Agriculture Reference
In-Depth Information
were 13°C for the autumn-sown crops and 17.5°C for the spring-sown. Direct
evidence for this comes from experiments where onion cv. 'Hysam', which is very
similar to cv. 'Robusta' of Table 4.3, was grown along a gradient of temperatures.
Bulb yields approximately halved with an increase in mean temperature from 12
to 19°C (Daymond
et al
., 1997). This decrease in yield corresponded with a
decrease in the interval between onset of bulbing and maturity from about 117 to
55 days (see Fig. 4.4). LAI at the start of bulbing, and therefore light interception,
did not differ over the temperature range. Therefore, the decrease in bulb yield at
warmer temperature can be attributed to a shorter duration of bulb growth,
causing less PAR to be converted into bulb dry matter.
When the atmospheric CO
2
concentration was increased from 374 to 532
mol/mol there was a large yield increase at all temperatures, but little effect
on the rate of crop development (see Fig. 4.4a). These results indicated that the
yield increase caused by this rise in atmospheric CO
2
would more than
compensate for the lower bulb yield resulting from the temperature increase
predicted for the 'global warming' effect of such a CO
2
increase. Under UK
conditions it appears that elevated atmospheric CO
2
and the consequent global
warming would benefit bulb onion production.
As Table 4.1 shows, leaf canopy light interception may be increased by
growing at higher plant densities but, although yields increase, individual bulb
size is drastically reduced, and this is not normally acceptable. Most markets
require bulbs of 5-7 cm diameter and in fertile conditions this dictates a plant
density in the range 50-100 plants/m
2
(see Fig. 6.8). Therefore, to achieve a
high yield of medium to large bulbs, a cultivar with the capability of producing
sufficient leaf area to intercept a high proportion (> 60%) of the incident light
must be sown at the appropriate time.
Fig. 4.4.
(a) The relationship between the rate of crop development from
transplanting to maturity and mean temperature for onion cv. 'Hysam' grown at
elevated (
mol/mol CO
2
, respectively).
(b) The relationship between bulb dry matter yield at harvest and mean temperature
from transplanting to harvest maturity at elevated (
●
,
) and normal (
,
) CO
2
(532 and 374
) CO
2
for onion
cv. 'Hysam' (a) from Fig. 1 and (b) from Fig. 4 of Daymond
et al
., 1997. Courtesy of
Journal of Horticultural Science
).
●
) and normal (
