Interaction of Fusarium oxysporum f. sp. ciceri and Meloidogyne javanica on Cicer arietinum

Interaction of Meloidogyne javanica and Fusarium oxysporum f. sp. ciceri was studied on Fusarium wilt-susceptible (JG 62 and K 850) and resistant (JG 74 and Avrodhi) chickpea cultivars. In greenhouse experiments, inoculation of M. javanica juveniles prior to F. oxysporum f. sp. ciceri caused greater wilt incidence in susceptible cultivars and induced vascular discoloration in roots of resistant cultivars. Nematode reproduction was greatest (P = 0.05) at 25 °C. Number of galls and percentage of root area galled increased when the temperature was increased from 15 °C to 25 °C. Wilt incidence was greater at 20 °C than at 25 °C. Chlorosis of leaves and vascular discoloration of plants did not occur at 15 °C. The nematode enhanced the wilt incidence in wilt-susceptible cultivars only at 25 °C. Interaction between the two pathogens on shoot and root weights was significant only at 20 °C, and F. o. ciceri suppressed the nematode density at this temperature. Wilt incidence was greater in clayey (48% clay) than in loamy sand (85% sand) soils. The nematode caused greater plant damage on loamy sand than on clayey soil. Fusarium wilt resistance in Avrodhi and JG 74 was stable in the presence of M. javanica across temperatures and soil types.     

Ice nucleation temperature of individual leaves in relation to population sizes of ice nucleation active bacteria and frost injury

Ice nucleation temperatures of individual leaves were determined by a tube nucleation test. With this assay, a direct quantitative relationship was obtained between the temperatures at which ice nucleation occurred on individual oat (Avena sativa L.) leaves and the population sizes of ice nucleation active (INA) bacteria present on those leaves. In the absence of INA bacteria, nucleation of supercooled growth-chamber grown oat leaves did not occur until temperatures were below approximately −5°C. Both nucleation temperature and population size of INA bacteria were determined on the same individual, field-grown oat leaves. Leaves with higher ice nucleation temperatures harbored larger populations of INA bacteria than did leaves with lower nucleation temperatures. Log₁₀ mean populations of INA bacteria per leaf were 5.14 and 3.51 for leaves with nucleation temperatures of −2.5°C and −3.0°C, respectively. Nucleation frequencies (the ratio of ice nuclei to viable cells) of INA bacteria on leaves were lognormally distributed. Strains from two very different collections of Pseudomonas syringae and one of Erwinia herbicola were cultured on nutrient glycerol agar and tested for nucleation frequency at −5°C. Nucleation frequencies of these bacterial strains were also lognormally distributed within each of the three sets. The tube nucleation test was used to determine the frequency with which individual leaves in an oat canopy harbored large populations of INA bacteria throughout the growing season. This test also predicted relative frost hazard to tomato (Lycopersicon esculentum Mill) plants.     

Thermal Acclimation of Photosynthetic Electron Transport Activity by Thylakoids of Saxifraga cernua

Thermal acclimation by Saxifraga cernua to low temperatures results in a change in the optimum temperature for gross photosynthetic activity and may directly involve the photosynthetic apparatus. In order to test this hypothesis photosynthetic electron transport activity of S. cernua thylakoids acclimated to growth temperatures of 20°C and 10°C was measured in vitro. Both populations exhibited optimum temperatures for whole chain and PSII electron transport activity at temperatures close to those at which the plants were grown. Chlorophyll a fluorescence transients from 10°C-acclimated leaves showed higher rates in the rise and subsequent quenching of variable fluorescence at low measuring temperatures; 20°C-acclimated leaves showed higher rates of fluorescence rise at higher measuring temperatures. At these higher temperatures, fluorescence quenching rates were similar in both populations. The kinetics of State 1-State 2 transitions in 20°C- and 10°C-acclimated leaf discs were measured as changes in the magnitude of the fluorescence emission maxima measured at 77K. Leaves acclimated at 10°C showed a larger F730/F695 ratio at low temperatures, while at higher temperatures, 20°C-acclimated leaves showed a higher F730/F695 ratio after the establishment of State 2. High incubation temperatures also resulted in a decrease in the F695/F685 ratio for 10°C-acclimated leaves, suggesting a reduction in the excitation transfer from the light-harvesting complex of photosystem II to photosystem II reaction centers. The relative amounts of chlorophyll-protein complexes and thylakoid polypeptides separated electro-phoretically were similar for both 20°C- and 10°C-acclimated leaves. Thus, photosynthetic acclimation to low temperatures by S. cernua is correlated with an increase in photosynthetic electron transport activity but does not appear to be accompanied by major structural changes or different relative amounts in thylakoid protein composition.     

Variation among Species in the Temperature Dependence of the Reappearance of Variable Fluorescence following Illumination

The relationship between the thermal dependence of the reappearance of chlorophyll variable fluorescence following illumination and temperature dependence of the apparent Michaelis constant (K_m) of NADH hydroxypyruvate reductase for NADH was investigated in cool and warm season plant species. Brancker SF-20 and SF-30 fluorometers were used to evaluate induced fluorescence transients from detached leaves of wheat (Triticum aestivum L. cv TAM-101), cotton (Gossypium hirsutum L. cv Paymaster 145), tomato (Lycopersicon esculentum cv Del Oro), bell pepper (Capsicum annuum L. cv California Wonder), and petunia (Petunia hybrida cv. Red Sail). Following an illumination period at 25°C, the reappearance of variable fluorescence during a dark incubation was determined at 5°C intervals from 15°C to 45°C. Variable fluorescence recovery was normally distributed with the maximum recovery observed at 20°C in wheat, 30°C in cotton, 20°C to 25°C in tomato, 30 to 35°C in bell pepper and 25°C in petunia. Comparison of the thermal response of fluorescence recovery with the temperature sensitivity of the apparent K_m of hydroxypyruvate reductase for NADH showed that the range of temperatures providing fluorescence recovery corresponded with those temperatures providing the minimum apparent K_m values (viz. the thermal kinetic window).     

The Influence of Dark Adaptation Temperature on the Reappearance of Variable Fluorescence following Illumination

The effect of chilling temperatures (5°C) on chlorophyll fluorescence transients was used to study chilling-induced inhibition of photosynthesis in plant species with differing chilling sensitivities. A Brancker SF-20 fluorometer was used to measure induced fluorescence transients from both attached and detached leaves of chilling-sensitive cucumber (Cucumis sativus L. cv Ashley) and chilling-resistant pea (Pisum sativum L. cv Alaska). The rate of reappearance of the variable component of fluorescence (F_v), following a period of illumination at 25°C, was dependent on the temperature at which the leaf was allowed to dark adapt in chilling-sensitive cucumber, but not in chilling-resistant pea. In cucumber, dark adaptation at 25°C following illumination resulted in a much faster return of F_v than dark adaptation at 5°C following illumination. However, F_v reappearance during the dark adaptation period in chilling-resistant pea was temperature independent. The difference in the temperature response of F_v following illumination correlated with temperature sensitivity of these two species. The process responsible for the difference in F_v may represent a site of chilling sensitivity in the photosynthetic apparatus.     

Effect of cold hardening on sensitivity of winter and spring wheat leaves to short-term photoinhibition and recovery of photosynthesis

Photoinhibition of photosynthesis and its recovery were studied in wheat (Triticum aestivum L.) leaves grown at nonhardening (20°C) and cold-hardening (5°C) temperatures. Cold-hardened wheat leaves were less susceptible to photoinhibition at 5°C than nonhardened leaves, and the winter cultivars, Kharkov and Monopol, were less susceptible than the spring cultivar, Glenlea. The presence of chloramphenicol, a chloroplastic protein synthesis inhibitor, increased the susceptibility to photoinhibition, but cold-hardened leaves still remained less susceptible to photoinhibition than nonhardened leaves. Recovery at 50 μmol m⁻² s⁻¹ photosynthetic photon flux density and 20°C was at least biphasic, with a fast and a slow phase in all cultivars. Cold-hardened leaves recovered maximum fluorescence and maximum variable fluorescence in the dark-adapted state during the fast phase at a rate of 42% h⁻¹ compared with 22% h⁻¹ for nonhardened leaves. The slow phase occurred at similar rates (2% h⁻¹) in cold-hardened and nonhardened leaves. Full recovery required up to 30 h. Fast-recovery phase was not reduced by either lowering the recovery temperature to 5°C or by the presence of chloramphenicol. Slow-recovery phase was inhibited by both treatments. Hence, the fast phase of recovery does not require de novo chloroplast protein synthesis. In addition, only approximately 60% of the photochemical efficiency lost through photoinhibition at 5°C was associated with lost [¹⁴C]atrazine binding and, hence, with damage to the secondary quinone electron acceptor for photosystem II-binding site. We conclude that the decrease in susceptibility to photoinhibition exhibited following cold hardening of winter and spring cultivars is not due to an increased capacity for repair of photoinhibitory damage at 5°C but reflects intrinsic properties of the cold-hardened photosynthetic apparatus. A model to account for the fast component of recovery is discussed.     

Frost injury and heterogeneous ice nucleation in leaves of tuber-bearing solanum species : ice nucleation activity of external source of nucleants

The heterogeneous ice nucleation characteristics and frost injury in supercooled leaves upon ice formation were studied in nonhardened and cold-hardened species and crosses of tuber-bearing Solanum. The ice nucleation activity of the leaves was low at temperatures just below 0°C and further decreased as a result of cold acclimation. In the absence of supercooling, the nonhardened and cold-hardened leaves tolerated extracellular freezing between −3.5° and −8.5°C. However, if ice initiation in the supercooled leaves occurred at any temperature below −2.6°C, the leaves were lethally injured.

To prevent supercooling in these leaves, various nucleants were tested for their ice nucleating ability. One% aqueous suspensions of fluorophlogopite and acetoacetanilide were found to be effective in ice nucleation of the Solanum leaves above −1°C. They had threshold temperatures of −0.7° and −0.8°C, respectively, for freezing in distilled H₂O. Although freezing could be initiated in the Solanum leaves above −1°C with both the nucleants, 1% aqueous fluorophlogopite suspension showed overall higher ice nucleation activity than acetoacetanilide and was nontoxic to the leaves. The cold-hardened leaves survived between −2.5° and −6.5° using 1% aqueous fluorophlogopite suspension as a nucleant. The killing temperatures in the cold-hardened leaves were similar to those determined using ice as a nucleant. However, in the nonhardened leaves, use of fluorophlogopite as a nucleant resulted in lethal injury at higher temperatures than those estimated using ice as a nucleant.

     

Auxin-induced leaf blade expansion in Arabidopsis requires both wounding and detachment

Elevation of leaf auxin (indole-3-acetic acid; IAA) levels in intact plants has been consistently found to inhibit leaf expansion whereas excised leaf strips grow faster when treated with IAA. Here we test two hypothetical explanations for this difference in growth sensitivity to IAA by expanding leaf tissues in vivo versus in vitro. We asked if, in Arabidopsis, IAA-induced growth of excised leaf strips results from the wounding required to excise tissue and/or results from detachment from the plant and thus loss of some shoot or root derived growth controlling factors. We tested the effect of a range of exogenous IAA concentrations on the growth of intact attached, wounded attached, detached intact, detached wounded as well as excised leaf strips. After 24 h, the growth of intact attached, wounded attached, and detached intact leaves was inhibited by IAA concentrations as little as 1 µM in some experiments. Growth of detached wounded leaves and leaf strips was induced by IAA concentrations as low as 10 µM. Stress, in the form of high light, increased the growth response to IAA by leaf strips and reduced growth inhibition response by intact detached leaves. Endogenous free IAA content of intact attached leaves and excised leaf strips was found not to change over the course of 24 h. Together these results indicate growth induction of Arabidopsis leaf blade tissue by IAA requires both substantial wounding as well as detachment from the plant and suggests in vivo that IAA induces parallel pathways leading to growth inhibition.     

Survival of F-Specific RNA Coliphage, Feline Calicivirus, and Escherichia coli in Water: a Comparative Study

The relationship between the survival of enteric viral pathogens and their indicators (coliform bacteria and coliphages) is not well understood. We compared the survival rates of feline calicivirus (FCV), Escherichia coli, and a male-specific RNA coliphage MS2 at 4, 25, and 37°C for up to 28 days in dechlorinated water. The survival rates of E. coli and FCV, a surrogate of noroviruses (NV), had a high degree of correlation at 4 and 25°C, while MS2 phage survived significantly longer (P < 0.05) at these two temperatures. At 37°C, the survival rates for all three organisms were highly correlated. Decimal reduction values indicating the number of days needed for 90% reduction in titer (D values) decreased for all three organisms as storage temperatures increased. FCV had the shortest D value among all three organisms at all temperatures investigated. These findings indicate that F-specific RNA phages may be useful indicators of NV in the environment.     

Gramine Accumulation in Leaves of Barley Grown under High-Temperature Stress

The indole alkaloid gramine is toxic to animals and may play a defensive role in plants. Under certain conditions, shoots of barley cultivars such as `Arimar' and CI 12020 accumulate gramine (N,N-dimethyl-3-aminomethylindole) and lesser amounts of its precursors 3-aminomethylindole (AMI) and N-methyl-3-aminomethylindole (MAMI); other cultivars such as `Proctor' do not. When grown at optimal temperatures (21°C/16°C, day/night), Arimar contained a high level of gramine in the first leaf (approximately 6 milligrams per gram dry weight), but progressively less accumulated in successive leaves so that the gramine level in the shoot as a whole fell sharply with age. In Arimar and CI 12020 plants transferred at the two- to three-leaf stage from 21°C/16°C to supra-optimal temperatures (≥30°C/25°C), there was massive gramine accumulation in leaves which developed at high temperature, so that gramine level in the whole shoot remained high (about 3-8 milligrams per gram dry weight).

Proctor lacked both constitutive gramine accumulation in the first leaf and heat-induced gramine accumulation in later leaves. The following evidence indicates that this results from a lesion in the pathway of synthesis (tryptophan →→ AMI → MAMI → gramine) between tryptophan and AMI. (a) Proctor and Arimar leaves readily absorbed [¹⁴C]gramine, but neither cultivar degraded it extensively. (b) Arimar leaf tissue incorporated [¹⁴C]formate label into the N-methyl groups of gramine and MAMI, and converted [methylene-¹⁴C]tryptophan to AMI, MAMI, and gramine; Proctor leaf tissue did not, even when a trapping pool of unlabeled gramine was supplied. (c) Proctor converted [¹⁴C]MAMI to gramine as actively as Arimar. (d) Proctor incorporated [¹⁴C]formate label into gramine and MAMI when supplied with AMI; the ratio [¹⁴C]gramine/[¹⁴C]MAMI fell with leaf age, suggesting that the two N-methylations involve different enzymes. Inasmuch as Proctor leaf tissue did not methylate added tryptamine or tyramine, the N-methyltransferase(s) of gramine synthesis may be substrate specific.

In sterile culture at optimal temperatures, 10 millimolar gramine did not affect autotrophic growth of Arimar or Proctor plantlets or heterotrophic growth of callus. At supra-optimal temperature, plantlet growth was reduced by gramine although callus growth was not. We speculate that gramine-accumulating cultivars may suffer autotoxic effects at high leaf temperatures.

     

Pushed to the limit: consequences of climate change for the Araucariaceae: a relictual rain forest family

Background and Aims

Under predicted climate change scenarios, increased temperatures are likely to predispose trees to leaf and other tissue damage, resulting in plant death and contraction of already narrow distribution ranges in many relictual species. The effects of predicted upward temperatures may be further exacerbated by changes in rainfall patterns and damage caused by frosts on trees that have been insufficiently cold-hardened. The Araucariaceae is a relictual family and the seven species found in Australia have limited natural distributions characterized by low frost intensity and frequency, and warm summer temperatures. The temperature limits for these species were determined in order to help understand how such species will fare in a changing climate.

Methods

Experiments were conducted using samples from representative trees of the Araucariaceae species occurring in Australia, Agathis (A. atropurpurea, A. microstachya and A. robusta), Arauacaria (A. bidwilli, A. cunninghamii and A. heterophylla) and Wollemia nobilis. Samples were collected from plants grown in a common garden environment. Lower and higher temperature limits were determined by subjecting detached winter-hardened leaves to temperatures from 0 to –17 °C and summer-exposed leaves to 25 to 63 °C, then measuring the efficiency of photosystem II (F_v/F_m) and visually rating leaf damage. The exotherm, a sharp rise in temperature indicating the point of ice nucleation within the cells of the leaf, was measured on detached leaves of winter-hardened and summer temperature-exposed leaves.

Key Results

Lower temperature limits (indicated by FT₅₀, the temperature at which PSII efficiency is 50 %, and LT₅₀ the temperature at which 50 % visual leaf damage occurred) were approx. –5·5 to –7·5 °C for A. atropurpurea, A. microstachya and A. heterophylla, approx. –7 to –9 °C for A. robusta, A. bidwillii and A. cunninghamii, and –10·5 to –11 °C for W. nobilis. High temperature damage began at 47·5 °C for W. nobilis, and occurred in the range 48·5–52 °C for A. bidwillii and A. cunninghamii, and in the range 50·5–53·5 °C for A. robusta, A. microstachya and A. heterophylla. Winter-hardened leaves had ice nucleation temperatures of –5·5 °C or lower, with W. nobilis the lowest at –6·8 °C. All species had significantly higher ice nucleation temperatures in summer, with A. atropurpurea and A. heterophylla forming ice in the leaf at temperatures >3 °C higher in summer than in winter. Wollemia nobilis had lower FT₅₀ and LT₅₀ values than its ice nucleation temperature, indicating that the species has a degree of ice tolerance.

Conclusions

While lower temperature limits in the Australian Araucariaceae are generally unlikely to affect their survival in wild populations during normal winters, unseasonal frosts may have devastating effects on tree survival. Extreme high temperatures are not common in the areas of natural occurrence, but upward temperature shifts, in combination with localized radiant heating, may increase the heat experienced within a canopy by at least 10 °C and impact on tree survival, and may contribute to range contraction. Heat stress may explain why many landscape plantings of W. nobilis have failed in hotter areas of Australia.     

Temperature-dependent water and ion transport properties of barley and sorghum roots : I. Relationship to leaf growth

Root temperature strongly affects shoot growth, possibly via “nonhydraulic messengers” from root to shoot. In short-term studies with barley (Hordeum vulgare L.) and sorghum (Sorghum bicolor L.) seedlings, the optimum root temperatures for leaf expansion were 25° and 35°C, respectively. Hydraulic conductance (L_p) of both intact plants and detached exuding roots of barley increased with increasing root temperature to a high value at 25°C, remaining high with further warming. In sorghum, the L_p of intact plants and of detached roots peaked at 35°C. In both species, root temperature did not affect water potentials of the expanded leaf blade or the growing region despite marked changes in L_p. Extreme temperatures greatly decreased ion flux, particularly K⁺ and NO₃⁻, to the xylem of detached roots of both species. Removing external K⁺ did not alter short-term K⁺ flux to the xylem in sorghum but strongly inhibited flux at high temperature in barley, indicating differences in the sites of temperature effects. Leaf growth responses to root temperature, although apparently “uncoupled” from water transport properties, were correlated with ion fluxes. Studies of putative root messengers must take into account the possible role of ions.     

Infection Behavior and Overwintering Survival of Foliar Nematodes, Aphelenchoides fragariae, on Hosta

We studied the pathogenicity and overwintering survival of the foliar nematode, Aphelenchoides fragariae, infecting Hosta spp. Nematodes applied to either lower or upper sides of noninjured and injured hosta leaves were able to infect and produce typical symptoms on nine cultivars. Leaves of only four cultivars (Borschi, Fragrant Blue, Patomic Pride, and Olive Bailey Langdon) showed no symptoms of nematode infection. The nematodes overwintered as juveniles and adults in soil, dry leaves, and dormant buds, but not in roots. Nematode winter survival was higher in dormant buds and soil from the polyhouse than in an open home garden. Of the nematodes found in the dormant buds, 35% to 79% were located between the first two outside layers of the buds. The nematodes tolerated 8 hr exposure to 40°C and −80°C in leaf tissues. Relative humidity influenced nematode migration from soil to leaves. The presence of nematodes only on the outer surface of foliage (leaves and petioles) confirmed the migration of A. fragariae on the surface of the plants. Of the total number of nematodes found on the foliage, 25% to 46% and 66% to 77% were alive at 90% and 100% relative humidity, respectively, suggesting that high moisture is required for the survival and upward movement of nematodes. We conclude that A. fragariae can overwinter in soil, infected dry leaves, and dormant buds and migrate in films of water on the outer surface of the plant during spring to leaves to initiate infection.     

Temperature Dependence of Photosynthesis in Agropyron smithii Rydb. : II. CONTRIBUTION FROM ELECTRON TRANSPORT AND PHOTOPHOSPHORYLATION

As part of an analysis of the factors regulating photosynthesis in Agropyron smithii Rydb., a C₃ grass, the response of electron transport and photophosphorylation to temperature in isolated chloroplast thylakoids has been examined. The response of the light reactions to temperature was found to depend strongly on the preincubation time especially at temperatures above 35°C. Using methyl viologen as a noncyclic electron acceptor, coupled electron transport was found to be stable to 38°C; however, uncoupled electron transport was inhibited above 38°C. Photophosphorylation became unstable at lower temperatures, becoming progressively inhibited from 35 to 42°C. The coupling ratio, ATP/2e⁻, decreased continuously with temperature above 35°C. Likewise, photosystem I electron transport was stable up to 48°C, while cyclic photophosphorylation became inhibited above 35°C. Net proton uptake was found to decrease with temperatures above 35°C supporting the hypothesis that high temperature produces thermal uncoupling in these chloroplast thylakoids. Previously determined limitations of net photosynthesis in whole leaves in the temperature region from 35 to 40°C may be due to thermal uncoupling that limits ATP and/or changes the stromal environment required for photosynthetic carbon reduction. Previously determined limitations to photosynthesis in whole leaves above 40°C correlate with inhibition of photosynthetic electron transport at photosystem II along with the cessation of photophosphorylation.     

Control of Continuous Irradiation Injury on Potatoes with Daily Temperature Cycling

Two controlled-environment experiments were conducted to determine the effects of temperature fluctuations under continuous irradiation on growth and tuberization of two potato (Solanum tuberosum L.) cultivars, Kennebec and Superior. These cultivars had exhibited chlorotic and stunted growth under continuous irradiation and constant temperatures. The plants were grown for 4 weeks in the first experiment and for 6 weeks in the second experiment. Each experiment was conducted under continuous irradiation of 400 micromoles per square meter per second of photosynthetic photon flux and included two temperature treatments: constant 18°C and fluctuating 22°C/14°C on a 12-hour cycle. A common vapor pressure deficit of 0.62 kilopascal was maintained at all temperatures. Plants under constant 18°C were stunted and had chlorotic and abscised leaves and essentially no tuber formation. Plants grown under the fluctuating temperature treatment developed normally, were developing tubers, and had a fivefold or greater total dry weight as compared with those under the constant temperature. These results suggest that a thermoperiod can allow normal plant growth and tuberization in potato cultivars that are unable to develop effectively under continuous irradiation.     

Overwintering Strategy and Mechanisms of Cold Tolerance in the Codling Moth (Cydia pomonella)

Background

The codling moth (Cydia pomonella) is a major insect pest of apples worldwide. Fully grown last instar larvae overwinter in diapause state. Their overwintering strategies and physiological principles of cold tolerance have been insufficiently studied. No elaborate analysis of overwintering physiology is available for European populations.

Principal Findings

We observed that codling moth larvae of a Central European population prefer to overwinter in the microhabitat of litter layer near the base of trees. Reliance on extensive supercooling, or freeze-avoidance, appears as their major strategy for survival of the winter cold. The supercooling point decreases from approximately −15.3°C during summer to −26.3°C during winter. Seasonal extension of supercooling capacity is assisted by partial dehydration, increasing osmolality of body fluids, and the accumulation of a complex mixture of winter specific metabolites. Glycogen and glutamine reserves are depleted, while fructose, alanine and some other sugars, polyols and free amino acids are accumulated during winter. The concentrations of trehalose and proline remain high and relatively constant throughout the season, and may contribute to the stabilization of proteins and membranes at subzero temperatures. In addition to supercooling, overwintering larvae acquire considerable capacity to survive at subzero temperatures, down to −15°C, even in partially frozen state.

Conclusion

Our detailed laboratory analysis of cold tolerance, and whole-winter survival assays in semi-natural conditions, suggest that the average winter cold does not represent a major threat for codling moth populations. More than 83% of larvae survived over winter in the field and pupated in spring irrespective of the overwintering microhabitat (cold-exposed tree trunk or temperature-buffered litter layer).     

H-NS Regulates DNA Repair in Shigella

We report a new role for H-NS in Shigella spp.: suppression of repair of DNA damage after UV irradiation. H-NS-mediated suppression of virulence gene expression is thermoregulated in Shigella, being functional at 30°C and nonfunctional at 37 to 40°C. We find that H-NS-mediated suppression of DNA repair after UV irradiation is also thermoregulated. Thus, Shigella flexneri M90T, incubated at 37 or 40°C postirradiation, shows up to 30-fold higher survival than when incubated at 30°C postirradiation. The hns mutants BS189 and BS208, both of which lack functional H-NS, show a high rate of survival (no repression) whether incubated at 30 or 40°C postirradiation. Suppression of DNA repair by H-NS is not mediated through genes on the invasion plasmid of S. flexneri M90T, since BS176, cured of plasmid, behaves identically to the parental M90T. Thus, in Shigella the nonfunctionality of H-NS permits enhanced DNA repair at temperatures encountered in the human host. However, pathogenic Escherichia coli strains (enteroinvasive and enterohemorrhagic E. coli) show low survival whether incubated at 30 or 40°C postirradiation. E. coli K-12 shows markedly different behavior; high survival postirradiation at both 30 and 40°C. These K-12 strains were originally selected from E. coli organisms subjected to both UV and X irradiation. Therefore, our data suggest that repair processes, extensively described for laboratory strains of E. coli, require experimental verification in pathogenic strains which were not adapted to irradiation.     

Accumulation of heat shock proteins in field-grown cotton

Cotton (Gossypium hirsutum L.) plants grown under field water deficits exhibited an 80 to 85% reduction in leaf area index, plant height, and dry matter accumulation compared with irrigated controls. Midday photosynthetic rates of dryland plants decreased 2-fold, and canopy temperatures increased to 40°C at 80 days after planting compared with canopy temperatures of 30°C for irrigated plants. Leaves from dryland plants which had exhibited canopy temperatures of 40°C for several weeks accumulated stainable levels of polypeptides with apparent molecular weights of 100, 94, 89, 75, 60, 58, 37, and 21 kilodaltons. These polypeptides did not accumulate in leaves from irrigated plants.

Addition of [³⁵S]methionine to leaves of growth chamber-grown cotton plants and subsequent incubation at 40°C for 3 hours radiolabeled polypeptides with molecular weights similar to those that accumulate in dryland cotton leaves. These data suggest that the proteins which accumulate in water-stressed cotton leaves at elevated temperatures (40°C) are heat shock proteins and that these proteins can accumulate to substantial levels in field-stressed plants.

     

Effect of Storage Temperature and Duration on Survival and Infectivity of Steinernema innovationi (Rhabditida: Steinernematidae)

Entomopathogenic nematode species differ in their optimum storage temperature; therefore, we conducted a study on the survival and infectivity of the recently described Steinernema innovationi from South Africa at five storage temperatures (5°C, 10°C, 15°C, 20°C, and 25°C) over 84 d using 20,000 infective juveniles (IJ) in 25 ml aqueous suspension containing 0.1% formalin. Our results showed that survival was highest and most stable at 15°C, ranging from 84% to 88% after 84 d. Infectivity of IJ against Galleria mellonella larvae was >90% for all temperatures except for 5°C at which survival decreased to 10% after 84 d. In addition, we stored 2.5 million IJ on a sponge formulation in 15 ml of 0.1% formalin solution for 84 d at the optimum 15°C followed by 2 wk storage at 25°C. Storage of the IJ on a sponge formulation for 14 d at 25°C post 15°C storage for 84 d did not have a detrimental effect on IJ survival (87%) or infectivity to G. mellonella (95%).