Yield and berry quality of red raspberry as affected by cultivar and fertilisation regimes

This study was carried out in a commercial plantation located in Gliječa village near Ivanjica city, western Serbia in 2017. The study area is located at 809 m above sea level, 43°34’52” N latitude and 20°10’56” E longitude. The plantation was established in 2002 with ‘Willamette’ and ‘Meeker’ (summer-bearing) red raspberry cultivars. Plants were spaced 0.25 m within rows and 2.0 m apart. Vertical I-trellis (i.e. espalier) consisted of posts (spaced 5 to 6 m apart) and two horizontal wires at 0.8 and 1.7 m above ground level with 2 to 3 cross arms. Floricane training was done from mid-February to end of March. Floricanes were shortened to 2 to 3 buds above the top wire (total floricane height was about 1.9 m on average). Conventional and traditional management practices under Serbian conditions for red raspberry such as pruning, training, floor management, pest, disease and weed control were conducted without irrigation.

A randomized complete block design with 20 m of I-trellis, i.e. espalier, per each cultivar and fertiliser treatment in four replicates (n = 80 m) was used. Each cultivar/fertiliser treatment was repeated in four blocks. Fertilisation treatments were as follows (Tab. 1):

Fertor (FulviChina Co., Tianjin, China) is an organic fertiliser in the form of pellets. It is produced from 100 % chicken manure, with the addition of other organic substances of plant origin, which increase and improve the nutritional value of the manure. Kalk (ground limestone) (Schaefer Kalk GmbH & Co., Diez, Germany) is solid, powdery fertiliser and soil improver, with declared CaCO₃ content (min. 90 %). It is used to reduce soil acidity. NutriMAP (FulviChina Co., Tianjin, China) is a complex mineral fertiliser with a balanced ratio of N, P, Ca, S and Zn. Foliar fertilisers commercially named Fitofert Cristal Berry (Fertico DOO, Inđija, Serbia), Nutriamino Plus (FulviChina, Tianjin, China), Nutri Fos-K (AMC Chemical, Sevilla, Spain), Ferticare™ (Yara Int., Oslo, Norway) and Fertileader Elite (Timac Agro, Novi Sad, Serbia) contained essential macro- and microelements and amino acids in different ratios and concentrations. Fertor, Kalk and NutriMAP were applied to the soil at end of winter (after binding of the canes to the wire) in amounts of 1,500 kg, 3,000 kg and 300 kg per hectare, respectively. CAN as N fertiliser was applied at onset of vegetative cycle (1/3), just before flowering (1/3) and at the beginning of berry ripening (1/3) in amount of 200 kg per hectare. Organic and mineral fertilisers and ground limestone were applied to the soil parallel to the row, 0.5 m from the row center using field rotocultivator.

Foliar fertilisers were applied in the following order: Nutriamino Plus at the end of April in order to prevent abiotic stresses and create optimal conditions for the growth of fruiting laterals and flowering; Nutri Fos-K at mid-May in order to improve quality of flowering and good berry formation (fruit set); Fitofert Cristal Berry, Ferticare™ and Fertileader Elite at the beginning and mid-June, respectively in order to improve the growth, development and ripening of berries and increase their size.

Average annual temperature, humidity and total precipitation were 9.3 °C, 78.2 % and 920.4 mm, respectively, in the research area in 2017, which is the experimental year. Average temperature during vegetative cycle was 11.9 °C. The soil is brown on Paleozoic shale as a sole. It has a very acidic reaction (soil pH in KCl was 3.68). Soil contained a high amount of organic matter (3.24 %) and total N (0.28 %), 5.59 and 20.16 mg available K₂O and P₂O₅, respectively without CaCO₃ in 0-30 cm soil depth. Results revealed that the soil had an unfavourable pH value, a very high and high contents of organic matter and total N, respectively, and moderate levels of available P₂O₅ and K₂O.

Measurements were performed at 20 floricanes in four replications per each cultivar/fertiliser treatment and control (n = 80). Number of fruiting laterals per floricanes and their length (cm) were enumerated. Yield per floricane and per m of trellis (both in g) was measured using digital balance OEM (Quebec, Canada) after each harvest. For physical and chemical evaluation, 20 fully mature berries in four replicates (n = 80) per each cultivar/fertiliser treatment were randomly handpicked. Berries were assessed at mid of harvest for berry weight (BW, g) and their linear dimensions (length – L and width – W, both in cm). The BW was measured using digital balance VK-TECH (Daejeon, South Korea) whereas linear dimensions were measured with caliper gauge Starrett 727 (Athol, MA, USA). Length/width ratio (L/W ratio), also called berry elongation index, was calculated.

Soluble solids content (SSC, °Brix) was determined by hand refractometer Milwaukee MR 200 (ATC, Rocky Mount, USA) at 20°C, from juice extracted from the berries. Titratable acidity (TA, % of citric acid) was measured by titration of the same solution with 0.1N sodium hydroxide until reaching pH 8.1 using phenolphthalein as indicator. The ripening or maturity index (RI) was calculated based on the SSC/TA ratio.

The data obtained were analyzed using the Microsoft Office Excel 2003 program (Redmond, WA, USA). A two-way analysis of variance (ANOVA), model 2 × 4, was performed, and differences between means were considered significant at P < 0.05 by LSD test. Source of variations were cultivar (A) and fertiliser (B).

Data in Tab. 2 showed that the number of fruiting laterals and their length were significantly affected by fertiliser treatments. Impact of cultivar on number of fruiting laterals was random whereas impact on their length was significant, being higher in ‘Meeker’ than in ‘Willamette’. Our results for both parameters were much lower than those found by Glisic et al. (2009), Bećirspahić et al. (2014) and Stojanov et al. (2019) for both ‘Willamette’ and ‘Meeker’, probably due to better weather and soil conditions and cultural management in their locations. Otherwise, ‘Meeker’ is a genetically more vigorous plant (both primocane and floricane) than ‘Willamette’ (Finn et al., 2001); the long fruiting laterals of ‘Meeker’ require additional support in the plantation so that they do not break and/or shrivel under the weight of biomass i.e. leaves and berries, and need more space than other red raspberries in plantations (Petrovic and Milosevic, 2005; Petrović et al., 2020).

Control treatment, T₃ and T₄ induced statistically similar and higher numbers of fruiting laterals per floricane in comparison with T₂. Length of fruiting laterals was the highest under T₄ and the lowest in control (without fertilisation). These data are in agreement with Stojanov et al. (2019) who reported that multi-nutrient compound mineral fertiliser and organo-mineral fertilisers with adequate content of macro- and micronutrients and amino acids improved properties of fruiting laterals in ‘Meeker’. Dale and Daubeny (1985) reported that high yield in raspberries was closely related to high numbers of fruiting laterals which confirmed our data.

Effect of cultivar on yield per floricane and yield per meter of trellis was not significant whereas influence on yield per hectare was significant (Tab. 3). Namely, yield per hectare was higher in ‘Willamette’ than in ‘Meeker’. As known, yield is a complex genetic category and varies widely between cultivars under the same environmental conditions, cultivation management and care treatments as previously reported by Petrovic and Milosevic (2005), Hansen et al. (2021) and Valentinuzzi et al. (2022). Namely, growth potential of plant cultivars, ability to form berry, ripening date or plant structure may affect the yield (Sawicka et al., 2023).

In the present study, the yield performances of raspberry were statistically higher under T₄ treatment in comparison with the unfertilized control variant and the other two treatments (Tab. 3). Similarly to T₄, T₃ also induced higher yield per floricane than control, whereas T₄ induced the highest yield per hectare in comparison with the other treatments and control. These results were expected, because red raspberry, apart from the requirement for organic and mineral fertilisers applied to the soil, especially rich in N (Strik, 2008; Unuk et al., 2023), responds very well to the application of foliar fertilisers (Wojcik, 2005). In addition, Ochmian et al. (2008) and Ičanović and Handanović (2022) have described significant yield increases after the use of biostimulants. However, the interaction cultivar × fertiliser was significant, showing the inconsistent reaction of cultivars to applied fertilisers. For example, yield traits of ‘Willamette’ were different between T₃ and T₄ applications although the general tendency is that both these treatments had a statistically similar effect. Our range values for yielding were lower, similar and/or higher than those found by different authors. So, Finn et al. (2001) reported similar yield for ‘Willamette’ under Oregon conditions, whereas Stanisavljević et al. (2002) and Glisic et al. (2009) found much higher yield for both ‘Willamette’ and ‘Meeker’ cultivars grown under western Serbian environment. These discrepancies among results can be connected with fact that the yield of raspberry depends to a large extent on the course of the weather (low winter temperatures, drought in spring and early summer), the age of the plantation and cultural practices including fertilization (Sawicka et al., 2023) and irrigation (Petrovic an Milosevic, 2005).

Although ‘Willamette’ and ‘Meeker’ are considered very productive raspberries under Serbian conditions (Petrović et al., 2020), yield was poor in the present study. Earlier studies and analyses showed that raspberry yield lower than 10 t ha⁻¹ is not profitable and economically justified and viable for Serbian growers (Petrovic and Milosevic, 2005). According to these authors, in the 1980s of the 20th century, not far from Ivanjica city (i.e. in Arilje municipality), the yield of ‘Willamette’ was 35 t ha⁻¹. Since ‘Meeker’ and also ‘Willamette’ are susceptible to drought (Morales et al., 2013), especially under global warming conditions in recent decades, constant irrigation during vegetation is imperative in the dry season in order to obtain higher yields as previously reported by Petrović et al. (2020).

The results of berry dimensions (L and W) as well as BW and berry shape index are shown in Tab. 4. Absence of statistically significant differences between cultivars for BW and L was evident, whereas W was higher in ‘Meeker’ than in ‘Willamette’. Results also revealed that ‘Willamette’ had more elongated berries than ‘Meeker’. Hence, berry of ‘Willamette’ is conical in shape, whereas berry of ‘Meeker’ is spherical (Stojanov et al., 2019). Bobinaitė et al. (2016) reported that BW in both summer-bearing and fall-bearing raspberries varied from 2.5 to 5-6 g, and, under certain conditions, the berry can exceed 10 g. Differences in berry size can be explained by a very high variability within individual genotypes and is affected by several auxin response factors through modulating the genes transcription (Liu et al., 2015). In addition, main factors affecting berry shape, especially near spherical shape, were sufficient carbohydrates and water, due to mitosis and cell expansion by accumulation of water and secondary metabolites (Tanksley, 2004).

Soil application of organic and mineral fertilisers supplemented with ground limestone and foliar fertilisers (T₄) induced the best values of BW and berry dimensions, whereas fertilisation treatments and control variant had no capacity to change berry shape. As expected, the smallest berries were observed in the control variant. For example, T₄ induced more than 25 % higher BW compared to the control, which is in agreement with results of Krok and Wieniarska (2008), Ičanović and Handanović (2022) and Unuk et al. (2023) who reported a higher berry weight treated with fertilisers containing biostimulants – alone or mixed. Also, many local and foreign researchers stated that fertilisation of raspberries, especially with N fertiliser, increases the physical properties of the berries, especially their weight and size (Stojanov et al., 2019; Unuk et al., 2023; Sawicka et al., 2023). However, the results of our study showed that the interaction cultivar × fertiliser had a significant effect on the examined physical berry properties. This indicates a specific reaction of the cultivar to applied fertilisers, i.e. a deviation from the shown tendencies. Hence, genetic background is a key factor that determines the size and shape of the berries as previously reported by Sawicka et al. (2023). In general, our data for BW and berry dimensions were much lower than those found by Stanisavljević et al. (2002, 2003) and Glisic et al. (2009) but much higher than those obtained by Finn et al. (2001), Alibabić et al. (2018), Stojanov et al. (2019) and Yu et al. (2022). Discrepancies in results among authors can be attributed to specific cultural practices, weather and soil conditions, as well as age of plantation.

Results from Tab. 5 show that SSC, TA and RI were not affected by fertiliser treatment, but values significantly varied between cultivars. ‘Meeker’ had statistically higher values than ‘Willamette’. The absence of significant interaction cultivar × fertiliser on the above traits indicates a very strong influence of the cultivar per se.

In the available literature, contradictory data can be found regarding the impact of fertilisation management, especially N rates, on SSC and TA in berries (Prange and Del, 1998). The most common claims were that higher N rates reduce the SSC (Strik, 2008) and increase the acidity (Stojanov et al., 2019) which is not the case in our study. In general, our results showed that soluble solids, acidity and RI were within data previously reported by other authors for the same cultivars (Alibabić et al., 2018; Milinković et al., 2021; Murtić et al., 2022). However, Stanisavljević et al. (2002), Milivojević et al. (2011) and Yu et al. (2022) reported lower TA values than ours, whereas Stanisavljević et al. (2002), Milivojević et al. (2012) and Stojanov et al. (2019) noted higher TA and SSC, respectively for both ‘Willamette’ and ‘Meeker’ raspberries. On the basis of these discrepancies, it can be said that geographic region i.e. site with specific pedoclimatic conditions also play an important role in the biosynthesis of above compounds. The specific nature of the response of different berry species and their cultivars to fertilisation management in different sites and soil types has been previously reported (Hargreaves et al., 2008). However, according to the many different interpretations that can be found in literature, the most important factor determining the raspberry quality is the genetic background of cultivar (Dale and Daubeny, 1985; Valentinuzzi et al., 2018), which was confirmed by our results.