Improving the performance of calcium-containing spray formulations to limit the incidence of bitter pit in apple (Malus x domestica Borkh.)

Abstract

Laboratory and field experiments were carried out with apples (Malus x domestica Borkh.) cv. ‘Golden Reinders’, to assess the efficacy of sodium salt of carboxymethyl ether of cellulose (0.5%, CMC) as an adjuvant for Ca spray formulations containing either Ca-chloride or Ca-propionate as active ingredient (120 or 250 mM Ca). This additive significantly increased the retention of Ca-containing solutions by the apple skin and prolonged the process of drying of the solution at room temperature. Four days after immersion of apples in 0.5% CMC plus CaCl₂ or Ca-propionate solutions (120 and 250 mM Ca) significant Ca increases were recorded in the peel and cortex of treated fruits. Application to apple trees of in-season sprays containing 250 mM CaCl₂ plus 0.05% Tween 20, Ca-propionate (120 and 250 mM Ca) plus 0.5% CMC or 250 mM CaCl₂ plus 0.5% CMC had no impact on fruit yield and quality, but significantly limited the rate of bitter pit incidence during the following 3-month cold-storage period. Evidence is provided that addition of appropriate adjuvants to Ca sprays can favour the distribution of Ca into the apple fruit and helps to reduce the incidence of Ca-related disorders over the postharvest cold-storage period.

Introduction

Bitter pit remains as one of the main problems for apple growing industry around the world, particularly in areas where climatic conditions are generally dry. Such physiological disorder which develops during the period of fruit growth (Ferguson et al., 1999), has generally been related to calcium (Ca) deficiency in the fruit cortex (Ferguson and Watkins, 1989, Fallahi et al., 1997).

The low mobility of Ca in the plant poses serious problems to enhance the distribution of this element to the fruit via Ca application to the root system (Bangerth, 1979). Subsequently, treatment of aerial plant parts with Ca sprays, is recommended and applied in many fruit production areas of the world, either as routine applications to prevent the occurrence of localised Ca deficiency in the fruit or to improve fruit quality (T. Schlegel and J. Schönherr, 2002, T.K. Schlegel and J. Schönherr, 2002, Lötze et al., 2008, Fernández et al., 2009). The efficacy of Ca treatments supplied to leaves and fruits may be highly variable and currently, many factors involved on the penetration and distribution of Ca within the fruit remain unclear (Saure, 2005, Bai et al., 2008, Val et al., 2008).

The effects of in-season spraying and/or post-harvest dipping of apple fruits in Ca solutions have been evaluated in various studies in terms of e.g., bitter pit development, Ca content increase and improved fruit firmness. However, inconsistent results have been often reported (van Goor, 1971, Lidster and Porritt, 1978, Hewett and Watkins, 1991, Neilsen et al., 2005, Lötze and Theron, 2006, Lötze et al., 2008, Val et al., 2008). Recently, Val et al. (2008) showed that in-season CaCl₂ sprays led to increased Ca concentrations in the skin, while no significant changes were measured in the cortex of apples.

Some studies estimated the permeability of apples to Ca solutions either with intact fruits (van Goor, 1973, Mason et al., 1974), fruit discs (T. Schlegel and J. Schönherr, 2002, T.K. Schlegel and J. Schönherr, 2002) or cuticular membranes (Glenn and Poovaiah, 1985, Harker and Ferguson, 1988, Harker and Ferguson, 1991, Chamel, 1989). Regarding the penetrability of apples at different developmental stages, T. Schlegel and J. Schönherr (2002) and T.K. Schlegel and J. Schönherr (2002) observed that apples were highly permeable to CaCl₂ solutions until June drop, the fruits turning significantly less penetrable after such date. The authors suggested that this may be due to the potential contribution of existing stomata and lenticels in the surface of young apple fruits, which would disappear after June drop (T. Schlegel and J. Schönherr, 2002, T.K. Schlegel and J. Schönherr, 2002).

Penetration of the plant surface by a nutrient solution may occur via stomata, the cuticle, cuticular cracks and imperfections and through trichomes or specialised epidermal cells. Most research efforts in the last decades focused on investigating the diffusion of substances through the plant cuticle (Schönherr, 2006). To explain the mechanisms of cuticular penetration of apolar, lipophilic compounds the “diffusion–dissolution model” was proposed (Riederer and Friedmann, 2006). On the other hand, the penetration pathway of hydrophilic solutes through the cuticle is currently not fully understood (Fernández and Eichert, 2009) and the existence of “aqueous pores” as a parallel diffusion mechanism has been hypothesised (Schönherr, 2006). The occurrence of epidermal structures in plant surfaces such as lenticels, stomata or trichomes may significantly influence the rate of absorption of surface-applied agrochemicals. While the significance of the stomatal pathway on the absorption of foliar sprays has been a matter of controversy for many years, recent evidence shows that it can largely contribute to the uptake process (Eichert et al., 2008).

Using young apples and apple segments, T. Schlegel and J. Schönherr (2002) and T.K. Schlegel and J. Schönherr (2002) suggested the major contribution of stomata and trichomes to the penetration of CaCl₂ solutions only at early developmental stages. On the other hand, Harker and Ferguson, 1988, Harker and Ferguson, 1991 and Glenn and Poovaiah (1985) suggested that lenticels in mature apples were preferential sites for the uptake of Ca solutions through the fruit surface.

Several studies showed that surface treatment of cherry fruits with Ca compounds at different stages of development decreased the incidence of cracking (e.g., Glenn and Poovaiah, 1987, Brown et al., 1995, Wermund et al., 2005). Furthermore, trials developed by Glenn and Poovaiah (1987) showed that the integrity of the cell wall structure was better preserved in apples treated with Ca, which presented a greater cell-to-cell contact as compared to non-treated fruits.

The effectiveness of Ca sprays is largely influenced by the prevailing environmental conditions, particularly relative air humidity (Schönherr, 2000, Schönherr, 2001). In this regard, Schönherr (2001) pointed out the relevance of the point of deliquescence (POD) of Ca compounds in relation to the rate of diffusion through the cuticle. Bai et al. (2008) showed that foliar treatment with Ca-chloride and Ca-hydroxide in combination with two non-ionic surfactants led to transient changes in the rate of photosynthesis and stomatal conductance of apple and bean leaves. Recently, Kraemer et al., 2009a, Kraemer et al., 2009b assessed the permeability, drop and deposit characteristics of CaCl₂ and Ca-acetate formulations applied to tomato fruit and adaxial apple leaf isolated cuticles. They found that surfactants increased the spreading of Ca within the droplet, and also that the rate of penetration of CaCl₂ was always higher than that of Ca-acetate.

Apart from surface-active agents, few studies tested the effect of employing adjuvants to improve the penetration rate and performance of Ca formulations (Mason et al., 1974, Schönherr, 2001, Fernández et al., 2009). Addition of suitable adjuvants into spray formulations can help increase the rate of retention, spreading, penetration and drying of the solution, thereby, improving the performance of fertilisers. Sodium salt of carboxymethyl ether of cellulose (CMC) is a food additive used by the agro-food industry to improve e.g., moisture retention, as a thickener or as an emulsion stabiliser (Ghannam and Esmail, 1998, Nie et al., 2004).

Calcium-propionate could potentially be a good Ca source since it is a small organic salt molecule, which may provide anti-fungal properties. This chemical is also used by the food industry, and has been successfully tested as a fungicide in peach for canker (Biggs et al., 1994) and brown rot (Biggs et al., 1997), and in apple to provide both protective and curative effects against infections caused by Botrytis cinerea (Droby et al., 2003).

The goal of this investigation was to develop laboratory and field studies intended to enhance the uptake of Ca in apple and thereby reduce bitter pit development and improve storage quality of fruit. The working hypothesis was to evaluate whether the addition of 0.5% CMC as formulation adjuvant to be used in combination with Ca-propionate and CaCl₂ as active ingredients, provided beneficial effects in terms of formulation properties, fruit tissue Ca increases and fruit quality and storability.