Water availability affects tolerance and resistance to aphids but not the trade-off between the two

This study evaluates the effect of water availability on tolerance and resistance to the aphid Chaitophorus leucomelas by clones of the double hybrid [(Populus trichocarpa × P. maximowiczii Henry) × (P. trichocarpa × P. maximowiczii)] (TM × TM), a hybrid that was previously described as resistant to this aphid. In a 2 × 2 experimental design implemented in a nursery, we were able to assess growth (branch length, number of leaves and branch base diameter) in saplings reared in the natural presence of aphids (natural aphid damage) and aphid-controlled conditions (undamaged) under both well-watered and drought stress conditions. We found that resistance was reduced under drought stress conditions, while tolerance in branch length was increased. Cost of resistance was detected as clones displaying higher tolerance grew less in the absence of aphids, whereas no evidence of costs associated with tolerance was found in any of the growth traits measured, A genetic trade-off between tolerance and resistance was detected, but this trade-off was not affected by water availability. Considering the average response of both defence strategies, well-watered trees seem to allocate more resources to resistance than to tolerance, whereas drought-stressed trees allocate more to tolerance than to resistance. This suggests that tolerance would imply a lower cost than resistance, and the shift to either strategy could be modulated by resource availability.     

Assessing the heat tolerance of 17 beef cattle genotypes

Cattle production plays a significant role in terms of world food production. Nearly 82% of the world’s 1.2 billion cattle can be found in developing countries. An increasing demand for meat in developing countries has seen an increase in intensification of animal industries, and a move to cross-bred animals. Heat tolerance is considered to be one of the most important adaptive aspects for cattle, and the lack of thermally-tolerant breeds is a major constraint on cattle production in many countries. There is a need to not only identify heat tolerant breeds, but also heat tolerant animals within a non-tolerant breed. Identification of heat tolerant animals is not easy under field conditions. In this study, panting score (0 to 4.5 scale where 0 = no stress and 4.5 = extreme stress) and the heat load index (HLI) [HLI_BG<25°C = 10.66 + 0.28 × rh + 1.30 × BG – WS; and, HLI _{BG> 25°C} = 8.62 + 0.38 × rh + 1.55 × BG – 0.5 × WS + e^{(2.4 – WS)}, where BG = black globe temperature (^oC), rh = relative humidity (decimal form), WS = wind speed (m/s) and e is the base of the natural logarithm] were used to assess the heat tolerance of 17 genotypes (12,757 steers) within 13 Australian feedlots over three summers. The cattle were assessed under natural climatic conditions in which HLI ranged from thermonuetral (HLI < 70) to extreme (HLI > 96; black globe temperature = 40.2°C, relative humidity = 64%, wind speed = 1.58 m/s). When HLI > 96 a greater number (P < 0.001) of pure bred Bos taurus and crosses of Bos taurus cattle had a panting score ≥ 2 compared to Brahman cattle, and Brahman-cross cattle. The heat tolerance of the assessed breeds was verified using panting scores and the HLI. Heat tolerance of cattle can be assessed under field conditions by using panting score and HLI.     

Thermotolerance of the photosynthetic light reactions in two <Emphasis Type="Italic">Phaseolus</Emphasis> species: a comparative study

The common bean (Phaseolus vulgaris L.) is sensitive to high temperature, while an ecologically contrasting species (Phaseolus acutifolius A. Gray) is cultivated successfully in hot environments. In this study, the two bean species were respectively acclimated to a control temperature of 25 °C and a moderately elevated temperature of 35 °C in order to compare the thermotolerance capabilities of their photosynthetic light reactions. Growth at 35 °C appeared to have no obvious adverse effect on the photosynthetic activities of the two beans, but changed their thermotolerance. After a short period of heat shock (40 °C for up to 4 h), the photosynthetic activities of 25 °C-grown P. vulgaris declined more severely than those of P. acutifolius grown at 25 °C, implying that the basal thermotolerance of P. vulgaris is inferior to that of P. acutifolius. But after acclimating to 35 °C, the thermotolerances of the two species were both greatly enhanced to about the same level, clearly demonstrating the induction of acquired thermotolerance in their chloroplasts, and P. vulgaris could be as good as P. acutifolius. Temperature acclimation also changed plants’ resistance to photoinhibition in a manner similar to those toward heat stress. In addition, acquisition of tolerance to heat and strong irradiance would reduce the dependency of the two beans on xanthophyll pigments to dissipate heat, and also seemed irrelevant to the agents with antioxidant activities such as SOD.     

Inheritance of stomatal conductance in cotton (Gossypium barbadense)

Stomatal conductance in improved Pima cotton cultivars (Gossypium barbadense) has been previously shown to be positively associated with heat resistance and yield potential. In the present study we determined the mode of inheritance of stomatal conductance in crosses of six G. barbadense parents varying in origin, degree of agronomic development and stomatal conductance. Parents included a primitive tropical cotton (B368), two obsolete cultivars (St Vincent V135, Pima 32), one modern commercial line (Pima S-6) and two elite genotypes of the Pima germplasm (P70, P73). These lines showed distinct differences in stomatal conductance, under greenhouse and field conditions. The primitive B368 had the lowest conductance, and the elite lines the highest. Generation means analysis was used to quantify genetic effects in the crosses P70 × St Vincent V135, Pima S-6 × B368, Pima S-6 × Pima 32, P73 × Pima 32 and P73 × Pima S-6. Best-fit models of the inheritance of stomatal conductance varied in complexity from a simple additive-dominance model in the cross P70 × St. Vincent V135 to models displaying digenic epistatic interactions in the remaining crosses. Significant additive mean effects for stomatal conductance were detected in all crosses. Dominance mean effects were significant in the crosses P73 × Pima 32 and P73 × Pima S-6. Broadsense heritability estimates of stomatal conductance were relatively low (0.16–0.44) in all crosses except Pima S-6 × B368 (0.74). Results also show that the mode of inheritance of stomatal conductance is multigenic, and may have maternal as well as nuclear components. Recouping higher stomatal conductance levels from genetically wider crosses appears feasible and could proceed at a moderate rate. Fixing higher levels of stomatal conductance in populations from crosses of elite germplasm may be more difficult because of the presence of dominant mean effects and digenic epistatic interactions.     

Modulation of flowering responses in different Nicotiana varieties

We have identified and characterized a FLOWERING PROMOTING FACTOR 1(FPF1) gene from tobacco (NtFPF1). Over-expression of NtFPF1 leads to early flowering in the day-neutral tobacco Nicotiana tabacum cv. Hicks, and under inductive photoperiods also in the short-day Nicotiana tabacum cv. Hicks Maryland Mammoth (MM) tobacco and the long-day plant Nicotiana sylvestris. N. sylvestris wild-type plants remained in the rosette stage and never flowered under non-inductive short-days, whereas 35S::NtFPF1 transgenic plants bolted but did not flower. However, if treated with gibberellins, transgenic N. sylvestrisplants flowered much faster under non-inductive short days than corresponding wild type plants, indicating an additive effect of gibberellins and the NtFPF1 protein in flowering time control. The day-neutral wild type cv. Hicks and the short-day cv. Hicks MM plants exhibit an initial rosette stage, both under short- and long-days. In the transgenic lines, this rosette stage was completely abolished. Wild-type plants of cv. Hicks MM never flowered under long days; however, all transgenic lines over-expressing NtFPF1 flowered under this otherwise non-inductive photoperiod.     

Differential metabolic responses of perennial grass Cynodon transvaalensis×Cynodon dactylon (C₄) and Poa Pratensis (C₃) to heat stress

Differential metabolic responses to heat stress may be associated with variations in heat tolerance between cool‐season (C₃) and warm‐season (C₄) perennial grass species. The main objective of this study was to identify metabolites associated with differential heat tolerance between C₄ bermudagrass and C₃ Kentucky bluegrass by performing metabolite profile analysis using gas chromatography‐mass spectrometry. Plants of Kentucky bluegrass (Poa Pratensis‘Midnight’) and hybrid bermudagrass (Cynodon transvaalensis×Cynodon dactylon‘Tifdwarf’) were grown under optimum temperature conditions (20/15°C for Kentucky bluegrass and 30/25°C for bermudagrass) or heat stress (35/30°C for Kentucky bluegrass and 45/40°C for bermudagrass). Physiological responses to heat stress were evaluated by visual rating of grass quality, measuring photochemical efficiency (variable fluorescence to maximal fluorescence) and electrolyte leakage. All of these parameters indicated that bermudagrass exhibited better heat tolerance than Kentucky bluegrass. The metabolite analysis of leaf polar extracts revealed 36 heat‐responsive metabolites identified in both grass species, mainly consisting of organic acids, amino acids, sugars and sugar alcohols. Most metabolites showed higher accumulation in bermudagrass compared with Kentucky bluegrass, especially following long‐term (18 days) heat stress. The differentially accumulated metabolites included seven sugars (sucrose, fructose, galactose, floridoside, melibiose, maltose and xylose), a sugar alcohol (inositol), six organic acids (malic acid, citric acid, threonic acid, galacturonic acid, isocitric acid and methyl malonic acid) and nine amino acids (Asn, Ala, Val, Thr, γ‐Aminobutyric acid, IIe, Gly, Lys and Met). The differential accumulation of those metabolites could be associated with the differential heat tolerance between C₃ Kentucky bluegrass and C₄ bermudagrass.     

Comparative seed resistance in eighty cowpea accessions to the seed bruchid,Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae)

Eighty cowpea accessions comprising both improved and local were assessed for their resistance to Callosobruchus maculatus (F.) attack under laboratory conditions (29 ± 3?°C and 65 ± 5% R.H.). Sixty-nine accessions were derived from the International Institute for Tropical Agriculture breeding programmes, four were local cultivars, while seven were improved cultivars from national research institutes. Results showed that there were significant differences among the accessions in terms of number of initial bruchid eggs laid on them, adult emergence, number of seeds damaged and bruchid-induced seed weight loss at three months after artificial infestation with C. maculatus. On IT04K-334-2, IT04K-343-1, IT06K-141, IT99K-216-48-1, IT99K-494-6 and IT99K-529-2, fewer initial eggs were laid and no adult emergence, seed damage or weight loss and were considered highly resistant to C. maculatus. On the other hand, IT03K-337, IT04K-217-51, IT04K-332-1, IT04K-339-1, IT06K-188-49, IT81D-1137, IT845-2231-15, IT93K-452-1, IT04K-154-1, IT96K-610, IT97K-1069-6, IT97K-390-2, Ife Bpc, IT99K-573-2-1, Ife Modupe, IT845-2246, Drum, Ife Brown and Oloyin were the most susceptible with the highest damage indices values (p?<?0.05). The range of each of the resistance indices measured in the susceptible varieties was: number of initial eggs laid (61–100), adult emergence (40.0–62.10%), number of seeds damaged (53.0–85.5) and seed weight loss (58.0–66.25%).     

Inducible heat tolerance in Antarctic notothenioid fishes

Significant increases in heat tolerance (time of survival at 14°C) were observed for some, but not all, species of notothenioid fishes collected from McMurdo Sound, Antarctica (77°51′S) following acclimation to 4°C. The increase in thermal tolerance was rapid in Trematomus bernacchii, developing within 1–2 days of acclimation to 4°C. Long-term (6–8 weeks) acclimation to 4°C led to greater heat tolerance in Trematomus pennellii than in T. bernacchii. Unlike its demersal congeners, the cryopelagic notothenioid Pagothenia borchgrevinki did not increase heat tolerance during warm acclimation. A deep-living zoarcid fish, Lycodichthys dearborni, also failed to increase heat tolerance, but survived significantly (> threefold) longer at 14°C than the notothenioids.     

Apparent absence of viruses in most symptomless field-grown sweet potato in Uganda

Heat tolerance of groundnut (Arachis hypogaea L.) genotypes was evaluated by solute leakage and chlorophyll fluorescence techniques in heat-hardened and non-hardened plants. To determine the appropriate hardening treatment, 1-month-old plants of two groundnut genotypes, ICGV 86707 and Chico were conditioned at five combinations of hardening (37°C) and non-hardening (30°C) air temperatures over a 5-day period. Heat injury, was assessed through measurements of electrolyte leakage after stressing leaf discs to 55°C for 15 min. The relative injury was significantly influenced by the conditioning temperatures and by the temperature during 24 h prior to measurement if those involved non-hardening conditions. Relative injury and chlorophyll fluorescence were measured after stressing leaves of six genotypes at a range of temperatures between 49°C and 55°C. Significant genotype × hardening treatment interactions were observed in relative injury and chlorophyll fluorescence. Chico was susceptible to heat stress, the relative injury test identified ICGV 86707 as tolerant, and the chlorophyll fluorescence test identified ICGV 86707 as tolerant under hardened conditions and ICGV 87358 as tolerant when non-hardened. When expressed as percentage of control values, the relative injury and chlorophyll fluorescence measurements over the 49–53°C stress temperature range were strongly correlated. Chlorophyll concentrations were increased by hardening in all genotypes except Chico. In Chico, chl_b concentration was decreased and the chl_a/b ratio increased by hardening, and chlorophyll concentrations were correlated with chlorophyll fluorescence parameters. Chlorophyll concentration may therefore provide an alternative means of screening for heat tolerance.     

Isolation of four heat shock protein cDNAs from grapefruit peel tissue and characterization of their expression in response to heat and chilling temperature stresses

In order to continuously supply horticultural products for long periods, it is essential to store them after harvest in low temperatures. However, many tropical and subtropical fruits and vegetables, such as citrus, are sensitive to chilling. In previous studies, the authors have shown that a short hot water rinsing treatment (at 62°C for 20 s) increased chilling tolerance in grapefruit. In order to gain more insight into the molecular mechanisms involved in heat‐induced chilling tolerance, PCR cDNA subtraction analysis was performed which isolated four different PCR fragments whose expression was enhanced 24 h after the heat treatment, and that showed high sequence homology with various plant HSP18‐I, HSP18‐II, HSP22 and HSP70 genes. It was found that the short hot water treatment given at 62°C for 20 s, but not at lower temperatures of 20 or 53°C, increased the expression of the various HSP cDNAs in grapefruit peel tissue. However, when the fruits were kept at ambient temperatures, the increases in HSP mRNA levels following the hot water treatment were temporary and lasted only between 6 and 48 h. Similar temporary increases in the HSP mRNA levels were detected following exposure of the fruit to a hot air treatment at 40°C for 2 h. Nevertheless, when the fruits were treated with hot water but afterwards stored at chilling temperatures of 2°C, the mRNA levels of the various HSP18‐I, HSP18‐II, HSP22 and HSP70 cDNAs increased and remained high and stable during the entire 8‐week cold‐storage period, suggesting their possible involvement in heat‐induced chilling‐tolerance responses. The chilling treatment by itself increased the expression of the HSP18‐I cDNA, but had no effect on the mRNA levels of any of the other HSP cDNAs. Exposure of fruit to other stresses, such as wounding, UV irradiation, anaerobic conditions and exposure to ethylene, had no effect on the expression of the various HSPs. Overall, the study explored the correlation between the expression and persistence of various HSP cDNAs in grapefruit peel tissue during cold storage, on the one hand, and the acquisition of chilling tolerance, on the other hand, and the results suggest that HSPs may play a general role in protecting plant cells under both high‐ and low‐temperature stresses.     

Heat tolerance of strawberry tarsonemid mite Phytonemus pallidus

The heat tolerance of the strawberry tarsonemid mite Phytonemus pallidus ssp. fragariae was studied in northern Sweden by treating adult mites in hot water at temperatures 44°C, 46°C and 48°C. During the summer, the shortest exposure times required to kill all adult mites at these temperatures were 6, 2.5 and 1.25 min, respectively. It is concluded that hot‐water treatment of strawberry runners for 6.5 min at 46°C would suffice to kill all tarsonemid mites on infested runners. This exposure time includes the time needed to raise the temperature of the plant material to that of the water bath. In the autumn, from the middle of September and later, the mites became successively much more heat tolerant as they prepared for hibernation. It is expected that longer exposure periods are needed to kill mites on frigoplants.     

Effects of acclimation on the thermal tolerance of the brown planthopper Nilaparvata lugens (Stål)

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Heat sensitivity of eggs attributes to the reduction in Agasicles hygrophila population

Agasicles hygrophila has been introduced worldwide as a control agent for the invasive weed Alternanthera philoxeroides.However,global warming has potential impact on its controlling efficacy.The aim of this research was to explore the primary factors responsible for the greatly reduced A.hygrophila population in hot summers.To imitate the temperature conditions in summers,different developmental stages of hygrophila were treated with high temperatures from 32.5℃ to 45℃ for 1-5 h.Based on the survival rate,the heat tolerance of each developmental stage was ranked from lowest to highest as follows:egg,1st,2nd,3rd instar larva,adult and pupa.Eggs showed the lowest heat tolerance with 37.5℃ as the critical temperature affecting larval hatching.Heat treatment of the A.hygrophila eggs at 37.5℃ for 1 h decreased the hatch rate to 24%.Our results indicated that when compared with the control at 25℃,1 h treatment at 37.5℃ prolonged the duration of the egg stage,shortened the duration of oviposition and total longevity,and changed the reproductive pattern of A.hygrophila.The net reproductive rate,intrinsic rate and finite rate were all significantly reduced.The results suggest that low heat tolerance of the eggs was the major factor responsible for the reduction of A.hygrophila populations,and the key temperature was 37.5°C.Therefore,appropriate measures should be taken to protect eggs in order to maintain the efficacy of A.hygrophila in the biological control of A.philoxeroides in hot summers.     

The role of the photosynthetic apparatus in cold acclimation of <Emphasis Type="Italic">Lolium multiflorum</Emphasis>. Characteristics of novel genotypes low-sensitive to PSII over-reduction

During cold acclimation by higher plants, temperature perception via changes in redox state of Photosystem II (PSII) and subsequent acclimation of the photosynthetic apparatus to cold is very important for achieving freezing tolerance. These properties were studied in two groups (A and B) of the same backcross 3 (BC₃) progeny derived from a triploid hybrid of Festuca pratensis (2×) × Lolium multiflorum (4×) backcrossed three times onto diploid L. multiflorum cultivars. Leaves of Group A plants formed at 20°C at medium-low light were unable to acclimate their photosynthetic apparatus to cold. Compared to Group B, the Group A plants were also more frost sensitive. This acclimation ability correlated with the freezing tolerance of the plants. However, leaves of the same Group A plants developed at 20°C, but under higher-light conditions had increased ability to acclimate their photosynthetic apparatus to cold. It was concluded that Group A plants may have impaired PSII temperature perception, and this then resulted in their poor capability to cold acclimate.     

The effects of marine heatwaves on acute heat tolerance in corals

Scleractinian coral populations are increasingly exposed to conditions above their upper thermal limits due to marine heatwaves, contributing to global declines of coral reef ecosystem health. However, historic mass bleaching events indicate there is considerable inter- and intra-specific variation in thermal tolerance whereby species, individual coral colonies and populations show differential susceptibility to exposure to elevated temperatures. Despite this, we lack a clear understanding of how heat tolerance varies across large contemporary and historical environmental gradients, or the selective pressures that underpin this variation. Here we conducted standardised acute heat stress experiments to identify variation in heat tolerance among species and isolated reefs spanning a large environmental gradient across the Coral Sea Marine Park. We quantified the photochemical yield (F_v/F_m) of coral samples in three coral species, Acropora cf humilis, Pocillopora meandrina, and Pocillopora verrucosa, following exposure to four temperature treatments (local ambient temperatures, and + 3°C, +6°C and + 9°C above local maximum monthly mean). We quantified the temperature at which F_v/F_m decreased by 50% (termed ED50) and used derived values to directly compare acute heat tolerance across reefs and species. The ED50 for Acropora was 0.4–0.7°C lower than either Pocillopora species, with a 0.3°C difference between the two Pocillopora species. We also recorded 0.9°C to 1.9°C phenotypic variation in heat tolerance among reefs within species, indicating spatial heterogeneity in heat tolerance across broad environmental gradients. Acute heat tolerance had a strong positive relationship to mild heatwave exposure over the past 35 years (since 1986) but was negatively related to recent severe heatwaves (2016–2020). Phenotypic variation associated with mild thermal history in local environments provides supportive evidence that marine heatwaves are selecting for tolerant individuals and populations; however, this adaptive potential may be compromised by the exposure to recent severe heatwaves.     

Heat tolerance among different strains of the entomopathogenic nematode <Emphasis Type="Italic">Heterorhabditis bacteriophora</Emphasis>

Quality of biological control products based on entomopathogenic nematodes can be severely damaged due to exposure to high temperature surpassing 40°C. The study screened 36 natural populations and 18 hybrid or inbred strains of Heterorhabditis bacteriophora for their response to high temperature. Nematodes were tested with or without prior adaptation to heat at 35°C for 3 h. Five strains of H. indica and one of H. megidis were also included. Molecular identification using nuclear ribosomal DNA sequences confirmed the designation to the three Heterorhabditis spp. The mean tolerated temperature ranged from 33.3°C to 40.1°C for non-adapted and from 34.8°C to 39.2°C for adapted strain populations. H. indica was the most tolerant, followed by H. bacteriophora and H. megidis. No correlation was recorded between tolerance assessed with and without adaptation to heat, implying that different genes are involved. Correlation between heat tolerance and mean annual temperature at place of origin of the strains was weak. A high variability in tolerance among strains and the relatively high heritability (h2 = 0.68) for the adapted heat tolerance recorded for H. bacteriophora provide an excellent foundation for future selective breeding with the objective to enhance heat tolerance of H. bacteriophora.     

Storage of osmotically treated entomopathogenic nematode Steinernema carpocapsae

The infective juveniles (IJs) of Steinernema carpocapsae‘All’ were osmotically stressed by a mixture of ionic (fortified artificial seawater) and non‐ionic (3.2 mol/L glycerol) solutions to establish a method for osmotic storage of entomopathogenic nematodes. Seven combinations (termed solution A to G) with different proportions of these two solutions were tested, with sterile extra pure water (sepH₂O, termed solution H) as a control. The mortality of the IJs at a concentration of 5 × 10⁵ IJ/mL in the solutions A to G, and H were 13.2%, 16.2%, 16.7%, 13.5%, 25.2%, 31.6%, 44.6%, and 1.0%, respectively, after 21 days storage at 25°C. Most of the IJs shrunk and stopped motility after 6–9 hours incubation at 25°C in solutions A to D. Based on the results, solutions A to D and H were chosen to further test the osmotic survival of the IJs at different IJ concentrations (5 × 10⁵, 2.5 × 10⁵, 2 000 IJ/mL) and incubation temperature (30°C, 25°C, 10°C). The resulting IJs were exposed to a high temperature assay (45°C for 4 h, HTA). Osmotically stressed IJs showed improved heat tolerance. The mortality of the IJs increased with the increasing concentrations of the test IJs and the storage temperatures after exposing to the HTA. More than 88.4%, 62.3% or 2.4% of the treated IJs died at the above three IJ concentrations, respectively. At the three IJ concentrations (2 000 IJs/mL, 2.5 × 10⁵ IJs/mL or 5 × 10⁵ IJs/mL), the highest mortality was recorded in solution D (11.6%, 85.9% or 98.0%, respectively), and the lowest mortality in solution B (2. 4%, 62.3% or 86.6%, respectively). No untreated IJs survived after the heat treatment. During 42 days storage at 10°C, the IJs mortality in the solutions A to D and H were 7.19%, 5.97%, 4.41%, 4.34%, and 4.34% respectively, and showed no significant differences. In conclusion, osmotic treatment of the IJs of S. carpocapsae‘All’ in a mixture of ionic and non‐ionic solutions enhances the heat tolerance. The mortality of the IJs after HTA increased with the increasing concentrations of the test IJs and the storage temperatures after exposure to the HTA. The result is promising for the osmotic storage of the entomopathogenic nematodes.     

Effects of Thermotherapy Treatments on Systemic Agrobacterium vitis in Dormant Grape Cuttings

Agrobacterium tumefaciens biovar 3 (A. vitis) was eradicated or reduced below the level of detection in dormant cuttings of grape [Vitis vinifera cultivar (cv.) Thompson seedless] and rootstock NAZ3 (V. vinifera × V. rupestris) by hot‐water treatment (exposure to 50°C for 30 min). Strains of A. vitis varied in their sensitivity to heat, but were generally more sensitive than strains of A. tumefaciens biovar 1 or biovar 2. Populations of about 10³ CFU/ml in broth were killed by a 30‐min treatment. Biovar 1 strains were apparently unaffected by 50°C, even when exposed for 30 min. Non‐tumorigenic biovar 1 strains were recovered from hot‐water‐treated cuttings. An assessment was also made of the effect of treatment on growth parameters of cuttings in a field nursery after 9 months. The effect of hot‐water treatment on the vitality and growth of vines varied with different cultivars or rootstocks. The number and length of canes, root dry weight and length and diameter of trunks, increased in most instances. Treatment and time usually did affect bud survival and, in most cases, increased the level of callus formation at the base of the cuttings. Hot‐water treatment may offer a simple, effective, economical and environmentally safe means of eradicating A. vitis from dormant grape cuttings.     

COMPARISON OF DAYLENGTH AND TEMPERATURE RESPONSES IN NICOTIANA AND ITS TAXONOMIC SECTIONS

Steinberg , Robert A. (U.S.D.A., Beltsville, Md.) Comparison of daylength and temperature responses in Nicotiana and its taxonomic sections. Amer. Jour. Bot. 46(4): 261–268. Illus. 1959.—Fifty-seven of the sixty species of Nicotiana were grown in the greenhouse under long- and short-day regimes. Supplemental tungsten light of about 30 ft.-c. (bench) was used to extend natural illumination to 16 hr. daily. Short-day controls received natural illumination for 9.5–12 hr. daily from about September to March. Two temperature levels were also employed—one with temperature held uniformly at about 25°C. and the other with a day temperature of about 20°C. and a night temperature of about 10°C. Daylength behavior of the species ranged from long-day to day-neutral to short-day. All species were brought into flower and all, except N. acaulis and N. ameghinoi, formed viable seed in at least 1 of the 4 environments. A modified classification of photoperiodic flowering responses based on rapidity and not ability to flower was adopted to permit quantitative comparison of species responses to both daylength and temperature. Very few species flowered equally rapidly (day-neutral) in both the 10- and 16-hr. day-lengths. Temperature level caused modifications in response from long-day to day-neutral and vice versa, and from short-day to day-neutral and vice versa. Data for N. glauca and some other species would indicate that a greater spread between temperature levels could possibly lead to opposite classifications at upper and lower temperatures. Excellent agreement was found between daylength responses of the species and the 14 taxonomic sections of Goodspeed for the genus Nicotiana. Only 2 of the sections (Paniculatae and Undulatae) were heterogeneous in that both included short- and long-day species in the same section. The native habitat of all short-day species was South America. Certain of the species gave a compensatory response to variations in light duration and low temperature similar to that given by sugarbeets and other biennials. This phenomenon may therefore be of general occurrence. Use of a quantitative expression for photoperiodic flowering responses is proposed to avoid ambiguity. It is the quotient of days from sowing to first blossom on short-days divided by that on long-days. The value 0.6₂₀°C. (9–12) would read short-day at 20°C. with 9–12 hr. daylengths. Close agreement was found in daylength flowering ratios in successive tests in the greenhouse. The ratios alter under cold treatment with species susceptible to low-temperature stimulation or inhibition of blossoming.     

Acid and heat tolerance of persistent and nonpersistent Listeria monocytogenes food plant strains

Aims: Acid and heat tolerance of 17 persistent and 23 nonpersistent Listeria monocytogenes strains, recovered from three meat‐processing plants, were investigated. Methods and Results: The isolates were genotyped by pulsed‐field gel electrophoresis and categorized into persistent strains according to the frequency of the strain and duration of the contamination. The persistent and nonpersistent strains were challenged to acidic conditions (pH 2·4 for 2 h, 1 mol l^?1 HCl were used to acidify the suspension) and to heat (55°C for 40 min) to receive a reduction in cell count. Listeria monocytogenes strains showed large variation in acid tolerance (over 6 log units) and in heat tolerance (3 log units). The persistent strains showed higher tolerance to acidic conditions than the nonpersistent strains (Student’s t‐test, P = 0·02), but significant differences in heat tolerance between persistent and nonpersistent strains were not observed. Conclusions: The results indicate that acid tolerance may have an effect on the persistence of L. monocytogenes contamination. Significance and Impact of the Study: This study highlights the fact that there are great differences in acid and heat tolerances between L. monocytogenes strains, and the preventive measures should be designed to be effective against the most tolerant strains.