Plant–Water Relations and Irrigation Management of Potato - The Potato: Botany, Production and Uses - page 105

In response to cues from the environment such
as photoperiod, the subapical region develops
into a tuber, where over 70% of the dry matter
produced by the plant will accumulate (Cutter,
1978). When tuberizing potato plants are
exposed to drought stress, an alteration in parti-
tioning occurs that results in reduced starch
synthesis, increased sucrose levels, and reduced
tuber dry matter (Cutter, 1978; Geigenberger
et al ., 1997; Levy et al ., 2013).
Tuber growth is reduced when tuber water
potential falls below -0.4 MPa (Gandar and
Tanner, 1976), although it will resume when the
plant is rehydrated. However, the effects on total
and marketable yield are complex since they de-
pend, inter alia, on such variables as timing and
severity of the drought stress, soil characteristics,
tuber number, and the simultaneous occurrence
of other abiotic stresses (Iwama and Yamaguchi,
2006; Schafleitner, 2009). In addition, drought
stress will affect the interactions between devel-
oping tubers and metabolic events in aboveground
potato shoots, such as photosynthesis (Basu
et al ., 1999).

heat better than larger leaves (Nobel, 1999). In
addition, small leaves may possess a greater vein
density, and this feature has been associated
with increased drought tolerance in a range of
plant species (Brodribb and Holbrook, 2003;
Scoffoni et al ., 2011). Increased drought toler-
ance may be related to the observation that vein
density has been correlated with hydraulic con-
ductivity (Brodribb et al ., 2010) and maximum
rate of photosynthesis. Whether vein density is
optimized at specific levels of drought stress is
unknown (Noblin et al ., 2008), as is the role
played by stomata in the potato plant.

7.4 Stomata and the Cuticle

Stomata serve as major control points for uptake
of carbon dioxide and loss of water. For example,
on days of high evaporative demand, stomata
begin to close when the leaf water potential falls
below -0.1 MPa (Levy et al ., 2013). As stomata
close (e.g. midday depression of stomatal con-
ductance), water loss by transpiration is affected
more than carbon assimilation (Yoo et al .,
2009). This optimization of carbon gain in rela-
tion to water loss may be affected by such diverse
factors as xylem sap pH, decreased hydraulic
conductance, stomatal heterogeneity, circadian
rhythms, xylem-borne abscisic acid (ABA), or
may represent a stomatal response to excessive
solar radiation (Mott and Buckley, 1998; Brod-
ribb and Holbrook, 2003; Chaves et al ., 2003;
Buckley, 2005; Liu et al ., 2005). Understanding
the roles and timing of these factors in the po-
tato plant will be important research goals.
However, potato cultivars developed in the fu-
ture will have to exhibit increased yield per unit
of water used, such as increased water-use effi-
ciency (WUE), in conjunction with no signifi-
cant decrease in harvest index (Yoo et al ., 2009;
Levy et al ., 2013).
Potato cultivars and Solanum species
exhibit a wide range of stomatal conductance
responses that may be due to the size and fre-
quency of stomata as well as their distribution on
adaxial and abaxial leaf surfaces (Dwelle et al .,
1983; Ekanayake and De Jong, 1992; Chaves
et al ., 2003; Coleman, 2008). As a cautionary
note, increased CO 2 levels associated with cli-
mate change may have pronounced effects on
stomatal density and patterning (Brownlee, 2001;

7.3 Leaf Growth and Development

There is a close correspondence between leaf
and tuber water relations, with inhibition of
growth at tissue water potentials below -0.4 MPa
(Gandar and Tanner, 1976; Bethke et al ., 2009).
Maintenance of leaf turgor under mild to mod-
erate drought may be due to one or more changes
in such features as osmotic adjustment, altered
cell wall elasticity, and reduced cell size (Radin,
1983; Heuer and Nadler, 1998). However, under
extreme drought, when water potentials fell below
-2.0 MPa, researchers observed a loss of potato
leaf viability (Ackerson et al ., 1977; Shimshi
et al ., 1983).
In potato plants, the earliest observed re-
sponse to drought is stomatal closure. However,
with increasing drought stress, the first morpho-
logical effect observed is reduced leaf size (Jefferies,
1993; Jefferies and MacKerron, 1993). Other
effects include reduced leaf expansion rate, re-
duced leaf formation, and increased rates of leaf
senescence (Fleisher et al ., 2008a,b).
The production of smaller leaves is a posi-
tive adaptation to an arid environment. For ex-
ample, small leaves are able to dissipate sensible