| In this paper, the Burley21 and Maryland609 were chose as the experimental plant materials, and the heredity of yellowing leaf color and the variation of plastid pigment contents and the mechanism of this variation between Burley21 and Maryland609 were researched. The research results of this paper will give the experimental proof for the causes of burley leaf color, the aroma quanlity of white burley as well as green-deficient mutant in other higher plants. The main researches and conclusions of this paper were as follows:1. The leaf color heredity of white burley was studied by way of construction of field breeding groups. The results found that the F1 generation plants of reciprocal cross between Burley21 and Maryland609 showed the normal green type, and the BC1 generation, F2 generation have both normal green and white burley type. Statictical analysis showed that the ratio of normal green type and white burley type in F2 generation and BC1 generation were 12.5:1 and 4.9:1, respectively. Chi square test showed that the segregation ratio of F2 generation and BC1 generation were consistent with the inheritance law of double-recessive trait. These results further estimated that the leaf color in white burley was the double-recessive trait controlled by nucleus.2. Continuous observation found that the white vein trait in Burley21 can be seen during the whole growth and development period. There was no significant difference between Burley21 and Maryland609 in leaf color before the vigorous growing period, but the yellowing leaf color trait of Burley21 can be seen clearly from the vigorous growing period.3. Chlorophyll and carotenoid contents in the leaf mesophyll and vein tissue of Burely21 and Maryland609 were determined using spectrophotometer during the development of temporary planting period, rosette period, vigorous growing period, flower budding period and mature period, respectively. The results indicated that the contents of chlorophyll in the leaf vein of Burley21 was significantly lower than in that of Maryland609 at the earlier growing periods, but there was no obvious difference in the contents of carotenoids during all those five periods. There were no significant differences in chlorophyll and crotenoid contents before vigorous growing periods in the leaf mesophyll, but from vigorous growing period, the difference of chlorophyll content between the two tobacco varieties was significantly at the same growth and development period. The difference of chlorophyll contents in leaf mesophyll ranked the largest at the mature period, and the chlorophyll contents in Burley21 was 3.35 times as high as in that of Maryland609. The contents of crotenoid in the leaf mesophyll of Burley21 also lower than in that of Maryland609 at the same growh and development period from vigorous growing period, but these differences were much smaller than chlorophyll contents.4. Seven kinds of precursors in the biosynthesis of chlorophyll in leaf mesophyll of Burley21 and Maryland609 during viogous growing period were measured. The results showed that there were no significant difference in the contents of AlA, PBG, UrogenIII, Coprogen III in leaf mesophyll between Burley21 and Maryland609, but the contents of Protoâ…¨, Mg-Protoâ…¨and pchlide in Burley21 were significantly lower than in that of Maryland609. This indicated that the synthesis of chlorophyll was inhibited during these steps, and this might led to the lower chlorophyll contents and yellowing leaf phenotype of Burley21 from vigorous growing period. It can be supposed that the mechanism of yellwong leaf phenotype might relate with the mutation of gene in the chlorophyll biosynthesis.5. Chlorophyllase is the rate-limiting enzyme in the degradation of chlorophyll(Matile et al.,1999; Gan et al.,2002; Tang, 2002). The activities of chlorophyllase as well as its gene expression level in the leaf mesophyll of Burley21 and Maryland609 in vigorous growing period were measured using the technologies of spectrophotometer and real-time quantitative PCR, respectively. The results showed that the activities of chlorophyllase in leaf mesophyll of Burley21 and Maryland609 in vigorous growing period were 525.17nmolg-1h-1 and 257.60nmolg-1h-1. The relative activitiy of chlorophyllase in Burley21 leaf was 2.04 times as high as in Maryland609. The gene expression level of chlorophyllase analysized by real-time quantitative PCR showed 2.01 times higher in Burley21 than in that of Maryland609. These results indicated that the increased activities and its gene expression level of chlorophyllase in the leaf mesophyll of Burley21 in vigorous growing period resulted in the speed up chlorophyll degradation and yellowing leaf color of Burley21.6. The gene copies of chlorophyllase in Burley21 and Maryland609 were studied by using the Southern blotting method. The results indicated that the chlorophyllase was encoded by single-copied gene in both Burley21 and Maryland609. 7. Using Gas Chromatography-Mass Spectrometry (GC-MS) technology, the volatile aroma ingredients produced from the degradation of chlorophyll and caroteniod in the tobacco leaf of Burley21 and Maryland609 were analyzed. The results indicated that sixteen kinds of aromatic components detected from the degradation of these plastid pigments in all volatile aroma ingredients, and in which of sixteen kinds of aromatic components, five kinds were get from the degradation of chlorophyll and the other eleven kinds were from that of caroteniod. But the total contents of these eleven kinds degradated from carotenoid were lower than that of five kinds degradated from chlorophyll. The contents of neophytadiene was the highest in all these detected aroma compounds in both tobacco leaf of Burley21 and Maryland609. We might concluded from these results that the plastid pigments were degradated more fully in curing process in white burley than in that of Maryland tobacco, and this affected the the unique arom quality of white burley.
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