Agriculture Reference
In-Depth Information
Keeping the plant's source to sink ratio in
balance is important when trying to maximize
profits from crop production. Therefore, it is es-
sential that the relationship between foliar and
tuber growth be maintained in a manner that
will maximize tuber yield and quality.
Donald (1962) coined the term “harvest
index” (HI) to describe the source-sink rela-
tionship of wheat varieties as a ratio indicat-
ing the percentage of the plant's total weight
coming from grain yield versus straw yield. He
believed wheat varieties that produced a rela-
tively high grain yield compared to straw yield
were more photosynthetically efficient than
those where the straw yield was dominant.
The HI can be used as a tool for any plant to
describe the relationship between source and
sink. For potatoes, HI expresses tuber growth
(yield) as a percentage of total plant biomass
(tubers plus aboveground foliar growth):
Harvest index (%) = (tuber fresh wt/(foliar +
tuber fresh wt))
×100
Hence, a HI of 40% indicates that 40% of total
plant fresh weight is tubers. Following tuber set,
HI increases throughout the season, eventually
reaching
100
as vines fully senesce. The HI, source-
sink relationship, tuber bulking, and other crop
growth milestones are discussed later in the chap-
ter, in the section “Stages of Crop Growth and
Development: A Long-Season Cultivar Case Study”.
demand for carbon, water, and nutrients (Engels
and Marschner, 1986). The GA to ABA ratio in
the plant also regulates carbon partitioning be-
tween the source and sink. Excessive GA due to
high temperatures or long photoperiods can slow
tuber growth as a result of a reduction in the ac-
tivity of ADP-glucose (ADPG)-pyrophosphorylase
in the tuber (Dwelle, 1985). Alternatively, low
GA levels favor tuber bulking.
Tuber growth is a function of both cell
division and expansion; however, cell division
plays a larger role in determining final tuber size.
According to Plaisted (1957),
200-
g Cobbler
tubers had approximately 500-fold more cells
than their
37-
g counterparts, but only tenfold
more cell volume. Over the life of the growing
tubers, from tuberization to plant maturity,
tuber growth is typically sigmoidal; growth is
initially slow as tubers begin to swell, but is rapid
and linear soon after tuberization is complete. As
an example, tuber growth of Alpine Russet is lin-
ear between
70
and 120 days after planting (DAP),
when grown in the Columbia Basin of Washing-
ton, USA. During this period, tuber weight is in-
As the canopy begins to senesce, however, tuber
growth slows and eventually ceases prior to
complete foliar senescence.
Final tuber size is determined largely by
genotype, inter- and intra-row plant spacing (plant
population), management inputs, environment,
canopy size, and duration. Commercial produ-
cers are often able to increase average tuber size
by increasing the inter- and intra-row distance
between plants (e.g. reducing plant competition),
and vice versa (Pavek and Thornton, 2006).
However, decreases in plant population can lead
to yield reduction, even though average tuber
size may increase. Because growers are often
paid premiums for certain tuber sizes, yield re-
ductions may be acceptable if associated with an
increase in average tuber size. The commercial
production goal is to produce the largest yield
of the most valuable tubers, regardless of total
yield per hectare. To accomplish this, growers
must first identify what is the most valuable
tuber size profile for the intended market. Once
this is defined, the grower must find the right
combination of plant population and spatial ar-
rangement to produce the ideal tuber size profile.
Final tuber size may also be altered through a
deliberate change in stem number per plant.
5.8 Tuber Bulking
Once the canopy is well established and tuberiza-
tion finished, tuber bulking begins. Final yield is
ultimately determined by the rate and duration
of tuber bulking. The rate and duration of bulk-
ing depends largely on canopy health and size,
genotype, soil and air temperatures, the night to
daytime temperature differential, day length,
and incident radiation (Bodlaender, 1963). As
discussed below in the long-season cultivar case
study, agronomic inputs also play a major role
in tuber bulking and are often manipulated to
maximize economic and/or biological yield. Soon
after tuber set, source leaves begin partitioning
an increasing amount of dry matter to tubers.
The rapidly growing tubers soon become the
dominant sink and largely drive the partitioning
activity of the foliage through their increasing
