Blackberry (Rubus spp.) production and consumption is increasing worldwide (Antunes et al., 2014; Fachinello et al., 2011; Strik et al., 2007; Strik and Finn, 2012). The increase in consumption is mainly due to the high antioxidant capacity of phenolic compounds present in fruits, substances that make blackberry a fruit with nutraceutical properties that can help fight degenerative diseases (Lee et al., 2012; Vizzotto et al., 2012). Moreover, blackberry attracts producers because it requires few pesticides (Antunes et al., 2010, 2014; Botelho et al., 2009), an aspect that favors the implementation of organic systems.
Although native species of the genus Rubus (R. urticaefolius, R. erythroclados, R. brasiliensis, R. Sellowii, and R. imperialis) occur in Brazil, blackberry research did not gain importance until 1972 with the introduction of the cultivars Brazos, Cherokee, and Comanche (Antunes, 2002). Since 1975, the EMBRAPA blackberry breeding program, in collaboration with the University of Arkansas, has released six blackberry cultivars: Ébano (1981), Negrita (1983), Tupy (1988), Guarani (1988), Caingangue (1992), and Xavante (2004). Tupy is the most grown blackberry cultivar in Brazil, and also the most common in Mexico, mainly for export to the United States. ‘Tupy’ is a low-chill, semierect, vigorous, and thorny plant which produces large fruits (8 to 10 g) with soluble solid content between 8 and 10 °Brix. ‘Xavante’ is a low-chill, vigorous, erect, and thornless plant which produces medium size fruit (6 g) with a fruit soluble solids content around 8 °Brix (Antunes, 2002; Raseira and Franzon, 2012).
Many have suggested that the growth of the blackberry market depends on the study of various aspects of the production system, especially plant nutrition (Antunes et al., 2014; Castaño et al., 2008; Pereira et al., 2013b; Strik and Finn, 2012). Knowledge of nutritional requirements, nutrient uptake, and the rate and application method of fertilization are fundamental to obtaining higher yields in blackberry (Castaño et al., 2008). However, little research has been done on fertility and nutrient requirements for blackberry production in Brazil, and therefore remains a major constraint toward improving productivity.
Potassium performs various structural and metabolic functions in the plant. It has an important role in regulating the ψS of cells, and thus acts as an activator of many enzymes involved in respiration and photosynthesis (Castaño et al., 2008). It is a nutrient used in large quantities, especially in situations of high productivity. Depending on the cultivar and weed management strategy, nutrient loss from the fruit and floricanes may vary between 36 and 84 kg·ha−1 of K (Harkins et al., 2014). As the need is greatest during fruiting, K deficiency is common in years of high production, with symptoms including reduced growth, followed by chlorosis and necrosis in the leaves, and shortening of the internodes (Antunes et al., 2014; Castaño et al., 2008; Pereira et al., 2011). According to Pereira et al. (2013a), in an experiment where N was not evaluated, K was the element with the greatest uptake in ‘Tupy’ and ‘Xavante’ blackberries, with 6.8 and 5.4 kg·t−1 of fruit, respectively. In addition, K may influence fruit size, sugar content, and fruit firmness (Castaño et al., 2008; Hart et al., 2006; Rincon and Salas, 1987).
In blackberry, K is commonly provided by potassium chloride (KCl), potassium sulfate, or potassium nitrate (Hart et al., 2006). KCl is usually less expensive, but high rates of chloride can be harmful. Maintenance fertilization with K should be applied in the spring, before budbreak and flowering (Freire, 2007; Crandall, 1995).
In Brazil, recommended K rates range from 0 to 66.7 kg·ha−1, depending on the nutrient content of the soil (CQFS-RS/SC, 2004). Typically, soils show low or medium levels, and therefore the recommended rates are between 33 and 66.7 kg·ha−1 of K2O. In contrast, recommended rates of K fertilization in the United States may be as high as 112 kg·ha−1, depending on soil and primocane tissue K (Hart et al., 2006).
This study aimed to evaluate the vegetative growth, yield, and changes in floricane leaf nutrient concentration of two blackberry cultivars for three seasons in southern Brazil in response to annual K treatments.
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