Tesfay, Samson Zeray.
Abstract:
Onions are widely produced within the tropics, but little scientific research has been done
specifically on the Eritrean cultivars, like Hagaz Red 1 and 2 (HR I, and HR 2). Many onion
cultivars are limited in their range of adaptation due to the combined effects of photoperiod
and temperature. A priority for research on the crop was to elucidate the local crop's growth
response to environmental conditions, particularly temperature and daylength.
The Eritrean cultivars HR I and 2 and an American (Louisiana) cultivar Red Creole (RC)
grown in South Africa were grown in growth rooms under all combinations of three
daylengths (11.5h, 12h, 12.5h) and three day/night temperatures (25/12°C, 30/15°C and
35/18°C). Growth responses were determined at 108 days and by using a growing degree day
(GDD) base. A broken-stick regression model was used to determine the points of inflection,
indicating the initiation of bulbing. Based on leaf area and plant height data, mathematical
differentiation equations and coefficient of determination (R2) were applied to determine the
base temperature (6.4°C) for these particular cultivars.
All three cultivars needed at least 12 h daylength for bulb initiation when assessed by a
bulbing ratio >=2.0. A bulbing ratio >=2.0 characterizes the onset of bulbing. Under a 11.5 h
daylength, a temperature higher than 25/12°C decreased vegetative growth. Temperature in
this region may be a supra-optimal condition for the growth of these cultivars at this
daylength. However, the 25/12°C and 30/15°C temperatures were found to be ideal for onion
bulb production under 12 hand 12.5 h daylengths. The three cultivars (HR I, HR 2 and RC)
showed very similar growth response to the daylength and temperature interactions.
The thermal presentation of plant growth indicated that there were relationships between bulb
initiation and rate of leaf area growth under inductive conditions (12 hand 12.5 h). Under the
12 h daylength, cultivars needed 343, 482, and 597 GDD units before bulb initiation and 405,
432, and 431 GDD to increase the rate of leaf area development at 25/12°C, 30/15°C, and
35/18°C, respectively. Under a 12.5 h daylength, these cultivars needed 344, 423, and 432.2
GDDs to initiate bulbing and 140, 411, and 579 GDDs to increase leaf growth rates at
25/12°C, 30/15°C, and 35/18°C, respectively. In the 12 h daylength, bulbing was initiated
and followed by an increased rate of growth of leaf area. However, the reverse happened for
the 12.5 h daylength. Overall, where plant response to temperature can be expressed as the
rate of progress towards a morphogenetic change, GDD values can be used to predict a plant
developmental stage at a particular temperature. It must be concluded that temperatures
induced significant variations in growth components (leaf number, plant height, leaf area),
and affected bulbing response.
The findings in this study confirmed that the cultivars require only a certain fixed amount of
thermal time for their development at a particular temperature, and that, if anything, the slow
growth rate at the higher temperature must be due to supra-optimal temperatures. They also
require a minimum 12 h photoperiod for bulb formation.
