Exploring Differential Degreening Responses of Citrus Types

Degreening is a postharvest treatment in citrus during which fruit are exposed to ethylene to degrade chlorophyll content of the peel. Under cool nights (<12.5 °C) fruit are subjected to mild stress which triggers ethylene production and pigment synthesis. This results in degradation of chlorophyll, and unmasking and synthesis of natural pigments. Early season fruit are not subjected to cool nights and they remain green in color, while the internal quality has reached acceptable maturity standards for marketing. When the peel is completely or partially green in color, application of exogenous ethylene is required to degreen the fruit.

The response of fruit to ethylene varies between varieties. Though most of these differences are minor, some citrus types show extreme responses. In general, when fruit are removed from ethylene exposure, chlorophyll degradation stops and color change ceases. But in the commercial cultivar Fallglo, the peel continues to change color even after removing from ethylene. Conversely, the hybrid Lee x Orlando (developed by USDA) does not degreen completely after standard degreening treatment. These extreme responses could lead to various problems like increased cost during degreening treatment, rapid senescence and increased decay development due to over exposure to ethylene. Lee x Orlando may not be released as a commercial variety in part due to incomplete degreening.

We hypothesize that the differential response of citrus types to ethylene is due to differences in ethylene biosynthesis and/or ethylene signal transduction. The expression pattern of ethylene biosynthetic and perception genes after exposure to ethylene for different durations will be studied in peel of Fallglo and Lee x Orlando and their parents (Bower and Temple, Lee and Orlando, respectively). Changes in fruit ethylene evolution will be measured. Color, chlorophyll content, carotenoid level and Brix/acid ratio will be used to describe the maturity and physiological status of the fruit. For each citrus type, fruit will be harvested three times during the season (early, mid and late season). Fruit will be exposed to 8 ppm of ethylene for 0.5, 1 or 2 days, followed by storage in ethylene-free air (air) for periods up to 3 days after ethylene treatment. As a positive control, fruit will be stored in ethylene for up to 5 days. As a negative control fruit will be stored in air for up to 5 days. The sampling schedule shown in table 1 indicates that sampling will be done to appropriately compare equal duration exposures to ethylene and/or air and account for differences due to experiment length, if any. The fruit will be harvested from trees at various locations in central Florida and transported to the packinghouse at the Citrus Research and Education Center.  Fruit from each citrus type will be randomly assigned to exposure/duration treatments with 4 replications and 5 fruit per replication. Analysis of variance will be performed to test significant differences among treatments.

At the end of this study we will be able to describe the degreening responses of different citrus types and examine the contribution of ethylene biosynthesis and/or perception to these degreening responses. From this, more useful recommendations for successful commercial degreening of mature citrus fruit will be provided.

Table 1.  Schedule of sampling by days after start of experiment.  Mature fruit from the citrus types will be harvested and randomized into exposure/duration treatments.  Air, ethylene-free air; C₂H_4, ethylene; t, time; d, day.