Plant Growth and Development - The Potato: Botany, Production and Uses - page 77

often occurs closer to PM. Conversely, a crop grown
with a relatively low rate of seasonal nitrogen
will generally set tubers early and produce less
foliar growth; also, foliar senescence, tuber mat-
uration, and PM will occur before the season has
ended. Yield will be lower and the ability to re-
tain processing quality during storage can be
compromised if the maturation period under
dead vines is prolonged and harvest delayed
until late in the season (beyond PM) ( Fig. 5.5 ) .
Nitrogen rate and duration of application can
thus affect the retention of processing quality
significantly by influencing the timing of PM in
relation to harvest.

the tubers, and tuber dry matter content reaches
a maximum. The senescing leaves turn yellow
and abscise from stems, which also eventually
senesce. At this time, the tuber periderm (skin)
thickens and becomes stronger. Potato tuber
maturity is often discussed in terms of physio-
logical and physical maturity, but these two as-
pects of maturity do not always happen simul-
taneously (Sabba et al ., 2007).
As discussed earlier, tubers reach PM when
tuber yield and dry matter accumulation are at
maximum levels and tuber sucrose and reducing
sugar levels are at a minimum. Although not al-
ways possible, it is ideal to harvest healthy tubers
as near to PM as possible, to achieve high yields
and to maximize processing quality at harvest
and during long-term storage. Dry matter con-
tent is important to processors, because product
recovery and oil absorption are directly affected.
Because tubers with high dry matter contain
less water, they require less energy during frying
or dehydration and absorb less oil during frying.

5.11 Plant and Tuber Maturation

As potato plants mature, chlorophyll levels in
the leaves decline and the rate of photosynthesis
decreases. Carbohydrates and phloem-mobile
metabolites translocate from senescent foliage to

Retention of process quality - Alpine Russet

Nitrogen (kg/ha)

168 280 392 504

(8 days from harvest)

12 °C

(229 days from harvest)

9 °C

+2%* +12% +15%

7 °C

+5% +28%

*% change in lightness of fries over 168 kg/ha N

+7%

Fig. 5.5. Effects of nitrogen rate and storage temperature on retention of process quality of Alpine Russet
tubers following wound healing (8 days at 12°C) and 221 days of storage at 9 and 7°C (229 days total).
The percent values indicate the degree to which tubers fried lighter than those grown with 168 kg ha - 1
nitrogen (N). Physiological maturity (PM) (see Fig. 5.4) was delayed as N rate increased, resulting in the
high N crop being harvested closer to PM than the low N crop. Each fry plank is from a different tuber
selected to represent the average fry color in a 12- tuber sample.