Agriculture Reference
In-Depth Information
Fig. 4.2.
The change of total shoot (solid lines) and bulb (broken lines) dry weight
with time of irrigated, well-fertilized onion crops when grown at 400 (upper graph)
and 50 plants/m
2
(lower graph). A, autumn-sown, S, spring-sown (from Brewster
et al
., 1986. Courtesy of
Annals of Botany
).
will be low as harvest approaches in onions, since they have a large proportion
of the total weight in the bulb (see (iv) above). The overall maintenance
respiration rate of bulb onions close to the normal harvest time (i.e. with 81%
of the shoot dry in bulbs) was estimated as 0.0035 g CO
2
/g dry matter/day,
only 60% of the value of a beet crop at a comparable stage (Tei
et al
., 1996).
The net result of these processes, total dry biomass production, has been
investigated in onions. Well-irrigated onion crops produce, during bulbing, an
average of about 1.6 g of shoot dry matter per MJ of solar radiation intercepted
by the leaf canopy. Similar conversion efficiencies have been reported for
potatoes, sugarbeet and in cereals before anthesis. However, values of mean
conversion efficiency vary, and are low in conditions of high irradiance (high
light levels) and high temperature, and high in lower-irradiance, lower-
temperature conditions (Brewster
et al
., 1986).
As might be expected, lack of irrigation in dry weather decreases conversion
efficiency. In well-irrigated crops a range of conversion factors between 1.2 and
2.0 g/MJ has been observed. Tei
et al
. (1996) reported an efficiency of conversion
of absorbed PAR to dry-matter of 5.08 g/MJ in a well-irrigated and
-fertilized bulb onion crop during the bulbing phase. Efficiencies of red-beet and
lettuce in the same experiment were 71 and 48% of the onion value, respectively.
