Antioxidants in ‘Hass’ avocado

The potential involvement of C7 sugars as antioxidants in various avocado tissues was tested. Total antioxidant activity was found to be similar in leaf, exocarp and seed tissue; however, the concentration of total antioxidants in mesocarp tissue was lower than in other tissues at all three sampling dates. The various tissues examined had different predominant antioxidant systems; while leaf tissue contained high concentrations of anthocyanins, ascorbic acid was found to be the main antioxidant in the seed as well as the rind tissue. In mesocarp tissue, however, C7 sugars, and particularly mannoheptulose, played the major antioxidant role. As levels of both C7 sugars declined signifi cantly as harvest maturity was approached, the reduction in C7 sugars could be related to deterioration in post-harvest quality. Therefore, management practises should be established to increase the concentration of these sugars in the mesocarp prior to harvesting. INTRODUCTION The quest for the role of the abundant C7 sugars mannoheptulose and perseitol in avocado is ongoing. Previous research by Liu et al. (2002) indicated that these carbohydrates might play a role as ripening inhibitors. Our previous fi ndings (Bertling and Bower, 2005) suggest importance of these carbohydrates in sink strength of fruit and later on in fruit ripening. Cowan (2004) proposed that these C7 sugars have various important functions, amongst them protection of certain key enzymes essential for fruit growth and development, from damage by reactive oxygen species (ROS). Therefore, C7 sugars could form part of a pool of antioxidant systems which not only protect the fruit against damage but would also carry health benefi ts for the consumer of the fruit and hence could become an important marketing issue. Antioxidant (AO) systems are present in cells to guard cellular structures against naturally occurring, extremely reactive compounds (particularly ROS) such as the oxygen ions, the hydroxyl radical and other free radicals as well as hydrogen peroxide. Such compounds, which form as natural by-products of the normal oxygen metabolism, could, due to their high chemical reactivity, damage proteins, DNA and lipids – a damage which would eventually lead to cell death. The removal of such compounds from cells is afforded either by ROS scavenging enzymes (e.g. superoxide dismutase, ascorbate peroxidase and catalase) which will inactivate the ROS or by small scavenging molecules (e.g. ascorbic acid, glutathione, polyphenols) which combine with the ROS to form non-toxic compounds. ROS commonly occurring in plant tissue are the superoxide anion, which is scavenged by superoxide dismutase (SOD), as well as hydrogen peroxide, which is scavenged by catalase and ascorbate peroxidase, and using the ascorbate-glutathione cycle. Hydroxyl radicals and singlet oxygen are scavenged nonenzymatically by compounds such as carotenoids, ascorbate and α-tocopherol. Under optimal growing conditions the production of antioxidants in cells is low and ROS occurring in cells are scavenged by the AOs present. During normal cell metabolism, ROS and AOs exist therefore in equilibrium. Stress (drought, chilling, heat shock, UV, pathogens, light) can, however, increase the production of ROS dramatically (Mittler, 2002) which then would need to be counteracted by an increase in AOs. However, when exposed to stressful conditions, the present pool of antioxidants cannot scavenge all ROS produced, which will result in non-healthy, damaged fruit. Certain information on antioxidant activity of avocado is available. Vinokur and Rodov (2006) reported on the lipophylic versus hydrophilic radical scavenging activity of avocado, while Song and Barlow (2004) published that the antioxidant activity in avocado seeds is – like in most fruit – higher than in the edible portion. Furthermore, Ekor et al. (2006) reported that the administration of a methanolic extract of avocado leaves to rats increased the activity of the antioxidant enzymes SOD and catalase in these mammals. Certain C6 sugar alcohols (sorbitol and mannitol) have been found to act as antioxidants when added to produce (Faraji and Lindsay, 2004). Similarly, C7 sugar alcohols could form an important part of the pool of antioxidants in avocado, protecting the developing fruit from oxidative stress and, therefore, playing an important role in the development of healthy fruit. Therefore, we investigated if perseitol and mannoheptulose form part of the total AOs in avocado tissue, which AOs are present in various ‘Hass’ avocado tissues and if the pool of antioxidant systems and its main fractions changes during fruit development. MATERIALS AND METHODS Leaves as well as fruit were sampled from approximately eightyear-old ‘Hass’ avocado trees in the KZN Midlands during November 2005 (early fruitset), March 2006 and June 2006 (close to harvest date). Samples were transported to the laboratory and 1 to 5 g leaf or fruit tissues – the latter separated into rind, fl esh and seed – shock-frozen and subsequently freeze-dried. Total antioxidant capacity was determined according to Benzie and Strain (1996) using the FRAP assay, while ascorbic acid was analysed using the colour reaction with 2,4-dinitrophenylhydrazine (DNPH) according to Böhm et al. (2006). Anthocyanins were determined spectrophotometrically (Steyn et al., 2006) and sugars were determined via HPLC according to Liu et al. (2002). Statistical evaluations were carried out using GenStat® Version 7.1. RESULTS AND DISCUSSION With the maturation of new leaves from November’05 to March’06 the levels of total antioxidants in these tissues increased slightly