Influences of girdling and potassium treatments on fruit quality and some physiological characters of ‘Fremont’ mandarin variety

The citrus production of Turkey is 4,301,415 tons in the year 2019. Approximately 39.52% of production is oranges (1,700,000 tons), 33.71% (1,450,000 tons) is mandarins and 22.09% (950,000 tons) is lemons. Turkey exports of 3,785,731 tons of fresh fruit and 19.87% of this fresh-fruit export consists of mandarins (FAO, 2021). Citrus fruits contain unique flavour and minerals, phytochemicals and dietary fibre. For fresh consumption, the visual quality and marketability of fruits can be preserved by long storage (Kahramanoğlu et al., 2020). Among citrus fruits, mandarins comprise the fruit group that is mainly preferred since they can be easily peeled in fresh consumption. The ‘Fremont’ mandarin variety has high fruit quality, bears fruit early and is suitable for both transportation and storage; however, this variety exhibits periodicity. Its fruits are small, as it has excessive fruit retention; therefore, thinning is necessary. The importance of this fruit in citrus growing has become very high because of its very sweet and flavourful nature, high appeal, suitability for thick planting and early yield.

Fruit size is one of the most important quality traits, and this characteristic is desired by consumers in both domestic and export markets. The fact that consumers demand large fruits leads to an increase in prices, and fruit size is as importance as yield (Guardiola and Garcia-Luis, 2000). One issue that is occasionally encountered in the mandarin industry is the small fruit size, which dramatically reduces the profitability of the product.

There are many factors in citrus-growing that affect the production of quality, such as manual thinning, girdling, chemical and hormone treatments and synthetic auxin treatments and the use of plant growth regulators.

Girdling treatments are applied in citrus production mainly to increase the quality of fruit under various environmental constraints (Freeman and Robbertse, 2003). Girdling can be applied to different organs of trees, such as stems, main branches and smaller branches. Girdling, which increases fruit set and yield, is an agricultural practice that may improve carbohydrate balance and increase carbohydrate availability (Peng and Rabe, 1996). Spring girdling, i.e. pre-bloom removal of a wide strip of bark without injuring the xylem, is widely used in citrus species to increase fruit set and size as well as fruit quality (Mostafa and Saleh, 2006). Girdling treatments increase the fruit retention rate and also reduce the fruit drop before harvest, in addition to increasing parthenocarpy, which enables the production of seedless fruits.

Potassium (K) plays an important role and is considered a key element for fruit production and maintenance of fruit quality worldwide. Potassium is most important for external aspects of fruit quality. Low K levels result in small fruits, which are rejected by the fresh fruit and export markets, in spite of their thin rinds and good colour (Erner et al., 2005). Foliar application of nutrients has gained importance in recent years as a tool with which to rectify the deficiencies in nutrients, as sometimes soil application is not effective due to adaphic and environmental hazards. Availability of nutrients through foliar application is easy and quick to the plants. It has been found that foliar application of potassium after flowering increased fruit size and yield of citrus (Vijay et al., 2017).

In this view, we aimed to improve the fruit size and quality of the ‘Fremont’ mandarin variety by girdling and foliar K treatments as well as their combinations in the present study. Additionally, the effects of various girdling and K treatments on the leaf sugar content, nitrogen (N) content, leaf chlorophyll fluorescence and leaf chlorophyll concentrations of the ‘Fremont’ mandarin variety were investigated in this study.

The effects of different treatments on the bloom drop rate in ‘Fremont’ mandarins grown under the conditions in Dortyol were found to be statistically significant. The highest bloom drop rate was determined in control (C) trees (67.91%). The lowest bloom drop rate was obtained from the SG treatment (55.64%). In terms of the fruit retention rate, the SG treatment (44.36%) yielded the highest value while the C treatment (32.09%) yielded the lowest value (Table 2). The SG treatment had the highest fruit retention, which was dependent on its lowest bloom drop rate. The results obtained by Ibrahim et al. (2016) from the ‘Washington Navel’ variety, indicating that girdling treatment increased fruit retention, support our findings.

According to the observations performed on the tree following the June fruit drop, the remaining fruit rate was highest in the DG (56.18%) treatment and lowest in the SG + KNO₃ (25.65%), SG (26.53%) and KNO₃ (27.70%) treatments, which were statistically significant in the same group. The results revealed that the June fruit drop was the lowest in the DG treatment (Table 2). Yesiloglu (1999) also reported that the rate of fruits remaining after the June fruit drop as a result of the girdling treatment on ‘Clementine’ mandarins was highest in the DG treatment.

The fruit yield per tree was highest in the DG (337.50 kg · tree⁻¹) and SG (335.00 kg · tree⁻¹) treatments, whereas it was the lowest in the C (240.00 kg · tree⁻¹) treatment (Table 3). Tuzcu et al. (1992) indicated that DG at the beginning of anthesis on ‘Clementine’ mandarins increased yield when compared with trees that were not subject to girdling; and Rivas et al. (2006), indicated that girdling in ‘Fortune’ and ‘Satsuma’ mandarin varieties could increase the time-dependent impact and yield in varieties with different parthenocarpic abilities. Yesiloglu et al. (2017) analysed the impacts of certain cultural treatments on ‘Star Ruby’ grapefruit and reported that DG and DG + KNO₃ treatments increased yield relative to the control. Yilmaz et al. (2018) also reported that the DG treatment in ‘Robinson’ mandarins provided a greater yield than in the SG treatment and the control. In this study, we also obtained the highest fruit yield from the DG treatment, and it was determined that the yield increased at a rate of 40% per tree relative to the control, and the findings were similar to the abovementioned studies.

Fruit size is an important external quality factor in citrus marketing. In general, the fruit diameter classification of this mandarin variety is 3rd grade (54–64 mm) under the ecological conditions with which this study was conducted. The DG + KNO₃ (57.79 mm), KNO₃ (57.43 mm) and DG (57.28 mm) treatments yielded the highest values in terms of this characteristic (Table 3). In addition, Mostafa and Saleh (2006) reported that the girdling treatment increased fruit size; the findings of Yesiloglu et al. (2017), indicating the positive impact of K and 3,5,6-TPA + DG treatments on the fruit size of the ‘Star Ruby’ grapefruit variety relative to the control, supported our study results. The fruit rind was found to be the thinnest in the DG treatment (2.49 mm) and the thickest in the DG + KNO₃ treatment (3.58 mm). The combined effect of the girdling treatment and KNO₃ treatment increased the fruit rind thickness by 34%. Cicala and Catara (1994) reported that there was a positive correlation between the fruit weight, rind thickness and SSC content of KNO₃ treatments in the ‘Taracco’ orange variety. Similar to our findings, the result obtained by Yilmaz et al. (2018) as a result of using DG treatment in comparison with other treatments (SG, control) in the ‘Robinson’ mandarin variety was fruits with thinner rinds.

Girdling and KNO₃ treatments did not significantly affect juice content (%), SSC (%) and acid (%) content or SSC/acid ratio in the ‘Fremont’ mandarin variety (Table 3). Verreynne et al. (2001) reported that the stem girdling treatment did not impact the internal fruit quality in the ‘Marisol’ variety. Roussos and Tassis (2011) considered the impact of a girdling treatment on the titratable acidity and ripeness index in ‘Nova’ mandarins to be insignificant. Based on previous studies and our findings, it was determined that treatments did not have a significant effect on internal fruit quality characteristics.

During the study, while similarities were found between treatments in terms of L and a* characteristics of fruit rinds, differences in terms of b* and h° values were found to be statistically significant. The b* value was highest in the C treatment and lowest in the KNO₃ and DG + KNO₃ treatments. The h value was highest in C and lowest in KNO₃ (Table 4). Voss (1992) reported that the colour turned from yellow to red as the h° value decreased from 90° to 0°. Accordingly, the fruit rind was found to be close to deep red orange in colour in the KNO₃ and DG + KNO₃ treatments. Mataa et al. (1998) determined that girdling enhanced fruit rind colour during the full bloom period of the ‘Ponkan’ mandarin.

Chlorophyll fluorescence efficiency yielded a value between 0.6980 and 0.7560 in the various treatments done (Figure 3). Rivas et al. (2007) indicated that girdling did not have a significant impact on the PSII efficiency on mature leaves. Yilmaz et al. (2018) reported that a DG treatment on ‘Robinson’ mandarins displayed a tendency to reduce the PSII efficiency relative to the control. Further, such DG treatments resulted in a variation in the leaf chlorophyll content, and this variation was in the range of 62.08–70.67 (Figure 4). Yilmaz et al. (2018) reported that the impact of different treatments (SG and DG) varied the chlorophyll content between 52.13 and 68.84 in 22-year-old ‘Robinson’ mandarins. It is believed that the partially high SPAD values in our study could have been caused by the variety, ecology and leaves on which measurements were performed.

Figure 3

Figure 4

The leaf N content of the ‘Fremont’ mandarin variety significantly differed depending on the treatment. The N level was highest in the KNO₃ treatment (2.73%) and lowest in the C treatment (2.30%) (Table 5). Rivas et al. (2006) also determined that the distribution of mineral nutrients and growth regulators in the plant could vary after the performing girdling on trees. This study also found that October N values in ‘Fremont’ mandarins were higher in the treatments than in the control. Mostafa and Saleh (2006) reported that blooming and fruit retention increased in the section of plants that are above the girdling location, depending on the increases in the leaf N content and the C/N ratio.

Modern fruit growers attempt to manipulate the source–sink relationship to guarantee adequate fruit production and quality. The relationship between photosynthetic and non-photosynthetic tissues is less pronounced in fruit trees. Leaves are the most important organ for photosynthesis and leaves accumulate CH at a high rate. Simple sugars, such as glucose, are the principal products of photosynthesis. Soluble CH in fruit trees is composed of monosaccharides (normally glucose and fructose) and oligosaccharides (mainly sucrose). Different environmental factors influence the source–sink relationship (Fischer et al., 2012). Cultural treatments such as girdling may increase existing carbohydrate distribution, fruit retention rate and fruit yield (Goren et al., 2003; Rivas et al., 2004). In this regard, upon analysis of the leaf sugar contents in the study, significant differences were observed between treatments. However, the leaf sugar contents were impacted differently by the treatments. In this sense, it was seen that (1) the DG treatment increased the fructose content; (2) the DG + KNO₃ treatment increased the glucose content; and (3) the SG treatment increased the total sugar content significantly compared with other treatments (Table 5). Rivas et al. (2007) determined in their study that, in certain cases, girdling treatment statistically impacted leaf sugar contents at different branches and ages in the ‘Loretina’ mandarin variety relative to the control. This study also found that the DG treatment had a higher impact on fructose content than the other treatments. The varying impacts due to different treatments on the accumulation of other sugars in leaves could have been caused by the differences in the synthesis of sugars in the plant, their transformation into each other, fruit load of tree, vegetative development and climate conditions.

This study analysed the impacts of girdling and potassium applications on blooming and retention rate, fruit yield and quality, leaf sugar and N content. It was determined that the treatments performed under the scope of the study reduced the bloom drop ratio and increased the fruit retention rate relative to the control, and the girdling treatment yielded the highest value in this regard. It was also determined that the amount of fruit remained in a tree after the June fruit drops was the highest in the DG treatment. Single or DG treatments increased the fruit yield per tree by almost 40% relative to the control. It was determined that the highest values were yielded by the DG + KNO₃ treatment in terms of fruit weight and treatments involving separate and combined performances of this procedure in terms of fruit diameter. The impact of the SG treatment on yield was not reflected in fruit size relative to that in the C. In this sense, studies on fruit thinning may be required. In terms of fruit juice and quality, the treatments yielded similar results as the control. The impact of treatments on leaf N content was found to be statistically significant. The highest nitrogen content was found in the foliar KNO₃ treatment. The performed applications did not have any negative impact on the photosynthetic capacity of plants; therefore, these variables were included in the study. It was concluded that the SG treatment increased the total sugar content in leaves.