Agriculture Reference
In-Depth Information
the flower. The bud elongates to form a new
branch stem with a fixed number of leaves, and
terminates in another flower (secondary flower).
This process, known as sympodial branching,
can be repeated several more times to result in
many tiers of branch stems and leaves when
growing conditions are favorable (Allen and
Scott, 2001). Additional branches may arise
from buds in leaf axils along the stem, but these
axillary branches do not terminate in flowers.
The tendency for potatoes to produce long sym-
podial branches explains why potato leaves and
branches together are commonly referred to as
“vines”.
Axillary branches give the plant a bushy,
upright appearance, whereas sympodial branches
are spreading and prostrate. As an herbaceous
perennial, potatoes are inherently indeterminate;
however, cultivars are often classified, based on
the amount of foliage they produce, as “determinant”,
“semi-determinant”, or “indeterminant”, depending
on their growth habit. Determinant cultivars, also
known as early cultivars, produce only primary
flowers and have limited foliar growth. Examples
of determinant cultivars are Estima, Norland, and
Russet Norkotah. Indeterminant cultivars, or late
cultivars, continue to produce foliage and flowers
throughout the season. Examples of indetermi-
nant cultivars are Cara and Russet Burbank.
(Viola
et al
., 2001; Blauer
et al
., 2013c). The result
is displacement of axillary buds, as they are formed
from the apical meristem along the long axis of
the growing tuber (
Fig. 5.3b)
(Xu
et al
., 1998).
A mature tuber thus has a distinctly recog-
nizable apical or bud end, and a basal or stem
end where the tuber attaches to the stolon. The
eyes are typically concentrated at the apical end
of the tuber and are fewer in number and farther
apart on and near the stem end. As a result, an
inconsistent number of eyes will be found on cut
seed pieces, which may give rise to plants with
more variable stem numbers compared to those
produced from whole tubers. Stem-end seed pieces
produce fewer main stems than seed pieces de-
rived from the bud end, which, given the often-
times direct relationship between tuber set and
stem number per seed piece (Knowles and Know-
les, 2006), ultimately contributes to variation in
tuber size distribution in commercial production.
Tubers typically begin to form on the lower
stolons first (Plaisted, 1957). The majority of
tubers develop on stolons originating from the
lower one-third of the belowground stem, and
these tubers are usually larger than tubers that
form on stolons higher up the stem (Plaisted,
1957; Pavek and Thornton, 2009). Stolons do
not always produce a tuber, nor do they all form
tubers simultaneously. On occasion, excessive heat
or intruding light may cause a belowground sto-
lon near the soil surface to emerge and form a leafy
shoot. Stolons that develop into aboveground
shoots are called secondary stems or heat run-
ners. The term “tuber initiation” is more com-
monly thought of as a process that occurs over a
period of time rather than the mere beginning of
tuber development. Tuber initiation may con-
tinue for up to
2
weeks, but is often complete
within
3-
4 days of onset (O'Brien
et al
., 1998).
Tuberization is a short-day response that
can be modified by temperature and stress.
Phytochrome in the leaves perceives day length.
In response to environmental cues (day length
and day/night temperatures), a tuberization
stimulus is produced that can cross graft
unions (Rodriguez-Falcon
et al
., 2006). The
nature of the tuberization stimulus is not
fully understood; however, endogenous plant-
growth regulators, especially GA, cytokinin,
jasmonic acid, and ABA, likely play a role
(Krauss, 1985). The timing of tuber initiation
is controlled primarily by the ratio of GA to
5.6 Tuber Initiation
Soon after plants emerge, leaves and stolons
begin to develop at aboveground and below-
ground nodes, respectively. Several stolons may
form at each node, but the larger, primary stolon
is the one most likely to terminate in a tuber. Lat-
eral and branch stolons may terminate in tubers,
but tubers from these stolons do not usually be-
come as large as tubers produced on primary
stolons (O'Brien
et al
., 1998). Stolon morph-
ology is characterized by elongated internodes
and a hooked stolon tip prior to tuber initiation
(Viola
et al
., 2001).
Tuber initiation, also called tuberization,
begins when stolons cease to elongate and the
stolon tips begin to swell. During tuberization,
the tuber first appears after cells in the subapical
region of a hooked stolon begin to expand, the
hook opens, and growth is marked by much ra-
dial expansion relative to elongation (
Fig. 5.3a
)