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51.
Summary Powdery mildew development was assessed on squash (Cucurbita pepo) plants of a susceptible cultivar, a resistant accession, their F1, and their F2 in an early summer planting in the field, covered or not covered with a shading net. Three reaction types were observed: susceptible, powdery mildew on stems and on both, the upper and lower leaf surfaces, as in the susceptible parent; resistant, no powdery mildew on leaves or stems, as in the resistant parent; and partially resistant, powdery mildew on upper leaf surfaces only, as in the F1. Disease presence on the stem was associated with susceptibility. Shading hastened the appearance of powdery mildew and increased the severity of infection on partially resistant and susceptible plants, facilitating identification of resistant individuals in the F2 population.Contribution No. 1613-E from the Agricultural Research Organization, Bet Dagan, Israel 相似文献
52.
Summary
Lycopersicon pimpenellifolium L3707, resistant to the late blight oomycete Phytophthora infestans was crossed with the susceptible Lycopersicon pimpenellifolium 14377 or the susceptible Lycopersicon esculentum ZH. Progeny F1 and F2 generations were scored at the 5-leaf stage for resistance against 175 field and recombinant isolates of the pathogen. F1 plants exhibited various levels of moderate resistance and F2 plants segregated 3:6:7 resistant/moderately resistant/susceptible. The data support the hypothesis that race-non-specific resistance in L3707 is controlled by two independent genes: a partially-dominant gene and a dominant epistatic gene. 相似文献
53.
Summary Seedlings of Trifolium repens showed considerable variation with regard to the morphology and growth of their calli, and their ability for in vitro differentiation of shoots. One of the lines selected for regeneration in primary callus cultures also showed shoot formation from protoplasts. Somatic embryogenesis in callus cultures of T. pratense and T. arvense occurred only in selected seedling lines. This paper highlights the importance of screening a large number of plants within a cultivar of outbreeding species to achieve reproducible plant regeneration from tissue culture. 相似文献
54.
Summary Several varieties of Cucurbita moschata were found resistant to cucumber mosaic virus (CMV). Inter-specific hybridization between C. pepo and a CMV resistant variety of C. moschata resulted in partial fertile hybrids which set F2 seeds. Cotyledonary inoculation of F2 seedlings with CMV brought various results, depending on seasonal differences and developmental stages at which observations were made. Resistance to CMV generally declined under conditions of low temperature, short days and low light intensity. 相似文献
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56.
Cohen J 《Science (New York, N.Y.)》2007,318(5855):1360-1361
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Cohen J 《Science (New York, N.Y.)》2000,289(5478):368-369
Although basic science and clinical research typically dominate at this biennial event, by far the most riveting sessions at the XIII International AIDS Conference here revolved around the thicket of issues that prevent poor people in sub-Saharan Africa--where a staggering 24.5 million people are estimated to be infected with HIV--from receiving the 14 anti-HIV treatments now used in wealthy countries. Pharmaceutical companies and donors last week loosed a flood of promises about removing barriers. Conference attendees welcomed the offers of help, but many stressed that much more will be needed. 相似文献
59.
The ionospheric modification experiments provide an opportunity to better understand the aeronomy of the natural ionosphere and also afford the control of a naturally occurring plasma, which will make possible further progress in plasma physics. The ionospheric modification by powerful radio waves is analogous to studies of laser and microwave heating of laboratory plasmas (20). " Anomalous" reflectivity effects similar to the observed ionospheric attenuation have already been noted in plasmas modulated by microwaves, and anomalous heating may have been observed in plasmas irradiated by lasers. Contacts have now been established between the workers in these diverse areas, which span a wide range of the electromagnetic spectrum. Perhaps ionospheric modification will also be a valuable technique in radio communications. 相似文献
60.
The Holleford Crater, a circular depression in southern Ontario, is filled with Paleozoic sediments and underlain by brecciated Precambrian igneous and metamorphic rocks. The presence of coesite in two core samples of this breccia has been established by petrographic and x-ray diflraction methods. Shattered quartz in the coesite-bearing samples exhibits planar fractures. The shocked quartz is the result of great shock pressures and the association of coesite with the shocked quartz strongly suggests that Holleford Crater originated from a hypervelocity impact. 相似文献