USE OF EVAPORATIVE COOLING TO IMPROVE ‘ROSEMARIE’ AND ‘FORELLE’ PEAR FRUIT BLUSH COLOUR AND QUALITY

Abstract

In Pyrus communis ‘Rosemarie’, a potentially lucrative blushed pear cultivar, poor fruit colour has been ascribed to pre-harvest red colour loss during periods of high temperature. High temperatures combined with high irradiances also lead to sunburn on pome fruit. The use of pulsed overhead evaporative cooling (EC) to improve red colour and reduce sunburn in two blushed pear cultivars was evaluated over four seasons (2000/01 to 2003/04) in Stellenbosch, South Africa. ‘Rosemarie’ fruit blush was usually but not always improved under EC, with a late application (starting two weeks before harvest) yielding the best improvement. Peel anthocyanin concentrations were increased under EC. Incidence of sunburn was reduced during some years, but increased in EC-treated fruit during 2003/04 due to system failure on a hot day. EC initiated early in fruit development initially led to larger fruit with a lower TSS concentration and firmness, but this effect was gradually eliminated following reductions in EC water use. EC had no effect on ‘Forelle’ pear fruit colour or mass, but reduced firmness and TSS when started early in the season. Sunburn was only recorded in EC-treated fruit during 2002/03 due to system failure on a warm day. Though EC could be used to improve ‘Rosemarie’ fruit colour in warm production areas, its effect was relatively small compared to colour change in response to fluctuating temperature. The increased risk of sunburn during system ‘downtime’ highlights the requirement for a highly reliable system. INTRODUCTION Insufficient red colour has limited the profitability of blushed pears in the warm production areas of the Western Cape, South Africa (Steyn et al., 2004). Pyrus communis 'Rosemarie’, a locally bred early-season cultivar, is especially susceptible to colour loss prior to harvest. This is due to the net degradation of anthocyanin in response to high temperatures (Steyn et al., 2004). The extent of red pigmentation in fruit peel is primarily determined by light and temperature. High irradiances are a requirement for colour development, but the strong associated radiant heating may be detrimental. Anthocyanin accumulation generally requires low night and moderate day temperatures (Christie et al., 1994; Curry, 1997). High temperatures inhibit the induction and reduce the synthesis of anthocyanin (Curry, 1997; Reay, 1999). They also increase the rate of anthocyanin degradation (Steyn et al., 2004) and thus lead to poor fruit colour. High summer irradiances and temperatures also give rise to sunburn on exposed fruit surfaces (Bergh et al., 1980). The extent of damage may vary from bleaching to a yellow or bronze discolouration of the skin, and in serious instances to death and browning of the flesh underneath (Schrader et al., 2001). Apple fruit skin temperatures can be up to 8-10°C warmer than air temperatures (Bergh et al., 1980; Wiinsche et al., 2001) and sunburn occurs at temperatures of 46°C or higher (Schrader et al., 2001). It is not known at which skin temperatures pear fruit experience sunburn. Overhead evaporative cooling (EC) is used commercially to counteract the adverse effects of high temperature on fruit quality in warm production regions. EC