Production of low-potassium onions based on mineral absorption patterns during growth and development

The production of low-potassium onions (Allium cepa) for patients with kidney disease is an important horticultural goal because onion is one of the most highly consumed vegetables in the world. To advance progress toward this goal, we conducted two experiments. In Experiment 1, we monitored the potassium content of each organ of the onion cultivar ‘Kaizuka Wase Ki’ during growth and development to determine whether potassium accumulates in the bulb as a result of translocation from other organs or direct absorption from the roots by hydroponic culture using granular polyester medium. We found that the plants developed green leaves under a short-day (12 -h) photoperiod and thickened their leaf sheaths and formed bulbs under a long-day (16 -h) photoperiod. The total potassium content of the green leaves decreased from 3 weeks after the start of the long-day treatment, which was thought to be due to the transfer of nutrients to the bulbs during bulb formation. The potassium contents in bulbs increased by 42.9 mg from week 12–15 after transplanting, it is considered that 14.2 mg of potassium was translocated from the leaves to the bulbs, while the remaining 28.7 mg was directly absorbed by the roots. In addition, there appeared to be a homeostatic mechanism in the onion bulb that maintained the potassium content at approximately 100 mg / 100 g fresh weight, so a strategy to overcome this homeostasis is considered necessary for the production of low-potassium onions. In Experiment 2, the supply of potassium after a long-day photoperiod was reduced to 75 %, 50 %, 25 %, and 0% of the usual amount in an attempt to reduce the potassium content of the bulbs by the same culture method as Exp. 1. This showed that decreasing the potassium supply to 25 % and 0% reduced the potassium content of the bulbs to 184.0 g and 146.7 g, respectively, compared with 303.5 g for 100 % supply. These results show that the extreme restriction of potassium fertilization after the bulb enlargement period is effective for overcoming the homeostatic mechanism, resulting in the production of low-potassium onions.