Influence of ADEPT Agro.Bio and potassium humate on the breaking of dormancy in potato tubers under a double-cropping system

The issue of potato seed production in the Ukrainian Steppe is successfully addressed through the double-cropping method—planting freshly harvested tubers in summer after the main crop. However, the double-cropping system for potatoes has not been widely adopted due to the imperfection of existing methods for breaking the dormancy period of freshly harvested tubers. After studying the impact of mechanical methods for breaking dormancy in freshly harvested tubers over four years, we established that none of these methods were effective enough to be recommended for production.

Foreign and Soviet researchers have noted the stimulatory effect of gibberellin on breaking the dormancy period of freshly harvested tubers. However, both foreign and domestic literature report the negative effect of gibberellin on root system development. Unlike heteroauxin and other growth substances, gibberellin stimulates shoot formation and growth but inhibits root development. In mixtures, gibberellin and heteroauxin act as antagonists.

Substances containing sulfhydryl groups (SH) or capable of forming them through isomerization or chemical transformation, such as thiourea and salts of xanthogenic acids, exhibit relatively high activity in breaking plant dormancy. Among these substances, thiourea has gained the most significance for breaking dormancy. A 2% thiourea solution is used to treat sliced tubers for two hours, followed by germination in hotbeds with a sand-sawdust mixture (1:1).

Although this method is more advanced than mechanical ones, it has several drawbacks. Primarily, the wounds on sliced tubers heal slowly and form a dense, impermeable cork tissue poorly, which facilitates the penetration of rot-causing bacteria and leads to tuber spoilage. Additionally, tubers germinate with few eyes, unevenly, requiring two rounds of removal from hotbeds for planting. Root system development is poorly stimulated. Tubers planted with sprouts but without roots produce significantly delayed shoots, and the plants often fail to yield a harvest. This indicates that the success of the double-cropping system for potatoes primarily depends on the effectiveness of the dormancy-breaking method.

The aim of our study was to find a method to actively intervene in the biochemical processes occurring in freshly harvested tubers, induce enhanced tissue respiration, and promote uniform germination with multiple sprouts and simultaneous root system development. Therefore, in our experiments to break tuber dormancy, we used a range of physiologically active organic substances, such as humic acid, heteroauxin, petroleum growth substance (PGS), thiourea, and their mixtures.

According to reports, petroleum growth substance (PGS) significantly stimulates not only the growth of the aerial parts of plants but also the root system. Literature data indicate that humic acids have a significant impact on the growth and development processes of higher plants. It has been established that potato tubers respire primarily through the phenolase system. According to A.N. Bach’s theory, biological oxidation requires the activation of the oxygen molecule, achieved through the formation of intermediate peroxides—oxygenases. Polyphenols and their derivatives can function as oxygenases. According to L.A. Khristeva, humic acid in an ion-dispersed state enters the plant, where polyphenolic groups of humic acids are oxidized by atmospheric oxygen with the involvement of polyphenol oxidase, forming peroxides, i.e., transforming into oxygenases. Oxygenases, with the participation of the peroxidase enzyme, release atomic oxygen, which is catalytically transferred to the substrate, while the peroxide form of the polyphenol converts into quinones with a very high oxidative potential. This allows them to extract “activated” hydrogen from organic substances that have undergone appropriate enzymatic catalysis, reverting to polyphenols. Thus, L.A. Khristeva considers the primary cause of the physiological action of humic acid to be the involvement of additional amounts of molecular oxygen in oxidation reactions and the direct participation of its molecules in redox processes.

Based on these theoretical premises, we conducted a series of investigations on this issue.

Results of the Study

Methods for Breaking Dormancy in Freshly Harvested Tubers

In the experiment on breaking dormancy, freshly harvested potato tubers were cut lengthwise into two parts before germination and then treated for two hours with water, a 2% thiourea solution, a 0.001% heteroauxin solution, and a 0.02% potassium humate solution as a source of humic acid. The tubers were germinated for 20 days in sand at 70–90% of full moisture capacity and a temperature of 25–28°C. The results of the experiment are presented in Table 1.

The results of these studies clearly demonstrate the superiority of thiourea over heteroauxin and potassium humate. The low percentage of germinated tubers under the influence of heteroauxin can be easily explained by the fact that it primarily stimulates root system growth and inhibits the aerial parts. As for humic acid, the reason for its almost neutral effect on freshly harvested tubers was entirely unclear.

Subsequently, we conducted a series of experiments on the combined effects of potassium humate, PGS, and heteroauxin with thiourea. Additionally, gibberellin was included in the experiment on breaking dormancy in freshly harvested tubers. The conditions of these experiments were similar to those of the previous experiment. The results are presented in Table 2.

The combined action of thiourea with humic acid, as well as PGS with thiourea, significantly surpassed their individual effects. The favorable interaction of humic acid with thiourea and PGS with thiourea in accelerating the awakening of freshly harvested tubers, in our opinion, is due to the phenomenon of synergism, i.e., the action of these two components in the same direction. Humic acid significantly enhances the redox processes initiated by thiourea, promoting intense cell division at growth points, significantly accelerating the growth of sprouts and roots.

A similar phenomenon was observed by Academician Prat in his work, stimulating cuttings of various plants with humic acid. He noted that in the initial period under the influence of humic acid, plant growth does not differ from the control, but as soon as buds appear, their growth significantly accelerates. The reason for this effect of potassium humate is likely the activation of respiration and metabolism in general. The dynamics of peroxidase activity confirm this.

To investigate the reasons for the increased resistance of freshly harvested tuber pieces treated with a mixture of thiourea and potassium humate solutions against rot-causing bacteria during germination, we studied the degree of wound healing in freshly harvested tubers. The formation of cork tissue—suberin—on the cut surfaces was determined using the Lamalier method. Studies on the formation of cork tissue—suberin—on the cut surfaces of freshly harvested tubers of the Early Rose variety showed that tubers treated with a mixture of thiourea and potassium humate began to form suberin along the edges of the cuts within 6–8 hours, and the cut was completely covered with a thin layer of this tissue within 32–36 hours. In tubers treated with thiourea alone, the cuts began to form suberin within 10–12 hours and were fully covered with a thin layer within 50–52 hours.

No synergism was observed in the combined action of thiourea and heteroauxin on the awakening of freshly harvested tubers. The activity of gibberellin, in terms of the number of sprouted shoots, was equivalent to the combined action of thiourea with PGS and potassium humate, but it was significantly lower in terms of the number of tubers with roots.

Influence of Growth Stimulants on Potato Yield

The study of the influence of stimulating substances on the growth of vegetative mass and potato yield was conducted with the most promising concentrations of stimulants that ensured the highest percentage of awakening in freshly harvested tubers. For this purpose, potato tubers were treated with a mixture of thiourea with potassium humate, thiourea with PGS, and gibberellin. The control was a variant where tubers were treated only with a 2% thiourea solution. Potatoes were planted in July and harvested in October in all years of the experiments. The results of the assessment of vegetative mass growth and tuber yield are presented in Table 3.

These data show that the height of plants during the flowering period across all studied varieties was significantly higher in variants where tubers were treated with gibberellin, as well as a mixture of thiourea with PGS and potassium humate. The same phenomenon was observed in the number of main and lateral stems. However, the thickness of lateral stems (diameter at 5 cm above the soil surface) reached its greatest value in variants where tubers were treated with a mixture of thiourea with potassium humate or PGS.

Our studies showed that the acceleration of root system and vegetative mass growth, as well as the increase in tuber yield under the influence of humic acid, is explained not only by the enhancement of physiological processes occurring in the plant but also by changes in the anatomical structure of root and leaf tissues. The cells of all tissue elements of young roots (exodermis, cortex parenchyma, endodermis, pericycle, phloem, and xylem) were significantly elongated and had a more elongated shape compared to the root tissues treated with thiourea alone. Under the influence of potassium humate, the cells of the palisade parenchyma of the leaf were significantly elongated, a second layer of palisade parenchyma was formed, and the covering and glandular hairs were more developed and consisted of a greater number of cells. In contrast, the leaves of potato plants whose tubers were stimulated with thiourea alone had palisade parenchyma cells arranged in a single layer, which were shortened and partially expanded, with less developed covering and glandular hairs consisting of fewer cells. A similar phenomenon—the elongation of root cells and changes in the palisade parenchyma of the leaf under the influence of humic acids—was observed in wheat, sugar beet, and tomatoes.

The increased photosynthetic activity of potato plants whose tubers were treated with potassium humate is also due to the increased chlorophyll content in their leaves (Table 4).

High tuber yields over the years of the experiments were also observed in variants where dormancy was broken using a mixture of thiourea with PGS solutions. The yield was slightly lower with gibberellin, and the lowest yield was observed in the variant where tubers were treated with thiourea alone.

Influence of Growth Stimulants on Root System Development in Freshly Harvested Tubers

The data presented show that a mixture of thiourea with potassium humate and PGS solutions significantly accelerates the awakening of freshly harvested tubers, promotes germination with multiple sprouts, accelerates wound healing, reduces the number of tubers affected by rot-causing bacteria, increases the number of germinated tubers with roots, and enhances potato yield. However, root system development was not achieved in all germinated tubers under the influence of these growth stimulants.

Further exploration in this direction led us to the following idea. It is known that plant roots, isolated from the aerial parts, continue to grow in an artificial medium and, as a heterotrophic organ, respond more actively to various complex organic compounds than leaves, which synthesize them. It should be noted that humus, compost, humic soil, and raw manure accumulate significant amounts of antibiotics, biological activators, and humic acids produced by microflora and fungi. Humus contains nicotinic acid, pyridine derivatives, thiamine, biotin, riboflavin, folic acid, pantothenic acid, alpha-aminobenzoic acid, auxins, streptomycin, penicillin, terramycin, and other biologically active compounds. Auxins, thiamine (vitamin B1), humic acid, as well as succinic and fumaric acids, play a particularly positive role in stimulating root system growth. Based on these considerations, we replaced the previously used germination medium—sand and sawdust—with humus.

This approach yielded positive results. By the end of the germination period (on the 20th day), all tubers had several developed sprouts and a well-developed root system. Moreover, plants grown from tubers germinated in humus significantly outperformed those germinated in sand in terms of growth and development. The data indicate that plants from tubers germinated in humus had more developed vegetative mass, particularly leaf surface, and provided a tuber yield 11.9–12.7% higher for certain varieties compared to plants from tubers germinated in sand.

Addressing Oxygen Deficiency During Dormancy Breaking in Freshly Harvested Tubers

One of the main conditions for the normal germination of freshly harvested tubers is high humidity in the medium. Insufficient moisture inhibits germination, while excess moisture causes suffocation. Plant respiration is an aerobic process, and its intensity depends on the amount of oxygen the cell can absorb. In nature, there is often a discrepancy between the plant’s oxygen demand, its availability in the environment, and its biological uptake. Consequently, oxygen deficiency can cause various forms of plant stress. According to L.A. Khristeva, addressing oxygen deficiency enhances the plant’s ability to withstand adverse conditions, and humic acid, as well as certain vitamins involved in hydrogen transfer, can be used for this purpose.

The stimulating effect of humic acids is particularly noticeable at the beginning of plant development and during periods of intense biochemical processes, such as the formation of reproductive organs, as well as under suboptimal external conditions. We utilized this valuable property of humic acid during the germination of freshly harvested tubers, which requires frequent watering. To avoid oxygen deficiency, tubers placed for germination were watered three times with various growth stimulants of domestic and foreign origin at 5-day intervals. As shown in Table 7, the best results were achieved by germinating pieces of freshly harvested tubers treated with a mixture of humic acid and thiourea solutions in sprouting boxes layered with hotbed humus and watered three times with a 0.01% humic acid solution. With this method, 100% germination of all tubers was achieved within 18–20 days, with a large number of sprouts and a well-developed root system. No tuber loss due to suffocation or rot-causing bacteria was observed.

The highest tuber yield was 464.28 cwt/ha. The second highest in terms of vegetative mass development and tuber yield was the foreign preparation Mind Extra, which produced an average tuber yield of 446.92 cwt/ha.

Thus, by combining the treatment of potato tubers with a mixture of growth stimulants and germinating them in humus with periodic watering with physiologically active substances, and applying a specific agricultural technique, we managed to achieve stable potato yields over several years using post-harvest planting of freshly harvested tubers.

Conclusions

The most effective method for breaking dormancy in freshly harvested tubers is treating them with a mixture of a 2% thiourea solution and a 0.02% potassium humate solution, as well as a 2% thiourea solution and a 0.001% petroleum growth substance (PGS) solution, followed by germination in humus with periodic watering with the specified solutions. The combined action of these preparations on freshly harvested potato tubers manifests as synergism, i.e., mutual action in the same direction. The short-term effect of thiourea is enhanced by the subsequent action of humic acid or PGS, which significantly increases the activity of oxidative enzymes, leading to more uniform germination with a greater number of sprouts and roots. However, the absolute leader in potato yield remains the Adept Agro.Bio preparation.