RED COLOUR DEVELOPMENT AND LOSS IN PEARS

Abstract

The endogenous and environmental regulation of red colour development in blushed and fully red pears is reviewed. Colour development in pears has an underlying developmental component. Generally, highest anthocyanin concentrations are attained in immature pears and colour tends to fade towards harvest. This is contrary to most other crop species where maximum pigmentation and colour are attained in ripe fruit and may relate to the photoprotective ability of anthocyanins. Because of this pigmentation pattern, net anthocyanin degradation at high temperatures results in pre-harvest red colour loss in susceptible pear cultivars. Susceptibility depends on the capacity to accumulate anthocyanin and on whether low temperatures are required for anthocyanin synthesis. Unlike apples, where red colour development in all cultivars seems to require or benefit from low temperatures, not all pear cultivars seem to respond to low temperatures. Light appears to have two opposing effects in pears, being required for anthocyanin synthesis, but also apparently increasing red colour loss through increased degradation of anthocyanin. INTRODUCTION Blushed pear fruit are sought after by consumers and fetch higher prices than green or fully red fruit (Huysamer, 1998). Red colour of blushed fruit is due to the presence of two anthocyanin pigments, cyanidin 3-galactoside and cyanidin 3-arabinoside, in hypodermal layers of the skin (Dussi et al., 1995). The presence and extent of the red blush, which varies considerably between seasons and between cultivars, determines the profitability of blushed cultivars. Differences in the redness of pear peel primarily relate to differences in pigment levels (Steyn et al., 2004a). Hence, poor red colour and the downgrading of blushed pears can be attributed to low anthocyanin levels at harvest. In order to attain regular yields of sufficiently blushed fruit, producers need to be aware of the factors that influence pear red colour development. In contrast with numerous reviews on red colour development in apples, very little is known about the regulation of pear colour development. However, recent evidence indicates that the developmental regulation of red colour in pears differs considerably from that in other fruit kinds. While in most fruit kinds anthocyanin synthesis peaks during ripening (Macheix et al., 1990; Saure, 1990), pear red colour apparently fades towards harvest (Steyn et al., 2004a). This difference in developmental regulation influences the effect of environmental factors on red colour development. In apples, poor red colour at harvest can be attributed to insufficient anthocyanin accumulation during the ripening-associated peak in synthesis (Saure, 1990). Poor synthesis normally relates to low light levels within the tree canopy and/or high temperatures (Saure, 1990). In pears, high day temperatures and light may contribute to the fading of red colour by increasing anthocyanin degradation (Steyn et al., 2004b). In this review we discuss the endogenous and environmental regulation of red colour development in blushed and fully red pears as well as the contribution of anthocyanin degradation to pear colour at harvest. We also refer to the extensive literature available on the regulation of colour development in apples, the fruit kind of choice for