Inheritance Of Resistance to Mammalian Herbivores and of Plant Defensive Chemistry in an <Emphasis Type="Italic">Eucalyptus</Emphasis> Species

Hybridization in plants provides an opportunity to investigate the patterns of inheritance of hybrid resistance to herbivores, and of the plant mechanisms conferring this resistance such as plant secondary metabolites. We investigated how inter-race differences in resistance of Eucalyptus globulus to a generalist mammalian herbivore, Trichosurus vulpecula, are inherited in their F₁ hybrids. We assessed browsing damage of 3-year-old trees in a common environment field trial on four hybrid types of known progeny. The progeny were artificial intra-race crosses and reciprocal inter-race F₁ hybrids of two geographically distinct populations (races) of E. globulus north-eastern Tasmania and south-eastern Tasmania. Populations of trees from north-eastern Tasmania are relatively susceptible to browsing by T. vulpecula, while populations from south-eastern Tasmania are more resistant. We assessed the preferences of these trees in a series of paired feeding trials with captive animals to test the field trial results and also investigated the patterns of inheritance of plant secondary metabolites. Our results demonstrated that the phenotypic expression of resistance of the inter-race F₁ hybrids supported the additive pattern of inheritance, as these hybrids were intermediate in resistance compared to the pure parental hybrids. The expression of plant secondary metabolites in the F₁ hybrids varied among major groups of individual compounds. The most common pattern supported was dominance towards one of the parental types. Together, condensed tannins and essential oils appeared to explain the observed patterns of resistance among the four hybrid types. While both chemical groups were inherited in a dominant manner in the inter-race F₁ hybrids, the direction of dominance was opposite. Their combined concentration, however, was inherited in an additive manner, consistent with the phenotypic differences in browsing.     

NMR Metabolomics of Thrips (<Emphasis Type="Italic">Frankliniella occidentalis</Emphasis>) Resistance in <Emphasis Type="Italic">Senecio</Emphasis> Hybrids

Western flower thrips (Frankliniella occidentalis) has become a key insect pest of agricultural and horticultural crops worldwide. Little is known about host plant resistance to thrips. In this study, we investigated thrips resistance in F ₂ hybrids of Senecio jacobaea and Senecio aquaticus. We identified thrips-resistant hybrids applying three different bioassays. Subsequently, we compared the metabolomic profiles of these hybrids applying nuclear magnetic resonance spectroscopy (NMR). The new developments of NMR facilitate a wide range coverage of the metabolome. This makes NMR especially suitable if there is no a priori knowledge of the compounds related to herbivore resistance and allows a holistic approach analyzing different chemical compounds simultaneously. We show that the metabolomes of thrips-resistant and -susceptible hybrids differed considerably. Thrips-resistant hybrids contained higher amounts of the pyrrolizidine alkaloids (PA), jacobine, and jaconine, especially in younger leaves. Also, a flavanoid, kaempferol glucoside, accumulated in the resistant plants. Both PAs and kaempferol are known for their inhibitory effect on herbivores. In resistant and susceptible F ₂ hybrids, young leaves showed less thrips damage than old leaves. Consistent with the optimal plant defense theory, young leaves contained increased levels of primary metabolites such as sucrose, raffinose, and stachyose, but also accumulated jacaranone as a secondary plant defense compound. Our results prove NMR as a promising tool to identify different metabolites involved in herbivore resistance. It constitutes a significant advance in the study of plant–insect relationships, providing key information on the implementation of herbivore resistance breeding strategies in plants. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Relationship between Structurally Different Pyrrolizidine Alkaloids and Western Flower Thrips Resistance in F2 Hybrids of Jacobaea vulgaris and Jacobaea aquatica

Segregating plant hybrids often have more ecological and molecular variability compared to parental species, and are therefore useful for studying relationships between different traits, and the adaptive significance of trait variation. Hybrid systems have been used to study the relationship between the expression of plant defense compounds and herbivore susceptibility. We conducted a western flower thrips (WFT) bioassay using a hybrid family and investigated the relationship between WFT resistance and pyrrolizidine alkaloid (PA) variation. The hybrid family consisted of two parental (Jacobaea vulgaris and Jacobaea aquatica) genotypes, two F₁ genotypes, and 94 F₂ hybrid lines. The J. aquatica genotype was more susceptible to thrips attack than the J. vulgaris genotype, the two F₁ hybrids were as susceptible as J. aquatica, and susceptibility to WFT differed among F₂ hybrid lines: 69 F₂ lines were equally susceptible compared to J. aquatica, 10 F₂ lines were more susceptible than J. aquatica and 15 F₂ lines were as resistant as J. vulgaris or were intermediate to the two parental genotypes. Among 37 individual PAs that were derived from four structural groups (senecionine-, jacobine-, erucifoline- and otosenine-like PAs), the N-oxides of jacobine, jaconine, and jacoline were negatively correlated with feeding damage caused by WFT, and the tertiary amines of jacobine, jaconine, jacoline, and other PAs did not relate to feeding damage. Total PA concentration was negatively correlated with feeding damage. Among the four PA groups, only the total concentration of the jacobine-like PAs was negatively correlated with feeding damage. Multiple regression tests suggested that jacobine-like PAs play a greater role in WFT resistance than PAs from other structural groups. We found no evidence for synergistic effects of different PAs on WFT resistance. The relationship between PA variation and WFT feeding damage in the Jacobaea hybrids suggests a role for PAs in resistance to generalist insects.     

Evidence of Natural Hybridization Between Two Sympatric Sibling Species of <Emphasis Type="Italic">Bactrocera dorsalis</Emphasis> Complex Based on Pheromone Analysis

Bactrocera carambolae and B. papayae are major fruit fly pests and sympatric sibling species of the B. dorsalis complex. They possess distinct differences in male pheromonal components. In the 1990s, wild Bactrocera fruit flies with morphological traits intermediate between those of B. carambolae and B. papayae were often captured in traps baited with methyl eugenol (ME). Chemical analyses of rectal glands of ME-fed males revealed that the laboratory F₁, F₂, and backcross hybrids possessed ME-derived sex pheromonal components ranging from that typical of B. papayae to that of B. carambolae without any specific trend, which included a combination of pheromonal components from both parental species within an individual hybrid. ME-fed hybrids without any ME-derived pheromonal components were also detected. Further chemical analysis of rectal glands from wild Bactrocera males, after ME feeding in the laboratory, showed a combination of pheromonal components similar to that found in the ME-fed, laboratory-bred hybrids. These findings present circumstantial evidence for the occurrence of a natural hybrid of the two Bactrocera species.     

Inheritance patterns of phenolics in F1, F2, and back-cross hybrids of willows: implications for herbivore responses to hybrid plants

The aim of this study was to determine the inheritance pattern of phenolic secondary compounds in pure and hybrid willows and its consequences for plant resistance to leaf-feeding insects. F1, F2, and back-cross hybrids along with pure species were produced by hand pollination of pure, naturally-growing Salix caprea (L., Salicaceae) and S. repens (L.) plants. Leaf concentrations of condensed tannins and seven different phenolic glucosides were determined by using butanol-HCl and HPLC analyses. Insect herbivore leaf damage was measured on the same leaves as used for chemical analyses. We found hybrids to be approximately intermediate between the parental species: S. caprea with high levels of condensed tannins and no phenolic glucosides, and S. repens with low levels of condensed tannins and high levels of phenolic glucosides. We also found a negative correlation between concentrations of condensed tannins and phenolic glucosides, suggesting a trade-off in production of these two substances. F2 hybrids and the hybrid back-crossed to S. caprea were significantly more damaged by insect herbivores than the parental species and the F1 hybrid, indicating reduced resistance and possibly a selective disadvantage for these hybrid categories.     

Species by Environment Interactions Affect Pyrrolizidine Alkaloid Expression in <Emphasis Type="Italic">Senecio jacobaea</Emphasis>, <Emphasis Type="Italic">Senecio aquaticus</Emphasis>, and Their Hybrids

We examined the effects of water and nutrient availability on the expression of the defense pyrrolizidine alkaloids (PAs) in Senecio jacobaea and S. aquaticus. Senecio jacobaea, and S. aquaticus are adapted to different natural habitats, characterized by differing abiotic conditions and different selection pressures from natural enemies. We tested if PA concentration and diversity are plastic over a range of water and nutrient treatments, and also whether such plasticity is dependent on plant species. We also tested the hypothesis that hybridization may contribute to PA diversity within plants, by comparing PA expression in parental species to that in artificially generated F₁ hybrids, and also in later generation natural hybrids between S. jacobaea and S. aquaticus. We showed that total PA concentration in roots and shoots is not dependent on species, but that species determines the pattern of PA diversification. Pyrrolizidine alkaloid diversity and concentration are both dependent on environmental factors. Hybrids produce a putatively novel PA, and this PA is conserved in natural hybrids, that are backcrossed to S. jacobaea. Natural hybrids that are backcrossed several times to S. jacobaea are with regard to PA diversity significantly different from S. jacobaea but not from S. aquaticus, while F₁ hybrids are in all cases more similar to S. jacobaea. These results collectively suggest that PA diversity is under the influence of natural selection.     

Biochemical investigations of antibiosis material in leaf exudate of wildNicotiana species and interspecific hybrids

A model system involving severalNicotiana species containing novel nicotine alkaloids was used to study heritability and expression of alkaloid production in leaf trichomes. The three species that comprise the section Repandae (N. repanda, N. stocktonii, andN. nesophila) were hybridized with eitherN. tabacum orN. sylvestris (neither of which producesN-acylnornicotine). The progeny of the hybrid with sylvestris produced theN-acylnornicotines at a level found in the Repandae parent.Nicotiana repanda was crossed toN. tabacum, and the F₂ progeny produced the alkaloid at the same level as the original Repandae parent. Inheritance of the ability to acylate nornicotine in Repandae species is inherited in hybrids in a dominant manner These and other data obtained suggest that theN-acyltransferase that acylates nornicotine in Repandae species inherited in hybrids is in a dominant manner and that the regulatory sequence(s) for the gene is expressed in leaf trichomes when the gene is in a foreignNicotiana background.     

Extending Near Infrared Reflectance(NIR) Pulp Yield Calibrations to NewSites and Species

Abstract

Recently, we demonstrated that the accuracy of pulp yield predictions for wood samples from a site (Gog) new to their calibration (Tasmania‐wide Eucalyptus nitens) was greatly improved by adding five Gog samples to the calibration set. In this study we investigated the addition of Gog samples to the Tasmania‐wide E. nitens set, with the aim of further improving predictive accuracy. It was demonstrated that the addition of a single Gog sample to the Tasmania‐wide calibration set was sufficient to greatly reduce predictive errors and that the inclusion of at least 3 Gog samples in the Tasmania‐wide set was sufficient to give relatively stable predictive errors. The addition of different sets of 5 Gog samples to the Tasmania‐wide calibration, however, caused predictive errors to vary between sets. The standard deviation of pulp yield for the prediction set (20 Gog samples) was important, with sets having the largest standard deviations giving the best predictive statistics. Finally, the Tasmania‐wide E. nitens calibration was enhanced using samples from a different species (Eucalyptus globulus) and applied successfully to other E. globulus samples.     

Clade-Specific Plastid Inheritance Patterns Including Frequent Biparental Inheritance in Passiflora Interspecific Crosses

Plastid inheritance in angiosperms is presumed to be largely maternal, with the potential to inherit plastids biparentally estimated for about 20% of species. In Passiflora, maternal, paternal and biparental inheritance has been reported; however, these studies were limited in the number of crosses and progeny examined. To improve the understanding of plastid transmission in Passiflora, the progeny of 45 interspecific crosses were analyzed in the three subgenera: Passiflora, Decaloba and Astrophea. Plastid types were assessed following restriction digestion of PCR amplified plastid DNA in hybrid embryos, cotyledons and leaves at different developmental stages. Clade-specific patterns of inheritance were detected such that hybrid progeny from subgenera Passiflora and Astrophea predominantly inherited paternal plastids with occasional incidences of maternal inheritance, whereas subgenus Decaloba showed predominantly maternal and biparental inheritance. Biparental plastid inheritance was also detected in some hybrids from subgenus Passiflora. Heteroplasmy due to biparental inheritance was restricted to hybrid cotyledons and first leaves with a single parental plastid type detectable in mature plants. This indicates that in Passiflora, plastid retention at later stages of plant development may not reflect the plastid inheritance patterns in embryos. Passiflora exhibits diverse patterns of plastid inheritance, providing an excellent system to investigate underlying mechanisms in angiosperms.     

Behavioral Responses of a Generalist Mammalian Folivore to the Physiological Constraints of a Chemically Defended Diet

Mammalian herbivores, particularly browsers and folivores, encounter and consume a range of plant chemical defenses [plant secondary metabolites (PSMs)] on a regular basis. The physiological regulation of PSM ingestion and the resulting behavioral responses of mammalian herbivores directly affect their feeding decisions and the subsequent foraging strategies that they adopt. Generalist mammalian herbivores are hypothesized to consume a generalized diet because of physiological limitations of their detoxification systems. The consumption of a generalized diet is proposed to enable toxin (PSM) dilution through the use of multiple detoxification pathways. We tested the predictions of the detoxification–limitation hypothesis by offering two chemically different plant species, Eucalyptus regnans and E. globulus, to a generalist mammalian folivore, the common brushtail possum (Trichosurus vulpecula), as single- and mixed-species diets. By feeding more efficiently, brushtail possums benefited more, through increased intake, on the mixed-species diet than on either of the single-species diets. We argue that frequently switching between chemically diverse foliage reduces the physiological constraints imposed by a PSM-rich diet and enables more efficient feeding. The behavioral responses of brushtail possums were consistent with the proposed physiological constraints of a chemically defended diet, offering support for predictions of the detoxification–limitation hypothesis. We suggest that feeding behavior of herbivores may be a useful indicator of the physiological constraints imposed by a chemically defended diet.     

Stability of Genetic-Based Defensive Chemistry Across Life Stages in a <Emphasis Type="Italic">Eucalyptus</Emphasis> Species

Defensive chemistry is a key plant fitness trait, and the investigation of the expression of plant secondary metabolites across life stages is important in understanding the lifetime evolutionary selection pressures on a plant. The expression of genetic-based differences in foliar defensive chemistry, known to influence mammalian herbivore preferences, was studied across two contrasting life phases of the heteroblastic tree, Eucalyptus globulus. With plants from different subraces of E. globulus growing in a field trial, we compared the levels of seven chemical constituents in adult and juvenile foliage from related coppiced plants. Defensive chemistry was generally higher in more vulnerable coppice foliage than adult foliage. Significant, genetic-based differences among subraces were detected for two key defensive chemicals, a sideroxylonal and a macrocarpal, and these differences were stable across life phases. In contrast, significant differences among subraces in adult leaf condensed tannins were not evident in the coppice because of the absence of this group of tannins in this foliage. These findings lend support to hypotheses that suggest condensed tannins may have evolved for reasons other than mammalian herbivore defense.     

<Emphasis Type="Italic">Corymbia</Emphasis> Species and Hybrids: Chemical and Physical Foliar Attributes and Implications for Herbivory

Hybridization is an important biological phenomenon that can be used to understand the evolutionary process of speciation of plants and their associated pests and diseases. Interactions between hybrid plants and the herbivores of the parental taxa may be used to elucidate the various cues being used by the pests for host location or other processes. The chemical composition of plants, and their physical foliar attributes, including leaf thickness, trichome density, moisture content and specific leaf weight were compared between allopatric pure and commercial hybrid species of Corymbia, an important subtropical hardwood taxon. The leaf-eating beetle Paropsis atomaria, to which the pure taxa represented host (C. citriodora subsp. variegata) and non-host (C. torelliana) plants, was used to examine patterns of herbivory in relation to these traits. Hybrid physical foliar traits, chemical profiles, and field and laboratory beetle feeding preference, while showing some variability, were generally intermediate to those exhibited by parent taxa, thus suggesting an additive inheritance pattern. The hybrid susceptibility hypothesis was not supported by our field or laboratory studies, and there was no strong relationship between adult preference and larval performance. The most-preferred adult host was the sympatric taxon, although this species supported the lowest larval survival, while the hybrid produced significantly smaller pupae than the pure species. The results are discussed in relation to plant chemistry and physical characteristics. The findings suggest a chemical basis for host selection behavior and indicate that it may be possible to select for resistance to this insect pest in these commercially important hardwood trees.     

Preparation and applications of novel fluoroalkyl end‐capped sulfonic acid oligomers–silica gel polymer hybrids

Fluoroalkyl end‐capped 2‐methacryloxyethanesulfonic acid homo‐oligomer [R_F–(MES)_n–R_F] and 2‐methacryloxyethanesulfonic acid–N,N‐dimethylacrylamide co‐oligomers [R_F–(MES)_x–(DMAA)_y–R_F] reacted with tetraethoxysilane (TEOS) under acidic conditions to afford R_F–(MES)_n–R_F homo‐oligomer–SiO₂ polymer hybrid and R_F–(MES)_x–(DMAA)_y–R_F co‐oligomer–SiO₂ polymer hybrid, respectively. Thermogravimetric–mass spectra showed that the thermal stability of R_F–(MES)_n–R_F homo‐oligomer–SiO₂ polymer hybrid was superior to that of traditionally well‐known perfluorinated ion exchange polymers such as Nafion 112 (TR). The sol solutions of the fluorinated co‐oligomer–SiO₂ polymer hybrid were applied to the surface modification of glass to exhibit not only a strong oleophobicity imparted by fluorine but also a good hydrophilicity on the glass surface. On the other hand, R_F–(MES)_x–(DMAA)_y–R_F co‐oligomer reacted with TEOS in the presence of a variety of silica nanoparticles (mean diameters: 11–95 nm) under alkaline conditions to afford fluoroalkyl end‐capped oligomers–silica nanoparticles (mean diameters: 32–173 nm) with a good dispersibility and stability in methanol. Similarly, a variety of fluorinated oligomers containing sulfo groups–silica nanoparticles were prepared by the homo‐ and co‐oligomerizations of fluoroalkanoyl peroxides with 2‐methacryloxyethane sulfonic acid (MES) and comonomers such as N,N‐dimethylacrylamide (DMAA) and acryloylmorpholine (ACMO) in the presence of silica nanoparticles. Interestingly, these isolated fluorinated particle powders were found to afford nanometer size‐controlled colloidal particles with a good redispersibility and stability in aqueous and organic media such as methanol. These fluorinated nanoparticles containing sulfo groups were also applied to an excellent heterogeneous catalyst for Bronsted acid‐catalyzed transformations. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 110–117, 2007     

Leaf-to-Whole Plant Spread Bioassay for Pepper and Ralstonia solanacearum Interaction Determines Inheritance of Resistance to Bacterial Wilt for Further Breeding

Bacterial wilt (BW) disease from Ralstonia solanacearum is a serious disease and causes severe yield losses in chili peppers worldwide. Resistant cultivar breeding is the most effective in controlling BW. Thus, a simple and reliable evaluation method is required to assess disease severity and to investigate the inheritance of resistance for further breeding programs. Here, we developed a reliable leaf-to-whole plant spread bioassay for evaluating BW disease and then, using this, determined the inheritance of resistance to R. solanacearum in peppers. Capsicum annuum ‘MC4′ displayed a completely resistant response with fewer disease symptoms, a low level of bacterial cell growth, and significant up-regulations of defense genes in infected leaves compared to those in susceptible ‘Subicho’. We also observed the spreading of wilt symptoms from the leaves to the whole susceptible plant, which denotes the normal BW wilt symptoms, similar to the drenching method. Through this, we optimized the evaluation method of the resistance to BW. Additionally, we performed genetic analysis for resistance inheritance. The parents, F₁ and 90 F₂ progenies, were evaluated, and the two major complementary genes involved in the BW resistance trait were confirmed. These could provide an accurate evaluation to improve resistant pepper breeding efficiency against BW.     

Mechanisms of resistance in wild riceOryza brachyantha to rice leaffolderCnaphalocrocis medinalis (Guenée) (Lepidoptera: Pyralidae)

A wild rice,Oryza brachyantha, was rated as highly resistant to rice leaffolder,Cnaphalocrocis medinalis (Guenée), while its F₁ hybrid from a cross with a high-yielding, leaffolder-susceptible rice, IR31917-45-3-2, was rated resistant in a standard screening test. In comparison with IR31917-45-3-2,O. brachyantha and the F₁ hybrid were unsuitable for oviposition, survival, and growth ofC. medinalis. However, growth and survival of larvae in artificial diets containing lyophilized leaf powder of IR31917-45-3-2 andO. brachyantha were comparable. Studies on the feeding and settling preference of the larval stages suggested that the mechanism of resistance ofO. brachyantha is that of antixenosis type. Olfactometer tests with first-instar larvae and electroantennogram responses of adults indicated a lack or low levels of volatile attractants inO. brachyantha and the F₁ hybrid in comparison with IR31917-45-3-2. Bioassays of sequential solvent extracts of these plants indicated that larval preference for IR31917-45-3-2 and nonpreference forO. brachyantha and the F₁ hybrid were due partly to chemical factors present in hexane and methylene chloride extractables. Closer arrangement of silica cells in the epidermal layer ofO. brachyantha and a higher mandibular wear in larvae reared onO. brachyantha suggested that physical resistance due to silica may be an additional cause of resistance. It was hypothesized that the high levels of resistance observed inO. brachyantha may be due to an additive or synergistic action of the absence of attractants or feeding stimulants, the presence of deterrents, and the physical resistance offered by silica. The significance of these results to a successful wide hybridization program aimed at transferring resistance factors from wild rice to cultivated rice is discussed.     

Detoxification of Gramine by the Cereal Aphid <Emphasis Type="Italic">Sitobion avenae</Emphasis>

Secondary metabolites play an important role in host plant resistance to insects, and insects, in turn, may develop mechanisms to counter plant resistance mechanisms. In this study, we investigated the toxicity of gramine to the cereal aphid Sitobion avenae and some enzymatic responses of S. avenae to this alkaloid. When S. avenae fed on an artificial diet containing gramine, mortality occurred in a dose-dependent manner. The LC₅₀ of gramine was determined to be 1.248 mM. In response to gramine, S. avenae developed increased activities of carboxylesterase and glutathione S-transferase, two important detoxification enzymes. The activities of both enzymes were positively correlated with the concentration of dietary gramine. In addition, the activities of peroxidase and polypheolic oxidase, two important oxidoreductase enzymes in S. avenae, increased in response to gramine; however, catalase activity decreased when insects were exposed to higher levels of dietary gramine. The potential role of gramine in host plant resistance and S. avenae counter-resistance is discussed.     

Modification of polyethylene–octene elastomer by silica through a sol–gel process

In this study, tetraethoxysilane (TEOS) and a metallocene polyethylene–octene elastomer (POE) were chosen as the ceramic precursor and the continuous phase, respectively, for the preparation of new hybrids by an in situ sol–gel process. To obtain a better hybrid, a maleic anhydride‐grafted polyethylene–octene elastomer (POE‐g‐MAH), used as the continuous phase, was also investigated. Characterizations of POE‐g‐MAH/SiO₂ and POE/SiO₂ hybrids were performed by Fourier transform infrared (FTIR) and ²⁹Si solid‐state nuclear magnetic resonance (NMR) spectrometers, a differential scanning calorimeter (DSC), a thermogravimetry analyzer, and an Instron mechanical tester. The results showed that the POE‐g‐MAH/SiO₂ hybrid could improve the properties of the POE/SiO₂ hybrid because the interfacial force between the polymer matrix and the silica network was changed from hydrogen bonds into covalent Si? O? C bonds through dehydration of hydroxy groups in POE‐g‐MAH with residual silanol groups in the silica network. The existence of covalent Si? O? C bonds was proved by FTIR spectra. For the POE/SiO₂ and POE‐g‐MAH/SiO₂ hybrids, maximum values of the tensile strength and the glass transition temperature were found at 9 wt % SiO₂ since a limited content of silica might be linked with the polymer chains through the covalent bond. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 966–972, 2003     

Selected chemical constituents and deer browsing preference of Douglas fir

Douglas fir foliage from eight full-sib F₁ families was analyzed for chlorogenic acid and essential oils and ranked according to browsing preference of penned black-tailed deer. Foliage was obtained during the dormant season, and tissues for both the preference test and chemical analyses were collected from the same trees. Deer ranked the different families, and chlorogenic acid content was correlated with preference order. Families varied significantly in yield and composition of essential oils, but differences were not related to preference. Results may have application in selections for Douglas fir resistant to browsing.     

Organic–inorganic hybrid materials. II. Chain mobility and stability of polysiloxaneimide–silica hybrids

Polysiloxaneimide–silica hybrid materials (PSI‐SiO₂) were obtained using the sol–gel technique by polycondensation of tetramethoxysilane (TMOS) in a polyamic acid solution. IR, ²⁹Si‐ and ¹³C‐NMR spectroscopy, and thermogravimetric analysis were used to study hybrids containing various proportions of TMOS and hydrolysis ratios. The morphology, dynamics, and thermal stability of the hybrids were investigated. The chain mobility of the hybrids was investigated by spin–spin relaxation time (T₂) measurements. The apparent activation energy (E_a) for degradation of the hybrids in air was studied by the van Krevelen method. The T₂ value was independent of the silica content whereas that of the E_a decreased as silica content increased. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 965–973, 2001