Agriculture Reference
In-Depth Information
Using these data it was shown that, for cv. 'Keepwell':
Rate of bulbing = -0.079 + 0.0043
Photoperiod +
(Eqn 4.20)
0.0027
Temperature
and for cv. 'Hyton':
Rate of bulbing = -0.066 + 0.0032
Photoperiod +
(Eqn 4.21)
0.0018
Temperature
Here, rate of bulbing was day, photoperiod in h and daily mean
temperature in °C. These equations show that rate of bulbing, when measured
in this way, is proportional to both photoperiod and temperature, with the
response being more acute for 'Keepwell' than for 'Hyton'.
Substituting natural photoperiods and daily average temperatures, as
recorded at Wellesbourne, central England, into these equations gives the
prediction that cv. 'Hyton' will initiate bulb scales on 7 July whereas cv.
'Keepwell' will initiate bulb scales on 7 June if both are planted on 1 May. Leaf
blade growth will slow after bulb scale initiation. Substituting recorded
temperatures for Wellesbourne into Eqn 4.17a indicates that a crop emerging
on 1 May with a plant population of 60 plants/m 2 will achieve a LAI of 3.2 and
therefore a light interception of 60% (see Fig. 4.1) by 6 July in an average
season. Such a crop will therefore have the potential for a high yield provided
that leaf growth can continue until 6 July. Cv. 'Hyton' therefore seems well
adapted and will have a higher potential yield than cv. 'Keepwell' if both are
sown in spring.
In a trial comparing cv. 'Keepwell' and cv. 'Rijnsburger-bola' (a cv. very similar
to 'Hyton' in season of bulbing), both sown on 28 February, the latter yielded 41
t/ha, with 25 t/ha of bulbs greater than 45 mm diameter, and was mature on
20 August, whereas the former yielded only 24 t/ha, with none larger than
45 mm, and was fully mature on 9 July (Salter, 1976). This illustrates in practice
the critical importance of the response of bulbing to photoperiod and temperature
in determining the yield potential of a cultivar at a given location. The response
must be appropriate for the natural photoperiods and temperatures of the locality,
in that it must permit sufficient leaf growth to occur before bulbing starts.
Cultivar 'Keepwell' is grown as an overwintered crop and will typically
produce a leaf area of about 25 cm 2 per plant in early March from a sowing
made in mid- to late August at Wellesbourne. From Equation 4.17a we can
calculate that the leaf area per plant will reach 442 cm 2 by 31 May in an
average year at Wellesbourne, when Equation 4.20 indicates that bulbs will be
initiated. If the plant population is 60 plants/m 2 this implies a leaf area index of
2.65 which, according to Eqn 4.1, would intercept 54% of the incoming light.
Thus, when grown from a late-summer sowing, cv. 'Keepwell' is well adapted to
produce a satisfactory bulb yield.
The ranking of cvs in terms of the rate of bulbing remains the same in
different photoperiods. Hence, by comparing cvs in an extended photoperiod,
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