| Based on their geographical distribution, the maize landraces from Sichuan, Chongqing, Guizhou and Yunan in southwest China were used in the study. Genetic diversity of 54 maize landraces was tested by using microsatellite (SSR) loci and their B chromosomes were observed cytologically. In addition, a field trial was conducted to analysis agronomic and economic traits, and investigate main morphological and physiological changes of different maize landraces to low-P stress at the stage of seedling. The main results as following:1. Variances of maize landraces in all agronomic and economic traits were significant at 0.01 level. With regard to their ranges among landraces, plant height, ear height, total leaves, flowering period, silking and growth period were 213.25～322.98, 87.30～198.59, 15.50～24.38, 69.50～94.50, 70.50～93.50 and 114.00～142.00, respectively. The ranges of ear length, sterile length, ear diameter, axis ear, rows per ear, kernels per row, kernel weight per ear, 100-kernel weight and unit weight were 8.40～18.32, 0.30～2.28, 2.49～4.88, 1.51～2.74, 8.70～17.05, 16.17～31.39, 26.57～161.68, 9.00～36.03 and 245.00～739.00, respectively. In comparison of the variation coefficients between agronomic and economic traits, the latter was higher than the former. The ranges of ear height, total leaves, silking period, flowering period, plant height and growth period were, in turns, higher. Accordingly, those of sterile length, kernel weight per ear, 100-kernel weight, kernels per row, ear length, rows per ear, ear diameter, axis ear and unit weigh, in turns, higher. On the whole, maize landraces exhibited various strains, a large phynotypic varition, high plant individuals and resistance to disease. Based on the principal component of all agronomic and economic traits by Principal Component Analysis (PCA) as well as the goals of maize breeding, 10 landraces with excellent traits DP-11, DP-44, DP-42, DP-31, DP-65, DP-19, DP-60, DP-15, DP-57 and DP-13 were chosen from the materials studied. The results of the clustering analysis indicated that obvious differences existed in agronomic and economic traits of landraces with the same geographical origin, and there maight be similar traits in landraces with the different geographical origin.2. Genetic diversity of 54 maize landraces was tested by using bulk DNA samples and 42 microsatellite (SSR) loci distributed uniformly on 10 chromosomes of maize. A total of 256 alleles were detected among 54 landraces. At each locus, the number of alleles varied from 2 to 9, with an average of 6.1. Mean polymorphism information content was 0.78 ranging from 0.34 to 0.78. 54 landraces could be clustered into four groups by the clustering analysis based on the genetic similarity coefficients. The landraces collected from the same region could mostly be grouped together and their genetic similar coefficients were over 0.6. Comparison of the clustering result of agronomic and economic traits and that of SSR, no consistent relationship was found. This implied that DNA differences revealed by SSR were different from those by PCA. To reveal the genetic structure and genetic diversity within landraces, 165 individuals in total from 11 out of 54 landraces were analysed on the basis of the same 42 SSR loci. The analysis of individual DNA samples was proved superior to that of bulk DNA samples to identify genetic diversity of landraces. A total of 330 alleles were found in 11 landraces. Estimates of the mean number of alleles 'A', the effective allelic number 'A_e', the observed heterozygosity 'H_o' and expected heterozygosity 'H_e' were 7.86, 3.90, 0.69 and 0.37, respectively. An obvious genetic deviation from Hardy-Weinberg expectation was observed both among and within landraces and a considerable genetic variation was revealed within rather than among landraces. The results of Principal Component Analysis (PCA) were consistent with those of the clustering analysis and the genetic distance. Individuals within a region and a landrace were grouped more closely while the individuals from the different landraces and regions were located more distantly. According to genetic diversity within landraces, the landraces from Sichuan were the highest genetic variation, allelic frequency and gene heterozygosity. It indicated genetic diversity of landraces was more plentiful in Sichuan than in other 3 regions.3. B chromosomes (Bs) in 54 maize landraces from the four regions was tested by means of cytological observations. General B characteristics was found in the B chromosome of maize landraces. The aberrant distribution and separation of B chromosome might result in their differences in plant individuals and cells. Out of 54 maize landraces, 9 landraces with Bs were observed. the number of Bs was found variable. The number of Bs in the landraces ranged from 0 to 7 and varied from 0 to 3 in a single cell. The 12.96, 5.56 and 3.70% of the total landraces were found with 1B, 2B and 3Bs. It was indicated that southeastern Sichuan was the main distribution area of the landraces with Bs in southwest China. The relationship between B chromosomes and DNA polymorphism based on SSRs was not found in the study. However, the geographical distribution of B chromosomes, together with the genetic diversity of the maize landraces, supported that maize landraces in southwest China were firstly introduced to Sichuan from India via Tibet to a certain degree.4. Using two P treatment, a randomized complete block design with two replications was designed to investigate biological changes of different maize landraces. The results showed that P-deficiency significantly decreased root volume, total leaf area, and plant dry weight, but greatly increased density of root hairs and root top ratio. In addition, P-deficiency induced the significant enhancement of phosphorus utilization efficiency and the amount of proline, malondialdehye (MDA), acid phosphatase (APase), peroxidase ( POD) and superoxide dismutase (SOD), but the significant reduction of P uptake and soluable protein content. Since P-deficiency had smaller effects on the P-tolerant maize landraces DP-60, DP-02 and DP-40 as compared with P-sensitive landraces DP-36 and DP-27, it was demonstrated that differences of tolerance to P-deficiency existed among different maize landraces. Dry matter weight and P uptake were suggested as reliable screening standards to identify low-P intolerant germplasm in landraces and Apase activity as physiological one. The results based on correlation analysis also indicated that it was feasible to rate low-P intolerant levels by P-deficiency symptoms. According to the effects of low-P stress on economic traits and the results discussed above, low-P intolerant landraces DP-60, DP-02, and DP-41 were found. At the same time, medium-P intolerant DP-12, DP-48, DP-54, DP-59 and DP-65 were also selected.5. On the whole, maize landraces exhibited a large phynotypic varition and resistance to leaf blight, leaf spot as well as brown spot of corn. Using 42 SSR loci, high genetic variation was found among landraces with a total of 256 alleles being detected. At the same SSR loci, 330 alleles in total were found in 11 landraces with a high expected heterozygosity 'H_e' (0.67). There were various strains in landraces. Their growth period varied from the early to late maturing and the kernels had yellow, red, white and blue flint, as well as yellow, red, white and blue dent. Rich low-P intolerant germplasm was also found. 10, 7, 9, 3 and 5 landraces with excellent traits, high genetic variation, B chromosomes, low-P intolerant and medium-P intolerant, respectively, were chosen from the materials studied. However, most landraces exhibited poor in plant height and resistance to lodging. Their plant and ear height were 2.64 and 1.38 m, respectively. Since direct utilization on maize landrace germplasm is difficult in maize breeding, it is suggested that the germplasm is utilized indirectly by using genetic innovation of landrace populations, mass selection between landrace and exotic germplasm, and recurrent selection between the landrace and tropic germplasm.
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