1. Introduction
Potatoes play an important role in the global food security, nutrition and healthy diet [
1]. Potatoes provide a significant amount of high-quality protein, vitamin C (mainly L-ascorbic acid) and group B vitamins, minerals as well as other health-promoting compounds. Consumption of 250–300 g of boiled potatoes provides about 7–8% of the recommended daily intake of protein, 6–11% of carbohydrates, 50% of vitamin C, 30–40% of potassium and about 17% of fibre. Apart from nutrients, potato tubers also contain anti-nutritional substances such as nitrates or glycoalkaloids. The quality of edible potatoes is determined by their nutritional value and low content of anti-nutrients. An important quality characteristic is also sensory properties including after-cooking darkening [
2,
3,
4]. The chemical composition of potato tubers depends on the cultivar, tuber size and maturity, but may change under environmental (weather and soil conditions) and agronomic factors [
5,
6,
7].
In sustainable crop production focusing on high-value products, biostimulants have been gaining increasing importance. These natural products enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits. Plant biostimulant based on seaweed extracts and humic acids have the largest market share [
8,
9,
10].
Seaweed extracts are complex mixtures with multiple mechanisms of action. Bioactive compounds present in seaweed extracts improve plant growth, enhance nutrient use efficiency, improve plant defences against pathogens and improve crop quality. The bioactivity of seaweed extracts depend on the algae species and the extraction method. Most commercial seaweed products used as plant biostimulants in agriculture and horticulture are manufactured from brown seaweeds (
Phaeophyta).
Ascophyllum nodosum (
A. nodosum) and
Ecklonia maxima (
E. maxima) are dominant in this group [
11,
12,
13]. The
A. nodosum extract Bio-algeen S90 and the
E. maxima extract Kelpak SL are used worldwide as a biostimulants for a number of agricultural and horticultural crops. Biostimulants based on
A. nodosum extracts are used to improwe plant growth and to mitigate abiotic and biotic stresses. The effect of
A. nodosum extracts on plants are attributed to phytohormone, microelement, and/or alga-specific polysaccharides, betaines, polyamines and phenolic compounds contents [
14,
15].
E. maxima extract Kelpak SL contains auxins, cytokinins, polyamines, abscisic acid, gibberellin, brassinosteriods and a small amounts of macro- and microelements. The active compounds present in Kelpak SL, alone or in combination, bring contribute to enhance plant growth and yield, and improve biotic and abiotic stresses tolerance. Content of some active compounds in Kelpak is higher than in other commercial products based on
E. maxima extract [
16,
17]. Commercial seaweed products manufactured from the same seaweed source by different companies, generally marketed as equivalent products, may vary significantly in product composition and in efficacy to induce specific plant responses following appplication, especially under field conditions. Commercial
A. nodosum extracts Phylgreenmira, Algazone and Ultra-Kelp increased the dry matter and starch content in potato tubers, as well as slightly decreased protein content [
18], whereas Bio-algeen S90 did not affect the starch, total nitrogen content or the potato after-cooking darkening [
19,
20]. Other products based on
A. nodosum extract Primo increased nitrogen and protein content in potato tubers [
21]. Bio-algeen S90 did not affect dry matter content in carrot, but increased L-ascorbic acid and total sugar contents [
22].
E. maxima extract Kelpak SL did not affect starch or total nitrogen content in potato tubers, but increased vitamin C and nitrate contents [
20,
23]. Kelpak SL increased nitrate and nitrite content in carrot and protein content in beans, decreased starch content in wheat, but did not affect protein or fat content in winter rape [
24,
25,
26,
27].
The biostimulant effects of humic substances are characterized by both structural and physiological changes in plants related to nutrient uptake, assimilation and distribution, and changes in plant primary and secondary metabolism related to abiotic stress tolerance. The biological activity of humic substances depends on their source, chemical structure, and concentrations. Leonardite is the most common commercial source of humic substances [
28,
29,
30]. Humic substances extracted from leonardite stimulate plant growth, nitrogen metabolism and accumulation of phenolic compounds. The positive effects of humic substances on plant metabolism are attributed to phytohormone-like activity. Biostimulant activity of humic substances extracted from leonardites depends on the origin of the leonardite [
31]. Humic acids derived from leonardite applied to soil or introduced into the irrigation system increased dry matter, starch and protein content in potato tuber [
32,
33,
34]. Soil application of humic acids did not affect dry matter or starch content in potato tubers, whereas the foliar application of fulvic acids did not affect tuber dry matter but increased starch content [
35]. Foliar application of humic and fulvic acids in HumiPlant, a commercial extract from leonardite, increased the sugar and carotenoid contents and decreased the nitrate content in carrot [
36]. Soil and foliar application of humic acid increased the sugar content in pepper and cucumber [
37,
38], whereas foliar application of humic and fulvic acids increased the vitamin C content and reduced tomato acidity [
39].
To date, few studies have been focused on the effect of plant biostimulants on potato tuber quality. The current study aimed to determine the effect of foliar application of seaweed extracts and humic and fulvic acids on the quality of new potatoes. In the current study, it was hypothesised that seaweed extracts and humic acids could contribute to improving edible potato quality.
4. Discussion
The biostimulants used in the experiment slightly affected potato tuber quality. The dry matter content is one of the most important characteristics of new potatoes. When potatoes are harvested early, low dry matter content can result in a soggy texture and decrease the quality of new potatoes. The biostimulants used in the experiment had no effect on dry matter content in immature tubers of very early potato cultivars ‘Denar’, ‘Lord’ and ‘Miłek’. In a study carried out by other authors, the biostimulants based on
A. nodosum extracts (Phylgreenmira, Algazone, Ultra-Kelp) increased dry matter content in mature tubers of early (‘Arizona’, ‘Riviera’) and medium-early (‘Agria’) potato cultivars [
18]. In a previous research, foliar application of fulvic acids had no effect on tuber dry matter of medium-early cultivar ‘Atlantic’ [
35]. The dry matter content in potato tuber is determined by leaf assimilation area and chlorophyll content in leaves. Reducing the assimilation area along with increasing the chlorophyll
a content and simultaneously decreasing the chlorophyll
b content in leaves increased the dry matter content in tubers [
56]. The biostimulants used in the experiment enlarged the assimilation area of very early potato cultivars, but did not affect the chlorophyll content in leaves [
57].
The quality of edible potatoes is determined by their starch and sugar contents [
4]. Among the biostimulants used in the experiment, only Bio-algeen S90 (
A. nodosum extract) increased starch content in tubers of very early potato cultivars 75 days after planting, especially in a year with a low air temperature and a drought in the tuber growth period. In the three years of the study, following application of this biostimulant, the average starch content was higher by 4.8 g∙kg
−1 FW compared to the average for the untreated control tubers. Bio-algeen S90 has more bioactive compounds that may promote starch synthesis through the induction of carbon metabolism and activities of starch synthesis enzymes. In a study carried out by other authors, Bio-algeen S90 did not affect the starch content in tubers of the medium-early cultivar ‘Muza’ [
19]. Other biostimulants based on
A. nodosum extracts (Phylgreenmira, Algazone, Ultra-Kelp) increased starch content in tubers of early (‘Arizona’, ‘Riviera’) and medium-early (‘Agria’) potato cultivars [
18]. The bioactivity of seaweed extracts depends on the extraction method [
11,
12] and on the date and dose of application [
13,
21]. In the present study, humic and fulvic acids from leonardite in HumiPlant (fulvic acid 6% and humic acid 12%) did not affect starch content in immature tubers of very early potato cultivars. In a study carried out by other authors, humic acid from leonardite in Huma K (humic acid 56% and fulvic acid 30%) introduced into the irrigation system increased starch content in tubers of medium-early cultivar ‘Hermes’ [
33], and soil application of crude humic acids from leonardite in Agro-Lig (total humic acid 85%) increased starch content in tubers of late maturity cultivar ‘Caspar’ [
34]. Foliar application of Natural Canadian fulvic acids (80% liquid) increased starch content in tubers of the late cultivar ‘Atlantic’ [
35]. Humic substances affect the activity of the main enzymes involved in carbohydrate metabolism and ADP-glucose pyrophosphorylase (AGPase) has a major role in starch synthesis. The effect of humic substances on enzyme activities depends on humic molecular size, molecular characteristics and concentration [
28,
31]. The biostimulants used in the experiment did not affect the sugar content in immature tubers of very early potato cultivars. Starch content in potato tubers is associated with assimilation leaf area, the chlorophyll
a content in leaves and efficiency of the photosystem in the dark, whereas the sugars content depend on chlorophyll content in leaves and fluorescence yield [
56]. The biostimulants used in the experiment did not affect starch and sugar content in immature tubers of very early potato cultivars, except for Bio-algeen S90, although it enlarged the assimilation leaf area [
57].
Protein and vitamin C are very important nutritional compounds in potatoes. The protein present in potato tubers has a higher biological value than other crops due to the content of all exogenous amino acids, and especially a high content of lysine [
4]. The biostimulants used in the experiment did not affect protein or L-ascorbic acid content in immature tubers of very early potato cultivars. Protein accumulation in potato tuber is determined by leaf assimilation area and the chlorophyll
a content in leaves, while vitamin C content is only determined by chlorophyll
a content [
56]. In the present study, enlargement of the leaf assimilation area as a result of biostimulant application [
57] had no effect on the protein content in tubers. In a study carried out by other authors, Bio-algeen S90 and Kelpak SL also did not affect total nitrogen content in tubers of very early (‘Volumia’) or medium-early (‘Irga’, ‘Satina’, ‘Silvana’) cultivars [
20], but Kelpak SL increased the vitamin C content in tubers of a medium-late cultivar (‘Bryza’) [
23].
Although potato tubers accumulate small amounts of nitrates [
4], due to high potato consumption they can be a source of substantial quantities of these compounds in the human diet. Nitrates are accumulated in potato tubers when their uptake is greater than the possibility of the plant to utilise them in organic nitrogenous compounds. In general, immaturity in potato tubers has been connected with high nitrate levels. The later the potatoes are harvested, the lower the nitrate contents are in tubers, but the relationship between nitrate content and tuber maturity differs between genotypes [
58]. Bio-algeen S90 did not affect nitrate content in immature tubers of very early potato cultivars tested, whereas Kelpak SL and HumiPlant reduced the nitrate content only in tubers of the ‘Denar’ cultivar. Following the application of those biostimulants, the nitrate content in tubers of ‘Denar’ was lower, on average, by 8.50 mg∙kg
−1 FW. In a study carried out by other authors, Bio-algeen S90 had no effect on nitrate content in tubers of very early (‘Volumia’) and medium-early (‘Irga’, ‘Satina’, ‘Silvana’) cultivars, whereas Kelpak SL increased the content of these compounds [
20]. Seaweed extracts and humic substances may promote nitrogen metabolism. A positive dose-dependent effect of seaweed extracts and humic acids on the activities of the main enzymes involved in the reduction and assimilation of inorganic nitrogen (nitrate reductase, glutamate dehydrogenase and glutamine synthetase) was found [
12,
29]. Results of the present study suggest, that after the application of biostimulants, the amount of transcripts of regulatory of enzymes related to the nitrogen metabolism of Denar cultivar increased more than of Lord and Miłek cultivars.
There is a significant correlation between the vitamin C content and nitrate level in potato tubers. A higher content of vitamin C is accompanied by a lower nitrate content [
59]. The relative levels of ascorbic acid content and nitrate content in potato tubers may be expressed using the ascorbate-nitrate index (I
AN), which is one of the indicators of food safety. A higher index value reflected higher food safety [
54]. The biostimulants used in the experiment had no effect on the ratio of L-ascorbic acid amount-to-nitrate amount in tubers of ‘Lord’ and ‘Miłek’ cultivars, whereas Kelpak SL and HumiPlant increased this ratio in tubers of the ‘Denar’ cultivar, on average, by 0.29. Regardless of the treatment (with or without biostimulant), the ratio of L-ascorbic acid amount-to-nitrate amount (I
AN) in immature tubers of potato cultivars tested was about 2/1, which indicates that the new potatoes were safe for human health regarding the nitrate content [
54].
After-cooking darkening is an important quality characteristic of edible potatoes. It is caused by non-enzymatic oxidation of the chlorogenic acid-iron compound after cooking. The severity of the darkening depends on the ratio of chlorogenic acid-to-citric acid concentration in the potato tubers [
60]. The biostimulants used in the experiment did not affect the susceptibility of new potatoes to after-cooking darkening. This characteristics of table potato quality depends on the cultivar [
60], which was confirmed in the present study. In a study carried by other authors, Bio-algeen S90 did not affect after-cooking darkening of the ‘Muza’ medium-early cultivar [
19].
The nutrient content in potatoes depends on several factors, with cultivar being among the most important [
3]. The nutrient content in potatoes depends on the cultivar and weather conditions during potato growth, to a greater extent than on the biostimulants applied, which was confirmed in a study carried out by other authors [
20].
The results of our study showed, that the application of plant biostimulants Bio-algeen S90, Kelpak SL or HumiPlant improved the plant growth and early crop potato yield [
57,
61] without any negative effect on the nutritional value of new potatoes.