Agriculture Reference
In-Depth Information
Vries
et al
., 1974). The edible parts of alliums contain primarily simple sugars
and fructan storage carbohydrates (see Chapter 8), plus the structural carbo-
hydrates and protoplasmic constituents typical of most plant cells. They
comprise, therefore, carbohydrate-rich tissues and should have a high ratio of
final dry weight to photosynthate utilized.
As regards (iv), the extent of partition to harvestable material, in bulb onions
at the optimum time for harvesting, when 80% of plants have 'soft necks', about
80% of the shoot weight is in the bulb. Bulb weight will continue to increase after
this stage and the percentage of total weight in the bulbs will increase. For
example, 2 weeks after the 80% 'soft necks' stage, an average of 89% of shoot dry
weight comprised bulb in one series of experiments (Brewster
et al
., 1986). Bulb
onions therefore have a high 'harvest index' (the proportion of total yield in
harvested material). In non-bulbing alliums the harvest index will vary
according to how much green leaf is harvested in addition to the pseudostems.
In salad onions and chives nearly 100% of the shoot may be harvested. The
fraction of the leek crop that is harvested varies with marketing traditions and
demands. For example, in southern England the market requires the
pseudostem only, whereas in northern UK leeks are sold with an additional 'flag'
of green leaf above the pseudostem. Trimmed leek pseudostems comprise
33-50% of the total shoot weight, with a mean of about 40%. Leeks trimmed
with a green 'flag' would typically comprise 55-60% of the total shoot weight.
Clearly, the harvest index of leek crops is lower than for bulb onions.
Concerning crop respiration and decay (v), even in the absence of bolting
and in favourable weather conditions, leeks can reach a maximum or 'ceiling'
yield (e.g. Fig. 4.50). Theoretically, such a ceiling will be reached when dry
matter gains from photosynthesis are equalled by dry matter losses due to
respiration and tissue senescence. Such a situation has not been observed in
onion crops, because bulb ripening and leaf senescence terminate crop
development before a ceiling is reached (see Fig. 4.2).
Respiration results from two broad components - first, 'growth respiration',
the CO
2
output associated with the synthesis of new tissue and, secondly,
'maintenance respiration', the CO
2
output needed to maintain existing tissues
alive (McCree, 1970). The ratio of growth respiration to dry matter growth
determines the photosynthate conversion efficiency (iii above) and varies with
the biochemical make-up of the new growth, as indicated. Maintenance
respiration varies with tissue and temperature. Leafy shoots have a much higher
maintenance respiration than storage organs like bulbs (de Visser, 1994a; Tei
et
al
., 1996). Onion and garlic bulbs in store have very low respiration rates (see
Chapter 7, 'Bulb Respiration Rates').
Onion bulbs taken directly from plants still actively growing in the field had
maintenance respiration rates of 0.0010 and 0.0011 g CO
2
/g/day at 9 and
19°C, respectively - about 58% the rate of red-beet storage roots in the same
conditions and less than 5% of the maintenance respiration rates typical of
leaves and stems (Tei
et al
., 1996). Respiration rates per unit of crop dry weight
