Agriculture Reference
In-Depth Information
About 50% of total solar radiation energy is PAR (Squire, 1990), so these
efficiencies in terms of PAR should be halved for comparison with results based on
total solar irradiance.
ONION YIELDS
In a study comparing well-irrigated autumn- and spring-sown bulb onion
crops, yields varied by a factor of more than five depending on cultivar, sowing
date and plant density (Mondal, 1985). Such large differences in yield cannot be
explained solely by differences in the efficiency with which intercepted light is
converted into dry matter. In Table 4.1 we see that bulb yield increases with
plant density and that this correlates with the percentage light interception by
the crop leaf canopy. Table 4.2 shows how yields are reduced by delaying
sowing. Later-sown plants switched from leaf blade to bulb production while the
Leaf Area Index (LAI) and, consequently the leaf canopy light interception, was
lower than for earlier-sown plants. A similar effect, but this time caused by a
difference in maturity date between cultivars sown at the same time, is
illustrated in Table 4.3. The later-maturing cultivar had longer to develop a high
LAI before the switch to bulb growth. From Tables 4.1, 4.2 and 4.3 it is clear
that a high bulb yield is dependent on a high percentage of light interception by
the leaf canopy (see Fig. 4.3). This requires sufficient time in conditions
conducive to leaf blade growth before bulbing starts.
Although important, percentage light interception was not the only factor
involved since, in these experiments, autumn-sown onions produced a higher
yield than spring-sown at a given percentage interception (see Fig. 4.3). For
crops with 60% light interception the duration of bulbing was 55 days when
autumn-sown and 43 days when spring-sown, and consequently the former
intercepted more light in total during bulbing.
Longer duration of bulbing may be caused by cooler temperatures, giving
slower bulb ripening and leaf senescence. Mean temperatures during bulbing
Table 4.1.
The effect of planting density on bulb yield and light interception for
onion cv. 'Augusta' sown on 21 April 1982 at Wellesbourne, UK.
Planting density (plants/m
2
)
Effect
25
100
400
Bulb yield (kg/m
2
of dry matter)
0.46
0.71
2
1.02
Leaf Area Index (LAI) during bulbing
0.82
1.5
2.3
Percentage light interception by leaf
30.0
46.0
59.4
canopy during bulbing
2
2
Maturity date
14 Aug
2 Aug
24 July
