| Leaves determine the industrial yield of tobacco. The advance of reproductive growth (namely early flowering) and the early occurrence of axillary buds are unfavorable to the formation and growth of leaves. Meanwhile it seviously affects the yield and quality of tobacco leaves. Using tobacco cultivar K326 as the experiment material, we have focused on the main inducing factors causing early flowering and early occurring of axillary buds in middle and below axils of tobacco stem. The sensitive phase to low temperature, signal material of early flowering of tobacco, effect of roots on development of tobacco, sprouting and growing mechanism of axillary buds were studied in controlled-environment chambers by pot simulated experiments. We also went further into their regulative measures. The results were as follows:1.On the bases of our studies and field production situations, it is suggested that the flower bud appears before 1 8-20th leaf belongs to the early flowering. The main factor causing flue-cured tobacco early flowering is the low temperature stress at seedling stage and field early stage.2.Tobacco seedlings were not sensitive to low temperature induction before the 6th leaf. After 6th leaf tobacco reached the sensitive state to low temperature. The longer induction of low temperature on the tobacco plant during sensitive state, the fewer numbers of leaf when flower buds appearing. When Low temperature treatment lasted to 7th leaf, the leaf numbers decreased by 3-5 leaves when flower bud appeared; lasted to 9th leaf, decreased by 4-7 leaves; lasted to 12th, decreaed by 12-16 leaves. But tobacco plant must have the primary leaf numbers before flroal differentiation under low temperature. The cultivar K326 started floral differentiation after 13th leaf under 1 1-12 癈 low temperature stress.3. Induced by low temperature at seedling stage, the growth of tobacco roots was inhibited especially after the 12th leaf. Under the low temperature tobacco roots lacked of rapid growing stage, compared with control. So, the volume and fresh weight of roots were far smaller than those of the control. And the reproductive growth of tobacco was advanced. The low temperature induction on roots at the early field stage also inhibited the growth ofroots, and accelerated the development of tobacco. But the more noticeable influence on the time of floral transition was the induction on seedling at seedling stage. So the low temperature of roots was not the main cause of early flowering. Leaves and stem were more sensitive to low temperature induction than roots. It seemed that under low temperature both up ground and under ground parts of tobacco plant contribute to shorten vegetative growth cycle, and the up-ground was more responsible.4.After the 6th leaf stage, low temperature could cause the early flowering of tobacco in varying degrees. Under low temperature stress, the changes of internal substance of tobacco plant are given as following:(1) Changes of hormones level: In terminal buds, the IAA content substantially decreased; the ABA content was obviously higher than that of the control between the 10th leaf and the 12th leaf stage, the peak of ABA appeared at the 12th leaf stage; the contents of ZR and GA3 slightly lower, after the 13th leaf ZR content was higher than that of the control. In roots, IAA content became higher than that of the control after the 9th leaf stage; ZR apparently increased after the 12th leaf. It is suggested that the decrease of IAA and the increase of ABA promote the floral transition of flue-cured tobacco, and the higher ZR content accelerate the floral differentiation.(2)Changes of the metabolism of carbon and nitrogen: Under low temperature, the concentrations of soluble sugar, starch and total sugar of leaves declined. The sucrose declined in the early stage, then increased after the 10th leaf and higher than that of the control. The free amino nitrogen concentration was not seriously affected by low temperature in early stage, after the 10th leaf decreased. As a result the...
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