Agriculture Reference
In-Depth Information
the surrounding humidity depends on its previous history of wetting and drying
(Thamizharasi and Narasimham, 1991).
Water vapour loss from onion bulbs is largely through the skins rather than
the neck or base plate in well-ripened bulbs with tight necks (Thamizharasi and
Narasimham, 1988). The driving force for water loss is the difference in water
vapour pressure between inside the bulb and the surrounding air. The majority
of investigations on water loss from stored bulbs have compared losses at
different ambient air RHs rather than vapour pressure deficits (VPDs). Most
studies indicate that a store atmosphere of about 65-70% RH is optimal,
irrespective of temperature, as this maintains reasonable skin flexibility and
avoids the surface dampness that can promote disease or rooting. Rates of water
loss at a given RH increase as temperature increases (see Fig. 7.3b), as expected,
since VPDs for a given RH increase with temperature. Rates of water loss from
stored bulbs do not increase steadily as the RH of the surrounding air decreases,
as would be the case if the conductivity of skins to water vapour were constant
(see Fig. 7.3b). Onions stored at RHs between 55 and 75% lose water less rapidly
than those at higher or lower RH (see Fig. 7.3b).
Another experiment indicates that the conductivity of onion skins to water
vapour varies with their water content, since conductivity decreased sharply as
the surrounding air decreased from 60 to 50% RH (see Fig. 7.3c). Similarly, as the
RH of the surrounding air was subsequently increased, there was a sharp increase
in water conductivity of skins between 50 and 60% RH. In this experiment the
bulbs were well ventilated, so water loss would be controlled almost entirely by the
water conductivity of the skin. Because of this variation in skin conductivity, the
rate of water loss from bulbs was not a simple linear function of the VPD gradient
between bulb and surrounding atmosphere (Kopec and Eurda, 1989).
One possible clue to the mechanism underlying these changes in con-
ductivity comes from studies on the walls of single epidermal cells from onion
fleshy scales stripped of their waxy cuticles (Schonherr and Merida, 1981). Cell
wall water conductivity decreased by 50% when RH external to the membrane
decreased from 100 to 20%. The effect of RH on membrane water conductivity
reflects the balance of polar (water-attracting or hydrophilic) and non-polar
(water-repelling) molecular groups in the membrane. In addition, the higher
the preponderance of polar groups the greater the tendency of the membrane
to swell when wet. A similar explanation at a macroscopic scale may underlie
the changes in onion skin water conductivity - and also swelling (see below) -
as external RH varies.
At low RH skins are less flexible and tend to split, and this also causes
increased water loss (Apeland, 1971). However, the pattern of water-loss response
of bulbs differing in skin quality to external RH was essentially the same, although
those with inferior skins had more rapid water loss (see Fig. 7.3c).
THE STRENGTH OF ONION SKINS
Skin water content strongly affects the
flexibility of onion skins (see Fig. 7.4a; Hole
et al.
, 2000). Skins exposed to
