Interspecific hybridization between common and tepary beans: increased hybrid embryo growth,fertility, and efficiency of hybridization through recurrent and congruity backcrossing

Cultivated common bean (Phaseolus vulgaris L.) and tepary bean (Phaseolus acutifolius A. Gray) genotypes possessing desirable agronomic traits were hybridized. The F₁ hybrids were backcrossed twice with the common bean (i.e., recurrent backcrossing). Also, alternate backcrosses with common and tepary beans (i.e., congruity backcrossing) were carried out. Embryo culture was necessary for all initial interspecific crosses, and its requirement was proportionally lower when the common bean was used as the recurrent parent and as the last parent of congruity backcrosses. Modification of the embryo culture technique was necessary to produce congruity hybrids. Effects of both tepary and common bean genotypes on the success rate of hybridization were observed. Tepary accession G 40001 and common bean cultivar ICA Pijao facilitated interspecies hybridization. Growth of hybrid embryos before rescue, recovery of mature hybrid plants, and the vigor and fertility of F₁ hybrids all increased with increased recurrent and congruity backcrosses and intermatings between male-sterile F₁ and selected fertile F₂ plants of the third and fifth congruity backcrosses. Introgression of tepary genes was verified by means of seed protein electrophoretic analysis and morphological markers. The results suggest that congruity backcrossing can help to gradually reduce or overcome P. vulgaris x P. acutifolius hybridization barriers such as genotype incompatibility, early embryo abortion, hybrid sterility, and lower frequencies of hybridization.     

Molecular marker-assisted backcrossing breeding: an example to transfer a thermostable β-amylase gene from wild barley

Molecular marker-assisted backcrossing (MABC) is widely recommended for transferring favorable alleles from a donor to an elite variety. The question remains whether MABC is an effective approach to developing a competitive commercial variety. Here, we illustrate the transfer of a thermostable β-amylase allele Sd3 from wild barley into a commercial barley variety Gairdner. The elite lines were chosen for the Regional Crop Variety Test that followed a standard conventional breeding process. The results demonstrated that the Sd3 allele not only increased enzyme thermostability but dramatically enhanced diastatic power, an important malting quality trait. The BC₁F₁ individuals had a fundamental impact on the comprehensive agronomic and quality traits of the final progenies, demonstrating the importance of screening at the early stage of backcrossing in MABC. There was sufficient genetic variation in the BC₃F₃ families to select other malting quality and agronomic traits. Ten individual breeding lines with improved β-amylase thermostability also had improved yields and grain plumpness. Three elite lines with improved malting quality and agronomic traits were selected to provide a parental line to incorporate the wild barley allele for breeding a commercial variety. A new strategy should be considered that uses marker-assisted selection and backcrossing to transfer a favorable allele from a wild parent.     

Interspecific T-DNA transfer through plant protoplast fusion

Summary An attempt was made to transfer the T-DNA of Agrobacterium tumefaciens, previously introduced into plant cells, via protoplast fusion from one species into another. For the experiments two cell lines were used: firstly, a Nicotiana paniculata cell line transformed with the Agrobacterium strain B6S3. This cell line exhibits both hormone independent growth and synthesis of octopine as a result of the incorporated T-DNA from Agrobacterium. These two markers are dominant. The second cell line was the nitrate reductase deficient cnx-68 cell line of N. tabacum which contains an intracellular calcium oxalate druse. These two markers are recessive. Isolated protoplasts of the donor cell line N. paniculata B6S3 were mitotically inactivated by X rays and fused with protoplasts of the cell line cnx-68. Asymmetric somatic hybrids were selected on hormone free agar medium supplemented with 50 mM KClO₃. This compound is toxic for cells possessing nitrate reductase activity. From about 1.1×10⁷ cultivated protoplasts 18 cell lines survived the selection treatment. Of these seven exhibited the two dominant and the two recessive markers, whereas the others showed either only one or none of the recessive or only one of the dominant markers. In dot-blot experiments using species specific DNA clones of the donor and the recipient plant species it was confirmed that besides the T-DNA other nuclear genomic DNA of the donor species had also been transferred in various amounts. The possible consequences of these results for plant breeding programmes are discussed.     

Microsatellite mapping of a Triticum urartu Tum. derived powdery mildew resistance gene transferred to common wheat (Triticum aestivum L.)

A powdery mildew resistance gene from Triticum urartu Tum. accession UR206 was successfully transferred into hexaploid wheat (Triticum aestivum L.) through crossing and backcrossing. The F₁ plants, which had 28 chromosomes and an average of 5.32 bivalents and 17.36 univalents in meiotic pollen mother cells (PMC), were obtained through embryos rescued owing to shriveling of endosperm in hybrid seed of cross Chinese Spring (CS) × UR206. Hybrid seeds were produced through backcrossing F₁ with common wheat parents. The derivative lines had normal chromosome numbers and powdery mildew resistance similar to the donor UR206, indicating that the powdery mildew resistance gene originating from T. urartu accession UR206 was successfully transferred and expressed in a hexaploid wheat background. Genetic analysis indicated that a single dominant gene controlled the powdery mildew resistance at the seedling stage. To map and tag the powdery mildew resistance gene, 143 F₂ individuals derived from a cross UR206 × UR203 were used to construct a linkage map. The resistant gene was mapped on the chromosome 7AL based on the mapped microsatellite makers. The map spanned 52.1 cM and the order of these microsatellite loci agreed well with the established microsatellite map of chromosome arm 7AL. The resistance gene was flanked by the microsatellite loci Xwmc273 and Xpsp3003, with the genetic distances of 2.2 cM and 3.8 cM, respectively. On the basis of the origin and chromosomal location of the gene, it was temporarily designated PmU.     

Characterization of the expression and inheritance of potato leafroll virus (PLRV) and potato virus Y (PVY) resistance in three generations of germplasm derived from Solanum etuberosum

Potato virus Y (PVY) and potato leafroll virus (PLRV) are two of the most important viral pathogens of potato. Infection of potato by these viruses results in losses of yield and quality in commercial production and in the rejection of seed in certification programs. Host plant resistance to these two viruses was identified in the backcross progeny of a Solanum etuberosum Lindl. somatic hybrid. Multiple years of field evaluations with high-virus inoculum and aphid populations have shown the PVY and PLRV resistances of S. etuberosum to be stably expressed in two generations of progeny. However, while PLRV resistance was transmitted and expressed in the third generation of backcrossing to cultivated potato (Solanum tuberosum L. subsp. tuberosum), PVY resistance was lost. PLRV resistance appears to be monogenic based on the inheritance of resistance in a BC₃ population. Data from a previous evaluation of the BC₂ progeny used in this study provides evidence that PLRV resistance was partly conferred by reduced PLRV accumulation in foliage. The field and grafting data presented in this study suggests that resistance to the systemic spread of PLRV from infected foliage to tubers also contributes to the observed resistance from S. etuberosum. The PLRV resistance contributed by S. etuberosum is stably transmitted and expressed through sexual generations and therefore would be useful to potato breeders for the development of PLRV resistant potato cultivars.     

Effects of nitrogen rates on grain yield and nitrogen agronomic efficiency of durum wheat genotypes under different environments

Durum wheat is an important staple food crop in Tunisia and other Mediterranean countries and is grown in various climatic conditions. Production and yield are however severely limited not only by drought events but also by reduced levels of nitrogen fertilisation. A study was carried out at two locations in the sub‐humid area of Tunisia: Mateur in 2009–10 and 2010–11 and Beja in 2011–12 and 2012–13 under rainfed conditions. Four durum wheat genotypes (landraces: Bidi, Azizi; improved: Om Rabia, Khiar) were evaluated for nitrogen agronomic efficiency and related agronomic traits under various nitrogen rates: 0, 50, 100, 150, 200 and 250 kg N ha^?1, with three replications. There was a significant interaction effect (P ≤ 0.001) environments × genotypes × N treatments for grain yield (GY), biomass yield (BY), harvest index (HI), partial factor productivity of applied nitrogen (PFP_N) and nitrogen agronomic use efficiencies (NAE). GY was the most affected trait by nitrogen applied showing an increase of 94% under high N treatment (250 kg N ha^?1) compared to control plots without N treatments. A significant linear regression exists between GY (0 N) and GY for the different N rates (r = 0.70; P < 0.001). This effect was more pronounced for improved genotypes than landraces for all parameters excepting BY and NAE_BY. BY showed +11% increase in landraces than improved genotypes. PFP_N showed an average decrease of 65% under high‐N fertilisation with 10% prevalence for improved genotypes. Landraces tend to promote vegetative growth while grain filling efficiency was higher for improved genotypes.     

Dominant male sterility in sorghum: effect of nuclear background on inheritance of tissue-culture-induced mutation

Occurrence of genetic instability and formation of stable mutations are basic genetic processes. This study demonstrates that nuclear background may influence the formation of stable dominant nuclear gene of male sterility (MS) on the basis of unstable mutation, which was induced in tissue culture of the sorghum haploid (cv. Milo-145). The mutants with complete or partial MS segregated in variable ratios in the progenies of diploid regenerants were obtained from different experiments on cultivation of haploid tissues. In the Milo-145 genetic background the mutation demonstrated somatic instability and was gradually eliminated by self-pollination of partially sterile plants. Hybridization of the MS-plants with the sorghum line SK-723, a fertility-restorer of the cytoplasmic MS A1 (milo) type, maintained the induced mutation. By repeated backcrossing of MS-plants with SK-723, the male-sterile versions of this line (SK-723- Ms _tc ) have been created. In BC-generations, fertile, partially and completely sterile plants were observed. The MS-plants from BC-generations are proposed to contain a dominant gene Ms _tc while fertile plants were ms _tc / ms _tc homozygotes. Crossing the original MS-plants with SK-723 was a key factor in stabilization of the Ms _tc gene. Dominant expression of the Ms _tc was observed in male-sterile versions of other sorghum lines created by backcrossing to SK-723- Ms _tc . The lines fertility-restorers for this mutation have been revealed. In the crosses of restored F₁ hybrids with emasculated plants of the non-restoring line, the Ms _tc has been transferred through the pollen and manifested in the F₁ generation. The possibility of the Ms _tc originating as a result of interaction of an unstable allele of the Milo-145 with the SK-723 genome is discussed.     

Nitrogen use efficiency of sugarcane in relation to its BNF potential and population of endophytic diazotrophs at different N levels

The nitrogen fixing bacterial endophytes Gluconacetobacter diazotrophicus and Herbaspirillum spp. have been proposed to benefit sugarcane (Saccaharum spp. hybrids) growth. Variable populations of these endophytes exist depending upon ontogenic and climatic variations as well. This study investigates the effect of variable chemical nitrogen application in soil on the population of endophytic diazotrophs, acetylene reduction ability of excised roots, plant N-nutrient use efficiency and probable interactions among different parameters in eight commercial sugarcane varieties of subtropical India. Recovery efficiency (RE), agronomic efficiency (AE), partial factor productivity (PFP) and physiologic efficiency (PE) indicators were used for accounting N-nutrient use efficiency. The population of G. diazotrophicus was more at N₇₅ compared to N₀ and N₁₅₀, whereas Herbaspirillum population increased from N₀ to N₁₅₀. ARA was positively correlated with Gluconacetobacter population in rhizosphere and root, whereas it had poor correlation with Herbaspirillum population. Positive correlation of RE and AE with ARA of roots, Gluconacetobacter and Herbaspirillum populations in roots and stems indicate their positive contribution in total nitrogen uptake by the plant per kg of N applied. Average PFP was 808.9 at N₇₅ compared to 408.7 at N₁₅₀ indicating that N was utilized efficiently at low N input status in sugarcane. Strong positive correlations of AE₇₅ (agronomic efficiency from 75 kg N ha⁻¹ to 150 kg N ha⁻¹) with N-uptake (r ² = 0.615), cane yield (r ² = 0.758) and PFP (r ² = 0.758) and other parameters compared to AE (agronomic efficiency from 0 kg N ha⁻¹ to 75 kg N ha⁻¹ or 150 kg N ha⁻¹) correlations with N-uptake (r ² = 0.111), cane yield (r ² = 0.368) and PFP (r ² = 0.190) indicated that the AE of sugarcane was strongly directed towards producing more cane yield per unit of N fertilizer once the sugarcane plant has established using initial dose of nitrogen and thus AE₇₅ seems to be a more appropriate indicator for accounting N-nutrient use efficiency in sugarcane.     

Characterization of agronomic traits and markers of recombinant inbred lines from intra- and interracial populations of Phaseolus vulgaris L.

The value of intra- and interracial populations in common bean (Phaseolus vulgaris L.) needs to be determined in order to create useful genetic variation for maximizing gains from selection, broadening the genetic base of commercial cultivars, and making efficient use of available resources. Five large-seeded parents of race Nueva Granada (N), two small-seeded race Mesoamerica (M), and one medium-seeded race Durango (D) were hybridized to produce one intraracial (N x N) and three interracial (two N x M and one N x D) populations. Seventy-nine F₂-derived F₆ lines randomly taken from each population along with their parents were evaluated for agronomic traits and markers at Palmira and Popayán, Colombia, in 1990 and 1991. Variation for agronomic traits and for morphological, protein, and isozyme markers was larger in interracial populations than in the intraracial population. Mean seed yield of all lines as well as yield of the highest yielding line from two interracial populations were significantly higher than that of the intraracial population. The highest ( 0.80±0.15) heritability was recorded for 100-seed weight. Values for seed yield varied from 0.19±0.17 to 0.50±0.16. Gains from selection (at 20% selection pressure) for seed yield ranged from 3.9% to 11.4%. Seed yield was positively associated with biomass yield, pods/m², and days to maturity, but harvest index showed negative correlations with these traits and a positive value with 100-seed weight. Polymorphism was recorded for phaseolin, lectins, protein Group-1 and protein Group-2 fractions, and six isozyme loci. Lines with indeterminate growth habit had significantly (P < 0.01) higher seed yield than lines with determinate growth habit in a Redkloud x MAM 4 population. Also, 23 other associations of markers with agronomic traits other than seed yield were recorded. Of these associations, lines with T phaseolin, the Diap1 ² allele, and lilac flower color tended to possess greater seed weight.     

Green manure technology: Potential,usage, and limitations. A case study for lowland rice

The growing concern about the sustainability of tropical agricultural systems stands in striking contrast to a world-wide decline in the use of soil-improving legumes. It is timely to assess the future role that soil-improving legumes may play in agricultural systems. This paper reviews recent progress, potential, and limitations of green manure technology, using lowland rice cropping systems as the example.Only a few legume species are currently used as green manures in lowland rice. Sesbania cannabina is the most widely used pre-rice green manure for rice in the humid tropics of Africa and Asia. Astragalus sinicus is the prototype post-rice green manure species for the cool tropics. Stem-nodulating S. rostrata has been most prominent in recent research. Many green manure legumes show a high N accumulation (80–100 kg N ha^-1 in 45–60 days of growth) of which the major portion (about 80%) is derived from biological N₂ fixation. The average amounts of N accumulated by green manures can entirely substitute for mineral fertilizer N at current average application rates. With similar N use efficiencies, green manure N is less prone to loss mechanisms than mineral N fertilizers and may therefore contribute to long-term residual effects on soil productivity.Despite a high N₂-fixing potential and positive effects on soil physical and chemical parameters, the use of green manure legumes for lowland rice production has declined dramatically world-wide over the last 30 years. Land scarcity due to increasing demographic pressure and a relatively low price of urea N are probably the main determining factors for the long-term reduction in pre-rice green manure use. Post-rice green manures were largely substituted for by high-yielding early-maturing grain legumes. Unreliability of green manure performance, non-availability of seeds, and labor intensive operations are the major agronomic constraints. The recognition and extrapolation of niches where green manures have a comparative advantage may improve an often unfavorable economic comparison of green manure with cash crop or fertilizer N. Socio-economic factors like the cost of land, labor, and mineral N fertilizer are seen to determine the cost-effectiveness and thereby farmers' adoption of sustainable pre-rice green manure technology. Hydrology and soil texture determine the agronomic competitiveness of a green manure with N fertilizers and with alternative cash crops. In general, the niches for pre-rice green manure are characterized by a relatively short time span available for green manure growth and a soil moisture regime that is unfavorable for cash crops (flood-prone rainfed lowlands with coarse-textured soils).Given the numerous agronomic and socio-economic constraints, green manure use is not seen to become a relevant feature of favourable rice-growing environments in the foreseeable future. However, in environments where soil properties and hydrology are marginal for food crop production, but which farmers may be compelled to cultivate in order to meet their subsistence food requirements, green manures may have a realistic and applicable potential.     

The impact on biosafety of the phosphinothricin-tolerance transgene in inter-specific B. rapa×B. napus hybrids and their successive backcrosses

 There is strong evidence indicating that gene flow from transgenic B. napus into weedy wild relatives is inevitable following commercial release. Research should now focus on the transmission, stability, and impact of transgene expression after the initial hybridization event. The present study investigated the transfer of a phosphinothricin-tolerance transgene by inter-specific hybridization between B. rapa and two transgenic B. napus lines. The expression of the transgene was monitored in the F₁ hybrids and in subsequent backcross generations. The transgene was transmitted relatively easily into the F₁ hybrids and retained activity. Large differences in the transmission frequency of the transgene were noted between offspring of the two transgenic lines during backcrossing. The most plausible explanation of these results is that the line showing least transmission during backcrossing contains a transgene integrated into a C-genome chromosome. Approximately 10% of offspring retained the tolerant trait in the BC₃ and BC₄ generations. The implications of these findings for the stable introgression of transgenes carried on one of the chromosomes of the C-genome from B. napus and into B. rapa are briefly discussed. Received: 5 November 1996 / Accepted: 21 February 1997     

Nitrogen and carbon dynamics in C3 and C4 estuarine marsh plants grown under elevated CO2 in situ

Summary Carbon dioxide concentrations were elevated in three estuarine communities for an entire growing season. Open top chambers were used to raise CO₂ concentrations ca. 336 ppm above ambient in monospecific communities of Scirpus olneyi (C₃) and Spartina patens (C₄), and a mixed community of S. olneyi, S. patens and Distichlis spicata (C₄). Nitrogen and carbon concentration (% wt) of aboveground tissue was followed throughout growth and senescence. Green shoot %N was reduced and %C was unchanged under elevated CO₂ in S. olneyi. This resulted in a 20%–40% increase in tissue C/N ratio. There was no effect of CO₂ on either C₄ species. Maximum aboveground N (g/m²) was unchanged in S. olneyi, indicating that increased productivity under elevated CO₂ was dependent on reallocation of stored N. There was no change in the N recovery efficiency of S. olneyi in pure stand and a decrease in the mixed community. Litter C/N ratio was not affected by elevated CO₂ suggesting that decomposition and N mineralization rates will also remain unchanged. Continued growth responses to elevated CO₂ could, however, be limited by the ability of S. olneyi to increase the total aboveground N pool.     

Production and characterization of arboreous and fertile <Emphasis Type="Italic">Solanum melongena</Emphasis> + <Emphasis Type="Italic">Solanum marginatum </Emphasis>somatic hybrid plants

In crop plants the shift from being annuals to perennials may allow future agricultural systems requiring less energy inputs. The practicability of this was tested for Solanum melongena. Leaf protoplasts of S. melongena (2n = 2x = 24) and one of the related arborescent species Solanum marginatum (2n = 2x = 24) were electrofused and fertile somatic hybrids with arborescent habit regenerated. The magnetic cell sorter (MACS) technique was used for the selection of heterokaryons. The hybrid nature of 18 regenerated plants was assessed on the banding patterns generated by inter-simple sequence repeat PCR. When taken to maturity in the greenhouse, hybrids grew more vigorously compared to the parental species. Their morphological traits were intermediate between those of S. melongena and S. marginatum. Hybrids flowered and produced an average of 85% stainable viable pollen and fertile fruits. The somatic hybrids were maintained in the greenhouse for more than 3 years and continued to produce flowers developing into two types of fruits with plentiful seeds. Fruits were either striated green containing non-germinable seeds or yellow with fully germinable seeds. Their S₁ progenies showed common features with S₀ hybrids, including fertility and arborescent habit. Cytologically, somatic hybrids exhibited the expected chromosome number of 2n = 4x = 48, while chromosome pairing during microsporogenesis was associated with a low frequency of intergenomic pairing. It is concluded that an arborescent perennial species has been obtained by somatic hybridization. The usefulness of this species per se or in eggplant breeding will depend not only on the transmission of the arborescent habit to cultivated eggplant varieties, but also on the variability that should be created from backcrossing the S. melongena + S. marginatum hybrids to S. melongena.     

Tidal marsh plant responses to elevated CO2, nitrogen fertilization,and sea level rise

Elevated CO₂ and nitrogen (N) addition directly affect plant productivity and the mechanisms that allow tidal marshes to maintain a constant elevation relative to sea level, but it remains unknown how these global change drivers modify marsh plant response to sea level rise. Here we manipulated factorial combinations of CO₂ concentration (two levels), N availability (two levels) and relative sea level (six levels) using in situ mesocosms containing a tidal marsh community composed of a sedge, Schoenoplectus americanus, and a grass, Spartina patens. Our objective is to determine, if elevated CO₂ and N alter the growth and persistence of these plants in coastal ecosystems facing rising sea levels. After two growing seasons, we found that N addition enhanced plant growth particularly at sea levels where plants were most stressed by flooding (114% stimulation in the + 10 cm treatment), and N effects were generally larger in combination with elevated CO₂ (288% stimulation). N fertilization shifted the optimal productivity of S. patens to a higher sea level, but did not confer S. patens an enhanced ability to tolerate sea level rise. S. americanus responded strongly to N only in the higher sea level treatments that excluded S. patens. Interestingly, addition of N, which has been suggested to accelerate marsh loss, may afford some marsh plants, such as the widespread sedge, S. americanus, the enhanced ability to tolerate inundation. However, if chronic N pollution reduces the availability of propagules of S. americanus or other flood‐tolerant species on the landscape scale, this shift in species dominance could render tidal marshes more susceptible to marsh collapse.     

ORIGIN OF FRAGARIA POLYPLOIDS. II. UNREDUCED AND DOUBLED-UNREDUCED GAMETES

Offspring from natural hybrids between octoploid Fragaria chiloensis (2n = 56) and diploid F. vesca (2n = 14) backcrossed under natural conditions to F. chiloensis were studied. The natural F₁ hybrids themselves were of three kinds: (1) The expected pentaploids which resulted from the union of normally reduced gametes of diploid F. vesca and octoploid F. chiloensis; (2) A hexaploid F₁ hybrid which resulted from the union of an unreduced gamete from diploid F. vesca with a normally reduced gamete from octoploid F. chiloensis; and (3) A 9-ploid F₁ hybrid which probably arose from the union of an unreduced gamete of the octoploid F. chiloensis with a normally reduced gamete of diploid F. vesca. The progenies that resulted from the natural backcrossing of each of the three sorts of F₁ hybrids to F. chiloensis were as follows: The pentaploid F₁ hybrids (2n = 35) yielded mostly 9-ploid offspring from unreduced 5X gametes; a relatively high percentage of 14-ploid plants arising from doubled-unreduced 10 X gametes and a few 2N = ±46 aneuploids from reduced gametes. The hexaploid F₁ hybrid (2n = 42) on backcrossing yielded over 50% 10-ploid offspring with the rest 2n = ±50 aneuploids from reduced gametes. The 9-ploid F₁ hybrid (2n = 63) on backcrossing yielded mostly aneuploids normally distributed about a modal 2n = 59 chromosome class resulting from a 31 chromosome gamete, with a few 2n = 56 and 2n = 63 euploids. The 9-ploids may facilitate diploid Å octoploid introgression. Screening of the open-pollinated offspring from F. chiloensis revealed almost 2% 12-ploid (2n = 84) offspring from the union of the reduced and unreduced F. chiloensis gametes. The probable genomic constitution of the observed novel ploidy levels and those that theoretically may be generated from the known hybrids are presented. The origin of the existing polyploids from diploids through simple unreduction is postulated.     

Biology of Multi Cell Ponds Treating Municipal Wastes

Observations were made over a period of one year on the performance of multi stage stabilization ponds treating domestic waste waters. The unit is composed of Primary anaerobic (P₁) and two consecutive Secondary (P₂) and Tertiary (P₃) facultative ponds. The overall reduction in BOD, bacteria, PO₄, SO₄, NH₃N, Cl⁻ and total solids were 89.4, 99.8, 76.2, 47.7, 79.7, 18.6 and 74.0 percent, respectively. Elimination of H₂S was 84.8% in the final effluents. Chlorella vulgaris was found to be the dominant alga followed by Euglena, Ankistrodesmus, Phacus and Synedra as subdominant forms. 9 species of protozoa have been recorded in the first pond and their number declined in the successive ponds.     

Effects of N on Plant Response to Heat-wave: A Field Study with Prairie Vegetation

More intense, more frequent, and longer heat-waves are expected in the future due to global warming, which could have dramatic ecological impacts. Increasing nitrogen (N) availability and its dynamics will likely impact plant responses to heat stress and carbon (C) sequestration in terrestrial ecosystems. This field study examined the effects of N availability on plant response to heat-stress (HS) treatment in naturally-occurring vegetation. HS (5 d at ambient or 40.5 ℃) and N treatments (±N) were applied to 16 1 m2 plots in restored prairie vegetation dominated by Andropogon gerardii (warm-season C4 grass) and Solidago canadensis (warm-season C3 forb). Before, during, and after HS, air, canopy, and soil temperature were monitored; net CO2 assimilation (Pn), quantum yield of photosystem Ⅱ (φPsⅡ), stomatal conductance (gs), and leaf water potential (Ψw) of the dominant species and soil respiration (Rsolf) of each plot were measured daily during HS. One week after HS, plots were harvested, and C% and N% were determined for rhizosphere and bulk soil, and above-ground tissue (green/senescent leaf, stem, and flower). Photosynthetic N-use efficiency (PNUE) and N resorption rate (NRR) were calculated. HS decreased Pn, gs, Ψw, and PNUE for both species, and N treatment generally increased these variables (±HS), but often slowed their poat-HS recovery. Aboveground biomass tended to decrease with HS in both species (and for green leaf mass in S. canadensis), but decrease with N for ,4. gerardii and increase with N for S. canadensis. For A. gerardii, HS tended to decrease N% in green tissues with N, whereas in S. canadensis, HS increased N% in green leaves.Added N decreased NRR for A. gerardii and HS increased NRR for S. canadensis. These results suggest that heat waves,though transient, could have significant effects on plants, communities, and ecosystem N cycling, and N can influence the effect of heat waves.     

Improvement of transgenic alfalfa by backcrossing

Summary Two regenerative alfalfa genotypes were transformed withAgrobacterium tumefaciens with binary vectors containing the coding sequences for ?-glucuronidase (GUS) and npt II (kanamycin resistance). The regenerative genotypes and their transgenic populations were agronomically inferior, and one was a somaclonal variant for flower color. GUS was used as a dominant genetic marker in a model system for studying backcrossing to improve transgenic alfalfa. Agronomic yield deficiencies and somaclonal changes were corrected by one to three backcrosses to cultivar genotypes, depending on the vigor of the original transformant. Three backcrosses were considered optimal because progeny contain 94% cultivar germplasm and could be used as parents of a new cultivar. Use of different cultivar genotypes each generation of backcrossing minimized inbreeding and maximized the heterotic potential of backcross derivatives. The improvement of transgenic alfalfa by backcrossing using a dominant marker required only as much time as the original transformation experiment.     

Nitrogen fixation by subclover associations fertilized with sulfur

Summary The effect of S fertilization on symbiotic N₂ fixation was measured with the¹⁵N technique and the N difference method in a lysimeter study using Josephine loam (Typic Haploxurults). Nitrogen fixation by subclover (Trifolium subterraneum L.) was strongly enhanced by added S. The association of soft chess (Bromus mollis L.) or filaree (Erodium botrys (Cav.) Bertol.) with subclover increased the percentage of N in subclover that was fixed, with the results that N₂ fixation was increased beyond that due to the mere increase in subclover biomass. Nitrogen fixation estimates by¹⁵N dilution and N difference methods were highly correlated (r²=0.97), and S fertilizer did not result in any significant differences in N₂-fixation estimation by the two methods. Both methods were useful in distinguishing between soil N uptake and N₂ fixation where S applications produced highly significant increases in both uptake and fixation. Application of sulfur fertilizers to much annual rangeland has the potential to increase pasture productivity through enhanced N₂ fixation. Contribution of the University of California Hopland Field Station and Department of Agronomy and Range Science, Univ. of California, Davis, CA 95616.     

Root Dynamics of Cultivar and Non‐Cultivar Population Sources of Two Dominant Grasses during Initial Establishment of Tallgrass Prairie

Dominance of warm‐season grasses modulates tallgrass prairie ecosystem structure and function. Reintroduction of these grasses is a widespread practice to conserve soil and restore prairie ecosystems degraded from human land use changes. Seed sources for reintroduction of dominant prairie grass species include local (non‐cultivar) and selected (cultivar) populations. The primary objective of this study was to quantify whether intraspecific variation in developing root systems exists between population sources (non‐cultivar and cultivar) of two dominant grasses (Sorghastrum nutans and Schizachyrium scoparium) widely used in restoration. Non‐cultivar and cultivar grass seedlings of both species were isolated in an experimental prairie restoration at the Konza Prairie Biological Station. We measured above‐ and belowground net primary production (ANPP and BNPP, respectively), root architecture, and root tissue quality, as well as soil moisture and plant available inorganic nitrogen (N) in soil associated with each species and source at the end of the first growing season. Cultivars had greater root length, surface area, and volume than non‐cultivars. Available inorganic N and soil moisture were present in lower amounts in soil proximal to roots of cultivars than non‐cultivars. Additionally, soil NO₃–N was negatively correlated with root volume in S. nutans cultivars. While cultivars had greater BNPP than non‐cultivars, this was not reflected aboveground root structure, as ANPP was similar between cultivars and non‐cultivars. Intraspecific variation in belowground root structure and function exists between cultivar and non‐cultivar sources of the dominant prairie grasses during initial reestablishment of tallgrass prairie. Population source selection should be considered in setting restoration goals and objectives.