Agriculture Reference
In-Depth Information
restricting sulfate to produce mild onions can result in lowered bulb yields
(Randle and Lancaster, 2002).
Sulfate fertility also affects sugar and soluble solids accumulation in onions.
Cultivars with the potential for mild flavour increase sugar content with low
sulfate, whereas some other cultivars decrease their solids and sugars (Randle
and Bussard, 1993). Some onion cultivars are greatly affected by changes in
available sulfate while others show less response. When onions are grown under
high-sulfate fertility, more sulfur is retained in the leaves during bulbing than at
low sulfate. If leaves are allowed to senesce and dry on the bulbs, very little sulfur
remains in the leaves of plants grown under low-sulfate fertility. As sulfate
fertility increased, the amount of total bulb sulfur stored as sulfate increased
from approximately 10 to almost 50% (Randle
et al.
, 1999).
Pungent cultivars accumulate a lower percentage of sulfate in the bulb
compared with mild cultivars. Pungent cultivars have a greater metabolic
requirement for sulfur and more efficiently incorporate sulfur into the
pathway, leading to flavour precursors, whereas mild cultivars can partition a
greater proportion of absorbed sulfur to sulfate, thereby excluding it from the
ACSO pathway (see Chapter 8).
Sulfur fertility affects how organic sulfur accumulates and the latter is
metabolized through the various peptides and precursors of the flavour pathway.
Under high-fertility conditions, 1-propenyl cysteine sulfoxide accumulates in the
highest concentrations. As sulfur fertility decreases to near deficiency levels,
methyl cysteine sulfoxide increases in concentration and becomes the dominant
precursor (Randle
et al.
, 1995). Propyl cysteine sulfoxide, which is normally the
lowest-concentration precursor, was found in higher concentration than 1-
propenyl cysteine sulfoxide when S fertility approached deficiency levels. At these
levels, 95% of all sulfur in the bulb occurred as flavour pathway compounds
whereas, at high-sulfur fertility, less than 40% of bulb sulfur was in such
compounds.
Selenate, which is chemically analogous to sulfate and competes with it in
root uptake and biochemical reactions, can cause a decrease in pungency in
onions. Supplying sodium selenate to roots had a similar effect on flavour quality
to low-sulfur fertility levels, namely methyl cysteine sulfoxide content increased
and 1-propenyl cysteine sulfoxide levels decreased (Kopsell and Randle, 1999).
Chloride, an essential element for onions because it is the counter-ion to potassium
in regulating stomatal turgidity, is also a competitor with sulfate in absorption by
roots. Onions supplied with high calcium chloride had decreased pungency and
lower sulfur accumulation in bulbs (Randle, 2005).
Plant density, row spacing and the control of bulb size
From the physiology described in Chapter 4, it is clear that cultivars must be
well adapted to the photoperiods and temperatures of the locality and growing
