Agriculture Reference
In-Depth Information
to chlorophyll accumulation in the leucoplasts.
When tubers, or parts of tubers, become green
due to exposure to direct or indirect sunlight in
the field, the disorder is known as sunburn or
sun green. Greening due to postharvest expos-
ure to artificial light is often referred to as light-
burn, or the tubers have been light-struck. The
higher the light intensity and longer the expos-
ure, the more intense the green color. The skin
and flesh, from 2 mm to 10 mm in depth, are af-
fected (O'Brien and Rich, 1976; Reeves, 1988).
The outer tissues become deep green, while the
inner tissues may be yellowish-green or deep yel-
low. Long exposure to low light levels in storage,
at market or at home, produces a lighter, more
diffuse green color over the entire tuber. Rela-
tively low light intensities,
3-11
W/m
2
for as little
as
24
h, will induce greening (Gull and Isenberg,
1958). Greening occurs more rapidly at temper-
atures above 13°C (Yamaguchi
et al
., 1960;
Thornton, 2001a). All potato cultivars are sus-
ceptible to greening, but white-skinned cultivars
are more prone to greening than red or rus-
set-type tubers (Reeves, 1988; Strand, 2006).
Sunburn does not kill the affected tissue.
Chlorophyll itself is tasteless and harmless,
but green tubers also accumulate glycoalkaloids
(such as solanine), which are bitter, mildly toxic
to humans, and tend to make tubers unpalat-
able. The production of chlorophyll and gly-
coalkaloids in green tubers occur independently
of each other (Edwards
et al
., 1998). Green
tubers are unacceptable for fresh market or pro-
cessing, but green seed tubers may be planted
(van der Zaag, 1996).
Tubers protruding from the hill or growing
close to the soil surface become green from direct
sunlight or light penetrating through cracks in
the soil. Contributing factors to greening in the
field are shallow planting of seed pieces or incor-
rect hilling (Bohl and Love, 2005), hills eroded
by rain, cracks in dry soils, and tubers forced to
the surface due to high plant densities and
crowding (Wale
et al
., 2008). Cultivars that nat-
urally set tubers near the soil surface are more
prone to greening. In the marketplace, transpar-
ent packaging and indoor lights increase green-
ing, especially of immature tubers, which green
more rapidly than mature tubers (Yamaguchi
et al
., 1960).
Tubers exposed to intense light and high
temperatures develop sunscald. The damaged
external tissues have a blistered appearance
and metallic color. Internal underlying tissues
are watery and turn brown. Affected tissues
may be invaded by secondary bacteria, or may
dry and become granular or leathery (O'Brien
and Rich, 1976).
To prevent tuber greening, plant seed pieces
sufficiently deep at the correct plant population,
and form hills of proper shape and size to ensure
tubers remain covered by soil. In some cases,
fields are rolled to fill in soil cracks. After harvest,
store tubers in complete darkness and avoid long
exposure to artificial lights. Tubers accumulate
chlorophyll at a slower rate under fiber optic
lights than under fluorescent lights (Olsen and
Brandt, 2005).
14.5
Elephant Hide
Also known as alligator hide in russet pota-
toes, elephant hide is the thick, coarse russet-
ing, furrowing or cracking of the periderm.
The cracks are shallow, appear corky or scaly,
and may affect part of the tuber or the entire
surface (Okazawa and Iriuda, 1980). Interior
tissues are not affected. On smooth-skinned
cultivars, the disorder is known as fishy
skin or turtle back (Zimmerman-Gries and
Blodgett, 1963).
The exact cause of elephant hide is un-
known. Elephant hide has been associated
with salts/fertilizer near the tuber surface
(Hodgson
et al
., 1974), decaying organic mat-
ter in contact with the tuber, excessive soil
moisture, and improper timing of field-applied,
sprout-inhibiting chemicals (Hiller
et al
.,
1985). Tubers exposed to high temperatures
produce increased numbers of cork layers
(Okazawa and Iriuda, 1980; Ginzberg
et al
.,
2009). When these cells do not slough off, due
to increased pectin and hemicellulose levels,
the thick russeting develops. Ginzberg
et al
.
(2009) postulated that periderm thickening
due to high temperatures was a defense mech-
anism, with the heavy russeting a secondary
outcome when the inflexible skin cracked as
the tuber grew.
There is no known control for elephant
hide. Culling of affected tubers, especially for the
fresh market, is necessary.
