The effects of plant size and leaf number on the bulbing of tropical short-day onion cultivars (Allium cepa L.) under controlled environments in the United Kingdom and tropical field conditions in Sri Lanka

Summary

Experiments were conducted in controlled environment conditions in the UK and under tropical field conditions in Sri Lanka to determine the effects of plant size in similarly aged onion seedlings at transplanting on their subsequent growth and development. Results indicated that the size at transplanting, in terms of leaf number, leaf area and total weight, continues to affect the size of the plant until maturity. Under low light conditions (maximum PAR levels of 800 mmol m^±2 s^±1), and low night temperatures (minimum 118C) in the UK, transplant size was negatively correlated with time to onset of bulbing and maturity, and positively correlated with bulb weight, and percentage contribution from scale leaves to total bulb weight. However, under high light intensities (PAR level of >1500 mmol m^±2 s^±1) and high night temperatures (>208C) in Sri Lanka, transplant size was positively correlated with time to onset of bulbing and maturity, percentage of thick neck bulbs, and yield of good quality bulbs. It was negatively correlated to percentage bulbing and percentage contribution from scale leaves to total bulb weight. In practice it should, therefore, be possible to manipulate plant size during the nursery stage to optimize the timing of bulb production and final bulb size: under favourable conditions of light and temperature small transplants should bulb earlier and produce high quality bulbs whereas larger transplants may produce later and larger bulbs but with increased proportion of thick necks. Contrasting findings under low light and temperature relative to high light and temperature conditions are discussed in terms of a postulated requirement for carbohydrate and a critical concentration of a bulbing stimulus.