Agriculture Reference
In-Depth Information
Table 6.6.
Effect of irrigation on seed yield of late November-planted Yellow
'Sweet Spanish' onion grown at 11 plants/m
2
at Davis, California, USA (from
MacGillivray, 1948).
Water
Number of
Seed yield
Seed yield
applied (mm)
irrigations
(1944, kg/ha)
(1945, kg/ha)
0
0
524
434
125
2
628
528
375
5
705
783
LSD
0.05
-
NS
217
NS, not significantly different.
of the umbel (Millar
et al.
, 1971). There is a particularly sharp decrease in
water potential between the seed stalk and the umbel, indicating a high
impedance to water flow in this region, possibly because the xylem divides into
many small vessels to supply up to 1000 pedicels per umbel. Decreases in water
potential from -4 to -15 bar between dawn and mid-day were measured in
well-irrigated onion seed crops in Wisconsin, USA. Drought plus hot, dry
winds, sometimes aggravated by root destruction by pink root disease
(
Pyrenochaeta terrestris
), can lead to shrivelling and death of developing seeds at
the 'milk' and 'dough' stages of endosperm development (Harrington, 1974).
Adequate nitrogen fertilization is essential for maximum yields. In Italy,
where good yields (> 1000 kg/ha) are consistently produced without irrigation,
the effects of a range of six N fertilizer levels - from 0 to 150 kg/ha in 30 kg/ha
increments - was tested (Cuocolo and Barbieri, 1988). The nitrogen was applied
as ammonium sulfate, one-third at planting in mid-October, one-third in late
January and one-third in late March (seeds were harvested in July). Seed yield
increased linearly from 830 to 1100 kg/ha with increasing N. Nitrogen and
water requirements are interrelated: higher N is needed where rainfall or
irrigation make high yields possible.
The synchronization of flowering
It is important when producing F
1
hybrid seed that the male-fertile and male-
sterile lines flower simultaneously. There should always be plenty of pollen from
the male-fertile line available while stigmas of the male-sterile mother plants are
receptive, thereby minimizing the risk of contamination from pollen of other lines
carried by visiting insects, or from the occasional male-fertile mother plant. The
different lines of a hybrid do not necessarily bloom simultaneously when treated
identically. Techniques to overcome this problem include: (i) storing bulbs of the
two lines at different temperatures before planting - the closer the temperature
during storage to the optimum for inflorescence development (usually around
10-12°C) the earlier the inflorescence emerges after planting out; (ii) staggering
the planting dates of the two lines - the earlier the planting date the earlier
