Influence of temperature and net radiation on the natural degreening process of grapefruit (Citrus paradisi Macf.) cultivars Rio Red and Star Ruby

• Minimum temperatures are determinant for the degreening process to begin.
• Degreening starts when the mean minimum daily temperatures of the 14 days prior to measuring are below 15 °C.
• Degreening starts when there are two days, not necessarily consecutive, when the minimum temperatures falls below 12.5 °C.
• For new plantations, the thermal integral must be high but that temperatures below 15 °C are also needed during ripening.

This contribution analyses the influence of net radiation and temperature on the colour changes that take place in the peel of two cultivars of grapefruit (Citrus paradisi Macf.), Rio Red and Star Ruby. The study, which covered four growing seasons (2005/6, 2006/7, 2010/11 and 2011/12) shows that, while net radiation influenced the colour change in both varieties, it was not sufficient in itself to explain when the degreening starts. This process only started when the mean of the minimum daily temperatures for the 14 days preceding the measurement was 13–14 °C or when the daily minimum temperature was below 12.5 °C on two (not necessarily consecutive) days. This information shows that, for new plantations of these two grapefruit cultivars to be successful, not only must the thermal integral be high, but that temperatures must fall below 13 or 14 °C for degreening to begin during ripening. If these conditions are not met, grapefruit intended for commercial distribution will have to be subjected to artificial degreening in an ethylene chamber, thus increasing production costs.