MECHANISATION IN VEGETABLE PRODUCTION

Mechanisation in Vegetable Production

Of onion bulbs

Mechanisation in Vegetable Production Of Onion Bulbs

Introduction

Onion (Allium cepa) is a popular vegetable grown for its pungent bulbs and flavorful leaves (American Heritage Dictionary 2007). It is widely grown throughout the world.

The bulb is composed of concentric, fleshy, enlarged leaf bases or scales. The outer leaf bases lose moisture and become scaly and the inner leaves generally thicken as bulbs develop.

The green leaves above the bulb are hollow and arise sequentially from the meristem at the innermost point at the base of the bulb. The stem is very small and insignificant during vegetative growth. After vernalization at temperatures below 10°C, the stem elongates rapidly (Needham 2006), eventually producing compound umbels. Bolting has been reported to be related to the length of day. However, long days do not induce reproductive growth but tend to accelerate development of the seedstalk once it has been initiated by vernalization. Temperature has a major role in inducing bolting (Dillehay 2007).

The onion root system is fibrous, spreading just beneath the soil surface to a distance of 30 to 46 cm. There are few laterals, and total root growth is sparse and not especially aggressive. Therefore, in monoculture, onions tolerate crowding, particularly in loose, friable soils such as peat and muck. Competition from aggressive root systems (as from weed growth) severely limits onion growth (Needham 2006).

Cultivars differ substantially with respect to the threshold daylength required for bulbing. Other factors such as temperature may interact with daylength to modify the bulbing response. In all cultivars, bulbing is accelerated with increasing temperature (American Heritage Dictionary 2007).

Temperature extremes not only affect the rate of bulbing, but also affect the bulb shape. Thick and elongated necks are common in plants exposed to 6° or lower (Dillehay 2007).

Cultivars

At one time, all onion cultivars were open-pollinated, and many of these cultivars are still offered by seed companies. The discovery of male sterility in onion led to a rapid change to F1 hybrids, possibly due to simplicity and low cost of seed production (Needham 2006). Male sterility is a genic-cytoplasmic factor, and male fertility can be restored in plants carrying the sterility factor by introducing a single dominant allele. Any line carrying the sterile trait must be cross-pollinated, and seeds harvested from male sterile plants isolated with a normal pollen-bearing parent will be hybrid seed. Hybrids have higher yield, larger and more uniform bulb sizes than open-pollinated cultivars (American Heritage Dictionary 2007).

The bulb onion cultivars are grouped into short, intermediate, and long-day types. Short-day onions (12 to 13 hour threshold) are generally mild, soft fleshed, and suitable for storage. Long-day onions (over 14.5-hour threshold), if grown in the lower latitudes, will not form bulbs, and only green onions would be produced (Dillehay 2007). In contrast, short-day types grown in the higher latitudes will bulb very quickly and will be little more than sets in size.

Short-day onions include the Bermudas and Grano-Granex types; long-day cultivars include yellow, white and red ...