drought impacts mineral contents in andean potato cultivars
TRANSCRIPT
DROUGHT STRESS
Drought Impacts Mineral Contents in Andean PotatoCultivarsI. Lefevre1, J. Ziebel1, C. Guignard1, J.-F. Hausman1, R. O. Gutierrez Rosales2, M. Bonierbale2,L. Hoffmann1, R. Schafleitner2 & D. Evers1
1 Centre de Recherche Public – Gabriel Lippmann, Department EVA, Belvaux, Luxembourg
2 International Potato Center, Germplasm Enhancement and Crop Improvement Division, La Molina, Lima, Peru
Introduction
Potato is the fifth most consumed food crop in the world
and is an excellent source of nutrients including carbohy-
drates, proteins, vitamin C, several vitamin B and miner-
als (Camire et al. 2009, White et al. 2009). Amongst the
minerals of nutritional importance are potassium, magne-
sium, iron and phosphorus. Relatively high concentra-
tions of organic compounds, such as vitamin C present in
potato tubers, enhance the absorption of mineral
micronutrients by humans (White et al. 2009). Andean
landraces cover the largest part of the available genetic
diversity of cultivated potato; several authors previously
showed that their tuber mineral concentrations were
related to genotype (Andre et al. 2007, Burgos et al. 2007)
as well as, for a given genotype, to environmental condi-
tions (Burgos et al. 2007, Di Giacomo et al. 2007). Pro-
duction in areas with low mineral phytoavailability
decreases nutrient acquisition by plant tissues (White and
Broadley 2009). Consequently, cultivars with an increased
ability to acquire mineral elements could be valuable for
these non-optimal-cultivation lands.
Among the abiotic stresses, drought is one of the major
limitations for sustainable agriculture worldwide. Over
35 % of the world’s land surface is considered to be arid
or semi-arid, experiencing precipitation that is inadequate
for most agricultural uses (Wood 2006). The Intergovern-
mental Panel on Climate Change (IPCC 2007) estimates
that areas affected by drought have increased since the
1970s. Many already dry regions may experience a decrease
Keywords
drought; mineral nutrients; Solanum
tuberosum; tubers; yield
Correspondence
D. Evers
Centre de Recherche Public – Gabriel
Lippmann, Department EVA, 41, rue du Brill,
L-4422 Belvaux, Luxembourg
Tel.: 00352 47 02 61 441
Fax: 00352 47 02 64
Email: [email protected]
Accepted November 23, 2011
doi:10.1111/j.1439-037X.2011.00499.x
Abstract
Mineral micro- and macronutrients in tubers of 21 Andean potato cultivars
were investigated in a field trial under control and drought conditions. Mineral
concentrations in potato tubers were highly variable between genotypes; some
were significantly and positively correlated with each other, the most notewor-
thy associations being Na–Ca, Mn–Mg and Zn–Fe, in both control and
drought-stressed plants. Overall, increasing yields are related to decreased con-
centrations of some nutrients, albeit some higher-yielding cultivars also dis-
played important concentrations of nutrients in their tubers. The most striking
result was the increase in the concentration of the majority of the analysed
cations in a large number of cultivars in response to water depletion; some of
them, such as K, may be related to water homeostasis and/or to sucrose load-
ing and unloading in phloem sap. Tuber mineral concentrations were not
related to drought tolerance in terms of tuber productivity. Interestingly, yield
loss under drought was not correlated with yield potential under control condi-
tions. Identification of cultivars such as 703264 and 701106 able to maintain
good yield stability in association with high mineral contents under water
deprivation is of particular interest, especially in view of the importance of
potato as a staple crop and the expansion of its cultivation to non-optimal cul-
tivation areas in the context of changing climatic conditions.
J. Agronomy & Crop Science (2012) ISSN 0931-2250
196 ª 2012 Blackwell Verlag GmbH, 198 (2012) 196–206
in precipitation, and heat waves have become more fre-
quent over most agricultural lands, leading to restricted
water availability of variable duration (Wood 2006, IPCC
2007). Soil water is a major limiting condition for yield
and quality of potatoes, as shown by many irrigation
experiments. Thus, potato crop is classified as sensitive to
water stress (McKersie and Leshem 1994, Costa et al. 1997,
Yuan et al. 2003). Reducing water during tuber initiation
severely hinders plant physiological processes and penalizes
tuber yield (Costa et al. 1997). This yield decrease is the
consequence of a significant reduction in tuber number
(Deblonde et al. 1999, Yuan et al. 2003, Eiasu et al. 2007),
but also a consequence of a reduction in tuber size (Schaf-
leitner et al. 2007). Albeit it has been shown that yield loss
under drought may be correlated with the yield potential
under irrigated conditions – highest yielding clones having
the highest yield drop – some clones can present high yield
and good yield stability (Schafleitner et al. 2007).
Drought can affect tuber mineral content by acting on
mineral composition of different tissues and, as a conse-
quence, on the redistribution of minerals within the
plants. Actually, acquisition of many water-soluble nutri-
ents is above all determined by mass flow of soil water
to roots as a result of plant transpiration through the
leaves (Barber 1995). To the best of our knowledge,
the interaction between drought and modulation of
mineral concentrations in potato has not been investi-
gated previously.
Identification of potato cultivars with good yield stabil-
ity and high nutritional quality also in respect to mineral
nutrients under water deprivation would be of great ben-
efit for consumers in some areas where this crop is a die-
tary mainstay. In this respect, the aim of this study was
(i) to investigate the diversity of mineral nutrients in
tubers of a set of field-grown Andean potato cultivars and
(ii) to determine drought impact on the level of these
mineral nutrients in relation to yield loss.
Material and methods
Plant material and culture conditions
Sprouted seed tubers of 21 Andean potato cultivars were
planted on 9 October 2006 in four plots covered by rain
shelters at the International Potato Center (CIP) experi-
mental station Huancayo, Peru, at 3200 m above sea level,
in a randomized complete bloc design with four replicates
of five plants per cultivar. Each rain shelter was equipped
with plastic roofs and plastic barriers 60 cm below
ground to prevent uncontrolled water inflow and with
plastic nets to exclude insects. Plots were filled with
humic soil with pH 4 to a soil depth of 50 cm. They were
fertilized with 100/160/120 kg ha)1 nitrogen–phosphate–
potassium before planting and with 100 kg ha)1 nitrogen
at hilling, 28 days after planting. Soil analysis indicated a
high organic content (between 30.5 % and 38.7 %), and a
very high Cation Exchange Capacity (CEC) (from 51.20
to 54.72 meq 100 g)1) with a base saturation between
71 % and 81 %, situating the soil at a lower limit of satu-
ration. These data indicate a relatively rich soil horizon
with a potentially high microbial activity. Fungicide and
insecticide sprays (mancozeb, propineb, dimetofor, cym-
oxanil, permethrin and cypermethrin) were applied in
weeks 4, 6, 8 and 10 after planting according to suppliers’
recommendations. Plants were watered by drip irrigation,
and soil water potential was kept between 0 and
)0.02 MPa. In the drought plot, irrigation was stopped
on day 86 after planting and drought was applied for
58 days, while the control plot was continuously irrigated.
Soil water content was determined in each replicate plot
and treatment as described by Evers et al. (2010); 37 days
after the onset of drought, soil water content reached val-
ues of around 53 % in the control plot and 32 % in the
drought plot and decreased further to 29 % after 53 days
of water depletion and to 25 % after 58 days of water
depletion, while soil water content remained around
47 % in control plots 58 days after the onset of drought.
After the drought period, irrigation was resumed and
continued until day 156 after planting. On day 163, the
haulms were cut and tuber harvest took place on day 170
after planting. The CIP number, the related variety and
the taxonomic group are used for the description of the
21 potato cultivars of this study (see also Table 2).
Yield analysis
Tubers of the three central plants of a bloc were harvested
170 days after planting. Fresh weight was determined
immediately after harvest. An aliquot of the tubers was
dried in an oven for dry matter determination.
Yield maintenance under drought stress was assessed
by the drought susceptibility index (DSI) calculated
according to the following formula:
DSI = (1)yielddrought/yieldcontrol)/DII. The DII was cal-
culated as (1)Xs/Xi) (Ramırez-Vallejo and Kelly 1998),
where Xs is the mean yield of all cultivars grown under
stress, and Xi is the mean yield of all cultivars grown
under non-stress conditions.
Mineral determination
After thorough washing, whole tubers with skin were
sliced and dried at harvest. Tubers were processed with
their skin owing to the difficulty of uniformly peeling cer-
tain potato tubers with irregular shape. The material was
ground in an IKA A11 stainless steel mill prior to extrac-
Drought Impacts Mineral Contents in Potato Tubers
ª 2012 Blackwell Verlag GmbH, 198 (2012) 196–206 197
tion. Samples, 500 mg of dry weight, were digested in
7 ml HNO3 (Plasma Pure 67–70 %; SCP Science, Cour-
taboeuf, France) and 3 ml H2O2 (30 % in weight for
metal traces analysis; Fisher Scientific, Tournai, Belgium).
Acid digestion was performed in Teflon tubes in a micro-
wave oven (Anton Paar Multiwave 3000, Graz, Austria)
by increasing temperature and pressure until 200 �C and
30 bars. At the end of the procedure, samples were
diluted with H2O up to 50 ml and kept at 4 �C prior to
analysis. Blank and certified reference material (white cab-
bage, IAEA-359; LGC Standards, Molsheim, France) were
included at each mineralization cycle for quality control.
Samples were analysed by inductively coupled plasma
mass spectrometry (ICP-MS) (Perkin Elmer Elan DRC-e,
Waltham, MA, USA). For each cultivar, extraction was
performed on three samples, which consisted each of
three pooled tubers from the same plant. Each sample
was extracted in duplicate.
Statistical analyses
The data were analysed by two-way anova, with treatment
and accessions as main effects. Multiple comparison proce-
dure between means was performed with a Tukey test with
confidence limit of 95 % using SigmaStat (Systat Software
Inc., San Jose, CA, USA) for Windows version 2.03 soft-
ware. Principal component analysis (PCA) was performed
on centred and reduced data to compare mineral profiles
and minerals and yield relation with SPSS (v16) (SPSS Inc.
Chicago, IL, USA) and R version 2.8.0 (2008) (http://
www.r-project.org) with FactoMineR package (http://
cran.r-project.org/web/packages/FactoMineR/index.html).
Results
Yield and mineral concentrations upon drought
Susceptibility to drought stress was variable in the investi-
gated set of potato cultivars, as reflected by a wide range
in the DSI (Fig. 1). High DSI corresponds to high
drought susceptibility. Drought caused yield losses
between 39 % (cultivar 703264) and 92 % (cultivar
704022). No correlation existed between this yield loss (in
%) and yield under control conditions among the tested
cultivars (r2 = 9.10)6).
Differences in tuber water content were significant
among control cultivars (P < 0.001) (Fig. 2). Upon
drought, significant changes in tuber water content were
observed in 7 of the 21 cultivars: water content signifi-
cantly increased in tubers from cultivars 701570, 703264,
703312, 703415 and 704022, while it significantly
decreased in those from cultivars 703899 and 706191
under drought condition. No correlation occurred
between tuber water content and yield, neither under
control nor under drought conditions (r2 = 0.007 and
r2 = 0.0021, respectively).
Under control conditions, mineral concentrations
strongly varied between cultivars (Fig. 3). When reported
to an edible portion (150 g FW), micronutrient contents
were between 456.5 and 1199.3, 88.4 and 212.6, 54.9 and
143,7, 334.5 and 1312.5 lg per 150 g FW for Fe, Mn, Cu
and Zn respectively. Macronutrient contents ranged
Fig. 1 Drought susceptibility index (DSI) of potato cultivars. High DSI
indicates high drought susceptibility. DSI is calculated as
DSI = (1)yielddrought/yieldcontrol)/DII. The DII was calculated as (1)Xs/
Xi), where Xs is the mean yield of all cultivars grown under
stress, and Xi is the mean yield of all cultivars grown under non-stress
conditions.
Fig. 2 Water content in tubers of potato cultivars from control and
drought-stressed plants. Values are means ± S.D. For a given cultivar,
*indicates a significant difference (P < 0.05) between control and
drought-stressed plants.
Lefevre et al.
198 ª 2012 Blackwell Verlag GmbH, 198 (2012) 196–206
Fig. 3 Concentrations of some minerals in tubers from control and drought-stressed plants. Values are means ± S.D. For a given cultivar, *indi-
cates a significant difference (P < 0.05) between control and drought-stressed plants.
Drought Impacts Mineral Contents in Potato Tubers
ª 2012 Blackwell Verlag GmbH, 198 (2012) 196–206 199
between 439.5 and 966.59, 13.18 and 28.08, 5.2 and 18.9,
1.25 and 5.48 mg per 150 g FW, for K, Mg, Ca and Na
respectively. Correlations were found between concentra-
tions of some minerals and yield under control condi-
tions (Table 1); yield was negatively correlated to the
macronutrients Na, K and Ca (P < 0.01) and to the mi-
cronutrients Mn and Zn (P < 0.05). Similarly under
drought, variation was important within the cultivars
(Fig. 3), and yield was negatively correlated to the ma-
cronutrients Na, K (P < 0.01), Mg and Ca (P < 0.05),
and to the micronutrients Mn (P < 0.05) and Zn
(P < 0.01).
Concomitancy of mineral concentration changes upon
drought
The double logarithmic matrix correlation highlighted
many highly positive significant correlations (P < 0.01)
between pairs of measured variables (Table 1), pointing
to a strong relationship between the concentrations of
some minerals in tubers from control or drought-stressed
plants. In control plants, the concentration of the macro-
nutrient K was correlated with all the mineral elements,
Ca and Mg were correlated together and with Na and
Mn, and the micronutrients Zn, Fe and Mn presented
correlations. These correlations tended to increase in
response to drought application, indicating some modifi-
cations in mineral accumulation. The following correla-
tions were noteworthy because of their high correlation
coefficients (r > 0.65) (Table 1) and the similarity in
vector length and direction in PCA plot performed on
variables (Fig. 4a and b) in both control and drought
plants: Na–Ca, Mn–Mg and Zn–Fe. Copper presented a
different trend than the other minerals.
Principal component analysis performed on the vari-
ables DSI and mineral element concentration of the 21
potato cultivars under control (Fig. 4a) and drought con-
ditions (Fig. 4b) showed that the first two principal com-
ponents accounted for 57.08 % and 62.64 %, respectively.
The length of the variable vector indicated that mineral
concentrations were well represented by the PC1 and
PC2, but DSI was not, as confirmed by the Pearson corre-
lation coefficient determined on log-transformed data.
This result indicated that mineral concentrations were not
associated with drought tolerance, in terms of DSI
(Fig. 4a and b).
Plots of variables indicated differences between mineral
concentrations in tubers from control and drought-
stressed plants (Fig. 5). According to a one-way anova
over the whole sample set, drought treatment implicated
significantly higher Cu, Fe, Mg, K and Zn concentrations
in tuber dry matter as compared to control plants, K, Fe,
Cu and Zn presenting variations in the highest number of
cultivars (Fig. 3). Copper was found in higher concentra-
Table 1 Pearson correlation coefficient performed on mineral concentrations indicated in lg g)1 DW and yield. Double logarithmic matrix corre-
lation for all the samples (when P > 0.05, values are not shown in table).
LogNa LogMg LogK LogCa LogFe LogMn LogCu LogZn
Potatoes from control plants
LogYield )0.377** )0.428** )0.344** )0.286* )0.282*
LogNa 0.461** 0.374** 0.730** 0.352**
LogMg 0.506** 0.513** 0.756** 0.256*
LogK 0.412** 0.456** 0.488** 0.345** 0.515**
LogCa 0.438** 0.283*
LogFe 0.382** 0.436** 0.678**
LogMn 0.353**
LogCu 0.370**
LogZn
Potatoes from drought-stressed plants
LogYield )0.345** )0.315* )0.392** )0.276* )0.312* )0.351**
LogNa 0.675** 0.583** 0.834** 0.408** 0.546** 0.414**
LogMg 0.525** 0.674** 0.630** 0.686** 0.412**
LogK 0.646** 0.586** 0.482** 0.688**
LogCa 0.568** 0.634** 0.484**
LogFe 0.491** 0.308* 0.703**
LogMn 0.450**
LogCu 0.437**
LogZn
**Correlation is significant at the 0.01 level; *Correlation is significant at the 0.05 level in grey; correlation >0.500
Lefevre et al.
200 ª 2012 Blackwell Verlag GmbH, 198 (2012) 196–206
tion in drought-exposed tubers in nearly all genotypes. In
cultivars 701524, 701570, 702464, 703264, 703356, 703415
and 704647, the concentration of at least four mineral
elements was increased in response to drought stress.
Sodium concentration in tubers from drought-stressed
plants was significantly lower than in control ones,
whereas no significant changes were found for Ca and
Mn. These modifications induced a decrease in Na/K
ratio in all the tuber set, which was significant for six cul-
tivars, 701106, 702568, 703264, 703899, 703988 and
705548 (Table 2). The ratio monovalent/divalent cations
only increased in tubers of two cultivars (Table 2). As can
be seen in Figure 5, mineral concentrations could not be
associated with any particular Solanum tuberosum group,
as they were highly variable within the groups.
Discussion
Variability in mineral micro- and macronutrients in the
investigated set of potato cultivars
Mineral micro- and macronutrient concentrations in
tubers were highly variable (Fig. 3). Overall, the contents
of macroelements are in the range of those reported by
different authors for flesh and skin of raw potatoes. Only
the mean value of K content is slightly higher than usu-
ally reported in literature (Heseker and Heseker 1993,
Souci et al. 2000, U.S. Department of Agriculture 2009).
As for the mineral micronutrients, Fe contents were in
the range of values reported by Souci et al. (2000) but
lower than those reported by the food composition table
of the U.S. Department of Agriculture (2009) (1170 lg
per 150 g FW). Higher levels of Zn were found in this
study compared with the values found in literature, while
Mn and Cu contents were lower (Heseker and Heseker
1993, Souci et al. 2000, U.S. Department of Agriculture
2009). Selenium concentrations remained under the
threshold of detection. Differences in mineral element
contents with values found in literature may depend on
genotypes and environmental factors as already men-
tioned, but also on the potato sampling as reported by
Andre et al. (2007).
Among our set of potatoes, variations in the concentra-
tions of minerals are related to genotypes. Genotypic vari-
ations have previously been reported in Andean potato
varieties for the studied elements Ca, Fe and Zn (Andre
Fig. 4 Principal component analysis between mineral concentrations
indicated in lg g)1 DW and drought susceptibility index (DSI) in
tubers from control (a) and drought-stressed (b) plants.
Fig. 5 Principal component analysis between variables of mineral
concentrations indicated in lg g)1 DW in 21 control and drought-
stressed cultivars. Results from three plants are represented for each.
Drought Impacts Mineral Contents in Potato Tubers
ª 2012 Blackwell Verlag GmbH, 198 (2012) 196–206 201
et al. 2007, Burgos et al. 2007) and, according to numer-
ous studies, can be extended to other elements both
between and within Solanum species, as reviewed by
White et al. (2009). In our study, this variability occurred
for all the nutrients analysed. These variations could be
partly explained by the size of potato tubers; skin and
flesh can differ in their amount of certain elements
(Wszelaki et al. 2005, Burgos et al. 2007, Subramanian
et al. 2011), and variation in the ratio flesh/skin dry
weight may be the reason. Andre et al. (2007) explained
about 13 % of the variability in tuber Fe concentrations
by tuber size. However, other authors did not find any
association between tuber size and concentrations of ele-
ments, such as Ca, although its concentration was gener-
ally more important in skin than in flesh (Bamberg et al.
1993). The most important factor in mineral variability is
probably linked to the plant physiology itself, i.e., the
capacity the plant has to absorb and redistribute elements
in its tissues. Numerous factors can interfere, such as
plant development and morphology, which are related to
the cultivar phenotype under varying environments
(Cotes et al. 2002, Kadaja and Tooming 2004), transport
processes in the plant and ion specificity (see for review
Karley and White 2009). Some elements such as K or Mg
are easily translocated within the plant by mass flow,
while some others require chelators to be transported
over a long distance (White et al. 1981a,b). Moreover,
redistribution of some elements to sink organs is low; Ca
is present at low concentration in phloem, and thus, its
movement from leaves to fruits, seeds and tubers is lim-
ited (White and Broadley 2003, Karley and White 2009).
Despite the fact that Ca may move with water to tubers
via small roots growing from the base of the tuber buds
and on the stolon (Kratzke and Palta 1985, 1986), its
concentration remains low in this organ, as evidenced by
our results. These changes in the moving of mineral ele-
ments within the plant are supported by the study of
Subramanian et al. (2011) who observed a pattern of dis-
tribution in the tuber directly related to the importance
of element unloading from the phloem.
Even if cation mobility differs depending on ion speci-
ficity, positive correlation between some elements was
observed in our study; some potato cultivars thus pre-
sented high concentrations of a number of mineral nutri-
ents. A positive correlation between Zn and Fe was
already reported when calculated on a fresh weight basis
(Burgos et al. 2007) or dry weight basis (Rivero et al.
2003). The latter study also reported strong correlations
between Cu and Zn, Fe and Zn, Mg and Zn, Ca and Fe.
Some correlations were not observed in the present study,
Table 2 Ratio of some mineral concentration in potato tubers under control and under drought stress
CIP number Variety Taxonomic group
Na/K ·100 Monovalent/divalent cations
Control Drought Control Drought
700234 SA-2563 Andigena 0.24 ± 0.03 0.15 ± 0.05 24.09 ± 0.85 31.88 ± 6.67*
701106 Camotillo Amarillo Andigena 0.48 ± 0.33 0.14 ± 0.02* 23.93 ± 7.62 29.50 ± 4.61
701524 Puca Huayro 0.31 ± 0.14 0.26 ± 0.05 22.42 ± 3.87 22.88 ± 5.29
701570 Chaucha Phureja 0.60 ± 0.38 0.45 ± 0.16 18.97 ± 0.87 15.92 ± 3.99
702464 Natin Suito 0.43 ± 0.14 0.30 ± 0.04 25.68 ± 5.65 24.16 ± 1.48
702568 Pichea Papa Andigena 0.49 ± 0.23 0.25 ± 0.10* 23.00 ± 4.37 26.49 ± 1.94
703264 Puca Quitish Andigena 0.65 ± 0.30 0.36 ± 0.07* 18.95 ± 4.26 21.91 ± 2.50
703312 Morada Taruna Stenotomum 0.36 ± 0.20 0.22 ± 0.09 25.26 ± 5.35 27.02 ± 3.96
703356 Peruanita Goniocalyx 0.57 ± 0.21 0.43 ± 0.10 21.65 ± 3.37 23.22 ± 5.30
703387 Huamantanga Andigena 0.43 ± 0.22 0.25 ± 0.07 25.36 ± 4.26 32.15 ± 2.06*
703415 Raiz del Palo Andigena 0.32 ± 0.01 0.22 ± 0.09 25.19 ± 0.31 26.70 ± 5.50
703825 China Runtush Goniocalyx 0.59 ± 0.21 0.49 ± 0.06 19.56 ± 1.35 20.94 ± 0.58
703899 Chaucha Roja Andigena 0.51 ± 0.21 0.23 ± 0.06* 21.16 ± 3.63 26.88 ± 1.86
703988 Canchillo Andigena 0.36 ± 0.13 0.17 ± 0.12* 27.03 ± 3.75 31.04 ± 5.30
704022 Kellu Suito Stenotomum 0.41 ± 0.01 0.33 ± 0.02 22.20 ± 3.27 22.02 ± 4.19
704338 Violeta Andigena 0.54 ± 0.17 0.36 ± 0.10 17.00 ± 2.15 21.29 ± 3.95
704647 Unknown Andigena 0.35 ± 0.15 0.20 ± 0.07 18.72 ± 3.36 23.58 ± 1.20
705191 Unknown Andigena 0.26 ± 0.08 0.24 ± 0.01 24.02 ± 1.83 21.70 ± 0.57
705548 Yurac Ccompis Andigena 0.63 ± 0.03 0.24 ± 0.04* 24.22 ± 2.65 28.74 ± 2.61
705940 Larga Stenotomum 0.34 ± 0.10 0.20 ± 0.05 25.03 ± 3.19 27.27 ± 4.01
706191 Cuchi Chucchan Andigena 0.22 ± 0.06 0.20 ± 0.06 26.35 ± 2.28 25.17 ± 2.26
Values are means ± S.D.
*Indicates significant differences between treatments at the a = 0.05 level.
CIP, International Potato Center.
Lefevre et al.
202 ª 2012 Blackwell Verlag GmbH, 198 (2012) 196–206
which was performed on different cultivars, in different
soil and climate conditions. Mineral absorption and
retranslocation are partly related to plant architecture and
environmental conditions. Nevertheless, such associations
between minerals are also related to the selectivity of their
transport in plant organs and tissues. In plant plasma
membranes, channels catalysing K+ influx are K+-selective
and voltage-dependent, but some other voltage-indepen-
dent channels were observed in plasma membranes and
vacuolar membrane, and are reported as non-selective
cation channels because of their permeability to not only
other monovalent but also divalent cations (Demidchik
and Maathuis 2007). The uptake of some divalent cations,
such as Ca2+and Mg2+, for which no selective pathways
have been found in plant membranes at resting mem-
brane potentials, is widely considered to be catalysed by
the latter channels (Demidchik and Maathuis 2007).
Some ions such as Zn and Fe can be mediated by identi-
cal groups of transporters (White and Broadley 2009).
In the present investigation on a set of 21 Andean cul-
tivars, a correlation between higher-yielding cultivars and
lower concentration of some nutrients was observed
(Table 1), as was also reported by White et al. (2009).
However, it is not a rule; many interactions between plant
and soil composition, fertilizer application or crop geno-
type are likely to affect concentration of mineral elements
in tubers. Despite being significant, the negative correla-
tion between yield and nutrients found in our study is
not striking (r > )0.5) and higher-yielding cultivars can
contain important concentrations of nutrients. Moreover,
it is not excluded that under other environmental condi-
tions, this correlation disappears.
Drought effect on potato mineral concentrations
Our results show that yield loss (in %) under drought is
not correlated with yield in control conditions in the
studied cultivars. Some high-yielding cultivars can main-
tain tuber formation, which is economically of great
interest. Physiological explanation of this genotypic yield
maintenance still remains unclear. According to Schafleit-
ner et al. (2007), canopy size maintenance is an impor-
tant determinant for yield under drought, while Lahlou
and Ledent (2005) found that the drought tolerance index
might be significantly associated with root depth in the
field, but in stressed conditions only. Indeed, a weak can-
opy size is not necessarily related to a low dry matter
accumulation in tubers, as suggested by Tourneux et al.
(2003). These authors showed that a weak foliage expan-
sion, expressed by a low leaf area index and canopy cover,
could contribute to reduced water loss and a better yield-
ing under water depletion, in comparison with a high
foliage expansion.
Our results indicate an increase in most of the analysed
cations. As results are expressed on a dry weight basis,
this increase is not related to a modification of water con-
tent. Variations were not equivalent for all elements, some
increased such as K, Fe, Zn and Cu, some remained con-
stant such as Ca and Mg, and some decreased such as
Na, indicating that the accumulation of elements was not
only the result of a ‘concentration’ of elements linked to
the yield decrease. Moreover, an absence of correlation
between percentage of increase in the different ions and
yield decrease can exclude at least partly this hypothesis.
Some studies have suggested that some mineral elements
could be involved in osmotic adjustment in some tissues
(Farooq et al. 2009). A higher ratio of monovalent to
divalent cations is known to passively influence the
hydration of cell membranes and macromolecules (Mars-
chner 1995). However, our results did not display signifi-
cant trends concerning this ratio, except for two cultivars,
although they displayed inverse trends for K+ and Na+.
The increase in K+ concentration in response to drought
in our set of potatoes may be related to water homeosta-
sis and turgor regulation as already suggested by some
authors (Shabala and Lew 2002, Maathuis 2009, Zhu et al.
2010). These findings are supported in the potato crop by
the work of Khosravifar et al. (2008), who showed that
increasing K application under water depletion decreased
leaf water potential through osmotic regulation. Several
studies indicate strong relationships between K+ channel
activity and sugar loading into the phloem (Ache et al.
2001, Deeken et al. 2002); a higher supplementation of K,
in a range of adequate supply, is reported to increase
sucrose transport to tubers (Marschner 1995, Allison et al.
2001). Considering the fact that a K+ gradient in the
phloem has been evidenced (Vreugdenhil 1985), a
hypothesis is that a modification in this gradient in con-
ditions of water depletion may sustain redistribution of
sucrose to sink organs despite a lower photoassimilate
production in source organs. Further studies related to
sucrose production in leaves and its translocation in
phloem are needed to investigate this assumption. Some
high-affinity transporters of K+ may be involved in the
increase in this ion concentration (Szczerba et al. 2009),
which could explain the decrease in the Na/K ratio
observed in our study. Moreover, activation of K uptake
in response to low K availability in soil has also been
reported (Karley and White 2009, Szczerba et al. 2009).
As initial root responses to K deprivation, the activity of
K+ transporters in cellular membranes seems regulated by
post-translational regulation.
It is generally assumed that drought impairs mineral
availability in soil by different processes. Low soil mois-
ture reduces the soil diffusion capacity for some elements
(Sardans et al. 2008). However, in our experiment,
Drought Impacts Mineral Contents in Potato Tubers
ª 2012 Blackwell Verlag GmbH, 198 (2012) 196–206 203
despite a strong decrease in soil water content in drought
soil compared to control one, no modification of CEC
between these two different soil conditions was noticed in
soil analyses (51.84 ± 0.91 and 52.96 ± 2.49 meq 100g)1,
in control and drought soils, respectively). Soil enzymes
also play an essential role in the nutrient mineralization
and may correlate well with nutrient availability (Sardans
and Penuelas 2005), especially in soils with high organic
matter content. Drought affects plant growth by hinder-
ing photosynthesis and metabolic activities (Zhu 2002,
Evers et al. 2010) associated with a decrease in transpira-
tion rate and nutrient absorption, as observed in various
species (Hu et al. 2007, Chołuj et al. 2008, Sardans et al.
2008). These modifications are ion- and organ dependent.
However, reports by Mahouachi et al. (2006) as well as
our results show the opposite trend.
Nutritional values
As already reported, potatoes are an excellent source of K
(Camire et al. 2009, White et al. 2009). All the cultivars
presented more than 15 % of its recommended daily
allowance (RDA, 2000 mg of K supplied by 100 g),
according to the Council Directive 90/496/EEC on nutri-
tion labelling, which is the recommended allowance sup-
plied by 100 g. Three cultivars (703356, 703387 and
703825) displayed more than 30 % of the RDA under the
environmental control conditions presented in this study,
and this value increased under drought conditions
(Fig. 3). Considering the fact that some Peruvian popula-
tions can consume between 200 and 800 g of potatoes
per day (Burgos et al. 2007), tubers of some cultivars pro-
vide consistent contents of several other minerals, such as
Mg, Fe, Zn, Cu and Mn. For example, tubers from the
cultivar 703387 contained 9.6 and 14.2 % of the Cu RDA
(1 mg per 100 g) under control and drought conditions,
respectively. Tubers from the cultivar 703825 contained
5.7 % and 5.8 % of the Fe and Zn RDA (14 mg of Fe
and 15 mg of Zn supplied by 100 g), respectively, under
control conditions.
Conclusions
The investigated set of potato clones displayed a strong
variability in mineral concentrations that was not related
to any drought tolerance in terms of tuber productivity.
Despite the fact that correlations exist between higher-
yielding cultivars and lower concentration of some nutri-
ents, some cultivars such as 703264 and 701106 presented
high tuber yield and high mineral contents associated
with a maintain of productivity under drought stress.
Moreover, most of the analysed mineral nutrient contents
increased in response to water depletion, this trend being
not only the result of a ‘concentration’ of elements linked
to a yield decrease; increase in the macroelement K+ may
be related to water homeostasis and/or to sucrose loading
and unloading in phloem sap.
Acknowledgements
The authors would like to thank Laurent Solinhac and
Sylvain Legay for their excellent technical assistance. This
work was financially supported by the Ministry of Finance
(Luxembourg).
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