| Tropical and temperate maize germplasm and hybrids of them were used to study the value methods of utilization of tropical germplasm in high altitude and temperate region, to compare the heterosis and combining ability of different cross pattern. The results showed: 1. The effect of introducing tropical germplasm into temperate germplasm in north spring maize region was fine. Open pollination for three to four times of random could increase the chance to obtain elite inbred line in converted population. The frequency of selected elite inbred line decreased in the following order: tropical×(temperate)2 →(tropical×temperate A)×temperate B → tropical ×temperate. The development of small allied population could increase the ratio of obtaining elite inbrid line. The principle to select inbrid line from tropical converted population should be the use of large population and heavy selection presure. 2. Yield characters of temperate germplasm could be obviously improved by converting tropical germplasm through the increase of ear length, kernels per row, volume weight and 100 kernel weight. 3. Layers of aerial root were increased, root and stalk lodging, root resistance to pulling force, stalk phloem strength, root and stalk lodging resistance and stalk physical structure of temperate germplasm could be inhanced by converting tropical germplasm. 4. Although Fusarium moniliforme and Curvularia lunata of temperate germplasm could be obviously improved by converting tropical germplasm, but it was highly infected by Sphacelotheca reiliana. Resistance to Exserohilum turcicaum and Helmintho sporium maydis was as the same as that of temperate germplasm. 5. Content of chlorophyll, stay-green period of temperate germplasm could be obviously increased by converting tropical germplasm, so that there was enough leaf area for source in the late filling period. 6. The photosynthesis rate of ear leaf in the late filling period of temperate germplasm could be obviously increased by converting tropical germplasm. 7. Plant hight, leaf area per plant of temperate germplasm could be obviously increased by converting tropical germplasm, and there were more leaves and larger leaf area at upper level of a plant, the internode was also longer. 8. The heterosis of yield per plant was high. The heterosis in cross of tropical×temperate germlasm was higher than that of tropical×tropical and temperate×temperate ones, owing to increased the length of ear, kernels per ear, 100 kernel weight. 9. Plant hight was transgressive inheritance and over CK, the average of transgressive inheritance were: Tropical × tropical 38.0%, tropical×temperate 40.7%, temperate×temperate 0.5%. The range of over CK were: tropical×tropical cross were all, tropical×tropical were 90 percent, temperate×temperate were half. 10. The heterosis of branch of staminate inflorescence showed a trend of tropical × tropical > tropical × temperate > temperate ×temperate, and the involvement of some temperate germplasm such as B73, Mo17 and Dan 340 was effective in decreasing the plant hight. 11. The gene action for root resistance to pulling force and stalk phloem strength was additive and the heterosis values of those characters were all negative. 12. Most charactors of tropical inbred line CML116 demonstrated high GCA, it is a useful inbred line. 13. The tropical inbred lines CML92 and CML40 are in the same heterotic group, CML27 and CML107 belong to same group and CML116 and temperate inbred line Huangzao 4 are in another group. 14. Stalk phloem strength and root resistance to pulling force were closely correlate with the length of seeding to silking, length of seeding to pollen sheding. ASI, leaf area per plant and stalk diameter were less correlate with the filling rate during 10-20 days, 0-40 days and 0-20 days after insemination. 15. Plant height was correlate with the length of fourth, fifth, eighth, ninth node and the average length of node was not correlate with the second, eleventh, twelfth and thirteenth nodes. 16. Leaf area per plant was correlate with the sixteenth, seventeenth, eighteenth, nineteenth, twentieth and twenty-first leaf, was less correlate with the fourth, fifth, sixth, ninth, tenth, eleventh leaves.
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