Lychee Pericarp Browning Caused Heat Injury

Mature lychee (Litchi chinensis Sonn.) fruit were heat-treated at 60C for 10 min to study heat-induced pericarp browning. Polyphenol oxidase (EC 1.10.3.2) activity of the pericarp increased immediately, corresponding with rapid anthocyanin degradation, Tissue browning was observed 2 min after heating, with pigmentation distributed uniformly throughout the pericarp. The distribution of brown pigments was different than the highly localized browning observed under ambient desiccation. Although both ambient and heat-induced pericarp browning are visually similar, the anatomical distribution of brown pigmentation is quite distinct. The distribution of brown pigmentation was not consistent with anthocyanin localization. Following ambient desiccation, the mesocarp became colorless even though this represented the greatest concentration of pigment. Browning caused by heating may result from nonselective degradation of a range of compounds, including anthocyanin. Lychee pericarp browning is a general surface injury commonly observed after harvest (Nip, 1988). Injury is known to occur in response to a wide range of factors, including climatic conditions during ontogeny (Sharma et al., 1986), fungal infection (Huang and Scott, 1985), desiccation (Scott et al., 1982), fruit senescence (Huang et al., 1990), and heat injury (Wong et al., 1991). Once present, it is difficult to identify the initial stress that caused the injury. As a result of this seemingly uniform injury, lychee pericarp browning has been studied as a single degradation system, with little emphasis placed on the original stress. Most previous studies have involved browning from postharvest desiccation. Relatively little is known about injury mechanisms under other conditions, or their similarity to ambient desiccation browning. Browning is commonly attributed to degradation of the anthocyanin pigments by polyphenol oxidase (PPO) activity (Akamine, 1960). However, it is not clear whether the by-products of this degradation contribute to tissue browning. This study was undertaken to understand lychee pericarp browning resulting from heat stress. Emphasis was given to injury localization to determine similarities between heatand ambient desiccation-induced pericarp browning. Received for publication 6 July 1992. Accepted for publication 18 Dec. 1992. We acknowledge the financial support provided by the Rural Industries Research and Development Corp. and the Australian Centre for International Agricultural Research. Technical advice given by David Simons, The Univ. of Queensland, Gatton College, is also acknowledged. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1Dept. of Botany, The Univ. of Queensland, Queensland 4072, Australia. HORTSCIENCE, VOL. 28(7), JULY 1993 Litchi chinensis cv. Kwai May Pink syn. Gui Wei were obtained from a commercial orchard in southeastern Queensland (lat. 27°S), Fruit were immersed in hot water (60C) for 10 min, then kept at 25C for progressive assessment. We used 60C because it was known to cause rapid browning (Wong et al., 1991). To minimize further changes during sample preparation, fruit were placed in an ice slurry. Polyphenol oxidase activity was measured polarographically using a method modified from Tremolieres and Bieth (1984) and Wesche-Ebeling and Montgomery (1990). Pericarp tissue was sliced finely, divided into three replicates of 4 g each, and stored overnight in liquid N. Samples were ground in liquid N and homogenized with 25 ml 0.02 M sodium phosphate buffer (pH 6.8) containing 1% triton X-100 (v/v) and 1% polyvinyl pyrrolidone-40 (w/v). The suspension was centrifuged at 20,000× g for 20 min at 4C, and the supernatant solution used as the crude enzyme extract. Oxygen consumption was measured with a Clarke O2 electrode (Rank Brothers, Bottisham, U.K.) at 28C. Prewarmed, 2.4 ml of sodium phosphate buffer (pH 6.8) and 0.2 ml of pericarp extract were added to the reaction chamber. Once equilibrium was achieved, 0.4 ml of 0.04 M 4-methyl catechol in 0.02 M sodium phosphate buffer (pH 6.8) was added, and the rate of O2 consumption recorded. Anthocyanin concentration was determined using 1 g of pericarp tissue from each of five fruit and homogenized for 2 h in 50 ml methanol containing 1% HCl (v/v). The extract was filtered, diluted, and absorbance was determined at 530 nm. Anthocyanin concentration was then calculated according to Fuleki and Francis (1968). The anatomy of the pericarp was examined before and after heat treatment. Tissue was hand-sectioned, mounted unstained in 30% glycerol, and observed using an Olympus (Japan) BH02 light microscope. As controls, lychee fruit were also kept for 3 days at 26C and 60% relative humidity (RH) to induce postharvest ambient desiccation browning. PPO activity of the pericarp increased immediately after heat treatment (Fig. 1), which corresponded to a rapid browning of the pericarp. Where browning has been studied in other fruit, discoloration correlates well with PPO activity and phenolic concentration (Coseteng and Lee, 1987; Martinez-Cayuela et al., 1988). Consequently, resulting brown pigmentation has been attributed directly to PPO action. Heat treatment caused brown pigmentation throughout the pericarp (Fig. 2, top). This uniform distribution indicates that PPO was not restricted to specific zones within the pericarp. Under ambient desiccation conditions,