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- Author or Editor: J. L. Johnson x
In vitro propagation of Mammillaria elongata DC plants was successful using tubercle explants grown on a medium based on Murashige and Skoog’s high salts supplemented with various auxins and cytokinins. Optimum callus proliferation occured in response to 2,4-dichlor-ophenoxyacetic acid (2,4-D) (2-10 mg/liter) with either kinetin or 6-(dimethylallylamino)-purine (2iP) (1-2 mg/liter). Root initiation was optimized with either napthaleneacetic acid (NAA) or indolebutyric acid (IBA) (60 mg/liter). Shoot initiation was optimized by addition of 2iP (10 mg/liter) and indoleacetic acid (IAA) (1 mg/liter). The auxin:cytokinin balance required for shoot initiation appears to be unique for each species of Mammillaria studied. Shoots developed in vitro of M. elongata were successfully transferred to greenhouse conditions, where they rooted and continued to grow.
Spine primordia of Mammillaria elongata are initiated in acropetal sequence, beginning on the abaxial side of the spiniferous meristem that occupies the summit of a young tubercle and continuing along either side until a single whorl is completed. If a central spine occurs, it is the last to be initiated and the first to begin elongation. Elongation of the whorled spines follows a basipetal sequence, the inverse of initiation. All elongated spines are at first erect, but the lateral ones become radially oriented as they appraoch maturity. Central spines remain erect and tilted slightly toward the shoot apex. The distal ends of many epidermal cells on developing spines bear conic projections that are most prominent along the median portions.
Axillary meristems of Mammillaria elongata DC were studied with a scanning electron microscope using fresh tissue to determine their degree of development and suitability as explants in tissue culture investigations. Meristems were excised at regular intervals along the stem. The axillary meristem reached maximum size about 10 cm and maximum development about 15 cm below the stem tip. Axillary meristems closer to the base of the stem were less developed. In fully developed meristems, leaf primordia were initiated in a spiral acropetal sequence at the margins of the apical dome, resulting in a phyllotaxis of 8 + 13 contact parastichies of tubercles on the developed stem. Enlarged leaf primordia were 1- or 3-lobed and about 75 μm in length. No flower initiation or lateral branch development was observed; however, a corky region, the result of aborted or senesced flowers, was observed in several axils. Variability in development or absence of axillary meristems in several axils make M. elongata axillary meristems unsuitable for critical studies on factors influencing organogenesis in tissue culture.
Clematis socialis Kral, commonly known as the Alabama Leatherflower, is an endangered species with only six known populations in northeast Alabama and northwest Georgia. Cutting propagation of the species will aid in establishing additional self-sustaining populations and provide genetic material for future hybridization and genetic preservation. Such research would also benefit growers, especially native nurseries, who wish to produce this species commercially for its ornamental value. Several experiments were performed to determine the effects of four non-amended substrates on root initiation, root growth, and survival of C. socialis stem cuttings. The four substrates tested included sand, perlite, vermiculite, and a 1 peat (P): 1 pine bark (PB): 1 sand (S) mix (by volume). Some of the best results in the preliminary experiments in 2000 were observed when 2 to 3 node cuttings kept under shade and treated with higher IBA/NAA concentrations were used. In 2004, there was a correlation between root growth and cutting survival and particle size of the substrates. Cuttings rooted in the finer-particle substrates sand and vermiculite had higher cutting survival, root growth, root number, and root quality than those rooted in perlite and the 1 P: 1 PB: 1 S mix. Sand, perlite and vermiculite consistently outperformed the 1 P: 1 PB: 1 S mix which had some of the lowest growth data means. Sand was among the highest performing substrates in all years and it is the most inexpensive and readily available making it the most logical substrate for rooting C. socialis stem cuttings.
Clematis socialis Kral, also known as the Alabama Leatherflower, is an endangered species with only six known populations in northeast Alabama and northwest Georgia. Cutting propagation of the species would be beneficial for establishing additional self-sustaining populations and providing genetic material for future hybridization. A study conducted in 2000 and 2004 determined the effects of four nonamended substrates on root initiation and growth, as well as survival of C. socialis stem cuttings. Of the four substrates tested, including sand, perlite, vermiculite, and 1:1:1 (by volume) peat (P): pine bark (PB): sand (S), cutting survival was highest in sand in both 2000 and 2004. In 2000, sand also produced the longest roots and highest root quality. Vermiculite produced the longest and most roots and highest root quality in the 2004 study. In 2004, cuttings rooted in fine-particled substrates, such as sand and vermiculite, had higher cutting survival, root growth, root number, and root quality than those rooted in perlite and 1:1:1 (by volume) P:PB:S. The 1:1:1 P:PB:S substrate produced the lowest averages for all data collected in both the 2000 and 2004 studies. Sand was among the two highest performing media in both years, regardless of differences in IBA concentration, misting times, and environmental conditions, making it the overall best substrate for rooting C. socialis stem cuttings. Increasing the concentration of IBA in the rooting solution, providing a cooler environment, and decreasing the number and duration of misting cycles the cuttings received increased cutting survival, root length, root number, and root quality for all four substrates from 2000 to 2004.
Increased soil moisture stress reduced growth and transpiration rate of Ficus benjamina. Leaf drop during indoor phase was greater for plants previously watered during production at 3 day intervals than for plants grown under the 6- and 9-day water regimes.
In 1998, the USDA-ARS and Cornell Univ. instituted a cooperative agreement that mobilized the resources for a jointly managed apple rootstock breeding and evaluation program. The program is a successor to the Cornell rootstock breeding program, formerly managed by Emeritus Professor of Horticultural Sciences James N. Cummins. The agreement broadens the scope of the program from a focus on regional concerns to address the constraints of all the U.S. apple production areas. In the future, the breeding program will continue to develop precocious and productive disease-resistant rootstock varieties with a range of vigor from fully dwarfing to near standard size, but there will be a renewed emphasis on nursery propagability, lodging resistance, tolerance to extreme temperatures, resistance to the soil pathogens of the sub-temperate regions of the U.S., and tolerance to apple replant disorder. The program draws on the expertise available at the Geneva campus through cooperation with plant pathologists, horticulturists, geneticists, biotechnologists, and the curator of the national apple germplasm repository. More than 1000 genotypes of apple rootstocks are currently under evaluation, and four fire blight- (Erwinia amylovora) resistant cultivars have been recently released from the program. As a service to U.S. apple producers, rootstock cultivars from other breeding programs will also be evaluated for productivity, size control, and tolerance to a range of biotic and abiotic stress events. The project will serve as an information source on all commercially available apple rootstock genotypes for nurseries and growers.
Seedlings of ‘Topa Topa’ avocado (Persea americana Mill.) were grown in steamed loamy sand soil with no fertilizer, complete fertilizer (N, P, K, S, Ca, Mg, Cu, Zn, Mn, Fe, Mo, B), −P, −Zn, −P and −Zn, and −Zn+10 × P(640 ppm P). Seedlings were inoculated separately with one of 2 isolates of Glomus fasciculatus (Thaxter) Gerd. & Trappe (GF) or were inoculated with a water filtrate of the mycorrhizal inoculum plus autoclaved mycorrhizal inoculum. Growth of mycorrhizal seedlings was 49-254% larger than nonmycorrhizal avocados except at the −Zn+10×P regime where mycorrhizal and nonmycorrhizal avocados were of similar size. Both mycorrhizal isolates increased absorption of N, P, and Cu at all fertilizer treatments and absorption of Zn was increased with all fertilizer treatments by one mycorrhizal isolate. Fertilization with P did not alter P concentrations in leaves of nonmycorrhizal plants but increased P concentrations in leaves of mycorrhizal seedlings. Fertilization with 10×P increased P concentrations in both mycorrhizal and nonmycorrhizal seedlings. One GF isolate appeared to be superior to the other based on mineral nutrition of the host avocados. Differences between the isolates apparently were related to their rate of growth or ability to infect. Poor growth of avocado seedlings in steamed or fumigated soil can be related to poor mineral nutrition due to the destruction of mycorrhizal fungi.
An increased incidence of graft union failure of apple trees during high wind events has been noted by researchers participating in the NC-140 regional rootstock testing project for certain rootstock-scion combinations. By measuring the strength of graft unions in a survey of mature apple trees in multiple stock-scion combinations, we have determined that there are significant differences. These differences may be attributable to genotype specific characteristics of rootstocks, scions, and/or rootstock-scion interactions. We are presently exploring potential biophysical and anatomical differences related to weak graft unions of apple rootstock and scion varieties. As traits correlated with weak graft unions are identified, they will be useful to help growers avoid the rootstock-scion combinations that are particularly susceptible to tree failure.
A primary focus of the apple rootstock breeding and evaluation program at USDA-ARS/Cornell Univ. has been to develop screening protocols to identify genotypes resistant to the fire blight bacterium (Erwinia amylovora). Direct inoculation is a simple technique, but does not represent the only mode by which rootstocks become infected in the orchard. Selection based on direct inoculation screens may, however, enrich the population for resistant genotypes. Large breeding populations from controlled crosses are shoot-tip inoculated with E. amylovora, and the fraction showing the highest levels of resistance are retained for further evaluations. These survivors are again screened through direct inoculation in the field, and the less-resistant genotypes are discarded. Following selection for other pathogen tolerance and horticultural characters, elite genotypes are multiplied through asexual propagation. Replicated tests using direct inoculation with multiple strains of E. amylovora are then used to estimate the level of fire blight resistance of elite genotypes. A final screen utilizes mature, grafted orchard trees to verify that the resistance of rootstock genotypes to fire blight is maintained under conditions simulating natural infection. Direct inoculation screening and selection have resulted in a high frequency of strong resistance to severe fire blight epidemics in recent orchard inoculation trials.