Physiological studies on the bulbing and dormancy of onion plant. V.

The relations between the metabolism of carbohydrates, nitrogen compounds, and auxin and the bulbing phenomenon

Abstract

In this study, metabolism and effects of external supplies of carbohydrates, nitrogen compounds and auxin were examined to clarify the physiological mechanism of buling in onion plant.
1. Changes in carbohydrates and nitrogen compounds were followed in relation to bulbing process. It was shown that nitrogen content in leaf blades was high during rapid increase in plant height, then decreased as the bulb developed, and that carbohydrates, on the contrary, accumulated in leaf sheathes and their concentration reached a maximum in the early stage of bulb development, then decreased gradually towards the end of bulb thickening, though total amount of carbohydrates per plant was increasing.
Top-dressing of nitrogen fertilizer increased nitrogen content in leaf blades, enhanced photosynthesis, and promoted bulb thickening. Shading reduced photosynthesis, inhibited bulb thickening, and resulted in low content of nitrogen in leaf blades.
2. Increase in carbohydrate content and decrease in nitrogen content were induced by long day condition, and vice versa by short day condition.
3. External supply of sucrose by injection into leaf blades promoted bulb development under long photoperiod, while it induced vigorous vegetative growth under short photoperiod.
4. Changes in auxin of terminal part of buds and leaf blades of onion plant under long photoperiod correlated with the rate of increase in plant height. Following transfer to long day, rate of increase in both plant height and auxin content increased and reached maximum on tenth day after transfer, then falled to the values below the contents of plant grown under short photoperiod.
5. Auxin content in leaf blades of Aichi-shiro, an early variety, increased earlier than that of Senshu-Ki, a medium variety, but general trend in auxin content was the same in both varieties.
6. External supply of auxin by injection in leaf blades retarded plant growth and did not stimulate bulb formation under short photoperiod. Under long photoperiod, bulb developed irrespective of the auxin treatments, though bulb fulb formation was retarded a little as auxin concentration increased. These results seemed to show that auxin does not have direct relation to bulb formation.