Agriculture Reference
In-Depth Information
Crops sown in autumn develop bulbs in the following spring as photo-
periods and temperatures increase. As Eqns 4.20 and 4.21 and Fig. 4.30 show,
this means that the stimulus for bulbing - and, therefore, the rate of bulbing -
increases as the season progresses. Spring-sown crops, especially in cool
regions, develop bulbs well after midsummer, when photoperiod and mean
temperature are decreasing. It follows from Eqn 4.20 that the rate of bulbing
will then be declining with time. Furthermore, experiments show that onions
will revert to leaf blade growth if they are transferred to sufficiently short days,
even if they have reached an advanced stage of bulb development (see Fig.
4.28). Therefore, it is not surprising that, sometimes, bulbing fails to reach
completion in spring-sown crops and they revert to leaf blade production,
resulting in 'thick-necked' plants (see Fig. 6.11).
The role of red:far-red ratio in light in bulbing: effects of LAI
Table 4.1 shows that bulb maturity is advanced by higher-density planting and
that this is associated with a high Leaf Area Index, LAI. Experiments show that
any agronomic factor, like high plant density, early sowing or the use of
'starter' fertilizer (Brewster
et al
., 1992 and Fig. 6.5), that tends to increase LAI
will also accelerate bulb maturity date. Figure 4.32 shows the relationship
between maturity date and percentage light interception by the leaf canopy for
a range of cultivars. An early maturity date is the result of an early initiation of
bulb scales.
Table 4.6 shows that neighbouring plants, which increase LAI and hence
light interception by the leaf canopy, advance the date of bulb scale initiation. The
relationships between rate of bulbing and photoperiod and temperature of Eqns
4.20 and 4.21 were derived from experiments on widely spaced plants where LAI
was low, and this effect was not a complication. These relationships need to be
modified by a 'leaf canopy effect' at higher LAIs (Mondal
et al
., 1986c).
The onions recorded in Table 4.6 were grown hydroponically and supplied
with complete nutrient solution, so competition for water or nutrients from
Table 4.6.
The effects of shading by neighbouring plants on the mean day of the
year of bulb scale initiation by onion cv. 'Prospero' grown hydroponically (from
Brewster, 1990c, Table 7).
Date of bulb scale
Incident light
Red:far-red ratio (RFR)
initiation (day of
intercepted (%)
under leaves relative
Treatment
the year)
by the leaves
to value above leaves
Unshaded
230
14
0.94
Shaded by large onions
207
66
0.52
Shaded by carrots
202
61
0.40
SED (df = 4)
2.7
3.1
0.07
