Turnip mosaic virus (TuMV) is the most important virus of commercially grown cole crops in many Asian countries, affecting both yield and quality. TuMV-infected Chinese cabbage becomes unmarketable because of the presence of black spots and necrosis often induced by the virus . Resistance breeding is complicated by the existence of five strains of the virus, one of which was discovered in 1985 for the first time in Taiwan . Resistance to strains C1 to C3 is readily available among the Chinese cabbage germplasm at AVRDC, whereas resistance to strains C4 and C5 is rarely found . To elucidate the inheritance of resistance to TuMV, P,, P 2 , F,, F2 and BC, generations of crosses between the resistant line`0-2' and three susceptible lines,`E-7', `E-9' and 'FL-9', were inoculated with strains C4 and C5 . Segregation ratios obtained by visual observation and enzyme-linked immunosorbent assay (ELISA) indicate that two recessive genes confer resistance to both TuMV-C4 and TuMV-C5 .
Variations with regard to heat tolerance, in terms of heading ability, and water consumption under high temperatures were studied in various Chinese cabbage (Brassica campestris spp . pekinensis Rupr .) varieties .Total water consumption did not differ among entries ; however, heat tolerant varieties had greater water uptake than heat sensitive ones at the onset of head formation . Heat tolerant varieties also possessed thicker leaves, higher leaf sap electrical conductivity and chlorophyll content, and lower stomatal number . These characteristics appeared to facilitate water transport to the leaves and reduce transpiration, thus enabling the heat tolerant plants to maintain leaf turgidity during the heading stage at high temperature . Vigorous root growth also seems essential to supply adequate water to the leaves so that they can maintain good turgor at high temperature . Breeding for heat tolerance in Chinese cabbage may be accomplished indirectly by selecting for plants which possess relative thick leaves, high levels of electrolytes, high chlorophyll content, few stomata, and vigorous root growth .
This was a 5-year study that included surveys during the first 2 years, followed by field evaluations of improved tomato lines over the next 3-year period. Surveys were conducted in 12 regions of Tanzania from 1993 to 1995 to identify the causal nematodes of tomato root-knots. Eighty-seven samples were collected and assayed for presence of Meloidogyne spp. M. hapla, M. incognita, and M. javanica were detected at incidences of 1, 19, and 89%, respectively. Species mixtures were common within samples. Results indicated the presence of races 1 and 2 of M. incognita in tomato-growing regions of Tanzania. Reactions of eight advanced tomato breeding lines to the three species in greenhouse and field tests indicated that three lines, namely ARP 365-2, 367-1, and 367-2, were resistant to M. incognita races 1 and 2 and to M. javanica, but all lines were susceptible to M. hapla.
Early generation breeding lines of Chinese cabbage (Brassica campestris L., Pekinensis group) produced easily discernible matroclinal diploids in crosses with radish (Raphanus sativus L.). Heterozygosity for a single gene marker among a number of these maternally derived plants precluded post-meiotic doubling or fusion of haploid nuclei within the same embryo sac followed by parthenogenesis as their modes of origin. Selfed progenies descended from matroclinous plants were also observed to be relatively more variable for metrical characters than related conventional inbreds. The matroclinal diploids examined in this experiment were therefore heterozygous. Consideration of this and other limitations led to the conclusion that these plants as well as the system to secure them do not have any direct useful application in Chinese cabbage breeding.
F2 and backcross segregation for heat tolerant × heat sensitive crosses in Chinese cabbage (Brassica campestris L. Group Pekinensis) indicated that heat tolerance was controlled by a single recessive gene.
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