The impact of various methods of apple-tree mineral nutrition on biochemical content of fruits and transformation thereof during storage

Introduction

Fruits are grown to be consumed as food in one way or another: as fresh fruits immediately after yielding and storage or after processing. Human physiological need for fruits is about 100 kg per year, although nowadays much less fruits are grown in Russia that is 18-20 kg per year (Trunov et al., 2011). The principal share of consumed fruits falls on apples — up to 95%. Apples are valuable food products containing various sugars, organic acids, vitamins, micronutrients, etc. (Saveliev, 2010). The biochemical content of apple-tree fruits is a very important indicator of their quality, their value for storage, consumption and processing.

Various agricultural practices including the ones toward the optimization of the plants' mineral nutrition, influence the content of organic substances in fruits and their quality. Foliage nutritions are especially interesting in this regard. By using them properly, there is a great potential for manage yield and fruits' quality with relatively low expenses and high eco-friendliness of the conducted measures (Tagliavini et al., 2002). Nowadays we have more information about the research of different medicaments, but new questions keep on arising. Quite disputable issue, in particular, is using of biostimulators, Borium- and calcium-containing medicaments. These substances have a considerable effect on the processes of reproduction, growth, development and metabolism of the plants in general.

The value of fruits as food products in winter is mostly determined by the content of vitamins, especially ascorbic acid. Ascorbic acid is very important for human nutrition. 70-100 mg is a daily amount of the ascorbic acid necessary for the optimal metabolic process, to ensure the health of all the connective tissues, strength and elasticity of blood vessels, blood coagulability regulation and capillary permeability. Vitamin C increases resistance to cold, diseases and negative environmental factors. Ascorbic acid plays an important role in the regulation of oxidation-reduction processes, takes part in collagen and procollagen synthesis, metabolism of folic acid and iron as well as in hormones synthesis.

Ascorbic acid doesn't withstand the thermal treatment and humans receive it only when they consume fresh fruits and vegetables. That's why apples as a long-term storage product can be the powerful source of this vitamin in winter period.

But ascorbic acid is also very important for plants, for their right growth and development, for the right metabolism. Ascorbic acid is the unique semi-functional species. With the ability to convertible acidification and recovery, it takes part in the most important energy processes of a plant cell: photosynthesis and respiration. Its participation in processes of growth, blooming, vegetative and reproductive differentiation, in water metabolism, enzymic activity regulation, and metabolic reactions stimulation is doubtless (Bokhinsky, 1987).

In a drought condition the effect of ascorbic acid on the moisture status is especially important. The literature contains information that ascorbic acid may delay water supply, decrease water balance in cells, change the agility of intracellular water, accelerate or slow down transpiration (Chupakhina, 1997). Under the influence, correlation between free and bonded water due to increasing the latter is changed. The mechanism of ascorbic acid effect on the water supply and loss is explained by changed colloid-chemical features of protoplasm as well as by depolarization of protoplasm membranes.

Ascorbic acid is extremely important to get over the oxidative stresses (Noctor, Foyer, 1998). Browning pulp doesn’t reveal until ascorbic acid is destroyed (Ponting, Joslyn, 1948; Veltman et al., 1999; De Castro et al., 2008). The ascorbic acid content correlates with the fruit susceptibility to browning pulp. Such biochemical mechanism is not clear but ascorbic acid decreases the oxidative stress on membranes and prevents the destruction of lipids by hyperoxides (Shalata, Neumann, 2011). The good supply of plants by Calcium (Picchioni, 1995) and Borium (O'Neill, York, 2003) also plays an important role in these processes. All of this accentuates the importance of foliage nutrition by Calcium, Borium and Megafol to provide the high level of ascorbic acid content during storage, which is important for the consumers, as well as for the improvement of fruits storability.

The most important factor for consumers isn't prophylactic or technological features of fruits, but primarily their consumer qualities. The ascorbic acid and sugar content mainly determine the prophylactic qualities of fruits, but consumers evaluate fruits primarily by their taste. The taste of fruits is determined by the balanced combination of sugars and organic acids. The sugar-acid ratio is used for quantitative estimation of taste harmonicity. It is believed that fruits with the sugar-acid ratio of 15-25 have the most harmonious taste (Sedov et al., 2011). The varieties with the sugar-acid ratio above 25 have the blank taste, usually have poor degustation estimation being consumed as fresh fruits and are hardly suitable for technical processing (Shirko, Yaroshevich, 1991).

Preservation ability of fruits and their valuable features depend not only on storing conditions, but also on the mineral element content in fruits and leaves (Gudkovski et al., 1990), i.e. on the conditions of mineral nutrition provision during the vegetation period.

The main purpose of our research is to study the effect of various foliage nutritions and methods of the soil fertilization on the formation of fruits' biochemical content both during yielding and removal from storage.

Research methods

The research was made in the experimental orchard of SSU of the ARSRIA named after I. V. Michurin of the Russian Agricultural Academy. The experiment was established according to «Methodological guidelines for establishing and carrying out experiments with fertilizers in fruit and berry plantations» (Kondakov, Pastukhova, 1991; Program, 1972). The allotment consisted of 5 trees, the replication was a three-stage one. The research objects were fruits and trees of the Zhigulevsk/62-396 apple-tree variety. The plantation scheme was 4,5x1 m. Treatments were made in the following phases: «the green cone», «the pink bud», in full bloom, just after blooming, «the walnut», upon reaching the fruit size of more than 3 cm in diameter and 4 and 2 weeks before yielding. Treatments were made according to the following schemes: 6* master + 3 boroplus + 2 megafol + 1 calbit — the system recommended by the distributor (complex master), experiment variants: system 1: +4 megafol, system 2: + 5 calbit, system 3: + 4 megafol + 5 calbit in tank mixtures. (* the number of treatments)

Fertilizers were applied once in the beginning of April, fertigation watering was performed 5 times within the period since the beginning of May until the end of July. Ammonia nitrate was used as the source of nitrogen fertilizers, superphosphate as the source of phosphorus, the potassium chloride as the source of potassium. Fertilizing standards were determined upon the results of the soil-and-leaf diagnostics.

Dry substances content was determined via multiple drainage of the plant material until a stable mass was achieved (Yagodin, 1987), the general acidity was determined by the titrimetric method, the ascorbic acid content was determined by the iodimetric method and the sugar content was determined by Bertrand's method (Pleshkov, 1986). Fruits were stored in the fruit storage of the SSU of the ARSRIA named after I.V. Michurin of the Russian Agricultural Academy and in the laboratory of advanced storage methods in the Research Centre of the Michurin State Agrarian University.

Results and discussion

On average, over a 2 year period, the application of different plant nutrition methods generally led to the increased ascorbic acid content at yielding time (Table 1).

Against the background of fertigation, the ascorbic acid content was high in almost all variants where additional Calcium and Megafol treatments were used. The best option was the combination of Borium + Calcium + Megafol (system 3). There was no such high effect on the ascorbic acid concentration at the surface fertilization.

The most ascorbic acid content was concentrated without fertilizers by using the system 1 and due to the recommended treatment system.

Mineral fertilizers had no effect on the sugar content: only by using combinations Borium+Calcium and Borium+Calcium+Megafol without fertilizers and Borium+Calcium in connection with fertigation gave higher sugar content in comparison with the control group.

The titrable acidity in our experiments was higher in all variants with fertilizers, but the application of the system 3 (Borium+Calcium+Megafol) was the most stable, when the ratio was the highest regardless of the background for foliage nutritions using.

Sugar-acid ratio, on the contrary, was higher in only one case, i.e. when the recommended treatment system without fertilizers was applied. In all other variants it was either at the control level or lower (Figure 3). Mineral fertilizers also didn't have any noticeable effect on the content of dry substances in fruits over 2 years of research.

Table 1

The effect of foliage nutritions and fertilizers on the biochemical content of apple-tree fruits Zhigulevskoe/62-396 variety in August at the yield, 2011-2012

Variants		Ascorbic acid, mg/%	General sugar, mg%	Titrable acidity, %	Sugar-acid ratio	Drained weight, %
Control (without fertilizers)	Without nutrition	11.67	12.58	0.57	22.1	14.22
	Complex master	17.02	12.60	0.52	24.2	13.84
	system 1	17.90	11.20	0.71	15.8	14.53
	system 2	13.65	13.27	0.59	22.5	14.06
	system 3	10.91	14.09	0.70	20.1	15.17
Fertilizer (N90P30K0)	Without nutritions	14.27	12.22	0.59	21.3	13.66
	Complex master	15.54	12.62	0.64	19.7	13.89
	system 1	14.23	12.13	0.70	17.3	13.96
	system 2	13.76	12.57	0.67	18.8	13.84
	system 3	12.65	12.49	0.74	16.9	14.39
Fertigation (N30P10K0)	Without nutritions	16.17	14.59	0.72	21.8	14.07
	Complex master	11.77	11.29	0.63	17.9	13.23
	system 1	15.29	12.32	0.67	18.4	14.29
	system 2	16.04	13.94	0.68	20.5	15.18
	system 3	18.34	12.30	0.75	16.4	14.27

As fruits packed for storage are mainly used for consumption in fresh, the losses of the ascorbic acid content are important not only because of their physiological condition, but also due to their consumer value as the vitamin source in the winter period. There are different mentions in literature concerning the decrease of the ascorbic acid content during storage.

Some sources inform about the decrease of its content by 80% during 4 months of storage in the standard environment at 40С in case of the varieties Golden Delicious and Jonathan (Avramiuc et al., 2012). The same authors report that the high sugar content (mainly the content of sucrose) allows mitigating the ascorbic acid losses to some extent. According to other data, the ascorbic acid losses for 6 months were 50% (Lee, Kader, 2000). In the experiments of Viškelis and his colleagues (2011) the ascorbic acid content decreased by 53,1% in Startis Variety and by 47,1% in Cortlend Variety during the 6 month storage period. After 4 months of storage in the standard environment, the ascorbic acid content in experiments of De Castro (2008) and his colleagues in California decreased by 70% (2004 yield), but in the next year those losses were at the level of 20-25%.

In our experiments, average decrease of the ascorbic acid content during storage period of 4,5 months amounted to 51,9-61,2% during 2 years in different variants (Figure 1).

Figure 1. Ascorbic acid losses during the period of fruit storage.

We’ve had the smallest ascorbic acid losses in our experiments by using Borium+Calcium+Megafol without fertilizers (51,9%), the recommended treatment system in connection with fertigation (52,8%) and Borium+Calcium+Megafol in combination with the surface fertilization with embedding into the soil (55,4%). The greatest losses (61,2%) were observed in the variant when Borium+Calcium+Megafol in connection with fertigation was used; however, in this variant the residual ascorbic acid content was one of the highest — 7,11 mg%. It was higher only by using Borium+Metafol without fertilizers (7,22 mg%). When Borium+Calcium in connection with fertigation was used, the ascorbic acid content was also relatively high at the removal from storage (6,76 mg%).

The usage of foliage calcium treatments both in combination with Megafol and without it allows optimizing the ascorbic acid content in fruits, to a certain degree. Similar results were obtained by Drake and Sprayd (1983) in case of variety Golden Delicious. The literature says a lot about the significance of high content of this substance during storage period. Ascorbic acid and calcium work together to preserve the cell membrane stability (Veltman et al., 1999).

Figure 2. Change of the sugar content during storage.

There is some information that the decrease of the ascorbic acid content is accompanied by gas injury development (De Castro et al., 2008). It is mostly related to the excessive concentration of hydrogen dioxide in fruits, which causes the oxidative intracellular stress that can lead to membrane damage (Larrigaudiere et al., 2001).

The literature contains the information about sugar content increase during storage (Robert, Loeska, 1960; Asif Ali et al., 2004); however, during our research it could be observed only in few variants: by using the recommended treatment system and the system 1 without fertilizers and by using the recommended treatment system, the system 1 and the system 3 in combination with fertigation. In other variants the sugar content during storage decreased.

It is obvious that the consequences of the extreme drought in 2010 affected the storability of fruits in 2011, causing variability of the sugar content. It is also obvious that fertigation had the positive influence on the sugar balance in combination with foliage nutritions by Megafol, Borium- and Calcium-containing medicaments.

The sugar-acid ratio at the yield in our experiments was within above-mentioned limits for all the variants (Figure 3).

Figure 3. The change of sugar-acid ration during storage, 2011-2012.

However, different changes took place during storage. There is information about clear increasing of the sugar-acid ratio during storage in a standard environment. According to experimental research of Blažek and his colleagues (2004) it increased 2,5 times during 150 days of storage of Golden Delicious fruits (from 24 to 60). In our work, the change of the sugar-acid ratio at storage wasn't so definite. It increased only in 5 variants and exceeded 25 only in one of them — by using the system 1 without fertilizers. In 5 variants the ratio was below 15 — by the systems 1 and 3 without fertilizers, the recommended nutrition system, systems 2 and 3 in combination with the surface fertilization with embedding into the soil. In combination with fertigation, the sugar-acid ratio at all nutrition systems was within the standard limits.

Conclusions

1. The ascorbic acid content in fruits at the yield depended both on applied treatment systems and the fertilizing method. Among the studied treatment systems, its content was the highest by the complex Borium, Calcium and Megafol using with fertigation.

2. Mineral fertilization via fertigation brought the ascorbic acid content to increase by using all the studied foliage nutrition systems, except for the recommended one.

3. Combined application of Borium-, Calcium-containing medicaments and Megafol had a positive effect on the preservation ability of ascorbic acid during storage.

4. Fertigation was affected positively the sugar-acid ratio level that, at the moment of the removal from storage in January, was within the optimal limits in all variants against its background.