Quality: Potassium Management is Critical for Horticultural Crops

Quality, What is it?
Potassium is frequently referred to as the “quality” nu­trient for plants. Quality has many characteristics and the most important aspects of quality will depend on the spe­cific crop. For example, with citrus, it may be the thickness of the peel and Vitamin C concentration, for apples, sug­ar concentrations, while for tomatoes, the development of uniformly red fruit rich with lycopene. The specific quality parameters for each crop will vary and should be well un­derstood to maximize crop nutritional practices and market profitability (Kumar et al., 2006).

While many “quality” benefits are generally understood, it can be difficult to define and quantify the exact benefits of K (Lester et al., 2010a). Most notably, the lack of quali­ty is frequently observed when the plant K supply becomes
 deficient. An inadequate K supply becomes especially im­portant for horticultural crops where the visual appearance of the fruit and leaves is critical for marketing. Although the total yield may be reduced with insufficient K, it is possible that the entire crop may be unsalable due to poor quality and visual appeal.

The growth and longevity of cut flowers and ornamen­tals can also be diminished by a lack of adequate K. Ship­ping, handling, and freshness are particularly important for ornamental horticulture.

Consumer Preference
Consumers have a strong preference for fresh fruits and vegetables with appealing appearance and texture. Quality and freshness of fruits and vegetables are often cited as the primary characteristics for making purchase decisions.

Potassium plays a critical role in many of the metabolic processes that enhance the quality, nutrition, flavor, appear­ance, and longevity of fresh food crops. These beneficial im­provements clearly are desirable for farmers and will add to the marketability of crops.

 Vitamin C
Application of K to the soil or plant foliage has been shown to increase the concentration of Vitamin C in a va­riety of fruit crops. While citrus is the most frequently cited example, increased Vitamin C has been reported in crops such as cucurbits, cauliflower, onion, banana, guava, and papaya (Imas, 2013). Muskmelon also had higher concen­trations of Vitamin C as a result of foliar K sprays (Lester et al., 2010b).

Nitrate Assimilation and Protein Synthesis
Potassium plays an important role in converting nitrate into amino acids and proteins. An insufficient supply of K may result in both lower nitrate uptake from the soil and slower nitrate assimilation into amino acids and proteins. Potassium deficiency can result in accumulation of low mo­lecular weight sugars and carbohydrates, along with solu­ble-N compounds in the plant.

Nitrate accumulation in K-deficient plants can be a con­cern where limits have been established (such the Europe­an Union nitrate limit for leafy vegetables). When nitrate is rapidly converted to protein, the concern for healthier food is satisfied.

Appearance of Fruits and Vegetables
An adequate K supply has been linked to improved vi­sual appearance of many horticultural crops. For example, banana is a crop that frequently responds favorably to K fertilization. Sufficient K improves banana fruit weight and number of fruits in each bunch, increases soluble solids, sug­ars, and starch. Low K results in thin and brittle bunches with a shorter shelf life. A lack of K has been linked with premature color development and harder, dry fruit sacs in citrus. Potassium-deficient grapes are less firm and have less juice.

An adequate supply of K increased mar­ketability traits of muskmelon fruit (maturity, yield, firmness, and sugars) and quality param­eters (ascorbic acid and β-carotene) (Lester et al., 2010a). The yield, quality, and shelf-life of tomatoes are improved with an adequate K supply. A lack of sufficient K results in uneven ripening, yellow shoulder fruit, and irregularly shaped fruit with poor internal quality (Hartz et al., 2005).

Extending Shelf-life and Reducing Food Waste
Potassium has been shown to have a bene­ficial impact on properties that improve shelf life, storage, and shipping of many fruits and vegetables. Some of this occurs as an adequate K supply generally increases the firmness and strength of skins, allowing greater resistance to damage during trans­port and storage. Extending the longevity of freshness pro­vides immediate benefits to both the farmers and the con­sumers.

The positive impact of K on fruit storage has been re­ported on many crops, including bananas (shelf-life), citrus (decreased post-harvest mold and rot), potatoes (storage longevity), carrots (crispness), pineapple (greater vitamin C leading to reduced browning and rot), figs, and apples.

Disease and Insects
Plants that are deficient in K are likely to be more sus­ceptible to infection and insect damage than when sufficient K is present. In a significant literature review, Perrenoud (1990) examined 2,449 scientific citations and concluded that the use of K reduced the incidence of fungal diseases by 70%, bacterial infection by 69%, insects and mites by 63%, viruses by 41%, and nematodes by 33%. Reducing these pathogens and insects had a large benefit of allowing higher yields to be achieved.

A review by Wang et al. (2013) presented an excellent summary of how optimal K nutrition imparts significant plant resistance to both biotic and abiotic stresses. They re­viewed the important role of K in protecting plants against diseases, pests, drought, salinity, cold and frost, and water­logging.

Consumers are sensitive to the use of plant protection chemicals in production of horticultural crops. This sensi­tivity partially accounts for the growth of the organic farm­ing sector (Mikkelsen, 2007). Whenever possible, providing adequate K should be used as a first line of protecting plant health. Decreased damage to harvested fruits and vegeta­bles from pathogens and stresses will also result in a more attractive, marketable, and hence profitable crop.

Nutrient Composition
Fruits and vegetables are the most important sources of dietary K in the human diet. However, a trend for a decline in the mineral concentration of many foods has been sug­gested for over 75 years (Davis, 2009). A decline of 5% to 40% or more in minerals, vitamins, and proteins has been measured in many foods, especially vegetables. The cause for this decline may be due to dilution, changes through plant breeding, and changes in farming cultural practices. Recent reviews indicate that the decline in nutrient concen­tration of fruits and grain may not be as severe as earlier claimed (Marles, 2017).

Whatever the cause of this dilution, clearly there is a need to reexamine how the K concentration of food can be enhanced to better meet the dietary and health needs of consumers.

Functional Foods
“Functional food” is a term used to describe foods that provide health benefits in addition to the regular vitamins and minerals contained in common foods. Including them in a human diet is often considered to promote health be­yond a more typical diet. Lycopene found in tomatoes, alli­cin present in garlic, and resveratrol in grapes are examples of nutraceutical compounds in functional foods that may provide health benefits.

The concentrations of all these functional food com­pounds listed above have been shown to increase in the pres­ence of an adequate or abundant K supply to plants. The direct metabolic link between K and these functional food compounds is not always clear, but the trends are consistent.

Human Health
Animals and humans have an absolute requirement for K for proper growth and health. Potassium is involved in many essential functions in nerves, biochemical reactions, muscle function, heart health, and water balance. However, almost all human diets are quite low in K compared with the recommendations for health (Weaver, 2013). For exam­ple, in the United States the average daily K consumption is only 55% of the recommended dietary intake.

A diet rich in fruits and vegetables is one of the best ways to increase K intake, with potatoes being one of the highest sources of dietary K. Increasing the K concentra­tion of the harvested portion of fruits, vegetables, and other plant-based products would make an important contribu­tion to improving human health.

Conclusions
Potassium is essential for sustaining both the yield and the quality of many horticultural crops. Enhanced quality is frequently observed in many vegetables and fruits from an abundant supply of K. This quality can be observed in different ways for each species, but includes parameters such as size, appearance, longevity of storage, sugar and acidity, soluble solids, and nutritional benefits. Damage from dis­ease, insects, and environmental stresses are frequently re­duced when adequate K is present. All these considerations combine to underline the importance of maintaining an adequate supply of K for the production of high quality horticultural crops.

References
Davis, D.R. 2009. HortScience 44:15-19.
Hartz, T.K. et al. 2005. HortScience 40:1862-1867.
Imas, P. 2013. Potassium - The quality element in crop production. Intern. Potash Institute: Horgen, Switzerland.
Kumar, A.R. et al. 2006. Agric. Rev. 4:284-291.
Lester, G.E. et al. 2010a. Better Crops 94(1):18-21.
Lester, G.E. et al. 2010b. Plant Soil 335:117-131.
Marles, R.J. 2017. J. Food Comp. Anal. 56:93-103.
Mikkelsen, R.L. 2007. HortTechnology 17:455-460.
Perrenoud, S. 1990. Potassium and Plant Health, 2nd ed., International Potash Institute: Bern, Switzerland.
Wang, M.Q. et al. 2013. Int. J. Mol. Sci. 14:7370-7390.
Weaver, C.M. 2013. Adv. Nutrition 4:368S-377S.

This article originally appeared in IPNI's magazine: Better Crops in 2018.  It is linked here
potassium potash health vegetable fruit quality consumer nitrate shelf waste functional human animal quinic sulphoraphine indole carotenoid echinacoside polysaccharide lignans allicin flavonoids ginsenoside serveratrol quercirtin anthocyanidins lycopene carotenoid beta glucans saporins terpenoids phytic robert mikkelsen rob plant nutrition