The Bulbing Process in Onion in Response to Nitrogen Nutrition

Bulb onion (A ilium cepa L.) is the most important species of the Allium genus. It is the third most highly produced vegetable in the world (Brewster, 1994) and the fourth in Kenya (Anon, 1989). The demand for onions worldwide is fairly constant. It is a potential export crop from tropical regions including Kenya (Currah and Proctor, 1990; Brewster, 1994). Onion is a physiologically long day plant with respect to bulbing (Jones and Mann, 1963), temperate cultivars having a relatively longer daylength requirement than tropical ones (Brewster, 1994; Currah and Proctor, 1990). Bulbing is a process in the growth and development of onion when the plant changes from the bladed leaf production phase to the scale formation phase. During this period translocation of assimilates to the leaf bases occurs in response to the stimulus of long days (Heath and Hollies, 1965). Longer daylengths enhance bulb formation (Butt, 1968, Steer, 1980). For a given daylength however, high temperatures accelerate bulb formation (Wiles, 1989) while soil nutrients may delay the process (Currah and Proctor, 1990).