We have evaluated the effect of a heat-conditioning treatment (3 days at 37 degreesC) on respiratory rate, soluble carbohydrate and starch content of 'Fortune' mandarin fruit harvested at different maturity stages and stored at a chilling (2 degreesC) and a non-chilling (12 degreesC) temperature. The treatment was highly effective, increasing the tolerance of detached 'Fortune' fruit to chilling injury (CI). Changes in carbohydrate levels in response to the high temperature treatment, and during exposure of the non-conditioned and heat-conditioned fruit to 2 and 12 degreesC, appear to be mainly related to the consumption of carbohydrate reserves for respiration. The highest respiratory rate was found in fruit exposed to 37 degreesC. The respiration of non-conditioned fruit stored at 2 degreesC was lower than that of control fruit stored at 12 degreesC. Therefore, anomalous high respiratory activity did not occur during chilling in 'Fortune' mandarins. Sucrose appeared to be the most accessible sugar as a respiratory substrate in non-conditioned fruit. No relationship between the susceptibility to CI and changes in carbohydrates during storage of non-conditioned fruit at 2 degreesC was found. The heat treatment prevented the decline in sucrose content during chilling. After 30 days at 2 degreesC, the sucrose content of non-conditioned fruit harvested in December, January and March was about 79, 57 and 68% that of their respective heat-conditioned fruit. In contrast, heating the fruit favoured the loss of glucose, fructose and starch in fruit kept at 2 inverted perpendicularC. These data suggest that sucrose but not glucose, fructose or starch could be involved in the heat-induced chilling tolerance of citrus fruit detached from the tree. The effect of the heat treatment preventing sucrose decline during storage of fruit at 2 degreesC was less relevant in fruit from December than in those harvested later in the season, although it was very effective in protecting fruit harvested at all maturity stages against chilling. Therefore, sucrose appears not to be a limiting factor for the heat-induced chilling tolerance in citrus fruit. (C) 2002 Elsevier Science B.V. All rights reserved.252181191