Effects of nitrogen supply on bulb development in onion Allium cepa L

Onions were sub-irrigated with 0.6, 2.4, 4.8 and 12 mol m-3 nitrate while growing in constant photoperiods of 14, 15 or 16 h. The lower the nitrogen, the later bulb scale initiation occurred and in 14 h photoperiods bulb scale, initiated only at the highest N level. In the early stages of the experiment, the ratio bulb width/neck width was higher in low N than in high N treatments, but the reverse was true at the later harvests, by which time bulb scales had formed. In a second experiment onions were sub-irrigated continuously with 0.6 mol m-3 or 12 mol m-3 nitrate or transferred from the low to high, or from high to low N after 36 d of growth in 14, 15.25 and 17 h photoperiods. An initial period of low N delayed bulb scale development and ripening relative to high N throughout, particularly under 14 h photoperiods. Late N deprivation did not prevent or delay bulb scale initiation or ripening. In a field experiment, plants not fertilized before sowing but given the total quantity of N late in the growing season formed mature bulbs later than plants fertilized with N before sowing, or given half the quantity of N before sowing and the second half late in the growing season.