Potato Research (2010) 53:267–275
DOI 10.1007/s11540-010-9163-0
Improving Potato Tuber Quality and
Production by Targeted Calcium Nutrition:
the Discovery of Tuber Roots Leading to a New
Concept in Potato Nutrition
Jiwan P Palta
Received: 30 September 2010 / Accepted: 1 October 2010 /
Published online: 21 October 2010
# EAPR 2010
Abstract Calcium plays a major role in plant growth and development and in the
maintenance and modulation of various cell functions, especially related to
membrane structure and function and to cell wall structure. Calcium stabilizes cell
membranes by bridging polar head groups of phospholipids at the membrane
surface. Calcium is also an integral part of the cell wall where it provides stable
intra-molecular linkages between pectin molecules, resulting in cell wall rigidity. A
change in the cytosolic calcium concentration is also known to provide a cellular
signal that regulates metabolism and mediates plant responses to stresses.
Calcium deficiency is pervasive among fruit and tuber crops because calcium
moves with water in the xylem and very little water moves to fruit and tuber
tissues as compared to leaves. The water potential gradient within the potato
plant favors xylem transport to the foliage since tubers are surrounded by moist
soil. As a result, the calcium concentration is much higher in foliage than in the
tuber. In our early work, we demonstrated that water and calcium taken up by the
main root system bypass the tubers and are delivered to the above-ground portion
of the plant, and roots arising from the stolons and tubers supply calcium to the
tubers. Tuber calcium concentration can therefore be increased by selectively
feeding tubers with calcium nutrients during tuber bulking period. The discovery
of tuber roots has led to the development of a new concept in potato nutrition.
We have demonstrated that in-season fertilization with calcium increases tuber
calcium and lowers incidence of physiological disorders such as internal brown
spot, hollow heart, and bruising. Localized tissue calcium deficiencies are
implicated as mechanisms initializing cell death and tissue necrosis leading to
internal brown spot and hollow heart in potatoes. There is also strong evidence
This paper is based on: Palta JP (2010) Improving potato tuber quality and production by targeting calcium
nutrition: the discovery of tuber roots leading to a new concept in potato nutrition. In: Çalişkan ME
& Arslanoğlu F (ed), Potato AgroPhysiology, Proceedings of the International Symposium on Agronomy and
Physiology of Potato. 20-24 September 2010, Nevşehīr, Turkey, pp. 11–18
J. P. Palta (*)
Department of Horticulture, University of Wisconsin, Madison, WI 53706, USA
e-mail: jppalta@wisc.edu
268
Potato Research (2010) 53:267–275
for reducing storage rot by increasing tuber calcium. Finally, tuber calcium is
important for the health of the sprout and of the tuber skin.
Keywords Apical necrosis . Calcium transport . Heat stress . Hollow heart . Internal
brown spots . Soft rot . Storage quality . Water uptake
Introduction
Our recent studies provide compelling evidence that potato tuber quality can be
improved by increasing the calcium (Ca) content of tubers. Benefits from
supplemental calcium application include reduced incidence of internal defects such
as internal brown spots and hollow heart. Furthermore, data from several studies also
suggest that higher calcium tubers store better and have reduced incidence and
severity of soft rot. We have also obtained some evidence that in some cultivars, the
seed piece tuber quality can be improved through calcium application. In this study,
we found that seed piece tubers given calcium during their development performed
better in the following season. More recently, we have found that potatoes given
calcium fertilization have reduced injury by bruising. We have also found that root
zone calcium can mitigate the impact of cold and heat stresses on the potato plant. In
addition to these issues, in our work, we have also investigated practical means of
delivering soluble Ca products, the timing and source of application of calcium.
In this article, I have attempted to answer the three following questions:
1. Why is calcium important?
2. Is it feasible to improve tuber calcium on a practical level?
3. Is there a benefit of improving tuber calcium?
Why is Calcium Important?
Calcium is Important for Healthy Cell Membranes and Strong Cell Walls.
In Addition, Ca Acts like a Hormone
Calcium plays an important role in the growth and development of plants. The cell
membrane health is very crucial to the survival and health of the plant cell. The health of
the cell membranes can only be maintained in the presence of sufficient Ca around the
membranes (Hanson 1984, Palta 1996, Hirschi 2004). If the level of Ca associated with
the membranes is reduced, the membranes become leaky, resulting in loss of cellular
salts and organic compounds (Arora and Palta, 1986, 1988, 1989). Such loss, if not
reversed, leads to the eventual death of the cell. Calcium is also an integral part of the
cell wall where it provides stable but reversible intra-molecular linkages between pectin
molecules, resulting in cell wall rigidity (Marschner 1995, Matoh and Kobayashi 1998).
In addition to its role in the membrane and cell wall, Ca is now regarded as a
signaling molecule mediating plant response to environmental stresses and hormones
(Bush 1995; Sanders et al. 1999; Hirschi 2004). In this regard, changes in the
cytosolic Ca levels can help a plant react to the impact of environmental (drought,
Potato Research (2010) 53:267–275
269
heat, and cold) and biotic (bacteria and fungus) stresses. Environmental stresses are
also known to cause perturbation of the cell membrane functions (Palta 1996). Soft
rot-causing bacteria also cause membranes to be leaky and digest cell walls by using
hydrolytic enzymes. Early work of McGuire and Kelman (1984) demonstrated
reduced severity of soft rot and improved storage quality of potatoes with increased
calcium concentration. Since Ca is able to protect the cell membranes and gives
strength to the cell walls, Ca plays an important role in tuber quality and plant
growth when plants are subjected to abiotic and biotic stresses. We have evidences
supporting the idea that the impact of environmental stresses on potatoes can be
reduced by improving calcium nutrition (Palta 1996; Tawfik et al. 1996; Vega et al.
1996; Kleinhenz and Palta 2002). Furthermore, our studies also suggest that Ca can
modulate hormone type of responses in potatoes such as influencing the tuberization
signal (Ozgen et al. 2003; Ozgen and Palta 2004; Vega et al. 2006).
Tubers Being Low-Transpiring Organs are Naturally Deficient in Calcium
Tuber is botanically a stem tissue. As compared to the above-ground stem portion of
the plant, tubers contain very little calcium. Transpiration is the main driving force
for calcium transport in plants (Kirby and Pilbeam 1984, Busse and Palta 2006).
Calcium therefore moves along with water in the xylem. Low-transpiring organs
such as fruits and tubers are known to suffer from Ca deficiency (Bangerth 1979).
Potato tubers, being surrounded by moist soil, will have much less transpiration than
above-ground parts of the plant. Consequently, low-transpiring tubers accumulate much
less calcium per unit fresh weight than leaves and stems. Deficiency of calcium in tuber
tissue is even greater for potatoes grown in sandy soil because of the very low level of
exchangeable Ca in these soils. Moreover, with constant irrigation, water-soluble Ca is
leached from the hill. Thus, the soil surrounding the tubers will contain very low soluble
calcium, especially during the later part of the season when tubers develop.
Is it Feasible to Improve Tuber Calcium on a Practical Level?
Discovery of Tuber Root: Application of Ca Around the Tuber Area Can Increase
Tuber Ca Uptake
Over 20 years ago, we provided evidence for the existence of functional roots on the
tuber and at the tuber–stolon junction (Kratzke and Palta 1985). In a follow-up study,
we showed that these tuber roots displayed normal root anatomy and they appeared to
originate from parenchyma cells adjacent to the vascular tissue. By feeding a watersoluble dye, we demonstrated that these roots were able to supply water to the tuber
whereas the main root system supplied water to the top part of the plant (Fig. 1).
Since water and calcium are known to move together, we suggested that these
tubers and stolon roots are able to supply calcium to the tuber. In a follow-up study,
we found that the addition of Ca to the main root system did not increase the Ca
concentration of the tuber tissue (Kratzke and Palta 1986). However, application of
Ca to the tuber and stolon areas resulted in a threefold increase in Ca concentration
in the tuber peel and medullary tissue (Kratzke and Palta 1986). These results
270
Potato Research (2010) 53:267–275
Fig. 1 Transport of a water soluble dye (Safranin O) from various roots present on the potato plant.
Description of various types of roots present on a potato plant. Insert showing tuber roots (top left panel). Dye
is transported from the main roots to the shoot and not to the tuber (top right panel). Uptake of dye from the
stolon roots in close proximity to a tuber (bottom left panel). Tuber after 30 min of dye exposure (bottom
right panel). The dye is transported from the stolon roots into the tuber. Source: Kratzke and Palta (1985)
showed that tuber Ca content can be increased by placing Ca in the tuber and stolon
areas. Thus, on a practical level, these results indicated that placement of Ca is
important for enhancing Ca uptake by the tuber. More recently, we have provided a
direct evidence for the transport of Ca to the tubers from the stolon roots closely
associated with the tuber (Busse and Palta 2006). This study also provided evidence
that main root system does not supply calcium to the tubers.
Spoon-Feeding Potatoes during Bulking, a New Concept in Potato Nutrition
Our discovery of tiny roots on tubers has changed the concept of potato nutrition.
Previously, it was believed that the potato plant’s main roots supplied all the water and
nutrients to the leaves, and the leaves in turn fed the tubers. In contrast, our results clearly
show that potato tubers are like ‘underground plants’ that draw their water and nutrients,
such as calcium, from the soil and not from the foliage. Since tubers are surrounded by
relatively moist soil, they cannot compete with leaves for transpirational water uptake.
Tubers have to rely on the roots that are in their close proximity (tuber roots, tuber–
stolon junction roots, and stolon roots) to transport water from the soil. Since calcium
moves in the xylem along with the water, it follows that potato tubers must transport
calcium from the soil in their close proximity.
These results have led to the development of a concept of ‘spoon feeding’ the tubers
with calcium during the tuber bulking period. Tubers produced in the same hill vary in
their calcium concentration. This means that each tuber is transporting different amounts
Potato Research (2010) 53:267–275
271
of calcium. These results also support our concept that potato tubers draw calcium from
soil. Since calcium concentration and water vary at different places in the soil, it is not
surprising that each tuber has different calcium levels.
Targeted Calcium Application in the Tuber Area During Tuber Bulking Period
is Most Effective at Increasing Tuber Calcium Concentration
To enhance calcium concentration in the tubers, it is important that calcium be added in
the upper portion of the hill, where tubers develop, during the tuber bulking period. Prior
to our research, potato growers used to complete fertilization at hilling. This was a
necessity, since no nutrient application could be made by tractor after hilling without
damaging the plants. Our results show that the application of calcium needs to be made
much later in the season and that this can be easily achieved by injecting water-soluble
calcium fertilizer directly into the irrigation line. Since tubers develop during the later
part of the season, it would be important to add supplemental calcium during tuber
bulking, which is very critical in sandy soils. Due to low moisture holding capacity,
sandy soils are irrigated 2–3 times a week. Thus, the top portion of the hill is
continuously washed by the irrigation and rain, with water moving soluble nutrients to
the lower portion of the hill. These nutrients remain accessible to vegetative growth via
the main root system. However, the tubers developing during late season will not have
access to these nutrients via the tuber and/or stolon roots.
Is There a Benefit of Improving Tuber Calcium?
Impact of Ca Application on Internal Defects
Internal defects such as brown center, internal brown spot, and hollow heart produce
no external symptoms on affected tubers and therefore cannot be removed before
sale. We have examined the impact of Ca fertilization on internal defects over
several seasons. Individual tubers were analysed for this purpose (Ozgen et al.
2006). Split applications (1/3 at hilling, 1/3 at 3 weeks after hilling, and 1/3 at
6 weeks after hilling) of calcium at 100–200 kg Ca/ha resulted in significant
reduction in internal defects in Russet Burbank potatoes.
Both peel and medullary calcium concentrations were improved by these calcium
applications. We found that as the average calcium concentration in tuber increased from
about 100–250 ppm (dry weight basis), the incidence of internal defects was reduced
from over 20 to 5% (Fig. 2). However, it is important to point out that at a given level
of tuber calcium, there is a large variability (especially at low Ca concentration) among
tubers in the incidence of internal defects. In other words, in addition to Ca, there are
other factors that contribute to the development of internal defects.
Incidence of Bruising (Mechanical Injury Resulting from Impact During Harvest,
Transport and Storage) is Reduced by Increasing Calcium Concentration in the Tuber
Potato tubers suffer from mechanical injury from the physical impacts during
harvesting, transporting, and storage. This results in a bruise which results from
Potato Research (2010) 53:267–275
% IBS
272
30
25
20
15
10
5
0
0
50
100
150
200
250
Non-periderm Calcium (ppm, dry weight basis)
300
Fig. 2 Scatter plot of the relationship between non-periderm tissue Ca and internal brown spot (IBS).
Each point indicates the mean Ca level in 15 tubers within a given replication and the corresponding IBS
in that replication in 1997. Significant at P<0.05. Source: Ozgen et al. (2006)
impact and compression injury. Bruises are usually found in the perimedullary tissue
just beneath the vascular ring. They can also be found in the cortical area. Pigment is
produced following injury, giving tissue a black, brown, or gray appearance. It
appears that the extent of bruise injury is related to the cell wall strength and
membrane health. We have conducted systematic studies on the relationship between
bruising incidences and tuber calcium levels among several cultivars. The
susceptibility of the tubers varies among cultivars which appears to be related, in
part, to the specific gravity (Karlsson et al. 2006). The cultivars Atlantic and
Snowden had low tuber calcium concentration and high incidence of bruise. On the
other hand, the cultivars Superior and Dark Red Norland had much higher calcium
concentration and had lower incidence of bruise. Taken together for all cultivars
studied, we found that as the tuber calcium concentration increased, the bruise
incidence decreased (Fig. 3).
Role of Calcium in Heat and Frost Stresses
Heat stress is known to reduce potato plant growth and reduce partitioning of
photosynthate to the tubers. Although there are differences among cultivars in their
response to heat stress, in general, heat tends to increase stem length and branches
while reducing the leaf size and total leaf area. In addition, high temperatures also
60
% Black Spot Bruise
50
40
30
20
10
0
0
50
100
150
200
250
300
350
400
Tuber Tissue Calcium Concentration (ppm dry weight basis)
Fig. 3 Relationship between tuber calcium concentration and incidence of black spot bruise. Data from
three seasons (1999–2001) and five cultivars are plotted. Each point in this figure represents incidence of
bruise in about 100 tubers. Source: Karlsson et al. (2006)
Potato Research (2010) 53:267–275
273
reduce the net photosynthesis. The overall result of the effects of heat stress on
potato plants is a decrease in plant growth and tuber yield.
The summer of 1998 was unusually warm and dry in Wisconsin. On average,
there was 25% decrease in tuber yield in the potato-growing area of the state. In our
field trials, we found a 20–30% increase in tuber yields where soluble calcium was
applied during tuber bulking period. These results suggested that calcium
fertilization during bulking could mitigate the adverse impact of heat stress on
tuber yield. Our controlled-environment study verified these results (Tawfik et al.
1996). In this study, plants were grown in large pots under simulated heat stress
using field soil. Plants given supplemental calcium fertilization during heat stress
gave about 30% higher yield. This beneficial effect of calcium was absent under
nonstress (normal and cool) conditions. In addition, under heat stress, plants
receiving supplemental calcium had higher leaf calcium contents and higher stomatal
conductance. These results show that application of calcium during heat stress can
mitigate heat stress effects and that the maintenance of a certain level of calcium in
leaf tissue is important under heat stress.
Although we do not know the mechanism by which calcium is able to
mitigate heat stress effects on potatoes, our results provide some insight. For
example, we found that stomatal conductance was higher in calcium-treated than
control plants under heat stress. Maintenance of stomatal opening could be
important in avoiding heat stress effects via enhanced transpirational water loss.
We found a decrease in the calcium concentration in leaves of potato plants
exposed to heat stress but the Ca concentration was maintained at the same level
as prior to heat stress in the leaves of plants given calcium fertilization during
heat stress. Thus, our results suggest that maintenance of a certain level of Ca
during heat stress is essential for the normal function of the stomata. In a followup study, we provided direct evidence that root zone calcium can modulate the
response of the potato plant to heat stress (Kleinhenz and Palta 2002). In this
study, we demonstrated that heat stress damages the apical meristem and inhibits
cell expansion. By increasing the calcium concentration in the root zone, we were
able to eliminate these injuries by heat stress to the potato plant.
In addition to heat stress, we have found evidence that the impact of frost on potato
plant can be mitigated by calcium nutrition. Injury to plants by frost is known to be
initiated at the cell membrane level (Palta 1996). In our early studies, we provided
evidence that in the initial stages of frost injury, a loss of cellular/membrane calcium
occurs (Arora and Palta 1988). By reversing this loss and by providing calcium, it is
possible to promote recovery of injury from stress. In 1996, we investigated the
possibility of improving frost survival of potato plants by supplemental application of
calcium during the season. Potato plants were subjected to frost under field conditions. In
general, plants given soluble calcium showed improved frost survival (Vega et al. 1996).
Calcium Deficiency can Result in Injury to the Tuber Sprout Thus Effecting Plant
Growth and Health
Using an in vitro shoot culture system, we have investigated the development of
injury due to calcium deficiency at the microscopic level. Our results show that the
primary injury is localized in the expanding pith cells below the shoot apical
274
Potato Research (2010) 53:267–275
meristem, and this injury results in a collapse of cell wall of the subapical cells
(Busse et al. 2008). This injury also results in lack of apical dominance, promoting
the growth of axillary shoots.
Genetic Variability Exists for Tuber Calcium Accumulation Ability Among Wild
Potato Species as Well as Among Cultivars and Tuber Calcium is a Heritable Trait
Our studies have shown significant variability in tuber calcium concentration among
cultivars grown under same environment (Karlsson et al. 2006). We have also
screened the US Potato Genebank collection for the ability to accumulate tuber
calcium. We found a large variation among wild Solanum species for tuber
accumulation ability (Bamberg et al. 1993). Some species, such as Solanum
microdontum, accumulated almost five times more tuber calcium than Solanum
tuberosum cultivars, whereas others such as Solanum kurtzianum accumulated even
lower levels than common cultivars. We are currently investigating the genetic
mechanism for tuber calcium uptake and studying the possibility of improving
potato tuber quality by breeding and selecting for tuber calcium.
References
Arora R, Palta JP (1986) Protoplasmic swelling as a symptom of freezing injury in onion bulb cells: its
simulation in extracellular KCl and prevention by calcium. Plant Physiol 82:625–629
Arora R, Palta JP (1988) In vivo perturbation of membrane-associated calcium by freeze-thaw stress in
onion bulb cells: simulation of this perturbation in extracellular KCl and alleviation by calcium. Plant
Physiol 87:622–628
Arora R, Palta JP (1989) Perturbation of membrane calcium as a molecular mechanism of freezing injury.
In: Cherry JH (ed) Environmental stress in plants. Springer, Berlin, pp 281–290
Bamberg JB, Palta JP, Peterson LA, Martin M, Krueger AR (1993) Screening tuber-bearing Solanum
(potato) germplasm for efficient accumulation of tuber calcium. Am Potato J 70:219–226
Bangerth F (1979) Calcium-related physiological disorders of plants. Annu Rev Phytopathol 17:97–122
Bush DS (1995) Calcium regulation in plant cells and its role in signaling. Annu Rev Plant Physiol Plant
Mol Biol 46:95–122
Busse JS, Palta JP (2006) Investigating the in vivo calcium transport path to developing tuber using 45 Ca:
a new concept in potato tuber calcium nutrition. Physiol Plant 128:313–323
Busse JS, Ozgen S, Palta JP (2008) Influence of root zone calcium on subapical necrosis in potato shoot
cultures: localization of injury at the tissue and cellular levels. J Amer Soc Hort Sci 133:653–662
Hanson JB (1984) The functions of calcium in plant nutrition. In: Tinker TB, Lauchli A (eds) Advances in
Plant Nutrition. Praeger Pub, New York, pp 149–208
Hirschi KD (2004) The calcium conundrum. Both versatile nutrient and specific signal. Plant Physiol
136:2438–2442
Karlsson BH, Crump PM, Palta JP (2006) Enhancing tuber calcium by in-season calcium application can
reduce incidence of black spot bruise injury in potatoes. HortScience 41:1213–1221
Kirby EA, Pilbeam DJ (1984) Calcium as a plant nutrient. Plant Cell Environ 7:397–405
Kleinhenz MD, Palta JP (2002) Root zone calcium modulates the response of potato plants to heat stress.
Physiol Plant 115:111–118
Kratzke MG, Palta JP (1985) Evidence for the existence of functional roots on potato tubers and stolons:
significance in water transport to the tuber. Am Potato J 62:227–236
Kratzke MG, Palta JP (1986) Calcium accumulation in potato tubers: role of the basal roots. HortScience
21(4):1022–1024
Marschner H (1995) Mineral nutrition of higher plants, 2nd edn. Academic, New York
Matoh H, Kobayashi M (1998) Boron and calcium, essential inorganic constituents of pectic
polysaccharides in higher plant cell walls. J Plant Res 111:179–190
Potato Research (2010) 53:267–275
275
McGuire RG, Kelman A (1984) Reduced severity of Erwinia soft rot in potato tubers with increased
calcium content. Phytopathology 74:1250–1256
Ozgen S, Palta JP (2004) Supplemental calcium application influences potato tuber number and size.
HortScience 40:102–105
Ozgen S, Kleinhenz MD, Palta JP (2003) Influence of supplemental calcium fertilization on potato tuber
size and number. In: Yada RY (ed) Proc. XXVI IHC─Potatoes—healthy food for humanity. Acta Hort
619:329–335
Ozgen S, Karlsson BH, Palta JP (2006) Response of potatoes (cv. Russet Burbank) to supplemental
calcium and nitrogen application under field conditions: tuber calcium, yield and internal quality. Am
J Potato Res 83:195–204
Palta JP (1996) Role of calcium in plant response to stresses: linking basic research to the solution of
practical problems. Hort Science 31:51–57
Sanders D, Brownlee C, Harper JF (1999) Communicating with calcium. Plant Cell 11:691–706
Tawfik AA, Kleinhenz MD, Palta JP (1996) Application of calcium and nitrogen for mitigating heat stress
effects on potatoes. Am Potato J 73:261–273
Vega SE, Bamberg JB, Palta JP (1996) Potential for improving freezing stress tolerance of wild potato
germplasm by supplemental calcium fertilization. Am Potato J 73:397–409
Vega SE, Bamberg JB, Palta JP (2006) Root zone calcium can modulate GA-induced tuberization signal.
Am J Potato Res 83:135