Vol. 29, No. 2, March 1996

Influence of Dibble Shape and Depth on the Germination and Seedling Establishment of Burley Tobacco in the Float System

B. Pearce, M. Cui, and L. Bush


    Direct seeding of pelleted tobacco seed into the float
transplant system has become common in Kentucky.  Direct
seeding reduces labor compared to the plug and transfer
method, but it  increases the risk involved and requires
more management by the producer.  Uniform germination, and
ultimately a high percentage of useable transplants are the
keys to success with direct seeding.
     Tobacco, like all plants, requires proper conditions
for germination.  The process of germination begins with the
dry seed taking up water, thus the seed must have a moist
environment.  However, germination also requires oxygen, so
a balance between adequate moisture and aeration must be
achieved for high rates of germination.
     Gemination is only the first step in producing a usable
transplant.  The root is the first part of the seedling to
emerge from the seed.  Under normal conditions the root will
enter growing medium and will soon be taking up water and
nutrients for the seedling.  Occasionally the young root of
a tobacco plant does not grow into the medium, but stays at
the surface.  This condition is commonly called "spiral
root".  Spiral root is thought to occur due to a lack of
oxygen caused by excessively wet conditions in the growing
medium.  A spiral root plant rarely develops into a useable
transplant.  Trays with 10 to 20% spiral root
plants have been reported.
     In order to make a micro-environment that is more
conducive to germination and growth, it is common to
"dibble" the growing media prior to placement of the seed.
Dibbling refers to the making of a small indentation in the
surface of the media where the seed will be placed.  Many
producers are using dibble boards consisting of rounded or
pyramid shaped objects fastened to a base with handles, that
allows entire trays to be dibbled at one time.  Dibblers
consisting of rounded or pyramid shapes on a rolling
cylinder are also becoming popular among tobacco producers.
     The objective of these studies was to determine the
importance of dibble shape and depth on the germination and
spiral root development of burley tobacco seedlings.

Materials and Methods

     Two studies were conducted in the spring in a glass
greenhouse at the University of Kentucky in Lexington.  In
study 1, polystyrene trays with 200 cells/tray were filled
with fortified Speedling1 tobacco media.  The trays were
carefully filled to avoid over packing of the media.  Wooden
pegs were carved into a pyramid or domed shape with a
maximum width of 1/2 inch.  Dibble marks were made by hand
using either the pyramid or domed pegs at depths of 1/4,
1/2, or 3/4 inch.  Each shape by depth treatment was
replicated five times with one tray per replication.
Pelleted seed (var. Ky14 x L8) was seeded over the trays
using a vacuum seeder.  All trays were floated in a common
14' x  6' water bed at a depth of five inches.
     A second study was conducted using only the domed
shaped dibble at depths of  0, 1/4, 1/2, 3/4, and 1 inch.
The trays were filled with unfortified Carolina's Choice1
media. Pelleted seed (var. NC BH129) was seeded with vacuum
seeder and the trays were floated in a common water bed as
in study 1.
     Germination was considered to be emergence of the root
and the first two leaves. Germination was counted at 17 days
for study 1, and at 7 and 14 days after seeding for study 2.
Germination is expressed as % seed germinated based on the
total number of cells seeded.  Evaluations of spiral root
formation were made at the same time as the germination
counts.  The spiral root percentage is based on the number
of germinated seed, not on the total cell number.


     An interaction between dibble shape and depth was
observed for Ky 14 x L8 germination at 17 days after seeding
(figure 1).  A shallow pyramid shaped dibble had much lower
gemination than any of the other treatments.  The domed
dibble created a dibble mark with rounded sides in which the
pelleted seed was more likely to roll to the bottom of the
dibble.  At the shallow dibble depth, a seed near the bottom
of the dibble mark was more likely to germinate.  At deeper
dibbling depths, the pyramid shaped dibble had slightly
higher germination than the domed dibble.  Regardless of
shape, there was a trend for greater germination with deeper
dibbling to 3/4".  This is most likely due to a more
favorable moisture environment in a deeper dibble.
     Both dibble shape and depth influenced the occurrence
of spiral root development, but there was no significant
interaction between these factors (figure 2).  The domed
shaped dibble had a significantly lower incidence of spiral
rooting.  Reasons for this response are unclear, but it is
thought that spiral rooting is caused by low oxygen in the
media, and may be related to gas exchange.  The rounded
dibble marks may promote better air exchange around the seed
than the pyramid shaped dibbles. Alternatively it may be
argued that because the seed was more likely to rest at the
bottom of a rounded dibble, the contact between seed and
media was more consistent than where a pyramid shaped dibble
was used.   Increasing dibble depth to 3/4"significantly
reduced the incidence of spiral root.
     The germination of NC BH129 decreased slightly with
increasing dibble depth to 3/4 inch (figure 3).  At a dibble
depth of 1 inch, germination was much slower and the 14 day
germination percentage was well below the other treatments.
The 1 inch dibble resulted in a disruption of the media
structure in the cell, and resulted in reduced germination.
     The effect of dibble depth on spiral root occurrence
was similar to that observed in study 1 (figure 4).  Spiral
root incidence decreased with dibble depth, with a minimum
amount of spiral root at a depth of 3/4 inch.  Even though
germination was relatively high in the undibbled treatment,
the incidence of spiral root was also quite high.  Dibbling
at 1/2 to 3/4 inch depth apparently provides a favorable
balance between moisture and aeration for proper germination
and growth of burley tobacco seedlings.
     Based on the results of these studies the best results
were obtained using a rounded shape dibble at a depth of 1/2
to 3/4 inch.  This treatment gave good germination with the
least spiral root incidence.

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