the molecular mode(s) of plant tolerance to heat stress (HS) is crucial since HS is a potential threat to sustainable agriculture and global crop production. Polyamines (PAs) seem to exert multifaceted effects in plant growth and development and responses to abiotic and biotic stresses, presumably via their homeostasis, chemical interactions and contribution to hydrogen peroxide (H2O2) cellular ‘signatures’. Downregulation of the apoplastic POLYAMINE OXIDASE (PAO) gene improved thermotolerance in tobacco (Nicotiana tabacum L.) transgenics. However, in the present work we show that transgenic tobacco plants with antisense-mediated S-ADENOSYL-L-METHIONINE DECARBOXYLASE silencing (AS-NtSAMDC) exhibited enhanced sensitivity and delayed responses to HS which was accompanied by profound injury upon HS removal (recovery), as assessed by phenological, physiological and biochemical characteristics. In particular, the AS-NtSAMDC transgenics exhibited significantly reduced rate of photosynthesis, as well as enzymatic and non-enzymatic antioxidants. These transgenics suffered irreversible damage, which significantly reduced their growth potential upon return to normal conditions. These data reinforce the contribution of increased PA homeostasis to tolerance, and can move forward our understanding on the PA-mediated mechanism(s) conferring tolerance to HS that might be targeted via traditional or biotechnological breeding for developing HS tolerant plants.

The impact of light-emitting diodes (LED) on developmental, physiological, and phytochemical characteristics of pomegranate (Punica granatum L.) seedlings was investigated. Five LED lights [L20AP67 (moderate blue and red, high green), AP673L (moderate blue, high red and red:far-red), G2 (low blue, high red and far-red), AP67 (moderate blue, high red), and NS1 (high blue and green, low red, high red:far-red, 1% UV) with wide and continuous radiation spectra were used. Fluorescent [FL (high blue and green, low red)] tubes constituted the control treatment. Among treatments, seedlings grown under L20AP67 exhibited the best morphological and growth characteristics (rapid height increase, longer roots, greater fresh and dry weight, greater leaf area). Root activity was comparable among the various treatments, with FL exhibiting the lowest arithmetic value. A variant effect on pigments and secondary metabolites was recorded with the highest chlorophyll and carotenoid content detected under FL, flavonoid under G2 and AP67, and anthocyanin under G2, AP67 and NS1. In addition, NS1 produced the highest total phenolic compound content. Root growth capacity (RGC) was monitored in specialized chambers, then assessed to evaluate the transplanting capacity. Transplanted seedlings of AP67 had the longest, of L20AP67 the heaviest newly formed roots, thus securing a successful transplantation. The herein undertaken study demonstrates that certain LEDs promoted developmental characteristics in pomegranate more efficiently than conventional FL. In particular, the use of L20AP67 was proven a promising tool for producing robust pomegranate seedlings, while some of the other LEDs tested apparently imposed a stressful situation on plants.

Improvement of soil characteristics through the use of renewable inputs is fundamental to environmentally friendly farming systems. In the present study, the potential of improving soil properties and, consequently, growth of tomato (Lycopersicon esculentum L.) seedlings through a direct incorporation of aromatic plant tissues into seedbeds is assessed. Dried spearmint (Mentha spicata L.) and sage (Salvia fruticosa Mill.) tissues are incorporated at different rates into the soil of experimental field plots. At 0, 20, 40, 60, and 90 days following incorporation, soil samples are removed from the plots and used as substrates in tomato seedbeds. Growth and physiological parameters of tomato seedlings (emergence, size of the most robust leaf, shoot length, dry weight, net photosynthetic rate, stomatal conductance, photosynthetic yield) as well as soil attributes (pH, nitrogen and organic carbon content, organic matter decomposition rate, microbial populations, changes in essential oil content) are monitored. Spearmint incorporation into the soil improved emergence, physiology and growth of tomato seedlings. This was not the case with sage. Soil microbial populations and organic matter decomposition increased with increasing rate of incorporated aromatic plant tissues, especially in the case of spearmint which exhibited a more prominent increasing trend. Soil pH was not affected, remaining within the range for optimum tomato growth. Further, C:N ratio increased, yet it did not inhibit tomato growth. Lastly, the observed decrease with time of the essential oil content in soil was dependent on the aromatic plant incorporated, and is discussed in relation to the beneficial effects of spearmint on tomato growth. The herein undertaken study demonstrates that incorporating intact spearmint tissues into the soil is a promising tool for improving tomato seedling production. This practice circumvents the arduous composting process and, therefore, it can be more cost-and-time-effective compared to the currently applied techniques.

Polyamines (PAs) and hydrogen peroxide (H2O2), the product of PA oxidation by polyamine oxidase (PAO), are potential players affecting plant growth, development and responses to abiotic/biotic stresses. Genetically modified Nicotiana tabacum plants with altered PA/H2O2 homeostasis due to over/underexpression of the ZmPAO gene (S-ZmPAO/AS-ZmPAO, respectively) were assessed under heat stress (HS). Underexpression of ZmPAO correlates with increased thermotolerance of the photosynthetic machinery and improved biomass accumulation, accompanied by enhanced levels of the enzymatic and non-enzymatic antioxidants, whereas ZmPAO overexpressors exhibit significant impairment of thermotolerance. These data provide important clues on PA catabolism/H2O2/thermotolerance, which merit further exploitation.

One hundred spearmint (Mentha spicata) plantlets were regenerated from apical shoot segments of ten field- grown donor plants. Although the accumulation of rosmarinic acid and total phenolics in vitro was almost half than in vivo, regenerants demonstrated a tenfold-higher hydrogen peroxide compared to the donor plants. This finding may have been associated with the increased activity of hydroxyphenylpyruvate reductase, a key enzyme of the phenolic biosyn- thetic pathway and the increased production of yet unidentified phenolic compounds in vitro. This process of in vitro cul- ture associated with a reduction of rosmarinic acid and total phenolics and with an increase of the antioxidant capacity in- dicated the possible promotion of in vitro-specific biosynthetic pathways.

Polyamine (PA) homeostasis is associated with plant development, growth and responses to biotic/abiotic stresses. Apoplastic PA oxidase (PAO) catalyzes the oxidation of PAs contributing to cellular homeostasis of reactive oxygen species (ROS) and PAs. In tobacco, PAs decrease with plant age, while apoplastic PAO activity increases. Our previous results with young transgenic tobacco plants with enhanced/reduced apoplastic PAO activity (S-ZmPAO/AS-ZmPAO, respectively) established the importance of apoplastic PAO in controlling tolerance to short-term salt stress. However, it remains unclear if the apoplastic PAO pathway is important for salt tolerance at later stages of plant development. In this work, we examined whether apoplastic PAO controls also plant development and tolerance of adult plants during long-term salt stress. The AS-ZmPAO plants contained higher Ca2+ during salt stress, showing also reduced chlorophyll content index (CCI), leaf area and biomass but taller phenotype compared to the wild-type plants during salt. On the contrary, the S-ZmPAO had more leaves with slightly greater size compared to the AS-ZmPAO and higher antioxidant genes/enzyme activities. Accumulation of proline in the roots was evident at prolonged stress and correlated negatively with PAO deregulation as did the transcripts of genes mediating ethylene biosynthesis. In contrast to the strong effect of apoplastic PAO to salt tolerance in young plants described previously, the effect it exerts at later stages of development is rather moderate. However, the different phenotypes observed in plants deregulating PAO reinforce the view that apoplastic PAO exerts multifaceted roles on plant growth and stress responses. Our data suggest that deregulation of the apoplastic PAO can be further examined as a potential approach to breed plants with enhanced/reduced tolerance to abiotic stress with minimal associated trade-offs.

Polyamines (PAs) are nitrogenous molecules that are indispensable for cell viability and with an agreed-on role in the modulation of stress responses. Tobacco plants with downregulated SAMDC (AS-SAMDC) exhibit reduced PAs synthesis but normal levels of PA catabolism. We used AS-SAMDC to increase our understanding on the role of PAs in stress responses. Surprisingly, at control conditions AS-SAMDC plants showed increased biomass and altered developmental characteristics, such as increased height and leaf number. On the contrary, during salt stress AS-SAMDC plants showed reduced vigor when compared to the WT. During salt stress, the AS-SAMDC plants although showing compensatory readjustments of the antioxidant machinery and of photosynthetic apparatus, they failed to sustain their vigor. AS-SAMDC sensitivity was accompanied by inability to effectively control H2O2 levels and concentrations of monovalent and divalent cations. In accordance with these findings, we suggest that PAs may regulate the trade-off between growth and tolerance responses.

Polyphenolic compounds produced by plants can chelate iron, reducing its bioavailability to plant-associated bacteria. In response to limited iron levels, most bacteria produce siderophores to acquire needed iron quantities. The amount of phenolic compounds detected in methanolic washings of leaves of different plant species varied greatly, being nearly sevenfold higher in Viburnum tinus than in Phaseolus vulgaris. In species with high levels of total phenolics (e.g. Pelargonium hortorum), tannin concentration of leaf washings was also high and accounted for up to 85% of total phenolics. Both stimulation of production of the siderophore pyoverdine in Pseudomonas syringae strain B728a and inhibition of growth of an isogenic mutant I-1, deficient in pyoverdine production were associated with plants harbouring high levels of leaf surface phenolics. Levels of tannic acid sufficient to inhibit growth of the pyoverdine mutant in culture in an iron-reversible fashion were similar to tannin levels found on leaves of plants such as P. hortorum. Additionally, the amount of pyoverdines produced by P. syringae and quantified in leaf washings from a variety of plants was directly related to the concentration of tannins released from the leaf, indicating that tannins were responsible for sequestering iron. Phenolic compounds, principally tannins, may thus play an important role in plant–microbe interactions.

The effect of essential oils and individual monoterpenoids on soil-borne fungi, in pure and mixed cultures, in growth media and in the soil environment, was investigated. Essential oils were extracted from lavender (Lavandula stoechas), oregano (Origanum vulgare subsp. hirtum), sage (Salvia fruticosa) and spearmint (Mentha spicata). The monoterpenoids tested were fenchone, carvacrol, 1,8-cineole, carvone, α-pinene and terpinen-4-ol. Their effect was examined on growth and sporulation of Aspergillus terreus, Fusarium oxysporum, Penicillium expansum and Verticillium dahliae isolated from an organic cultivation of tomato. All tested essential oils and individual monoterpenoids inhibited mycelial growth in all fungi and conidial production in most fungi. The strongest inhibitory activity on mycelial growth was exhibited by oregano and spearmint oils and by carvacrol and carvone, respectively their main constituents. The inhibitory activity was clearly fungistatic in A. terreus and F. oxysporum but fungicidal in V. dahliae. On sporulation, clearly stimulatory effects were observed alongside inhibitory ones. Conidial production was always promoted by α-pinene in P. expansum and by sage oil in F. oxysporum. At certain dosages it was promoted by cineole and carvone in F. oxysporum, and by lavender oil in A. terreus and V. dahliae. Experiments with carvone and carvacrol against mixed fungal cultures in a soil environment showed that V. dahliae was the most sensitive and A. terreus the most tolerant of the four fungi. Our results demonstrate strong but divergent effects and selectivity of action of the lower terpenoids on fungal strains that can become serious pests of tomato. Of special importance is the complete inhibition of growth and conidial production of V. dahliae, a pathogen otherwise very resistant to chemical control.

We investigated the effect of inorganic salt concentration on the in vitro organogenesis of spearmint (Mentha spicata L.) by using Murashige and Skoog culture medium of different strength (full, half- and quarter strength). The highest number of shoots and roots induced per explant (3.5 and 10, respectively), as well as the maximum average shoot length (16 cm) was observed on half strength medium. The maximum leaf number (35 per explant) and average root length (7 cm) were associated with regeneration on full medium. The highest average phenolic content was observed on shoot extracts (7.20 mg/g f.w.) and root extracts (5.93 mg/g f.w.) cultured onto quarter and half strength medium, respectively. The strength of the culture medium was inversely correlated with the antioxidant activity of plant extracts. Therefore, for spearmint organogenesis, half strength MS medium offers a compromise between optimum growth in vitro and antioxidant phenolic accumulation.

Τhe potential use of the aromatic plants Mentha spicata L. (spearmint) and Salvia fruticosa Mill. (sage) as soil amendments was evaluated. For this purpose, tomato seeds were sown in pots that had been filled with composts made from these plants and mixed with soil collected from an organically cultivated tomato field. A 2 × 2 × 4 [two types of fertilizer (synthetic and organic), two types of compost (M. spicata and S. fruticosa) and four compost rates (0%, 2%, 4% and 8%, w/w)] factorial experiment was used; the experiment was conducted twice in a growth chamber and lasted 60 days. At 0, 20, 40 and 60 days, after the establishment of the experiment, the soil bacterial and fungal abundance, the growth of nitrifying bacteria, the number of emerging weeds and the shoot length of tomato plants were measured in all treatments; at the end of the experiment, the above and belowground biomass of tomato plants was also determined. Soil microbial density increased with increasing compost rate of both species; the highest fungal and bacterial densities were recorded at 40 and 60 days, after the establishment of the experiment, respectively. Nitrifying bacteria were present in all treatments and at all sampling times. Both composts had a stimulating effect on tomato growth, which was remarkably pronounced with M. spicata. In contrast, weed emergence was reduced, but only in soils amended with M. spicata. The results suggest that M. spicata compost added at a rate of 4% to 8% is a very promising soil amendment, since it stimulates tomato growth, increases soil bacterial and fungal abundance and inhibits weed emergence. Further research is needed to elucidate the mode of action of M. spicata compost, its effect under field conditions and its possible use in mixed crop, rotational crop or cover crop systems.

We studied the epiphytic bacterial communities of the summer leaves of eight perennial species naturally occurring in a Mediterranean ecosystem. The species differ in essential-oil content (from rich in essential oil to non-producers) and composition, and also in life form (from herbaceous species to tall shrubs). We compared the epiphytic bacterial communities on the basis of (i) their abundance, (ii) their metabolic profile (derived by use of the BIOLOG Ecoplate system) and (iii) richness and diversity of substrates that they use, as a measure of functional diversity. Among all species, the aromatic Melissa officinalis was the most abundantly colonized. The bacterial communities on the leaves of the aromatic Myrtus communis, Calamintha nepeta and Melissa officinalis, and also of Cistus incanus catabolized all 31 substrates offered; those on the evergreen-sclerophyllous species, Arbutus unedo and Quercus coccifera, catabolized only 14 and 17 substrates, respectively. Carbohydrates were consistently used abundantly by all communities, whereas carboxylic acids were most variably used. On average, the group of aromatic plants scored higher regarding bacterial abundance, and richness and diversity of substrates used by the bacterial communities on their leaves; the lowest values for both substrate-use indices were recorded in A. unedo. Bacterial abundance or richness or diversity of substrates used did not vary with leaf oil content. Abundance was positively correlated with both substrate-use indices. Results support claims that the antimicrobial effects of essential oils are not exerted so much under natural conditions as reports based on biassays with pathogens usually show. Although essential oils play a part in the microbial colonization of the phyllosphere, it is not likely that inhibition of phyllosphere bacteria is essential oils’ primary role, at least in the Mediterranean environment.

Field experiments were conducted in northern Greece during 2001 and repeated in 2002 and 2004 to evaluate the effects of pronamide on sugar beet. Total leaf area, leaf area index (LAI), leaf and root dry weights, photosynthetic yield (quantum yield of photochemical energy conversion in photosystem II), chlorotic index, and yield components of sugar beet were monitored after pronamide application. Three sugar beet cultivars, ‘Avantage’, ‘Dorothea’, and ‘Bianca’, requiring short, intermediate, and long vegetative periods, respectively, were subjected to treatment. Pronamide was applied on sugar beet either as a double application of 0.63 kg ai ha−1at the two- to four-leaf and 0.63 kg ai ha−1at the four- to six-leaf stage or as a single application of 1.26 kg ai ha−1performed at the latter leaf stage. Both application procedures were combined with a split application of phenmedipham at 0.04 kg ai ha−1plus desmedipham at 0.04 kg ai ha−1plus metamitron at 0.70 kg ai ha−1plus ethofumesate at 0.10 kg ai ha−1plus mineral oil at 0.50 L ha−1applied POST at the cotyledon–to–two-leaf as well as at the four-leaf growth stages. Pronamide (both single and double application) initially caused chlorosis and reduction of sugar beet growth. LAI and photosynthetic yield were also significantly affected for a 2-mo period following the final application, after which the negative effects caused by pronamide were ameliorated. At harvest, sugar beet root and sugar yield, sucrose, K+, Na+, and N-amino acid concentrations were not affected by the herbicide treatments compared with those produced in weed-free and herbicide-free plots, indicating that all cultivars managed to overcome the transient pronamide stress. Regarding sugar beet cultivars, root and sugar yield of Avantage and Dorothea at harvest were higher than that of Bianca, whereas sucrose concentration of Avantage was the lowest. There was not an apparent relationship between the order of sugar yield per cultivar (Dorothea > Avantage > Bianca) and the length of the vegetative period (Avantage < Dorothea < Bianca).

Cultivation of roses in various soil-less media was studied with the aim to identify the optimum soil condition for rose production. Madelon roses grafted on rootstock of Rosa indica var. major were transplanted to polyethylene bags containing zeolite and perlite (at ratios of 25z:75p, 50z:50p, 75z:25p and 100z:0p, v/v) in a climate-controlled greenhouse. Net photosynthesis ( A net ), stomatal conductance ( g s ) and water use efficiency (WUE) of roses were followed for 5 months. Flower production and quality were recorded in three flowering flushes during a 5-month period. Analysis of variance of repeated measurements showed that even though the overall A net did not differ among treatments (average 18.7 μmol m −2 s −1 ), trends in A net seasonality for roses in 25z:75p substrate differed significantly from those in 50z:50p, 75z:25p or 100z:0p. Stomatal conductance did not show any significant seasonality or trends in response to substrate mixtures, averaging 0.89 mol m −2 s −1 . Water use efficiency was significantly lower for roses in 25z:75p than in 100z:0p mixtures (1.8 ± 0.15 and 2.0 ± 0.13 μmol m −2 s −1 CO 2 /mmol m −2 s −1 H 2 O, respectively). Cumulative production of rose plants did not differ among substrate mixtures. Productivity significantly differed among flower stem classes. Stem class I (>70 cm) and class V (≤30 cm) exhibited the least production, contributing to only 7.6 and 3.7% of the total production, respectively. The highest productivity was observed in classes III (51–60 cm) and IV (31–50 cm), contributing to the bulk of productivity (68.4%). Class II contributed a 20.3% of the production. Results showed that zeolite and perlite acted as inert materials. Zeolite did not exert any positive effect on productivity, in contrast to what has been reported in literature recently. Use of perlite resulted in a little improvement in photosynthesis, however this improvement was not reflected by a significant increase in production.

The scope of this work was to examine whether leaf constitutive secondary metabolites play a role in determining bacterial colonization of the phyllosphere. To this aim, we surveyed nineteen native or cultivated plant species that share a common bacterial pool in a North Mediterranean area, and estimated the size of total and ice nucleation active (INA) bacterial populations on their leaves. Large differences in the colonization of their phyllosphere were found; the population size of epiphytic bacteria ranged from 7.5 × 102 to 1 × 106 CFU/g fresh weight, in eucalypt and celery, respectively. Species native in Mediterranean-type climate areas, particularly those belonging to the group of aromatic plants, are characterized by scarce presence of INA bacteria. The antibacterial activity of essential oils, surface phenolics and leaf tissue extracts was also estimated against the INA strains P. syringae and E. herbicola, isolated from two of these plant species. E. herbicola proved more sensitive than P. syringae. Of the species examined, oregano [Origanum vulgare L. subsp. hirtum (Link.) Ietswaart], an aromatic plant, had the highest antimicrobial activity, whereas six species showed no activity at all. Further experiments were performed with oregano and bean (Phaseolus vulgaris L.) that represent two extremes in their secondary metabolite content. Both plants were inoculated with P. syringae. By the end of incubation, the bacterial population on bean plants was about 100 times higher than that on oregano leaves. Scanning electron micrographs showed that bacterial growth on oregano leaves was confined to sites away from glandular hairs. Results from the bacterial colonization survey together with those from the toxicity tests showed that all species rich in antibacterial secondary metabolites harbored low leaf bacterial populations. These results provide substantial evidence that leaf secondary metabolites function as constitutive defense chemicals against microbial invasions. However, the fact that species with non- or moderately active leaf secondary metabolites are not always highly colonized suggests mediation of other unknown factors, the contribution of which requires further investigation.

The simultaneous, long-term effects of three substrates (zeolite, perlite and their 1:1 mixture) and of two heating systems (lower and upper) upon the diurnal changes in photosynthesis of two gerbera cultivars (Cyprus and Heart Breaker) were investigated at different times of year. ‘Cyprus’ revealed higher net CO2 assimilation rate and stomatal conductance than ‘Heart Breaker’, while transpiration rate was almost the same. Based on both stomatal conductance and transpiration rate, the zeolite–perlite mixture substrate exerted a more beneficial effect on the soil–plant water system. No change in chlorophyll levels, either time-wise or between gerbera cultivars, was observed in response to different substrates and heating systems. Diurnal net CO2 assimilation rate, followed only for ‘Cyprus’, was reduced significantly from morning to late afternoon, possibly as a result of stomatal conductance reduction. The practical outcome with respect to the combined effect of different substrates, heating systems and cultivars was that ‘Cyprus’ grown on the mixture medium and subjected to an upper heating system (i.e. with heating pipes installed at the culture bag level) exhibited the best physiological responses.

Responses of tobacco and cotton seedlings to two chemical forms of Ag at various concentrations were studied. Cotton exhibited higher resistance than tobacco to Ag. The application of 1mgr;g L(-1) Ag, as AgI or AgNO(3), induced no toxicity symptoms in the growth and photosynthesis of cotton. However, the corresponding concentration negatively affected the growth and physiology of tobacco. On exposure to higher Ag concentrations (10(3) and 5 x 10(3)mgr;g L(-1) as AgNO(3)), root growth and concentrations of Ca, Mg, K, and Fe were significantly depressed, whereas concomitant Ag accumulation, especially in roots, was increased in both species. The loss of nutrients and chlorophyll was associated with disturbances in photosynthetic function. Because there was no apparent change in Fv/Fm, there is a possibility that the faster rise time was due to an increase in LHC size to the PSII reaction centers. The decreased Fv/Fo reflects the inactivation of chlorophyll associated with the PSII reaction center. Stomatal conductance was decreased less than the CO(2) assimilation, possibly due to the direct and/or indirect effects of Ag ions on stomatal guard cells. After the plants were transferred from the highest Ag concentration to a nutrient solution, recovery of tobacco, cotton morphology, and physiology were observed.http://link.springer-ny. com/link/service/journals/00244/bibs/37n4p480.html/HEA

This paper describes the development of a soxhlet extraction method for the extraction of silver as silver iodide from soil, and its subsequent determination by atomic absorption spectrometry. Silver iodide is extracted as a double salt with potassium iodide in acetone and, after condensation, silver is measured in the extract by flame atomic absorption spectrometry. The efficiency of this extraction was compared to a wet digestion method. Concentrated nitric acid was used for soil digestion and silver was determined in the acid digests by electrothermal atomic absorption spectrometry. In a series of soil samples, silver recovery varied between 90–110%. The detection limits were mass-dependent and, at the experimental conditions described in this work, these were found to be 2 μg/kg in extracted and 3 μg/kg in acid digested soil samples.

We studied the structure and diversity of the phyllosphere bacterial community of a Mediterranean ecosystem, in summer, the most stressful season in this environment. To this aim, we selected nine dominant perennial species, namely Arbutus unedo, Cistus incanus, Lavandula stoechas, Myrtus communis, Phillyrea latifolia, Pistacia lentiscus, Quercus coccifera (woody), Calamintha nepeta, and Melissa officinalis (herbaceous). We also examined the extent to which airborne bacteria resemble the epiphytic ones. Genotype composition of the leaf and airborne bacteria was analysed by using denaturing gradient gel electrophoresis profiling of a 16S rDNA gene fragment; 75 bands were cloned and sequenced corresponding to 28 taxa. Of these, two were found both in the air and the phyllosphere, eight only in the air, and the remaining 18 only in the phyllosphere. Only four taxa were found on leaves of all nine plant species. Cluster analysis showed highest similarity for the five evergreen sclerophyllous species. Aromatic plants were not grouped all together: the representatives of Lamiaceae, bearing both glandular and non-glandular trichomes, formed a separate group, whereas the aromatic and evergreen sclerophyllous M. communis was grouped with the other species of the same habit. The epiphytic communities that were the richest in bacterial taxa were those of C. nepeta and M. officinalis (Lamiaceae). Our results highlight the remarkable presence of lactic acid bacteria in the phyllosphere under the harsh conditions of the Mediterranean summer, the profound dissimilarity in the structure of bacterial communities in phyllosphere and air, and the remarkable differences of leaf microbial communities on neighbouring plants subjected to similar microbial inocula; they also point to the importance of the leaf glandular trichome in determining colonization patterns.

freezing behaviour and relative importance of heterogeneous ice nuclei in affecting supercooling of Citrus sinensis fruits were studied. The size of an ice nucleation active (INA) Pseudomonas syringae pool inhabiting fruits was positively correlated with the nucleation temperature (NT) of the plant tissue, with a mean of 3-69 log colony forming units (CFU) fruit"1 at NT~$> —2-5 °C. The INA bacterial pool was responsible for «23% of the nucleation events occurring at this temperature, and «29% were attributed to an additional nucleating source. The latter was sensitive to bacterial ice nucleation inhibitors, it occupied a different microniche from that of P. syringae nuclei, yet was neither a fungus nor any of the bacterial strains ever reported as active. Treatment with an ice nucleation inactive bacterium antagonistic to INA bacteria, a lentil lectin, a protease, and guanidine reduced mean nucleation temperatures (MNT) of fruits to ^ —2-58, —2-66, —4-21, and —4-52 °C, respectively, compared to a MNT^; — 1-67 CC for the controls. Thus, the citrus-associated nucleator apparently contained active proteinaceous components but was void of carbohydrate-like groups reportedly encountered at or near the bacterial ice nucleating site. Despite the different origins of citrus nuclei, bacterial nucleation inhibitors reduced nucleation in the field by 50% at NT^ —2-5 °C, an effect probably exerted through the proteinaceous site possessed by both nuclei.

Elm and black locust samples were collected in the city of Thessaloniki, Greece from a number of locations with different pollutant concentrations and the hormonal content was determined using radioimmunoassay tests. In both tree species, a considerable decrease in endogenous cytokinins was detected in symptomless leaves from the polluted city center, suggesting latent senescence and thus latent injury. Similarly, cytokinin content was found to be lower in seedlings exposed to SO2‐O3 fumigation than in controls. Thus cytokinin levels may serve as useful indicators of the onset of pollution stress and/or of the degree of stress experienced by urban trees. Several environmental stresses were reported to promote an ABA increase. An increased ABA content in black locust samples from the polluted city center was also detected. This observation did not hold in elm, where comparable ABA levels were detected in trees in the city center, the suburbs, and in controls grown in filtered air. Hence ABA content cannot ...

The frost sensitivity of Citrus sinensis in relation to the presence of biogenic ice nuclei was studied. In commercially managed citrus groves the ice nucleation active (INA) bacterium Pseudomonas syringae reached 6 × 104 colony forming units (CFU) leaf−1, a population sufficiently high to catalyze ice formation. However, a transient loss of bacterial nucleation activity was noticeable at subzero field temperatures, followed by resumption as temperatures rose. This loss was apparently due to a temporary transition of INA to ice nucleation inactive (INI) bacteria. Field application of Bordeaux mixture, copper hydroxide, streptomycin, and 2-hydroxypropylmethanethiolsulfonate (HPMTS), resulted in reduction of INA bacterial populations to detectability (≤ 102 CFU leaf−1) limits. However, the corresponding reduction in ice nucleation events in treated samples as compared to controls at nucleation temperature ≥−3°C was not as dramatic. It ranged from approximately 7% in samples treated with the bactericide HPMTS, to 35% in samples treated with chemicals possessing combined bactericidal - fungicidal action (coppers). Since a quantitative relationship exists between ice nucleation events on individual leaves and the INA bacterial populations harbored by these leaves, these results suggest the co-existence of a bacterial and a proteinaceous, yet non-bacterial ice nucleating source in citrus, both active at ≥−3°C.

Effects of water stress on phenology, growth, stomatal activity and water status were assessed from April to November 1996 in 2-year-old seedlings of Quercus frainetto Ten. (Quercus conferta Kit.), Quercus pubescens Willd., Quercus macrolepis Kotschy (Quercus aegilops auct.) and Quercus ilex L. growing in containers in northern Greece. All four species developed more than 50% of their total leaf area before the beginning of June--an adaptation to arid climates. Well-irrigated plants tended to develop greater individual leaf area, number of leaves per plant, total plant leaf area, height and root:shoot ratios than water-stressed plants, but the difference between treatments was not significant for any parameter in any species. Quercus macrolepis appeared to be the most drought-tolerant of the four species. It maintained the highest number of leaves of the smallest size and increased the proportion of fine roots during drought. In all species, drought caused significant decreases in stomatal conductance and predawn and midday water potentials from mid-July until the end of August, when the lowest soil water content and highest mean daily air temperatures and midday leaf temperatures occurred; however, the responses were species-specific. Among the four species, Quercus macrolepis sustained the highest stomatal conductance despite very low water potentials, thus overcoming drought by means of desiccation tolerance. Quercus ilex decreased stomatal conductance even before severe water stress occurred, thereby avoiding desication during drought. Quercus pubescens had the highest water potential despite a high stomatal conductance, indicating that its leaf water status was independent of stomatal activity. Quercus frainetto was the least drought-resistant of the four species. During drought it developed very low water potentials despite markedly reduced stomatal aperture.

A strong net downward flux of bacteria was measured in a soybean field during two different rain events. The ice nucleation-activity and phytopathogenic host range of strains of Pseudomonas syringae isolated from rainwater and aerosols present during rain differed from those of the bacteria present on the soybeans. Bacteria that produce fluorescent pigments on an iron-deficient medium were 3-8%, 5-9%, and 81% of those isolated from aerosols during rain, from rain, and from the soybean leaves, respectively (...)

The effects of SO2 (2 ppm for 6 h), O3 (0.9 ppm for 5 h), and SO2–O3 mixtures (2 ppm SO2 and 0.9 ppm O3 for 5 h followed by SO2 for 1 h) were studied on injury and growth of 4-month-old, actively growing and quiescent Ulmus americana L. seedlings. In actively growing seedlings visible injury to leaves from SO2–O3 mixtures was evident within 24 h; from O3 within 36 to 48 h; and from SO2 within 48 h after fumigation ceased. Sulfur dioxide and SO2–O3 treatments reduced expansion of new leaves ( 1 cm long at time of fumigation) was markedly inhibited by all treatments; it was inhibited most by SO2–O3, an intermediate amount by SO2, and least by O3. Numbers of emerging leaves were significantly reduced by SO2, and by SO2–O3, but only at the end of the 1st week. After 5 weeks, stem dry weights were reduced by O3 and SO2–O3, and root dry weights were reduced by SO2 and SO2–O3. In quiescent plants pollution injury to leaves was similar to that in actively growing plants. However, all fumigation treatments induced severe defoliation in quiescent plants.

The effects of SO2 (2 ppm for 6 h), O3 (0.9 ppm for 5 h), and SO2–O3 mixtures (2 ppm SO2 and 0.9 ppm O3 for 5 h followed by SO2 for 1 h) were studied on metabolite contents of 4-month-old, actively growing and quiescent Ulmus americana L. seedlings. Effects on metabolite contents in leaves, stems, and roots were examined 24 h, 1 week, and 5 weeks after exposure to pollutants for actively growing seedlings, and 24 h and 1 week for quiescent seedlings. Fumigations caused significant reduction in total nonstructural carbohydrate and protein contents of actively growing seedlings within 24 h after fumigation. By the end of 5 weeks, however, carbohydrate and protein contents of new ( 1 cm long) of actively growing seedlings persisted throughout the 5-week period. Changes in lipid contents of actively growing seedlings were small and in most cases not significant. Exposure to pollutants decreased lipid contents of quiescent seedlings (up to 28%); this decrease was much lower than reduction in carbohydrates or proteins (up to approximately 65%). Metabolite contents of both actively growing and quiescent seedlings were reduced most by SO2–O3, an intermediate amount by SO2, and least by O3 treatment.