Agriculture Reference
In-Depth Information
growth is restricted early in the season (e.g. by
lack of nutrients, water, or stress), HI at max-
imum foliar development will often exceed 50%,
favoring tuber growth over foliar growth, which,
for many cultivars, will limit the total amount
and duration of subsequent foliar growth. The
resulting source-sink imbalance can result in
early vine senescence, which in turn can com-
promise yield potential in a long-season area.
Conversely, too much nitrogen early in the sea-
son can promote excessive foliar growth and
delay tuberization (e.g. HI <40% at maximum
foliar development), limiting the available win-
dow for bulking and resulting in reduced yield
over the available growing season.
By definition, tuber growth equals foliar
growth when the HI is 50%. This index is thus
marked by the intersection of tuber and foliar yield
curves (point (B) in
Fig. 5.4
). If 50% HI occurs
prior to maximum foliar growth, the HI at max-
imum foliar growth will exceed 50% (e.g. tubers
will account for more than 50% of plant biomass).
Hence, the timing of 50% HI relative to maximum
foliar growth can also indicate a pending source-
sink imbalance that will potentially compromise
maximum yield, due to limiting foliar growth and
premature vine senescence later in the season. The
example given is for a late-season, indeterminate
cultivar (e.g. Alpine Russet, Ranger Russet, and
Alturas) grown in a long-season area. The rela-
tionships among these indices and specific index
recommendations to maximize yield and quality
will differ for production areas with shorter sea-
sons, production of late-season cultivars for early
markets, short-season cultivars, and cultivars with
a more determinate growth habit. The goal is to
build a robust canopy as early as possible and to
maintain it for as long as possible, within reason.
The potato plant needs time to mature naturally
for optimum yield, storability, and economic re-
turn. Plants with ideal foliage growth early to mid
season achieve spectacular yields if allowed
enough time to mature naturally
(Fig. 5.4)
. This
means cutting off and depleting the soil of nitro-
gen after vines have peaked, about late July or early
August in the Columbia Basin, to stimulate foliar
senescence and hasten crop maturation.
In addition to yield, optimizing source-sink
relationships to the growing season by synchron-
izing critical growth stages properly is important
for achieving tuber physiological maturity (PM)
at season end, which in turn affects tuber
quality at harvest and the retention of tuber
quality during storage. PM coincides with max-
imum dry matter (specific gravity) and min-
imum concentrations of sucrose and reducing
sugars in tubers. It equates to the average DAP to
achieve maximum tuber yield, maximum spe-
cific gravity, and minimum concentrations of
sucrose and reducing sugars in tubers (e.g. PM =
153
DAP in
Fig. 5.4
). Also, PM informs end-of-
season management and harvest timing. Healthy
tubers harvested within
7-
10 days of PM retain
processing quality the longest in storage (Know-
les
et al
., 2009, 2011). Delaying harvest well be-
yond PM exposes tubers to fluctuations in soil
temperature, which can accelerate tuber aging
and ultimately compromise retention of process-
ing quality during storage, but these effects are
cultivar dependent. For many long-season culti-
vars, reducing sugars (glucose and fructose)
often increase following PM and prior to harvest,
This response reflects overmaturation and sig-
nals the beginning of a progressive loss in qual-
ity. The result can be sugar ends at harvest,
increased propensity to develop sugar ends
during storage, earlier than normal increase
in reducing sugars throughout the entire
tuber during storage, and/or early sprouting.
Delaying harvest significantly beyond PM can
thus affect retention of postharvest quality for
many frozen processing or chipping cultivars.
In practice, the approximate timing of PM
should be part of the management recom-
mendations for new cultivars. It is best specified
by three criteria: DAP to PM, cumulative de-
gree days (7.2°C base) from planting to PM,
and the degree of vine senescence at PM. For
example, PM for Alpine Russet in the central
Columbia Basin is achieved at approximately
150-160
DAP, 1523-1643 cumulative de-
gree days (°C), and
70-
80% vine senescence.
Agronomic inputs such as nitrogen rate
and scheduling can effectively shift the relative
timing and duration of growth stages and the
various milestones of crop development to affect
final yield and quality. For example, as the nitro-
gen rate increases, foliar growth, duration, and
tuber bulking are prolonged, and the attainment
of tuber PM is delayed. In the highly productive
long-season growing areas of the Pacific North-
west, the result is usually high yields and im-
proved quality and storability, as harvest most
