**3. Physiological disorders**

Apart from the benefits that exogenous ethylene application during degreening treatment can provide to improve fruit coloration, it can lead to physiological alterations in fruit if the process is not properly carried out.

The negative effects associated with exposing the fruit to exogeneous ethylene are related to an accelerated senescence process, which can lead to major quality loss and a shorter shelf-life [26, 29]. These undesirable effects depend mainly on the concentration and duration of fruit exposure to ethylene, but also affect atmospheric conditions, such as low humidity, excessive temperature, and high CO2 concentrations. The incidence of these alterations also depends on the cultivar, preharvest conditions, and the treatment of the postharvest condition. In most cases, these disorders do not appear immediately after the degreening process but are manifested later during storage until marketing [16]. Special care must be taken when fruits are to be shipped at low temperatures, which occurs with exports to countries with quarantine requirements because low temperatures after degreening can accelerate the manifestation of these alterations. Therefore, to avoid possible fruit disorders, treatment must be carried out under careful conditions by considering all the factors involved in the process, as well as the conditions to which fruit are subjected after degreening.

The most frequent physiological disorders associated with incorrect degreening treatment are described below.

#### **3.1 Calyx senescence**

Calyx drop and browning are the main physiological disorders associated with incorrect degreening (**Figure 1**). Keeping the calyx fresh during the postharvest life of citrus fruit is a quality requirement because fruits are commercially more attractive, and it also protects the fruit from fungal infection upon abscission [30].

Ethylene is a vegetal hormone that promotes tissue abscission and senescence in fruit [31]. Low atmospheric ethylene levels (≤0.01 μL/L) reduce this disorder during long-term storage, and its accumulation during cold storage has been related to calyx senescence [32].

During degreening treatment, ethylene application has been reported to increase the activity of both polygalacturonic acid enzyme (PG) and cellulase (Cx), which promote browning and calyx abscission [33]. The incidence of calyx senescence triggered by degreening depends mainly on the ethylene concentration and process duration [26, 30, 34], and it is also very cultivar dependent. Clementine mandarins 'Marisol' and 'Oronules' have been shown to be more sensitive to calyx senescence than 'Clemenpons' and 'Clemenules' [35]. Other citrus cultivars, such as 'Satsuma' mandarin and 'Navelina' orange, are extremely sensitive to calyx senescence [6, 26]. Despite these calyx disorders appearing sometime after degreening, their incidence is accentuated with post-treatment storage time.

**Figure 1.** *Calyx senescence. Browning and drop of the calyx.*

#### *Quality of Postharvest Degreened Citrus Fruit DOI: http://dx.doi.org/10.5772/intechopen.105119*

Many studies have focused on finding solutions to prevent calyx senescence during degreening. Using auxins has been one of the effective treatments in controlling this alteration. Synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) has been extensively reported as a treatment to retard calyx abscission, drying, and browning. It occurs as a consequence of exposing the fruit to ethylene during degreening treatment [30, 34, 36]. This auxin's mechanism of action is to reduce PG and Cx activities and to increase the lignin and water contents of fruit peel [37]. Despite this auxin having been widely employed and proving useful, it has been restricted by current European Union legislation, hence the need to find other synthetic auxins to avoid physiological changes in the calyx [33, 35].

Carvalho et al. [35] tested four synthetic auxins (2,4-dichlorophenoxy propionic acid (2,4-DP); 3,5,6-trichloro-2-pyridyloxyacetic acid (3,5,6-TPA); 2,4-D isopropyl ester; 2,4-D-amine) for retarding the calyx disorders caused by degreening in different clementine cultivars. All the evaluated auxins reduced calyx senescence. In all the cultivars, the best results were obtained by applying 3,5,6-TPA, followed by 2,4-D isopropyl ester. In contrast, the treatment with 2,4-DP had a positive effect on avoiding calyx disorders, but only in 'Clemenpons' mandarins, and this effect was not observed in the other studied cultivars. Although auxin treatments can delay color evolution, they had no negative effect from a commercial point of view in all the studied cultivars because, after degreening, all the treatments had a commercially acceptable color index. Moreover, no auxin treatment affected the sensory quality of degreened fruit.

The application of 3,5,6-TPA at different concentrations (10, 20, and 40 ppm) under commercial degreening conditions has been well-studied with many varieties. The higher the 3,5,6-TPA dose, the lower the percentage of affected fruit with calyx alteration symptoms [38, 39]. In fact, 3,5,6-TPA is currently used in the postharvest industry at a dose of 40 ppm and is applied with drenchers [39].

Other plant growth regulators, such as HF-Calibra® (SIPCAM INAGRA, Spain) with active ingredient MCPA-thioethyl (S-Ethyl-4-chloro-o-tolyloxythioacetate), have been tested in the most important Spanish early-season citrus varieties subjected to degreening treatment at different concentrations (10, 20, 40, and 60 ml/L) [26]. That study revealed that this auxin contributes to decreasing calyx senescence triggered by exogenous ethylene. The higher the applied dose, the stronger the effect on preventing calyx abscission.

Another technique to avoid the negative effect of ethylene on calyx senescence is degreening treatments which combine different exposure periods with and without ethylene. This has been evaluated with mandarins and oranges to be exported to the USA from Spain [6, 24]. Optimal degreening process conditions have been established according to both the initial external color and variety [40]. These recommendations are provided in **Table 1**.

Recently, treatments with Oligochitosan and Chitosan (poly-β-(1,4)-Dglucosamine) have been demonstrated to reduce calyx browning caused by degreening treatment, which has been linked with the inhibition of protopectin, cellulose, and lignin degradation [33].

#### **3.2 Peel disorders**

Some citrus cultivars with very thin skin, especially mandarins, can manifest bruising symptoms when they pass along packing lines after being degreened.


#### **Table 1.**

*Recommendations for the degreening treatment of Spanish mandarins and oranges to be exported to the EU, USA, or Japan (adapted from Pássaro et al. [40]).*

This disorder is commonly called 'zebra skin' and is caused by mechanical abrasion during the brushing or rolling of fruit in an equatorial area and the cells darken and produce necrotic streaks in the center of fruit segments (**Figure 2**) [41]. This disorder can also be manifested in non-degreened fruit, but degreening treatment enhances susceptibility to bruising. When degreening treatment is carried out at low humidity, elevated temperature, and high CO2 levels, and for long ethylene exposure times, the susceptibility of fruit showing zebra skin symptoms after packing increases [42]. To avoid this peel alteration, keeping fruit in an ethylene-free atmosphere for at least 12 h after degreening and before packaging, and delaying harvest for 5–7 days after rainy periods, are highly recommended because skin turgidity can increase this damage [43].

Oleocellosis is one of the main postharvest skin disorders to occur in citrus. It is usually caused by mechanical injuries to cells in rind at harvest or on packing lines. Broken cells release the oil, which is phytotoxic to pericarp cells and causes browned areas on affected rind areas. Early-season citrus fruit is more susceptible to oleocellosis [44].

When fruit is subjected to degreening treatment, the mechanical damage caused at harvest and while transporting fruit from fields to packinghouses can lead to olleocelosis. Damaged cells, in which oil extravasation occurs, do not change color during degreening treatment, which leaves green areas on the rind (**Figure 3**). Moreover, the more turgid rind cells are, the more susceptible fruit are to mechanical damage and they, consequently, show oleocellosis. High environmental humidity, rainy days, or excess irrigation increase fruit peel turgidity, and it is advisable to avoid harvesting after rain or picking fruit early in the morning to avoid dew. Waiting for 24 h from harvesting before handling in containers is also recommended [45]. In addition, cultivar can be a relevant factor since as higher is the density of oil cavities, greater volume, and their position in the pericarp higher is the incidence of the disorder [46].

Another physiological disorder that can be accentuated by postharvest degreening is stem-end rind breakdown (SERB).

#### **Figure 2.**

*'Zebra skin' peel disorder caused by mechanical abrasion in packing line.*

**Figure 3.** *Oleocellosis symptoms after degreening treatment.*

SERB symptoms cause the peel tissue around the calyx to collapse, which becomes dark and sunken (**Figure 4**). SERB is due to excessive water loss and usually appears 2 and 7 storage days after packaging. The fruit that develops this disorder tends to rot more easily. Thinner-skinned fruit grown in humid growing environments, such as Florida, tend to be more prone to SERB than thicker-skinned fruit from arid environments [47]. Including a holding period for 12 to 24 h after degreening treatment and before packing is recommended to avoid SERB. Beneficial effects of ethylene on fruit have been reported; for example, ethylene has been related as a contributor to new wax formation. It also increases the total soft epicuticular wax

**Figure 4.** *Stem-end rind breakdown (SERB) damage.*

content in mature citrus fruit, which decreases transpiration, maintains the water balance, and regulates the gas exchange in plants [48].
