| Tuber
External Growth Defects: CHAINING
Tuber
Chaining also called Gemmation affects harvest yields. The small
tubers formed compete with the primary tuber for nutrients and thereby
lowers the weight of the marketable tuber. There is no effect on
fresh market quality of the primary tuber except for reduced tuber
size. For processing, the texture and starch content may not be
desirable. Tuber chaining may also interfere with harvest separation
of the primary tuber from stolon.
Development
and Appearance
Tuber
chaining refers to the initiation followed by limited growth of
tubers at the nodes of a stolon after the apical tuber at the end
of the stolon was initiated and has begun growing. The disorder’s
name refers to the chain appearance of the series of little tubers
along the stolon. The development or initiation of these is due
to a break down of apical dominance of the primary tuber. This is
a hormonal phenomenon associated with auxin (IAA) concentrations
basipetal from the primary tuber. The sizing of the primary tuber
is inhibited due to the lessened carbohydrate (sucrose) taken in
because of competition with the secondary tubers. The primary tuber,
in severe cases, can have a glassy or soft texture and have a low
specific gravity making it undesirable for processing.
A
related phenomenon is sprouting from buds on the nodes of the stolon.
The sprout does not affect quality but will compete for nutrients
much like tuber chaining, and is also related to loss of apical
dominance.
Causation
The
cause of tuber chaining is high soil temperature. Soil temperatures
around the daughter tubers that are above 75oF, especially early
to mid bulking or tuber growth (Potato Production Stages: Scheduling
Key Practices, Univ. Nebr. Coop. Ext. Circ. # 95-1249), will promote
tuber chaining. Once this disorder has begun, the return of cooling
soil temperature around tubers will not overcome the disorder. In
other words, the chaining and nutrient competition will continue
for the rest of the season, thereby yields are lowered. On a physiological
level, once apical dominance is lost by the primary tuber, it does
not return. Note, low soil moisture does not cause this disorder
but is usually associated with high soil temperature.
Susceptibility
Varieties,
in general, are susceptible but round tuber varieties, most white
chipping and red varieties are particularly prone to tuber chaining.
Cultural
Practices
Since
air temperature cannot be controlled, the best practices to avoid
over-heating of the soil is to plant deep, to hill and to maintain
a good trapezoidal structure to the row. Seed pieces should be at
least six inches below the surface; eight inches would not hurt.
In severe hot weather, cooling the ground with irrigation may be
necessary.
Summary:
External Disorders |
