Adult Plant Response Research Articles

Yield gains of coffee plants from phosphorus fertilization may not be generalized for high density planting

Inconclusive responses of the adult coffee plant to phosphorus fertilization have been reported in the literature, especially when dealing with application of this nutrient in high density planting systems. Thus, this study was carried out for the purpose of assessing the response of adult coffee plants at high planting density in full production (in regard to yield and their biennial cycle/stability) to the addition of different sources and application rates of P in the Zona da Mata region of Minas Gerais, Brazil. The experiment with coffee plants of the Catucaí Amarelo 6/30 variety was carried out over four growing seasons. Treatments were arranged in a full factorial design [(4 × 3) + 1] consisting of four P sources (monoammonium phosphate, simple superphosphate, natural reactive rock phosphate from Algeria (Djebel-Onk), and FH 550®), three P rates (100, 200, and 400 kg ha-1 year-1 of P2O5), and an additional treatment without application of the nutrient (0 kg ha-¹ year-¹). A randomized block experimental design was used with three replicates. The four seasons were evaluated as subplots in a split plot experiment. The P contents in soil and leaves increased with increased rates of P application. However, there was no effect from P application on the yield and its biennial cycle/stability regardless of the source used over the four seasons assessed.

Postulation of resistance genes and assessment of adult plant response variation for stripe rust in three international wheat nurseries

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is an important disease of wheat in cooler and humid geographic regions. It can cause significant yield losses. Three international nurseries, namely, 1st Australian Special Nursery (ASN), 22nd Semi-Arid Wheat Screening Nursery (SAWSN) and 12th High Temperature Wheat Yield Trial (HTWYT), were evaluated for seedling and adult plant response variation for stripe rust. A low proportion of these nurseries displayed stripe rust resistance in the seedling stage.Fifty one percent of 1st ASN, 54% 22nd SAWSN and 17% 12th HTWYT entries did not carry any seedling resistance genes. High level of APR to stripe rust was expressed in the field in all three nurseries. Stripe rust resistance genes Yr6,Yr7,Yr9, Yr17 and Yr27 were postulated either singly or in combinations across the three nurseries. Some entries carried these genes in combination with uncharacterised resistance. Based on Lr34 and Sr2 linked markers, Yr18 and Yr30 are inferred to be present in a high proportion of entries in all three nurseries. Due to the lack of robust marker, we were unable to postulate Yr29, this gene is however expected to be present in Pavon 76 derivatives. Results of this study can be used for strategic deployment of stripe rust resistance in new wheat cultivars.

Directed dimerization: an in vivo expression system for functional studies of type II phytochromes

Type II phytochromes (phy) in Arabidopsis form homodimers and heterodimers, resulting in a diverse collection of light-stable red/far-red (R/FR) sensing photoreceptors. We describe an in vivo protein engineering system and its use in characterizing the activities of these molecules. Using a phyB null mutant background, singly and doubly transgenic plants were generated that express fusion proteins containing the phyB-phyE N-terminal photosensory regions (NB-NE PSRs), a nuclear localization sequence, and small yeast protein domains that mediate either homodimerization or heterodimerization. Activity of NB/NB homodimers but not monomeric NB subunits in control of seedling and adult plant responses to R light is demonstrated. Heterodimers of the NB sequence with the chromophoreless NB(C357S) sequence, which mimic phyB Pfr/Pr photo-heterodimers, mediate R sensitivity in leaves and petioles but not hypocotyls. Homodimerization of the NC, ND and NE sequences and directed heterodimerization of these photosensory regions with the NB region reveal form-specific R-induced activities for different type II phy dimers. The experimental approach developed here of directed assembly of defined protein dimer combinations in vivo may be applicable to other systems.

First Report of Resistance of Wheat Line Avocet 'S' to Stripe Rust Caused by Puccinia striiformis f. sp. tritici (Pst) in Syria.

Seedling assays and field-based assessments are common approaches used in monitoring pathogenic variability of Puccinia striiformis f. sp. tritici (Pst). Due to the many candidate genes for resistance, and the occasional presence of uncharacterized or unexpected resistance genes, interpretation of seedling and adult plant responses of stripe rust differential genotypes can be confounded. To facilitate data interpretation and overcome growth habit differences, a set of near-isogenic lines (NILs) based on a susceptible selection of the widely susceptible Australian spring wheat cultivar Avocet was developed in Plant Breeding Institute, University of Sydney. During the 2011 rust survey of major wheat growing areas in Syria, we collected and recovered two isolates, one from bread wheat and one from durum, using susceptible cultivar Morocco. Fresh urediniospores collected from inoculated seedlings of cultivar Morocco under greenhouse conditions were used to infect a differential set of seedlings comprising the Avocet S NIL series with Yr1, 5, 6, 7, 8, 9, 10, 15, 17, 27, 32, A, SP and a supplementary set of Kalyansona (Yr2), Fielder (Yr6, Yr20), Thatcher (Yr7), Federation*4/Kavkaz (Yr9), TP1295 and TP981 (Yr25), Ciano 79 and Opata 85 (Yr27), Lemhi (Yr21), Anza (YrA), Tres/6*Avocet 'S', Cook, Avocet S, and Federation following standard protocol (1). Seedling infection types (IT) were recorded 15 to 17 days post-inoculation using a 0, ;(fleck), 1 to 4 scale, with ITs 3 and 4 considered to be high. According to European race nomenclature (1), these isolates would be classified as 6 E0 and assumes that differential cultivars Lee (Yr7) and Heines Kolben should be susceptible. A low infection of ;N (necrotic areas without sporulation) was observed on Avocet S and Avocet NILs Yr1, 5, 8, 10, 15, 17, 32, A, and SP. NILs Yr6, 7, and 9 responded with high infection types. Among the supplementary set, IT ;N was recorded for Anza, Cook, Tres/6*Avocet S, Opata 85, and Ciano 79 whereas IT 3+4 occurred on Kalyansona, Fielder, Thatcher, Federation*4/Kavkaz, TP1295 and TP981, and Lemhi. Federation and Morocco were susceptible to both isolates. Based on these data we concluded that 1) Avocet S carries at least one effective gene for resistance to the two isolates (temporarily designated as YrAvS and perhaps the same as the gene(s) in Anza and Cook); and 2) since Avocet S was resistant, it was expected that all the NILs should also be resistant unless one of the selected resistance genes was closely linked in repulsion with the gene in Avocet S. Since such a situation is possible for only one line, other explanations are that the recurrent parent, Avocet S, was not genetically homogeneous, or more likely, that the genetic background of Avocet S was not fully regenerated during the six backcross and subsequent selfing generations. This work illustrates the problems that can arise when sets of differential genotypes that perform in a satisfactory way in a particular (limited) geographical area are applied globally. Pst races carrying avirulence genes that are not present in the original area where a differential cultivar is characterized for its susceptibility may be present in other areas. Consequently, a differential cultivar which was susceptible in the original area may unexpectedly exhibit resistance in a new region. Therefore, host lines considered to be "universally susceptible" in one region may not be susceptible globally. The resistance of Avocet S is yet to be characterized. Reference: (1) R. Johnson et al. Trans. Br. Mycol. Soc. 58:475, 1972.

A new variant 5R9-7 of Puccinia triticina on emmer and durum wheats in India

Emmer and durum wheats are generally resistant to leaf (brown) rust in many countries including India, where pathotypes specialized to these wheats rarely occur. During 2006, widespread prevalence of leaf (brown) rust was recorded on Emmer wheat. A new pathotype, 5R9-7(DBBGB), of Puccinia triticina was associated with leaf rust on many of the popular cultivars, which was avirulent to most of the bread wheat accessions tested; including Agra Local, a susceptible host to wheat rusts in India. Pathotype 5R9-7(DBBGB) was found to be virulent to Lr2c, Lr14a, Lr18 and Lr20. Resistance sources to this pathotype based on evaluation of Indian and exotic wheat germplasm, its avirulence/virulence structure, adult plant response of important tetraploid wheat material and other attributes are described.

Identification and mapping of leaf, stem and stripe rust resistance quantitative trait loci and their interactions in durum wheat

Leaf rust (Puccinia triticina Eriks.), stripe rust (Puccinia striiformis f. tritici Eriks.) and stem rust (Puccinia graminis f. sp. tritici) cause major production losses in durum wheat (Triticum turgidum L. var. durum). The objective of this research was to identify and map leaf, stripe and stem rust resistance loci from the French cultivar Sachem and Canadian cultivar Strongfield. A doubled haploid population from Sachem/Strongfield and parents were phenotyped for seedling reaction to leaf rust races BBG/BN and BBG/BP and adult plant response was determined in three field rust nurseries near El Batan, Obregon and Toluca, Mexico. Stripe rust response was recorded in 2009 and 2011 nurseries near Toluca and near Njoro, Kenya in 2010. Response to stem rust was recorded in field nurseries near Njoro, Kenya, in 2010 and 2011. Sachem was resistant to leaf, stripe and stem rust. A major leaf rust quantitative trait locus (QTL) was identified on chromosome 7B at Xgwm146 in Sachem. In the same region on 7B, a stripe rust QTL was identified in Strongfield. Leaf and stripe rust QTL around DArT marker wPt3451 were identified on chromosome 1B. On chromosome 2B, a significant leaf rust QTL was detected conferred by Strongfield, and at the same QTL, a Yr gene derived from Sachem conferred resistance. Significant stem rust resistance QTL were detected on chromosome 4B. Consistent interactions among loci for resistance to each rust type across nurseries were detected, especially for leaf rust QTL on 7B. Sachem and Strongfield offer useful sources of rust resistance genes for durum rust breeding.

The response of genetically distinct bread wheat genotypes to salinity stress

With 4 figures and 3 tables AbstractSoil salinity reduces crop yield in many areas of the world. Ninety‐eight hexaploid wheat inbreds differing in geographic origin and belonging to six diverse adaptation groups were studied in control, EC‐10 and EC‐15 dS/m treatments to evaluate variability for salt tolerance and to assess the usefulness of early growth stage salinity assessment in the determination of adult plant salinity response. Significant differences (P ≤ 0.05) were observed among the 98 inbreds, three NaCl treatments and their interactions for relative growth rate (RGR), biomass, fertile tillers, grains/spike, 100‐grain weight and grain yield. A range of responses were observed with an increasing salinity for most characters among the inbreds tested. The three evaluation systems (solution culture, raised beds in the field with added salt, naturally saline field conditions) were effective in assessing variability for salt tolerance in wheat germplasm. Assessment of salinity tolerance at the seedling stage correlated well with adult plant response under field conditions, and several highly stable sources of salinity tolerance were identified.

Microsatellite mapping identifies TTKST-effective stem rust resistance gene in wheat cultivars VL404 and Janz

Wheat cultivar VL404 carries seedling resistance to Puccinia graminis f. sp. tritici pathotype TTKST. Monogenic segregation for seedling resistance was observed in a VL404/WL711 recombinant inbred line population and the resistance locus was temporarily designated SrVL. Bulked segregant analysis using Diversity Arrays Technology markers located SrVL on chromosome 2BL. Detailed simple sequence repeat mapping placed SrVL between gwm120 and wmc175, both at genetic distances of 3.3 cM. Based on adult plant responses of Janz and VL404 in India and Kenya, we expected these cultivars to carry the same gene against TTKST. A subset of Diamondbird/Janz doubled haploid (DH) population showed monogenic segregation, when tested against TTKST and the locus was temporarily named SrJNZ. SrVL-linked markers gwm120 and wmc175 flanked SrJNZ at a similar genetic distance, thereby confirming our hypothesis. Chromosome 2BL carries Sr9, Sr16 and Sr28. Sr9 is a multi-allelic locus and all known alleles of Sr9 and Sr16 are ineffective against TTKSK and its derivatives. A recombination value of 16.7 cM between Sr9g-linked stripe rust resistance gene Yr7 and SrJNZ in Diamondbird/Janz DH population suggested that SrJNZ is not an allele at the Sr9 locus. Based on comparison of published genetic distances between Lr13,Sr9, Sr28 and Sr16 with that observed in this study, we concluded SrVL and SrJNZ to be Sr28. This gene was contributed by a common parent Gabo, which also exhibited resistance against TTKST. Sr28-linked markers gwm120 and wmc175 confirmed the presence of this gene in a high proportion of Australian cultivars that showed stem rust resistance in Kenya. These markers can be used for marker-assisted pyramiding of Sr28 with other stem rust resistance genes.

Drought, climate change and vegetation response in the succulent karoo, South Africa

For the winter-rainfall region of South Africa, the frequency of drought is predicted to increase over the next 100 years, with dire consequences for the vegetation of this biodiversity hotspot. We analysed historical 20th century rainfall records for six rainfall stations within the succulent karoo biome to determine if the signal of increasing drought frequency is already apparent, and whether mean annual rainfall is decreasing. We found no evidence for a decrease either in mean annual rainfall or in the incidence of drought, as measured by the Standardised Precipitation Index (SPI) over the 20th century. Evidence points to a drying trend from 1900–1950 while no significant trend in rainfall and drought was found at most stations from 1951–2000. In a second analysis we synthesised the information concerning the response of adult succulent karoo biome plants and seedlings to extended drought conditions. General findings are that responses to drought differ between species, and that longevity is an important life history trait related to drought survival. Growth form is a poor predictor of drought response across the biome. There was a range of responses to drought among adult plants of various growth forms, and among non-succulent seedlings. Leaf-succulent seedlings, however, exhibited phenomenal drought resistance, the majority surviving drought long after all the experimentally comparative non-succulent seedlings had died. Our synthesis showed that previous studies on the impact of drought on succulent karoo biome plants differ greatly in terms of their location, sampling design, measured values and plant responses. A suite of coordinated long-term field observations, experiments and models are therefore needed to assess the response of succulent karoo biome species to key drought events as they occur over time and to integrate this information into conservation planning. : biodiversity hotspot, mortality, population dynamics, recruitment, Standardised Precipitation Index

Drought, climate change and vegetation response in the succulent karoo, South Africa

For the winter-rainfall region of South Africa, the frequency of drought is predicted to increase over the next 100 years, with dire consequences for the vegetation of this biodiversity hotspot. We analysed historical 20th century rainfall records for six rainfall stations within the succulent karoo biome to determine if the signal of increasing drought frequency is already apparent, and whether mean annual rainfall is decreasing. We found no evidence for a decrease either in mean annual rainfall or in the incidence of drought, as measured by the Standardised Precipitation Index (SPI) over the 20th century. Evidence points to a drying trend from 1900–1950 while no significant trend in rainfall and drought was found at most stations from 1951–2000. In a second analysis we synthesised the information concerning the response of adult succulent karoo biome plants and seedlings to extended drought conditions. General findings are that responses to drought differ between species, and that longevity is an important life history trait related to drought survival. Growth form is a poor predictor of drought response across the biome. There was a range of responses to drought among adult plants of various growth forms, and among non-succulent seedlings. Leaf-succulent seedlings, however, exhibited phenomenal drought resistance, the majority surviving drought long after all the experimentally comparative non-succulent seedlings had died. Our synthesis showed that previous studies on the impact of drought on succulent karoo biome plants differ greatly in terms of their location, sampling design, measured values and plant responses. A suite of coordinated long-term field observations, experiments and models are therefore needed to assess the response of succulent karoo biome species to key drought events as they occur over time and to integrate this information into conservation planning.

Characterization of Seedling Infection Types and Adult Plant Infection Responses of Monogenic Sr Gene Lines to Race TTKS of Puccinia graminis f. sp. tritici.

Stem rust, caused by Puccinia graminis f. sp. tritici, historically was one of the most destructive diseases of wheat and barley. The disease has been under effective control worldwide through the widespread use of host resistance. A number of stem rust resistance genes in wheat have been characterized for their reactions to specific races of P. graminis f. sp. tritici. Adult plant responses to race TTKS (also known as Ug99) of monogenic lines for Sr genes, a direct measurement of the effectiveness for a given gene, have not been investigated to any extent. This report summarizes adult plant infection responses and seedling infection types for monogenic lines of designated Sr genes challenged with race TTKS. High infection types at the seedling stage and susceptible infection responses in adult plants were observed on monogenic lines carrying Sr5, 6, 7a, 7b, 8a, 8b, 9a, 9b, 9d, 9g, 10, 11, 12, 15, 16, 17, 18, 19, 20, 23, 30, 31, 34, 38, and Wld-1. Monogenic lines of resistance genes Sr13, 22, 24, 25, 26, 27, 28, 32, 33, 35, 36, 37, 39, 40, 44, Tmp, and Tt-3 were effective against TTKS both at the seedling and adult plant stages. The low infection types to race TTKS observed for these resistance genes corresponded to the expected low infections of these genes to other incompatible races of P. graminis f. sp. tritici. The level of resistance conferred by these genes at the adult plant stage varied between highly resistant to moderately susceptible. The results from this study were inconclusive for determining the effectiveness of resistance genes Sr9e, 14, 21, and 29 against race TTKS. The understanding of the effectiveness of individual Sr genes against race TTKS will facilitate the utilization of these genes in breeding for stem rust resistance in wheat.

Relationship between Brassica napus seedling and adult plant responses to Leptosphaeria maculans is determined by plant growth stage at inoculation and temperature regime

Contradictory reports exist in the literature on the relationship of seedling and adult plant resistance to blackleg disease in Brassica napus (oilseed rape, canola) caused by Leptosphaeria maculans. Two repeat experiments were carried out to determine whether adult plant responses in certain cultivars of B. napus to a specific strain of L. maculans are evident in the response of the plants at the seedling stage and to evaluate whether the relationship between seedling and adult plant responses to L. maculans is determined by the growth stage of the plant at inoculation and the temperature of incubation prior to field planting. Point inoculations were made with pycnidiospores (conidia) on to cotyledons or youngest leaves of four B. napus cultivars (Westar, Surpass 400, Dunkeld and Grouse) at different growth stages under two temperature regimes (18/24 and 11/18 °C) which were rated for cotyledon or leaf lesions before they were planted out in the field and the severity of subsequent crown cankers assessed. Results across the four cultivars showed there was a significant effect of plant growth stage at inoculation ( P < 0.001), and cultivars ( P < 0.001) and a significant interaction between growth stage and cultivars ( P < 0.001) in relation to cotyledon lesions, leaf lesions, and crown cankers, with both temperature regimes. Overall, the cultivars differed in resistance to the strain tested in relation to cotyledon, leaf and crown canker symptoms, with cv. Dunkeld more resistant than Grouse > Surpass 400 > Westar. Different temperature regimes during inoculation, and for a minimum of 4 weeks post-inoculation, had significant effects on the growth rate of the plant as well as the development of disease symptoms. There was a high degree of positive correlation between the severity of cotyledon or leaf lesions with subsequent crown canker that developed for inoculations at nearly all growth stages at the 18/24 °C regime and the early growth stages (particularly up to and including growth stage 1,01, the first true leaf exposed) at the 11/18 °C regime. Overall, plant growth stage at inoculation clearly determined the expression of seedling and/or adult plant resistance. For all cultivars tested, at the higher temperature regime, expression of resistance between seedlings and adult plants, for nearly all growth stages at inoculation, could not be distinguished. In contrast, at the lower temperature regime, this relationship did not exist for inoculations carried out after growth stage 1,05 (the fifth true leaf exposed). These findings may explain the variable results obtained in previous studies attempting to define ‘adult plant resistance’ to blackleg and also indicates the need for standardizing screening processes for identification of host resistance.

A NEIGHBORHOOD ANALYSIS OF TREE GROWTH AND SURVIVAL IN A HURRICANE‐DRIVEN TROPICAL FOREST

We present a likelihood‐based regression method that was developed to analyze the effects of neighborhood competitive interactions and hurricane damage on tree growth and survival. The purpose of the method is to provide robust parameter estimates for a spatially explicit forest simulator and to gain insight into the processes that drive the patterns of species abundance in tropical forests. We test the method using census data from the 16‐ha Luquillo Forest Dynamics Plot in Puerto Rico and describe effects of the spatial configuration, sizes, and species of neighboring trees on the growth and survival of 12 dominant tree species representing a variety of life history strategies. Variation in size‐dependent growth and mortality suggests a complex relationship between size, growth, and survival under different regimes of light availability. Crowding effects on growth and survival appear to be idiosyncratic to each individual species, and with the exception of pioneers, there is little commonality among species that share similar life histories.We also explain the implications of differential susceptibility to hurricane damage on species' growth and survival and on their ability to respond to damage to neighboring trees. Tree species in the Luquillo Forest Dynamics Plot differ strikingly in both their susceptibility to hurricane disturbance and the nature of their recovery from wind disturbance, through response of both adult plants and juveniles to enhanced resource availability. At the stand level, intense competitive thinning of densely packed saplings that grew after hurricane damage accounted for the majority of post‐hurricane mortality, particularly for shade‐intolerant species. At the individual species level, effects of previous hurricane damage on growth and survival depended primarily on variation in the quantity and quality of hurricane damage sustained by target species and their interaction with life history characteristics of these individual species.Finally, we compare models that make different assumptions about the effects of competing species on tree growth and survival (e.g., equivalence of competitors vs. distinct species‐specific effects). Size effects alone could not account for growth and survival for the majority of target species. Our results also demonstrate that competing species have distinct per capita effects on growth of dominant target species. In contrast, we found moderate support for a model that assumed functional equivalence of competitors on survival.

Mapping and validation of the genes for resistance to Pyrenophora teres f. teres in barley (Hordeum vulgare L.)

Identification and deployment of disease resistance genes are key objectives of Australian barley breeding programs. Two doubled haploid (DH) populations derived from Tallon × Kaputar (TK) and VB9524 × ND11231 (VN) crosses were used to identify markers for net type net blotch (NTNB) (Pyrenophora teres f. teres). The maps included 263 and 250 markers for TK and VN populations, respectively. The TK population was screened with 5 pathotypes and the VN population with 1 pathotype of NTNB as seedlings in the glasshouse. In addition, the TK population was subjected to natural infection in the field at Hermitage Research Station, Qld. Analyses of the markers were performed using the software packages MapManager and Qgene. One region on chromosome 6H was strongly associated with resistance to NTNB in both populations (R2 = 83% for TK and 66% for VN). In the TK population, 2 more quantitative trait loci (QTLs) were identified on chromosomes 2H and 3H, with R2 values of 30% and 31%, respectively. These associations were consistent over all pathotypes studied during the seedling stage. The same QTL on chromosome 6H was also found to be highly significantly associated (R2 = 65%) with the adult plant (field) response in the TK population. There are several very closely linked markers showing strong associations in these regions. Association of the 4 markers on chromosome 6H QTL with resistance to the NTNB has been validated in 2 other DH populations derived from barley crosses Pompadour × Stirling and WPG8412 × Stirling. These markers present an opportunity for marker assisted selection of lines resistant to NTNB in barley breeding programs.

Leaf Rust Resistance of Spring, Facultative, and Winter Wheat Cultivars from China.

Leaf rust, caused by Puccinia triticina, is an important disease of wheat (Triticum aestivum) in China. Sixty-one spring and 102 facultative or winter growth habit wheat cultivars from China and a set of testers, carrying named Lr genes, were evaluated for resistance at the seedling growth stage with an array of Mexican Puccinia triticina races. Variation in seedling infection types of the cultivars was compared with that of the testers, and genes conferring low infection types were postulated. In total, nine named genes, Lr1 (in 13 cultivars), Lr3 (12), Lr3bg (2), Lr10 (1), Lr13 (4), Lr14a (1), Lr16 (49), Lr23 (9), and Lr26 (81), were identified. Thirty-one cultivars displayed intermediate reactions to one or more races that could not be attributed to any named gene. Twenty-eight spring cultivars were also evaluated at two field sites in Mexico using two common races. About half of them displayed good to moderate adult resistance that may be partly due to the presence of slow rusting gene Lr34 in at least seven cultivars. Diversity in adult plant responses of these wheats indicated the presence of additional slow rusting genes. Presence of 1B.1R translocation in 12 wheat cultivars, supposedly derived from intergeneric crosses involving T. durum, Haynaldia villosa, and Avena fatua, indicated that their pedigrees were incorrect.

Adventitious root initiation, plasticity, and response to plant growth regulator treatments of seedling, juvenile, and adult Elaeocarpus hookerianus plants

We characterise the propagation success of cuttings from juvenile and adult plants of the New Zealand endemic tree Elaeocarpus hookerianus, and describe the response of seedling, juvenile, and adult plants to various environmental and plant growth regulator treatments. Unexpectedly, more adult cuttings formed roots than juveniles when cuttings were taken during winter, but this was reversed when cuttings were taken during summer. Juvenile plants displayed more variability in leaf properties in response to shaded or glasshouse environments, and to gibberellic acid, benzyladenine, or paclobutrazol treatments than seedlings or adult plants. Juvenile plants displayed larger episodic changes in internode length along the vertical axis than seedling or adult plants. No substantial maturation of juvenile plants or rejuvenation of adult plants was observed following any of the treatments. Age of the shoot appeared to be more important than the size of the plant for control of the transition between juvenile and adult phases, and for the expression of morphological and physiological properties characteristic of each on‐togenetic phase.

Resistance to stripe rust in durum wheats, A-genome diploids, and their amphiploids

Stripe rust (caused by Puccinia striiformis Westend.) is a wheat disease of worldwide importance. Seedlings of 75 accessions of Triticum boeoticum, 12 of T. monococcum, 16 of T. urartu, 230 of durum wheat (T. turgidum L. var. durum), and 128 amphiploids (genome AAAABB) involving the crosses of the three diploid species (AA) with T. turgidum (AABB) were evaluated in the greenhouse for their reaction to P. striiformis race 14E14. Durum wheats and the amphiploids were also evaluated at two field locations in Mexico with the same race for their adult plant response. Resistant seedling reactions (infection types: 0-3 on a 0-9 scale) were seen for 10 (13%) accessions of T. boeticum, 19 (8%) accessions of T. turgidum and 32 (25%) amphiploids. The remaining accessions were either moderately resistant (ITs 4-6) or susceptible (ITs 7-9). The three amphiploids derived from the crosses of seedling resistant T. boeoticum and T. turgidum, were resistant as seedlings. Among the 51 amphiploids involving one resistant parent, 29 were resistant and the remaining 22 displayed intermediate to susceptible reactions. Suppressors for resistance were common in the A and AB genomes and suppression was resistance gene specific. Forty-five (20%) durums showed adequate field resistance (relative AUDPC <10% of the susceptible check ‘Morocco’). These included the 19 seedling resistant durums. Presence of genes involved in adult plant resistance was evident, because 26 of the remaining adult plant resistant durums had displayed intermediate-susceptible seedling reactions. Though the seedling reactions of the amphiploids varied from low to high, all involving the adult plant resistant durums possessed adequate field resistance. The resistant, newly produced, AAAABB amphiploids are useful genetic resources for stripe rust resistance which could be transferred to the cultivated T. turgidum.

Genes conferring low seedling reaction to Mexican pathotypes of Puccinia recondita f. sp. tritici, and adult-plant responses of recent wheat cultivars from the former USSR

Fifty-five spring bread wheat (Triticum aestivum L.) cultivars, mostly released between 1975 and 1991 in eight leaf rust-prone spring wheat growing regions of the former USSR, were tested in the seedling growth stage for reaction to 15 Mexican pathotypes of Puccinia recondita f. sp. tritici. In total, seven known and at least two unknown genes were identified, either singly or in combinations: Lr3 (7 cultivars), Lr10 (14), Lr13 (5), Lr14a (1), Lr16 (1), Lr23 (3); the unknown genes were identified in 14 cultivars. The first unknown gene could be either Lr9, Lr19, or Lr25; however, the second unknown gene in 9 cultivars was different from any named gene. Twelve of the 15 pathotypes are virulent for this gene, hence its use in breeding for resistance will be limited. The cultivars were also evaluated at two field locations in Mexico with two pathotypes in separate experiments. The area under the disease progress curve and the final disease rating of the cultivars indicated genetic diversity for genes conferring adult plant resistance. based on the symptoms of the leaf tip necrosis in adult plants, resistance gene Lr34 could be present in at least 20 cultivars. More than half of the cultivars carry high to moderate levels of adult plant resistance and were distributed in each region.

Host-pathogen interactions in natural populations of Linum marginale and Melampsora lini : III. Influence of pathogen epidemics on host survivorship and flower production.

The epidemiology of rust caused by the fungus Melampsora lini and the effects of infection by this pathogen on its host, the herbaceous perennial Linum marginale, were determined in the field and in garden experiments. There was considerable natural variability in disease levels over the four years (1986-1989) of the study. In two years (1986, 1989) major rust epidemics occurred. In the field, the main effect of disease was to reduce survivorship during the winter following infection. Plants which were heavily infected during the 1986 or 1989 growing seasons had reduced survivorship relative to more lightly infected plants. Melampsora lini infections did not appear to affect survivorship in either 1987 or 1988. Flowering was correlated with environmental factors and the number of stems a plant possessed. A severe drought in 1987 completely inhibited flowering. In the other three years the number of flowers produced by a plant was strongly positively correlated with the number of stems it possessed. Disease levels had no consistent effect on flowering. Controlled garden experiments were also used to examine the response of seedlings and adult plants to infection. These showed that both the timing and severity of disease appears to determine the effect of M. lini infections on L. marginale. Early, severe infection reduced growing season and overwintering survivorship as well as capsule production. However, plants in the field were most often infected only after flowering had begun, and the predominant effect of infection was a reduction in overwintering survivorship. The high variability in disease levels from year to year and the deferred nature of the effect of the rust on its host have significant implications for the design of experiments aimed at assessing the role of diseases in plant communities.

Seed and Seedling Biology in Relation to Modelling Vegetation Dynamics Under Global Climate Change

The distribution of many plant species will change with global climate change, depending on their ability to disperse into, and establish in, new communities. Past migrations of species under climate change have been an order of magnitude slower than the rate of predicted climate change for the next century. The limited evidence available suggests that chance long distance dispersal events will be critically important in determining migration rates. The JABOWA-derived gap replacement models and vital attributes/FATE models were examined, and the dispersal and establishment processes necessary to make improved projections of vegetation dynamics under climate change using these models were investigated. The minimum modifications of these models required to incorporate directional migration of species are described. To predict establishment success of species, it was suggested that a more fundamental understanding is needed of how establishment ability under different conditions relates to seed and seedling attributes and how this may be affected by elevated CO2. Finally, an examination was carried out of whether plant functional types based on vegetative attributes (used to model the response of adult plants) are correlated with functional types based on seed and seedling attributes. Available evidence suggests that the two sets of attributes are not strongly correlated; consequently, models of vegetation dynamics will need to incorporate seed biology explicitly.