Different Nutrition Approaches for Raspberry (Rubus idaeus L.) cv. 'Himbo-Top' (‘Rafzaqu’): Influence on Productivity, Fruit Quality and Storage Potential

The storage potential of raspberry fruits is considered to decrease rapidly with storage duration due to its high transpiration rate and fragile structure. In a short time, raspberry fruits lose their firmness, turn darker and begin to mould. Thus the shelf life of raspberries, stored at temperatures of 5 °C, is generally not longer than 4 days, the shortest of all fruit species (HAFFNER et al. 2002, KRÜGER et al. 2011). Problems during short storage are mainly caused by grey mould (Botrytis cinerea).

Among the physical characteristics of raspberry fruit that are important indicators of fruit quality, fruit firmness is a parameter that stands out and directly determines the sensitivity to mechanical damage (during harvesting and transport), to potential storage losses and, in particular, infections by pathogens, mostly fungi. According to SJULIN and ROBBINS (1987), a decrease of fruit firmness is directly linked to a severe reduction of the fruit quality.

Fruit quality is also determined by its mineral composition (BUSKIENE and USELIS 2008). Nitrogen overdoses often result in a lower fruit firmness (BUSHWAY et al. 2008) and poor fruit colour since nitrogen is preferably used for rapid growth and to a smaller extent to support secondary metabolism (HARGREAVES et al. 2008). The form of nitrogen also plays an important role, therefore the ammonium form is often used in raspberry production due to its lower effect on growth intensity (VALENTINUZZI et al. 2018b). Some contradictory data can be found in the literature regarding the effect of nitrogen rates on soluble solids content (SS) and titratable acidity (TA) in fruit (PRANGE and DELL 1998). The most common claims are that higher nitrogen rates reduce the soluble solids content of the fruit (STRIK 2008) and increase the TA (STOJANOV et al. 2019), which results in an altered sensory value of the fruit.

According to the many different interpretations that can be found in literature, the most important factor determining the quality and storability of fruit is the genetic background (DALE and DAUBENY 1985). In addition, RIAZ and BUSHWAY (1996) point out the significant influence of weather conditions at the time of harvest, the chemical composition of the fruit, and particularly the difference in temperature between day and night. Thus, biotic and abiotic environmental factors are significant indicators of fruit quality as well as having an impact on storage potential and shelf life (LEPOSAVIĆ et al. 2013). Due to the combinations and overlapping of various genetic factors, weather conditions (temperature, humidity, daily sun radiation, precipitation …) and agro-technical measures in the orchard the determination of the importance of the isolated effect of a single nutrient, such as nitrogen, becomes very demanding.

Plant biostimulants are compounds that, when applied to plants, seeds or growth substrates, can alter physiological processes, enhance plant and fruit growth and develop as well as influence (mainly improve) the plant’s response to stress conditions (HALPERN et al. 2014). Silicon (Si) is also found in some biostimulant formulations but is not recognized as an element, essential for plant growth and development. However, its biostimulating potential has recently been discovered, as it is thought to have a positive effect on the growth and development of many plant species, especially when they are exposed to various biotic and abiotic stressors (SAVVAS and NTATSI 2015).

The use of various biostimulants has been attributed to several additional positive effects: increased fruit weight and fruit firmness (GRAJKOWSKI and OCHMIAN 2007), increased fruit diameter and accelerated ripening (KROK and WIENIARSKA 2008). OCHMIAN et al. (2008) additionally reported about a positive effect of the algae Ascophyllum nodosum extract on plant growth, an index of leaf colour intensity and an increased content of anthocyanins in raspberry fruit (28 mg/100 g control and 39 mg/100 g algae). GRAJKOWSKI and OCHMIAN (2007) also pointed out the decrease of soluble solids (hereafter SS) content compared to the control (-0.7 °Brix), while the effect of biostimulants on the titratable acids (hereafter TA) was not confirmed. Some authors (MA 2004, SAVVAS and NTATSI 2015) have also reported positive effects of silicon on the control of a wide range of diseases and pests of fungal and bacterial origin in different plant species (vegetables, ornamentals, fruit plants, field crops…). These positive effects are attributed to the accumulation of silicates in the surface tissues, which improves the mechanical resistance and regulates the mobility of water and nutrients in the tissues. HAJIBOLAND et al. (2017) state that silicon does not only show positive effects under stress but also under optimal growing conditions. According to SAVVAS and NTATSI (2015), mainly ground applications of silicates are expected to be effective.

The main goal of the presented study was to define the effects of different fertigation approaches to raspberry nutrition as well as to clarify the role of nitrogen rate (20, 60 and 80 N kg/ha) and the use of a selected algae-based biostimulant with added Si on fertility, yield characteristics and storability.

The experimental year 2019 was the second warmest year since 1961. The average annual temperature in Maribor in 2019 was 11.9 °C, with 972 mm of precipitation. There were 2116 sunshine hours which is 6 % above the long-term average. The year of the experiment was marked by some weather extremes (ARSO 2020):

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  May: intensive precipitations along with low average temperatures;

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  June: unusually warm and dry weather with an exceptionally high number of sunshine hours;

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  July: intensive precipitations and high average summer temperatures;

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  Four prolonged heatwaves in summer (Table 1).

Soil in the plantation belongs to the eutric cambisol category. The organic matter content was 2.99 %, and the pH 6.55. The data in Table 2 show that before the trial the soil in the experimental plantation was well supplied with phosphorus (class C), while the potassium load was measured to fit in class B (Table 2).

The experiment was set up as a randomized block design and consisted of 4 treatments in 4 replications. Each replication consisted of 7 plants, similar in vegetative and generative development. Treatment designation is based on total N rate and use of biostimulant. Source of N was derived from different fertilisers and applied at different growth stages (table 3) with the use of the irrigation system. The experiment consisted of the following treatments:

• Control treatment (hereafter referred to as C; basic fertilization with pelleted farmyard manure with final rate of pure N up to 20 kg/ha; no fertigation, the so-called “extensive” or “low-input” approach).

• Additional fertigation with 40 kg N/ha (hereafter referred to as treatment 60 kg N/ha; basic fertilization with 20 kg N/ha + 40 kg N/ha via fertigation).

• Additional fertigation with 60 kg N/ha (hereafter referred to as treatment 80 kg N/ha; basic fertilization with 20 kg N/ha + 60 kg N/ha via fertigation).

• Additional fertigation with 60 kg N/ha + use of biostimulant (hereafter referred to as treatment 80 kg N/ha + BS; basic fertilization 20 kg N/ha + 60 kg N/ha via fertigation + addition of a seaweed-based biostimulant with added silicon via foliar application; the so-called “luxury nutrition”).

In all treatments the basic fertilization was carried out in spring (Table 3). Manure pellets were applied in an amount of 1 T/ha (20 kg N/ha) in the inter-row space. In the 60 kg N/ha, 80 kg N/ha and 80 kg N/ha + BS treatments, nitrogen addition up to the target rate was carried out by fertirrigation with the use of water-soluble NPK-type fertilisers in crystalline formulations (with use of 11 L of water per meter). In the 80 kg N/ha + BS treatment, the biostimulant was applied on leaves at the amount prescribed by the manufacturer.

In the experiment, different fertilisers were applied at 3 different stages of raspberry development. The fertilisers used had the following composition:

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  Pelletized manure: from an unknown producer, containing 2 % nitrogen with a C/N ratio of 24.

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  Fertiliser A (Solinure^® (Everris International, Netherlands)): 20 % total nitrogen, 20 % phosphorus (P₂O₅), 20 % potassium (K₂O), iron (0,06 %), manganese (0,05 %), boron (0,01 %), copper (0,01 %), molybdenum (0,005 %) and zinc (0,022 %).

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  Fertiliser B (Hakaphos Violeta^® (COMPO EXPERT GmbH, Münster, Germany)): 13 % nitrogen, 40 % phosphorus and 13 % potassium, iron (0,05 %), copper (0,01 %), zinc (0,01 %), magnesium (0,11 %) and manganese (0,05 %).

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  Fertiliser C (NovaTec^® Solub (COMPO EXPERT GmbH, Münster, Germany)): 14 % nitrogen, 8 % phosphorus and 30 % potassium, sulphur (5,6 %) and magnesium (0,7 %).

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  Biostimulant (BS) (Vitanica^® Si (COMPO EXPERT Italia, Cesano Maderno, Italy)): 5 % nitrogen, 3 % phosphorus, 7 % potassium, 1.7 % sodium, 10 % silicon, 22.7 % organic matter and an extract of the seaweed Ecklonia maxima (Table 3).

The harvesting season lasted 6 weeks (25^th July 2019 to 30^th August 2019). During this time, eight harvests were carried out. During and after harvesting, measurements of selected yield and quality parameters were performed (weight of the yield per plant, weight of 10 fruits; use of the Kern 442-43-N scale (Kern^®, Germany). Harvested fruit was immediately stored at low temperature (“fridge van”, normal atmosphere, 5 °C). Fruit sampling for the assessment of chemical composition and storability was carried out during the period of full bearing (August 5, August 10 and August 12).

Standard analysis of internal fruit quality parameters (SS content - refractometric value expressed in °Brix and TA value - titratable acid expressed in (g/L)) were carried out with a sample of 15 fruits immediately after harvesting. Fruit taste was determined with the Thiault index (SS/TA*10) (VARELA et al. 2005).

For the assessment of the storability fruits were kept in a cold storage with a normal atmosphere (NA) at 5 °C. Changes in the weight and volume were monitored daily. Volume changes were monitored using a piece of cardboard placed on the fruit in each container (plastic containers with 250 g of fruit); the level to which the cardboard lowered in the container while fruit collapsed under it served as an indicator of fruit volume decrease. Changes in mass and volume were expressed in percentages. After 7 days of storage, the incidence of mould was assessed visually and expressed as percentage of infected fruits.

The data analyses were performed using Statgraphics Centurion (version 15.2). The differences between treatments were determined using a one-way analysis of variance (ANOVA) (p < 0.05). Further analysis of data was performed by Duncan’s MRT pairwise comparison test at p < 0.05.

Table 5 shows the average SS and TA content, as well as the values of Thiault index, of the fruit sampled during the intensive ripening period. The SS content of the fruit ranged between 6.43 °Brix and 7.1°Brix depending on the nutrition program (nutrition approach). It showed a negative correlation to higher nitrogen rates, already visible when the rate of nitrogen was increased to 60 kg N/ha. The effect of the BS used on SS content was not confirmed (Table 5). In presented experiment, SS content of ‘Himbo-Top’ variety was about 40 % lower than expected, since GIUGGIOLIA et al. (2015), MILIVOJEVIĆ et al. (2011), PEANO et al. (2013) reported values between 9,5 °Brix and 11 °Brix.

There was no significant effect of nutrition approach on TA content. TA values varied from 11.7 g/L in the control to 13.08 g/L in the most heavily fertilised raspberries (Table 5). Based on information in the literature, the average TA levels in fruits for the cultivar 'Himbo-Top' range from 9.6 g/L (MILIVOJEVIĆ et al., 2011) to a very high 15.6 g/L (GIUGGIOLIA et al., 2015), while PEANO et al. (2013) recorded values comparable to the ones in our experiment.

The differences between the treatments (nutrition approach) were more pronounced in the Thiault index, a widely accepted parameter in practice that determines flavour, which is particularly important in terms of consumer acceptability (MAZUR et al., 2014). According to DROBEK et al. (2019), the quality of raspberry fruit is considered good if the ratio of SS to TA varies between 10 and 15, while KADER (1999) recommends a ratio of 10.

In the presented experiment it ranged between 4.96 and 6.10 and was relatively low (Table 5). It was influenced by the amount of nitrogen, in the way that intensifying nitrogen fertilization (increasing the nitrogen rate above 20 kg N/ha) significantly impaired the flavour of raspberries, so “the more "luxurious" the nutrition, the worse the flavour”. The use of BS, combined with higher N rates, could not compensate negative effects of high N dosages on organoleptic properties of fruits.

Table 6 shows the results in which 3 standard parameters of fruit storage potential were monitored: reduction in fruit weight and volume along with the occurrence of storage diseases. Loss of weight during storage is a factor that significantly reduces a fruit’s market value, mainly due to transpiration losses. In addition to temperature and relative humidity, it is influenced by the degree of fruit ripeness at the time of harvest and the variety of fruit.

After 7 days of storage at 5 °C, no significant effect of the nitrogen dose on storage parameters such as weight and volume loss was confirmed, nor were these parameters affected by the BS. The percentage of fruit weight loss ranged from 3.22 % to 3.54 %, and the percentage of volume loss ranged from 16.78 % to 22.67 %.

According to the results of presented study we cannot confirm the claim of VALENTINUZZI et al. (2018a) that the use of BS improves the quality of fruits in the period of harvest and prevents a significant weight loss during storage. NUNES and ÉMOND (2003) stated that the maximum weight loss of fruit, before it loses its market value, is 6 %.

The decision on the approach of plan nutrition seems to be important mainly regarding the occurrence of storage diseases of fruits, for the most part influenced by the evaluation of the activity of the fungus Botrytis c. After 7 days of storage, the percentage of mouldy fruit in cold storage ranged from 57.12 % to 75.61 %, so “the more "luxurious" the nutrition (higher nitrogen doses), the more pronounced was fungal activity”.

The severe decline in the storage capability of raspberries, linked to the application of high nitrogen rates, was also highlighted by ALI (2012) and ALI et al. (2012). The use of the chosen BS (foliar application) in our experiment did not play a significant role in preventing fruit collapse in cold storage under the given conditions, neither in terms of preventing physiological collapse nor in terms of preventing the occurrence of storage diseases, even though raspberries were exposed to a number of abiotic stresses during the growing season (4 heat waves, extremely high number of sunshine hours, etc.). So, the effect of the algae and silica-based biostimulant (in combination with higher doses of nitrogen) on fruit set and fruit quality remained unconfirmed. The results therefore do not support the thesis of HAJIBOLAND et al. (2017), who point out that silicon shows positive effects on quality under stress and under optimal growing conditions. The reason could partially originate from the application practice; according to SAVVAS and NTATSI (2015) foliar application of silicates seems to be less effective than application through the root system when it comes to reducing the negative effects of abiotic stress factors.

The so called 'luxury' raspberry nutrition (80 kg N/ha or 80 kg N/ha + BS) did not prove to be advantageous in terms of raspberry quality, yield and storability. On the contrary, raising the nitrogen rate above 60 kg N/ha in an orchard with optimal soil preparation and high organic matter content (approx. 3%) proved to be an inappropriate technological decision, with no effect on yield and with a clear negative effect on storage capability as well as sensory parameters of raspberries. That negative effect could not be compensated by the use of algae- and silica-based biostimulant.