62 AJCS 8(1):62-70 (2014) ISSN:1835-2707 Comparative changes in the rheological properties and cell wall metabolism in rind of healthy and creased fruit of Washington Navel and Navelina sweet orange (Citrus sinensis [L.] Osbeck) Basharat Ali Saleem † , Imran Hassan ‡ , Zora Singh * , Aman Ullah Malik § , Muhammad Aslam Pervez § Curtin Horticulture Research Laboratory, Department of Environment and Agriculture, School of Science and International Institute of Agri-Food Security (IIAFS), Curtin University, GPO Box U1987, Perth, 6845, WA, Australia * Corresponding author: Z.Singh@curtin.edu.au Present address: † Punjab Agriculture Department, Hill Fruit Research Station, Murree, Pakistan, ‡ PMAS Arid Agriculture University, Rawalpindi, Pakistan, § Institute of Horticultural Sciences, University of Agriculture Faisalabad, Pakistan. Abstract Creasing is a physiological disorder in navel oranges and causes serious economic losses. In 2007, the comparative changes in rheological properties of the rind, levels of starch, phenolics, pectins and the activity of pectinesterase (PE) in albedo and flavedo tissues of the healthy and the creased fruit of ‘Washington Navel’ and ‘Navelina’ sweet orange at ripe stage were investigated. During 2009, dynamics of the activities of PE, exo and endo polygalacturonase (exo-PG, endo-PG), and endo-1, 4-ß -D-glucanase (EGase) enzymes in albedo and flavedo tissues of the healthy as well as the creased ‘Washington Navel’ sweet orange fruit at different maturation and ripening stages were researched. The rind hardness, stiffness and tensile force were substantially lower in the creased fruit than the healthy ones in ‘Washington Navel’ and ‘Navelina’. The levels of starch, total phenolics, total pectins and water insoluble pectins decreased in the albedo and flavedo of the creased than the healthy fruit in both the cultivars. The levels of water soluble pectins increased in the albedo and flavedo of the creased fruit when compared to the healthy ones in both the cultivars. The activities of PE in albedo and flavedo tissues were higher in the creased fruit when compared to the healthy fruit of ‘Washington Navel’ and ‘Navelina’ in 2007. The activities of PE, exo and endo PG and Endo-1, 4-ß -D-glucanase were higher in the albedo tissue of creased fruit of ‘Washington Navel’ orange at different fruit maturation and ripening stages. In conclusion, the higher activities of pectinesterase, exo- polygalacturonase, endo- polygalacturonase, and endo-1, 4-ß -D-glucanase in the albedo of creased fruit at commercial harvest seem to be associated with the enhanced loss of pectins and starch in the cell walls of albedo tissue, leading to cell wall loosening and cracks formation consequently reducing hardness, stiffness and tensile force of the rind. Keywords: Creasing; EGase; PE; pectins, PG; rind texture; sweet orange. Abbreviations: CH_commercial harvest; DACH_days after commercial harvest; DBCH_days before commercial harvest; FW_fresh weight; EGase_endo-1, 4-ß -D-glucanase. Introduction Creasing in navel sweet oranges causes serious economic losses to the citrus growers. The creased fruit portion is characterized by multiple cracks in albedo (Bower, 2004), separation of cells in the albedo tissue resulting in channels in the rind (Storey and Treeby, 1994; Treeby et al., 1995), and separation of albedo from the flavedo causing formation of sunken areas on the peel (Agusti et al., 2001). Alquezar et al., (2010) reported that the disorder first appeared at the transition zone between flavedo and albedo, making the albedo cells flat and compact; later the affected cells extended progressively to outer albedo and inner flavedo. Various factors have been associated with creasing of sweet orange fruit such as lower levels of calcium in the albedo tissue (Treeby and Coote, 1997; Story et al., 2002), climatic changes (Zaragoza and Alonso, 1975; Agusti and Zaragoza, 2000), water stress (Zacarias et al., 2001), fruit position on the tree (Holtzhausen, 1981), crop load (Nagy et el., 1982), rootstocks (Treeby et al., 1995) and rind thickness (Koo and Reese, 1977; Holtzhausen, 1981; Ali et al., 2000). Earlier reports also indicate decreased pectin content in the cell wall (Yang et al., 2008; Li et al., 2009), hemicellulose and cell wall polysaccharides (Jona, 1989) and higher activity of pectin methylesterase in the creased tissues leading to increased water-soluble pectins (Monselise et al., 1976) as well as uronic acid oxidase enzyme as a gelling factor (Bower, 2004). The albedo and flavedo comprise of polysaccharides including pectic substances like pectin, protopectin, pectic acid and pectinic acid (Be Miller, 1986; Yang et al. 2008; Barany et al. 2010), which are degraded by pectic enzymes like pectinesterases (PE), polygalacturonases (PG) (Kashyap et al., 2001; Yoo et al., 2003) and Endo-1, 4-ß -D-glucanase (EGase) (Brummell et al., 1997). The enzymatic peeling of citrus fruit exhibited that albedo is efficiently degraded by the activity of enzyme which act on polygalacturonic acid (Pretel et al., 2005). Galacturonic acid is the main constituent of pectins in albedo of citrus fruit (Schroder et al., 2004). Phenolics and starch have also been involved in fruit firmness, and cell wall integrity, whilst increased soluble phenolics enhance cell wall endurance (Diaz et al., 2001). The starch contributes to fruit firmness