In order to establish the management system of onion plug seedlings for mechanical transplanting and to improve the onion bulb productivity, we evaluated the effects on plug seedling productions by different sowing environments (plug tray covering materials, plug cell size, sowing depth and position in cell, and loading rates of media soil, etc.), seedling environments (seedling places, treatments of growth regulators, cutting roots and leaves for controlling the plant length, and seedling age, etc.), and transplanting environments for root establishment of the mechanically transplanted plug seedlings.
In a selection test for plug tray covering materials, the treatment of double covering with the non-woven fabric and the reflecting sheet resulted in the lower change in media temperature and water content in cell than other treatments. Germination rate was 93.4% in the treatment, significantly higher than 88.2% of the shade sheet treatment (control). Plant growth parameters such as plant length, bulb diameter were higher than the control treatment. The labor demands for irrigation and tray covering work were 5.9 hour/ha in the treatment, shorter than the control treatment because of the reduction of labor demands for irrigation.
The deeper seeds were sown, the lower root exposure and root inversion ratio. Sowing in the middle of cell decreased the root inversion ratio and built up the development of root system as compared with the edge sowing. As the media loading frequencies increased, the root exposure ratio increased, but the water content of media and the stability of seedling plugs was higher for 3 days after irrigation than the control. The older seedling of the plug tray resulted in the better growth. In 348-cell tray, the growth parameters such as leaf number, plant length and plant sheath diameter were better than other trays.
Seedling growth along with tray positions was better in the bare-ground seedlings than in the bench one. Marketable bulb yield was also higher in the bare-ground seedlings. In a selection test for tray bedding material, high density polyethylene net decreased up to 88% in the labor forces for tray loading and root cutting than the control treatment of 51.9hr/ha. In the control and the treatment of high density polyethylene net, seedling growth was better, and the average bulb weight and the marketable yield were significantly higher than other treatments.
The growth retarding effects on the plant length as affected by irrigation intervals was not shown in the initial seedling stage, but was better in three days of the irrigation interval in the late stage. Root cutting at the first leaf stage decreased seedling height as compared with the control treatment or the late treatment, but it increased extra works to re-cut owing to the increased leaf drying. Chemical growth regulators such as Tebuconazole and Diniconazole decreased the number of leaves and height, but the leaf width and length were retarded as compared with the control treatment, when the regulators were applied 2 times at the cotyledon and first leaf stage. However, marketable yield of the onion applied with the regulators produced 15∼20% as low as the control.
Leaf cutting date and plant height did not affect the onion growth and bulb yield. During the seedling period, the leaf cutting at 5∼7 days interval from 20∼25 days of seedling showed the shortest shoot length. The longer seedling periods showed the better root length as well as increased root density. In addition, the root density of seedlings increased at the bed with the non-woven fabric as compared with the soil bed.
Plant height and root length increased at transplanting on 31 October, as compared with transplanting on 11 November. As onion seedlings were transplanted earlier and irrigated earlier after transplanting, the numbers of roots, root length as well as the marketable bulb yield increased.
In conclusion, to nurse the plug seedlings with the plant height of 15 cm adaptable for the mechanical transplanting, onion seeds should be sown in depth of 1 cm and at the center of the tray cell. Double covering with non-woven fabric and reflecting sheet as well as bedding with the high density net could be effective for the optimum seedlings. Additionally, the leaf cutting at 5∼7 days interval in 20∼25 days after sowing during the seeding period of 45∼55 days, and the irrigation at transplanting are necessary to achieve the successful mechanical transplanting.