Effect of Hydrogen Cyanamide on Vegetative Growth, Yield, and Fruit Quality of Fig cv. Zidi in a Warm Production Area

ABSTRACT

The fig tree (Ficus carica L.) is widely cultivated in the Mediterranean region under warm climates. Fig yield is governed by the previous crop load and is strongly dependent on shoot length. Special cultural practices are required to stimulate new vegetative growth that will ensure regular and high productivity. The present study focused on the use of the rest breaking agent hydrogen cyanamide (HC) as Dormex to enhance vegetative growth, which influences the yield. A field trial was held in northern Tunisia (36° 49′ N; 9° 48′ E) on mature fig trees cv. Zidi. In reference to untreated trees as control, two hydrogen cyanamide treatments at 1% and 1.5% HC were considered. Bud break rate, vegetative growth, starch content of shoots, yield, and fruit characteristics were assessed. Results indicated that 1.5% HC significantly increased shoot length, fruit number per shoot, and yield. It advanced bud break by 10 days and fruit ripening by 7 days compared to the control. However, leaf number, leaf area, and starch content seem to be unaffected by hydrogen cyanamide treatments. Regarding fruit quality, a 1.5% HC dose significantly increased dry matter content, while fruit diameter and total soluble solids remain unaffected. In conclusion, hydrogen cyanamide could be used under warm conditions to improve vegetative growth of fig trees cv. Zidi and consequently fig productivity. Application of 1.5% HC 40 days before bud break was the most effective in increasing shoot length and yield.

KEYWORDS:

• Ficus carica
• hydrogen cyanamide
• shoot length
• fig yield

Introduction

The fig tree (Ficus carica L.) is a subtropical species widely cultivated around the Mediterranean areas since ancient times. Tunisia is a favorable country for fig growing. Tunisian fig cultivars are numerous and well adapted to local agro-ecological conditions (Essid et al., 2015; Gaaliche et al., 2012). Some are of the Common type, which produce parthenocarpic figs without caprification (pollination), whereas others are the Smyrna type that require caprification (Gaaliche et al., 2011a). Fig cultivars may produce one or two crops a year. The first crop (also called Breba crop) is born laterally from buds produced at the leaf axils along the previous season shoot. The second crop (also called Main crop) is produced laterally at the axils of leaves along shoots of the current season (Flaishman et al., 2008). Fig yield strongly depends on the balance between vegetative growth and fruiting. The number of reproductive buds and fruits of Breba and Main crops are closely related to shoot length (Gaaliche et al., 2011c). Breba crop is dependent on one-year-old shoot length, whereas Main crop is highly correlated with shoot length of the current season and indirectly influences Breba crop for the next year (Gaaliche et al., 2011c). Therefore, to ensure regular and high productivity of fig trees, effective cultural practices, such as pruning, are required to increase fruit size and prevent alternate bearing (Puebla et al., 2003). Also, the caprification of Smyrna type fig trees is required and is a major factor affecting yield and fruit quality (Gaaliche et al., 2011a, 2011b; Pourghayoumi et al., 2012).

Rest-breaking agents were developed to temper the negative consequences of lack of chilling and to promote tree growth (Campoy et al., 2011). Among growth regulators, gibberellins and cytokinins were applied to break the dormancy and stimulate vegetative growth of deciduous fruit trees (El-Agamy et al., 2001; Mohamed and El-Sese, 2004; Shaltout and Unrath, 1983). Hydrogen cyanamide (Dormex) has been found to be the most useful dormancy breaking agent having a synergistic effect on bud break, flowering, vegetative growth, yield, and fruit quality (Ben Mohamed et al., 2010; Ghrab and Ben Mimoun, 2014; Pérez et al., 2008; Seif El-Yazal and Rady, 2012). Hydrogen cyanamide in the range of 1–2% HC was successfully applied on many fruit trees (Bartolini et al., 1997; Rahemi and Asghari, 2004; Shulman et al., 1986), however, higher concentrations lead to a marked advancement of leafing over bloom of stone fruit species, which may have negative effects on fruit set due to sink competition (Erez, 1995). Furthermore, stone fruit trees are prone to phytotoxicity when concentrations above 2% are used. Bound and Jones (2004) found that 1.5% HC advanced apple flowering when applied at 40 or 30 days before the estimated bud burst. Ghrab and Ben Mimoun (2014) reported that 2% HC sprayed 45 days before bud break was effective in improving pistachio productivity and preventing anomalies of lack of chilling. Similarly, 2% HC applied 45 days before bud break to 'Superior Seedless' grape vine advanced maturity and improved berry size (Ben Mohamed et al., 2010). In fig tree, the combination of HC and oil decreased bud break, thereby increasing shoot length and productivity (Gerber et al., 2010). Yablowitz et al. (1998) reported that early application of HC caused earlier bud break and harvest of the Breba crop, while also increasing the number of Breba figs.

In Tunisia, Zidi is the main fig cultivar of the Smyrna type grown commercially. It produces only one Main crop on current season shoots. Early vigorous shoot growth will give rise to a large number of syconia, which can give a large crop if caprification is done properly (Gaaliche et al., 2011a). Furthermore, HC can hasten fruit maturity allowing the grower to take advantage of the premium prices of early season. The purpose of this study was to investigate the effect of hydrogen cyanamide on bud break, shoot growth, and yield in fig trees cv. Zidi in a warm production region.

Materials and methods

Plant material

A field trial was conducted in the region of M’hamdia (36° 49′ N; 9° 48′ E) in northern Tunisia. The climate of this region is semi-arid with mild winters and hot summers. Average rainfall is 439 mm per year and the main precipitation occurs during late fall and winter. Average minimum and maximum temperatures are, respectively, 19 and 32 °C during the growing season. Annual mean temperature is around 21 °C. The soil is silty-clay, saline, and poor in organic matter. Twenty-year-old fig trees cv. Zidi planted at 6 m × 5 m spacing were used. The caprification of the trees was practiced at two times to ensure the main crop on Smyrna-type cv. Zidi (Gaaliche et al., 2011a). Other agricultural practices, such as pruning, fertilization, and irrigation, were done by fig growers according to standard practices in the area.

Chemical treatments

The chemical rest breaking agent Dormex, which contains 50% v/v HC, was assessed on the fig trees cv. Zidi described above. Three treatments were applied as: (i) untreated control, (ii) 2% Dormex (1% HC), and (iii) 3% Dormex (1.5% HC). The experimental design was completely randomized with three blocks. Each block consisted of nine single replicated trees for each treatment. The HC solution was sprayed on the whole tree in combination with mineral oil Agral. The spray applications were done on 24 Feb., approximately 40 days before bud break.

Agronomic survey

Four shoots of similar size (about 40 cm long) were selected on each tree of the trial during the dormancy period. Bud break percentage, apical shoot growth, number of laterally growing shoots and leaf number were recorded for each treatment. The leaf area evaluation was done by picking up 20 leaves per treatment using a planimeter (Type AM 200, ADC Bioscientific Ltd., Hoddesdon, UK). The productivity of fig trees was expressed by the average number of fruits counted on shoot portions of 20 cm (fruit number per 20 cm). The evolution of crop load of figs at different dates was followed and total yield (kg) per tree was determined. At harvest (commercial stage of maturity), 20 fruits per treatment were randomly collected to assess fig quality. Measurements concerned fruit diameter (mm), dry matter percentage, and total soluble solids TSS (°Brix) determined with a digital refractometer (PR-101 ATAGO, Norfolk, VA, USA). At the end of the growing season, current shoots were sampled during dormancy one year after Dormex treatments for starch analysis. Samples of current shoots were oven-dried at 70 °C for 48 h and then crushed separately. Starch content was determined following the Nielson (1943) method as modified by Pesis et al. (1978).

Statistical analysis

Data collected during the experiment were subjected to one-way analysis of variance (ANOVA) using PC software package SPSS (version 13.0; SPSS Inc., Chicago, IL, USA). The mean values were compared using Duncan’s multiple range test (P < 0.05).

Results and discussion

Dormancy release and vegetative growth

The hydrogen cyanamide treatments had consistent effects on bud break and vegetative growth of fig trees cv. Zidi. The application of 1.5% HC advanced the bud break date, which indicated the increase in the percentage of bud break by 10% on 29 Mar. compared to the control and 1% HC treatments (Figure 1).

Figure 1. Bud break rate of fig trees cv. Zidi in response to hydrogen cyanamide (HC) treatments (1% and 1.5% HC). Vertical bars represent S.D. (n = 4).

However, final bud break rates did not differ among the three treatments with a maximum rate of 40% (Figure 1). Similarly, a significant effect of 1.5% HC application on vegetative growth was obtained (Table 1).

Table 1. Effect of hydrogen cyanamide (HC) treatments on vegetative growth and starch content in fig trees cv. Zidi.

	Shoot length (cm)	Leaf number per shoot	Leaf area (cm^2)	Starch content (mg g^–1 DW)
Control untreated	18.5 ± 1.5b^z	12.5 ± 1.5	355 ± 9.5	7.0 ± 0.2
1% HC	20.0 ± 1.0b	12.5 ± 1.3	351 ± 8.2	7.3 ± 0.3
1.5% HC	28.8 ± 0.9a	14.8 ± 0.8	351 ± 9.0	8.0 ± 0.8
F-value	70.25^y	3.56 NS^x	0.20 NS	3.66 NS

^zMeans in each column followed by the same letters are not significantly different at P < 0.05 according to Duncan’s multiple range test.

^ySignificant at P < 0.01.

^xNS: not significant.

Treated trees with 1.5% HC produced the longest shoots (28.8 cm) compared to other treatments. By contrast, hydrogen cyanamide applications did not significantly affect the number of leaf per shoot, leaf area, and starch content (Table 1). Under the warm climate of northern Tunisia, HC application at 1.5% seems to be effective in advancing for bud break and increasing shoot length of fig trees cv. Zidi. Similarly, a rate of 1.5% HC has been successfully used in fig to force bud break and schedule fruit ripening over the season (Yablowitz et al., 1998). However, Theron et al. (2011) reported that 1% HC with mineral oil increased bud break in fig cv. Noire de Caromb but had a negative effect on shoot length. Previous findings were reported for early and uniform bud break in fig tree with application rates of 0.25–1.25% HC (Shulman et al., 1986). Other reports revealed conflicting results for starch content. No effect of hydrogen cyanamide on starch content was observed for pistachio trees with 1% and 2% HC (Ghrab and Ben Mimoun, 2014), while an increase of starch content of peach trees in response to 1% HC application was noted (Dbara and Ben Mimoun, 2009). These different results of the effect of cyanamide applications on bud break and vegetative growth could be related to cultivar and experimental conditions. Nevertheless, gaps remain in our knowledge regarding not only the physiology of cyanamide hydrogen action but also the timing and method of its application (Campoy et al., 2011). Generally, to reach maximum efficiency of rest-breaking agents, applications should be performed with other cultural practices, such as pruning (Puebla et al., 2003). In this context, Norberto et al. (2001) confirmed the beneficial effects of 2% hydrogen cyanamide and the anticipation of pruning time. Fig plants treated with hydrogen cyanamide, pruned in May, provided a first crop of green figs early in the off season, which allowed the anticipation of the pruning period, associated to the use of the compound.

Yield and fruit quality

Hydrogen cyanamide spray had a significant effect (P ˂ 0.01) on fruit number per shoot and yield. The fruit number per shoot varied according to the treatments with the highest fruit number (6.8 fruits per shoot) recorded with the 1.5% HC treatment (Table 2).

Table 2. Effect of hydrogen cyanamide (HC) treatments on productivity, yield, and fruit quality in fig trees cv. Zidi.

	Fruit number per shoot	Yield (kg)	Fruit diameter (cm)	TSS (°Brix)	Dry matter rate (%)
Control untreated	3.9 ± 0.2b^z	17 ± 0.6c	4.5 ± 0.5	20.2 ± 0.4	20.4 ± 0.6b
1% HC	4.1 ± 0.2b	20 ± 0.9b	4.6 ± 0.6	19.6 ± 0.6	24.7 ± 0.7a
1.5% HC	6.8 ± 0.3a	22 ± 1.0a	4.5 ± 0.5	20.8 ± 1.1	25.0 ± 0.4a
F-value	196.0^y	35.1^y	0.1 NS^x	2.0 NS	71.3^y

^zMean in each column followed by the same letters are not significantly different at P < 0.05 according to Duncan’s multiple range test.

^ySignificant at P < 0.01.

^xNS: not significant.

Regarding the evolution of crop load of figs, an earlier production was noted between 18 and 20 Aug. in treated trees compared to control, which reached the maximum production one week later (Figure 2).

Figure 2. Progression of the crop load of figs based on hydrogen cyanamide (HC) treatments (1% and 1.5% HC) in fig trees cv. Zidi. Vertical bars represent S.D. (n = 4).

The highest yield level (22 kg) was achieved with 1.5% HC (Table 2). This result corroborates with the previous finding, which report that the use of hydrogen cyanamide enabled a high Breba fig production in cv. Nazareth (Erez et al., 2003; Yablowitz et al., 1998). Similarly, Pasqual et al. (2003) reported that fig trees of cv. Roxo de Valinhos treated with 2% HC presented the highest number of fruits per shoot and a greater branch length, allowing a higher yield. In this context, Flaishman et al. (2008) showed that with main crop-producing cultivars, such as Kadota and Brown Turkey, application of hydrogen cyanamide is recommended. It will affect productivity by stimulation of new shoot growth that increases the Main crop. Concerning fruit quality, no significant effect of hydrogen cyanamide was observed on fruit diameter and TSS for fig trees cv. Zidi (Table 2). By contrast, the application of 1.5% HC increased significantly fruit dry matter compared to control and 1% HC treatments. Previous results do not corroborate with those of Leonel et al. (2015) who reported that fig trees of cv. Roxo de Valinhos sprayed with 2% HC showed a significant increase of fruit weight and diameter. In our study, it appears that yield and fruit quality performance of Smyrna-type cv. Zidi remain mainly dependent on the caprification efficiency, i.e., pollination intensity, frequency, and pollen source, as reported by Gaaliche et al. (2011a, 2011b).

Conclusion

In warm production areas, the use of hydrogen cyanamide seems to be an effective cultural practice to stimulate vegetative growth of fig trees cv. Zidi, resulting in a higher Main crop figs load and early fruit ripening. Application of hydrogen cyanamide at 1.5% increased significantly vegetative growth and consequently gave a higher yield. The efficiency of HC sprays should be checked also in Common type fig bearing two crop generations and it becomes imperative to extend HC applications on several fig cultivars. Furthermore, more investigations are required to assess the major role of other rest-breaking agents and their interactions with other cultural practices (pruning, fertilization, irrigation, etc.) in warm fig production regions.

Acknowledgment

We would like to thank the manager and technicians of the farm SMVDA ‘El Kheir’ for their efficient collaboration.