Investigating pre-symptomatic biochemical markers related to 'Marsh' grapefruit (Citrus × paradisi Macfad) susceptibility to chilling injury and rind pitting disorders

Fresh citrus fruit is usually exposed to -0.6°C for at least 14 d of cold treatment against pests such as fruit flies (Ceratitis sp.) during exports. However, 'Marsh' grapefruit (Citrus × paradisi MacFad) develops chilling injury and rind pitting disorders when exposed to such low temperatures. This study investigated the susceptibility of 'Marsh' grapefruit to chilling injury and rind pitting disorders by examining the relationships between the fruit's position within the tree canopy, rind physico-chemical profile and susceptibility to the disorders. The study was conducted in two major citrus producing provinces of South Africa (Limpopo and KwaZulu-Natal). Irrespective of location, the grapefruits from inside the canopy had a higher chilling injury incidence compared to those from outside the canopy (27 vs. 12 and 24 vs. 0%). Rind pitting was lower on the inside canopy grapefruit compared to the outside canopy ones (13 vs. 19 and 2 vs. 9%). The difference in disorders' incidences was associated with the difference in rind biochemical properties caused by the canopy position. The inside canopy fruit had higher antioxidant activity, fructose and sucrose, and lower dry matter, total antioxidant compounds, glucose and total phenolic compounds compared with the outside canopy fruit. The rind dry matter, total antioxidants, glucose and total phenolic content were positively correlated to the rind pitting but negatively correlated to chilling injury, indicating that the rind biochemical variables could be used as pre-symptomatic biochemical markers for determining 'Marsh' grapefruit susceptibility to chilling injury and rind pitting disorders.