Effect of Cross-cutting and Temperature on Shoot and Root Growth of Onion Bulb

Abstract

Several attempts have been made to stimulate sprouting of dormant onion bulbs. Lumis and Evans (1928) promoted sprouting by crosscutting, splitting, and injecting water into onion (Allium cepa L.) bulbs. However, in another study, bulbs with ≈50% of the upper portion removed by cross-cutting, planted in August in Texas and New Mexico for seed production, did not sprout (unpublished data). This observation implies that temperature may play an important role in shoot growth of onion bulbs. Therefore, we determined the effects of removing bulb tissues by crosscutting (i.e., removal of a portion of the bulb) and growing bulbs in various temperatures on shoot and root growth in dormant onion bulbs. ‘Texas Grano 1015Y’ onion bulbs (5.0 to 8.8 cm in diameter) were harvested on 25 Apr. and held at 24C for 2 weeks for curing. They were cross-cut at half (50% removal) or onequarter (75% removal) of their height (measured from the root plate). The control remained whole. The bulbs were planted in trays (38 × 58 × 14 cm) filled with wet vermiculite and covered by transparent plastic film to maintain humidity. Each removal treatment consisted of three trays with 20 bulbs each. Three trays, one from each treatment, were placed in growth chambers set at 15, 25, or 30C with 24-h fluorescent light (15 μmol•m•s) for 4 weeks. Leaf length, including the internal portion in the bulbs, and root fresh weight were measured. Shoot growth was promoted by removing portions of bulbs at 15 and 25C; 75% removal resulted in the largest shoots (Fig. 1, top). However, shoot growth was negligible at 30C regardless of the removal percentage. Shoot growth generally was less with higher temperatures. There was a highly significant removal percentage × temperature interaction. This interaction seemed to reflect the nonlinear response of shoot growth to temperature.