Changes in ratio of soluble sugars and free amino nitrogen in the apical meristem during floral transition of tobacco
Under a modification of the nutrient diversion hypothesis, we propose that an inequality in carbohydrate and nitrogen translocation to the apical meristem may be a controlling factor in floral transition. Experiments were conducted in controlled-environment chambers to determine the associations between microscopic characteristics of the transition from vegetative to floral stages of the apical meristem of flue-cured tobacco and to assimilate concentrations in the plant and apical meristem. Low temperature, nitrogen withdrawal, and restriction of nitrogen uptake were used as treatment variables. In all of these stress treatments, flowering occurred at a lesser number of leaves than in control treatments. Low temperature stress accelerated the time of transition to the floral stage as compared with a high temperature control; however, nitrogen stress did not accelerate the time of transition. All stress treatments affected the levels of nitrogen and carbohydrate in whole plants. Most notable was an increase in the percentage of starch and a decrease in the percentage of total soluble carbohydrate induced by the stress treatments. These data indicate that tobacco plants under stress accumulate excess carbohydrate in the form of starch. An apparent inequality in the relative concentrations of carbohydrate and nitrogen in the apical meristem was observed in all treatments at the time of floral transition and is in support of the nutrient diversion hypothesis.