Issue 1 2020
Keeping up appearances: how potato skin quality benefits from use of Polysulphate fertilizer
For potatoes, being good-looking is really important. Shoppers in grocery stores want to choose potatoes with clean, shiny skin and free from blemishes and other defects. Unfortunately, a variety of factors – such as disease, damage and deficiency – reduce the chances of potatoes having an attractive appearance. In addition to the cosmetic issues, potatoes with less than perfect skin are more likely to rot and have a reduced storage life. This is why there is so much interest in understanding the connection between balanced crop nutrition and potato skin perfection.

Keeping the potato skin appealing

The protective outer layer of a potato tuber is called periderm tissue. The outmost cell layers contain corky cell walls and are called ‘skin’. This skin, the part that is often peeled away before cooking, forms an effective barrier around the tuber. It protects the tuber from water loss and pathogen invasion and its smooth visual appearance is highly important in marketing the crop. Physiological skin blemishes – from disease or damage – are of great concern. However, there is a third potential cause: deficiency.

Previous research has shown that skin defects in potato are likely to be the result of calcium (Ca) deficiency. In other words, tuber calcium is genetically linked to tuber quality. Calcium has been proved to reduce the rate and severity of a potato skin problem known as ‘russeting’.

Beauty that is more than skin deep

The beneficial effect of Ca on the skin only occurs if this essential plant nutrient reaches the developing tubers. The exact effect of Ca on potato skin is not yet fully understood, although it has been connected with essential functions such as maintaining the structural integrity of cell walls.

Ca is transported with the uptake of water or transpiration stream via the xylem into the tuber, or from direct interaction of the tuber surface with the soil which surrounds it. The fact that calcium is an immobile nutrient in the plant and that potato tubers are low-transpiring organs, makes them prone to Ca deficiency. For this reason, the direct interaction of the tuber skin with Ca in solution in the soil may be one of the main sources of Ca and major contributor to the beneficial effect of Ca on the skin.
Lifting levels of calcium

Researchers from Agricultural Research Organization in Israel evaluated polyhalite, which is marketed as Polysulphate® fertilizer, as an alternative source of Ca for farmers to use on their potato crop. They particularly wanted to see the difference that an application of Polysulphate makes at the end of the growth period, assuming that providing the mineral at the last stages of skin development may enhance its quality.

Accordingly, Polysulphate was applied at three points in time: pre-planting; during growth but around 3–4 weeks prior to the usual pre-harvest task of destroying the green growth above soil known as haulm desiccation; and 2 days post-haulm desiccation. The experiments included several cultivars and locations.

Results indicated that the late application of Polysulphate, after haulm desiccation, resulted in improved tuber appearance. The tubers of the variety Rosanna showed less russeted skin with gray or faint pink color, and partially achieving the desired shiny appearance with dark-pink pigmentation. Tubers of Vivaldi variety exhibited less russeted skin with brownish and even more desirable shiny yellow appearance.
Testing in the lab the expressions of the genes involved in potato skin suberization by the qPCR method.

Understanding what goes on beneath the skin

To understand the observed positive effect of Polysulphate on skin appearance, an expression study of genes known to be involved with formation of potato skin was conducted. The aim was to check for any effect at the molecular level.

A laboratory technique of molecular biology – quantitative PCR (qPCR) – was used to measure the expressions of the genes involved in skin suberization, the conversion of the cell walls into corky tissue by deposition of suberin, the macromolecules that form a protective barrier.

Results showed that using Polysulphate fertilizer significantly uplifted the expression of FHT and CYP86A33, two indicator genes for skin suberization (see figure below).

These genetic studies have usefully revealed how applying Polysulphate fertilizer can reduce the incidence of skin disorders and improve the skin finish.
Expression of KCS6, FHT, and CYP86A33 indicator genes for skin suberization in the peel of Rosanna tubers. Peel samples were collected at harvest (135 DATP). C: control plants, not fertilized. PolyH: fertilized with Polysulphate at preplanting. PolyH": Late application of Polysulphate, 2 days after haulm desiccation, to both PolyH plants (PolyH+PolyH") and their controls (C+PolyH"). Different letters indicate significantly different values (P < 0.05).

Keeping up appearances

Knowing that skin finish is important to consumers and that farmers need ways to produce potatoes with clean, attractive skins to meet increasing demand is the context for contemporary potato farming in many parts of the world.

Looking within the potato, to the genes and the biological processes that create a robust and beautiful skin is key. Discovering what these processes are, knowing what each and every potato plant needs as nourishment with minerals such as Ca is essential.

Recognizing that Polysulphate fertilizer – which is a natural, organic mineral fertilizer with Ca and three other essential plant nutrients – has a beneficial effect on skin finish is a very important step. It is a step towards better quality and good-looking potatoes, contented consumers and satisfied farmers.


Evaluation of skin appearance of Rosanna tubers from the experiments. 
For full paper:

Keren-Keiserman A., Baghel R.S., Fogelman E., Faingold I., Zig U., Yermiyahu U. and Ginzberg I. (2019). Effects of Polyhalite Fertilization on Skin Quality of Potato Tuber. Front. Plant Sci. 10:1379. doi: 10.3389/fpls.2019.01379
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Mined in the UK, ICL is the first – and only - producer in the world to mine polyhalite, marketed as Polysulphate