Research Article

Horticultural Science and Technology. 30 April 2022. 121-133
https://doi.org/10.7235/HORT.20220012

ABSTRACT


MAIN

  • Introduction

  • Material and Methods

  •   Plant Material and Methodology

  •   Pollination

  •   Measured Parameters

  •   Statistical Analysis

  • Results

  •   Fruit Set and Retention Rate

  •   Effect of Pollination Techniques on the Pomological and Physicochemical Characteristics of Dates

  •   Effect of Pollination Methods on Date Palm Seeds

  •   Effect of Pollination Techniques on Dates Chemical Characteristics

  • Discussions

  • Conclusion

Introduction

The date palm (Phoenix dactylifera L.) is the main component of the oasis ecosystem allowing life sustainability in desert and creating microclimates favorable for the flora and fauna of the Saharan agriculture. Tunisia has nearly 4% of the worldwide date palm plantations representing 57 000 ha (APII, 2017). Palm trees constitute a tremendous source of food, commercial profitability, clothing, and craft equipment.

The date palm is a dioecious species, where pollination requires wind or manual labor involvement to pollinate the female flowers. For a along time, handmade traditional pollination techniques have been used to overcome dichogamy (Munier, 1973; Salomón-Torres et al., 2021) impacting date fruit quality. Hussein and Hussein (1983) and Hamood et al. (1986) have reported that pollination can be released either manually (traditional pollination) or mechanically. With more than 5 million palm trees and more than 200 varieties (Reynes et al., 1995; ONAGRI, 2016), Tunisia is recognized as one of the main date producers and exporters worldwide (ONAGRI, 2019). However, in these areas, the male trees, locally named as ‘Dokkars’, are not labelled and their multiplication is often released by seeds (El Kadri et al., 2019). However, in other phoenicultural countries ‘Dokkars’ are selected from the best female varieties and their multiplication is realized by offshoots (Babahani, 1991). Examples have been named as ‘Siwi’, ‘Sammani’ and ‘Zuegloul’ (Bacha, 2001).

The date palm yields are due to several factors such as, fertilization, irrigation and mainly traditional methods of pollination (Alikhani-Koupaei et al., 2018; Kadri et al., 2019). The date palm is naturally wind pollinated, but this method is not recommended, especially for ‘Deglet Nour’ variety. This method requires a very high number of pollinators and a very close distance between cultivars, in which the fruit setting rate does not exceed 30% (El-Mardi et al., 1998). In fact, the failure of pollination induces a high rate of parthenocarpic fruits (>70%) with no economic value (Zaid and De Wet, 2002). Moreover, date palm pollination is affected by several factors such as pollen sources, pollen quality (viability and germination), pollination time, pollination methods and receptivity of female flowers (Zaid and De Wet, 2002; Ahmed et al., 2016; Abdel-Sattar and Mohamed, 2017; Kadri et al., 2019; Kadri et al., 2022). It is worthy to note that date palm pollination methods are very important in improving fruit yield and quality, Khan and Khan (1993), Iqbal et al. (2010) and El-Dengawy (2017) have reported that pollination techniques have a direct effect on quality and yield. Indeed, artificial methods are the best compared to traditional and natural pollination in the date palm cv. ‘Dhakki’ concerning the percentage of fruit set, the number of fruits per bunch and the physical properties of the fruits. Likewise, Shaaban et al. (2019), Munir (2019) concluded that pollination by suspension of pollen grains or by dusting improved the yield and quality of date palm cv. ‘Said’.

The present research work aims to evaluate the effect of different pollination techniques on the agronomic characteristics (fruit set rate, retention rate and fruit yield) and also on the physicochemical characteristics of fruit at different stages of ripening (Kmiri, Khalal and Tamar) from the date palm cv. ‘Deglet Nour’ under oases agro-climatic conditions of Tunisia. The results obtained are meant to orient the farmers towards the most reliable technique in terms of quantity and quality.

Material and Methods

Plant Material and Methodology

The study was carried out at the Regional Research Center on Oases Agriculture of Dgueche (Latitude: 33°58'40.0"N, Longitude: 8°12'32.2"E, altitude 60 m) located in the south-west of Tunisia during 2020 cropping season. The climate of the study region is arid with hot summers. During the experiment season, the annual rainfalls in the area under study were about 50 mm, and the average temperature varied between 8.2 and 46.6°C (Table 1).

Table 1.

Climatic information of the research site during the experimental year 2020

Parameters Temperature °C Relative humidity
(%)
Wind speed
(m/s)
Precipitation
(mm)
Growing season max min Average
March 30.3 8.2 19.2 45.8 0.6 3.1
April 35.1 10.7 22.0 45.6 0.6 3.8
May 43.6 16.3 28.9 34.7 0.6 0.2
June 46.6 19.4 33.5 31.8 0.7 0
July 46.6 21.5 32.4 33.2 0.5 0.6
August 46.4 21.8 33.5 33.4 0.4 0
September 41.7 17.9 28.9 44.0 0.5 0.4
October 36.3 9.74 22.1 48.4 0.4 1.4

The male spathe samples were collected from one common pollinator (P7), which was selected on the basis of the following characteristics: (i) significant ability to produce inflorescences (more than 35 spathe male), (ii) Germination and pollen viability rate being higher than 85% and (iii) A high pollen yield. The female feet of the variety ‘Deglet Nour’ was 15-year-old and received standard and uniform cultivation techniques (irrigation and fertilization). The fresh male inflorescences were collected from the male spathe and then cut into separate spikelets and dried at room temperature (22–25°C) for 48 hours in the shade. The spikelets were then shaken manually to obtain the pollen powder which was collected in glass boxes and then stored at room temperature for imminent use. The two cultivars used (male and female) belong to the experimental plot of the regional oasis agriculture research center.

Pollination

The pollen was taken from the same pollinator (P7) to prevent any metaxenic effect. The pollination was done during the first week of April 2020, under full sun and low wind. Thus, one female date palms ‘Deglet Nour’ was selected with 15 years old and undergoing standard and uniform cultivation techniques (fertilization and irrigation). Four female spathes from the principal flowering flux were selected and covered with Kraft paper to avoid cross-contamination from other pollens, for each technique one spathe was used, the three techniques are applied on the same female feet to avoid the physiological effect. The fourth spathe is used as negative control (without pollination). Three pollination techniques were carried out in this study (Fig. 1), the first of which was the handmade traditional pollination technique (PT) which involved the attachment of 5 mature male spikelets (with an average of 5 mg of pollen per strand) in the opposite direction of the female spikelets. As for the second technique, it was the pollen dusting technique (PP), corresponding to the dispersion of a mixture of pollen grains / talcum (1:8) on to female spathe, where one gram of pollen was weighed and adjusted to a determined volume to which 7 portions of talcum carrying the same volume of pollen were added and well mixed. Concerning the third method, it was pollen suspension spraying (PS) which consists in spraying a 10 mL of water-pollen mixture (1g in 1l of deionized water) over the entire female spathe using handled sprayer.

https://static.apub.kr/journalsite/sites/kshs/2022-040-02/N0130400201/images/HST_40_02_01_F1.jpg
Fig. 1.

The different methods of pollination. (A) Traditional pollination: PT (B) pollen dusting: PP (C) Pollen suspension: PS.

Measured Parameters

The fruit set was determined two months after pollination (in the middle of the ‘Kimri’ stage) (Hussain et al., 2020). Thus, five spikelets were randomly selected and labeled with three repitions (total of 15 spikelets) per bunch and pollination technique. The numbers of attached and fallen fruits were counted and the ratio of fruit set was calculated according to El-Salhy et al. (2010). The fruit retention rate was calculated at the middle of stage ‘Khalal’ using the following formula : the number of retained fruit / total number of fruit set. The fruits of each pollinated bunch were weighed at the end of ‘Tamar’ fruit growth stage to evaluate date palm yield.

Date palm quality was measured on 15 randomly collected fruits as: total fruit weight, pulp weight and thickness, seed weight, length, width, fruit diameter and volume, seed weight, length and width. Fruit moisture content, in the flesh of the fruit was determined as the difference between the weight of fresh matter (FM) and the weight of dry matter (DM), until a constant weight was reached according to the following formula: H% = (FM –DM)/FM × 100 ). Besides, the pH was determined by heating a mixture of 10 g of date with 30 mL of distilled water for 30 min at 100°C, the mixture was then ground. The filtrate was used to determine the pH using a Consort C6010 pH-meter. The total soluble solids (°Brix) was recorded using the date juice filter obtained after dilution, the reading was made on Sopelem 7493 hand refractometer. Titratable acidity was determined according to standard methods (George, 2016). Water activity (AW) was measured at 25°C with a Novasina aw-meter.

Statistical Analysis

The analysis of variance (ANOVA) was performed for all collected data, according to a completely randomized design with five replicates per treatment. Besides, the least significant differences (LSD) at α=5% was used to compare the means of treatments. With respect to the statistical analyses, they were performed using STATISTICA 5.0 As for the box-plots of the different agronomic parameters; they were performed using R software (Version 1.2.5001). Furthermore, the PCA analysis was carried out using SPSS software (16.0 13 Sep. 2007, SPSS Inc.).

Results

Fruit Set and Retention Rate

The effect of pollination techniques on date palm fruit set are shown in Table 2. The different pollination techniques were proven to have significantly affected the fruit set percentage (p < 0.001).While the highest fruit setting rate of 82.21% was recorded for the dusting pollination method (PP), the lowest one of 40.04% was recorded for water spraying pollen suspension method (PS). Concerning the fruit retention was significantly affected (p < 0.001) by the pollination technique (Table 2). Whereas the (PP) method improved the fruit retention rate by 10% compared to the traditional pollination method (PT), the spraying method produced the lowest retention rate (15.97%). As regards the bunch weight at ‘Tamar’ stage, dusting pollination method gave the highest yield (5.5 kg per bunch) as a direct consequence of high fruit set and retention rates (Table 2). The traditional pollination techniques led to an average yield of 4 kg per bunch while (PS) recorded the lowest yield of 2.7 kg per bunch due to the high parthenocarpic fruit rate. The effect of pollination techniques on fruit types depending on pollination techniques is shown in Fig. 2. The number of seeded fruits obtained from mechanical pollination (PP) is much higher (97%) than those of traditional pollination (75%). Furthermore, water spraying technique induced a higher number of seedless fruits (more than 60% parthenocarpic fruits) than those obtained by mechanical and traditional pollination techniques (Fig. 2).

Table 2.

Influence of various pollination techniques on fruit set, fruit retention and yield of date palm cv ‘Deglet Nour’. PT: Traditional pollination, PP: pollen dusting, PS: Pollen suspension

Pollination method Fruit set (%) Fruit retention (%) Bunch weight (kg)
PP 82.21 a 68,34 a 5.50 a
PT 73.96 b 58,00 b 4.10 b
PS 40.04 c 15,97 c 2.71 c
P *********

Means within each column followed by the same letter are not significantly different at p < 0.05, P: level of effect significance.

***Significant at p < 0.001.

https://static.apub.kr/journalsite/sites/kshs/2022-040-02/N0130400201/images/HST_40_02_01_F2.jpg
Fig. 2.

Variation of fruit types according to pollination techniques. PP: dusting Pollination (Pollen / talcum), PS: Spray Pollination (Pollen / water), PT: Traditional (manual).

Effect of Pollination Techniques on the Pomological and Physicochemical Characteristics of Dates

The results obtained pertaining to the weight of the date is illustrated in Table 3 and in the Suppl. Fig. 1. It is noted that the weight of the fruits of the ‘Deglet Nour’ date was significantly influenced by the pollination method. Thus, the highest weights were recorded at the ‘Khalal’ stage with 13.5 g using the spray method, followed by the conventional method and dusting pollination with 11.68 g and 10.59 g, respectively. As for the ‘Tamer’ stage, the maximum weight was for the spray method (10.95 g), followed by pollination by dusting (8.16 g).The fruit length, width and diameter influenced by the pollination techniques at different fruit growth stages were recorded in Table 3 and in the Suppl. Fig. 1.6. The results showed that for fruit length indicated that pollination method had no significant difference at the three stages of fruit development stage. However, the width of the fruit was significantly influenced by the method of pollination during ‘Khalal’ stage with a maximum width obtained by suspension pollination technique (21.61 mm) (Table 3 and Suppl. Fig. 1.4). Regarding the fruit diameter, the analysis of variance showed significant variation during all stages of maturation. In fact, the dates pollinated by the (PP) and (PT) method were larger, longer and greater in diameter than those pollinated by the (PS) method. Besides, the statistical analysis revealed significant differences between pollination techniques for the volume of dates at the ‘Khlal’ and ‘Tamer’ stage. However, this difference was not significant for the ‘Kmiri’ stage. The highest fruit volume was obtained with the spray technique (33.98 mL) during the ‘Khalal’ stage (Table 3 and Suppl. Fig. 1.7). The maximum fruit flesh thickness was recorded in fruits developed from spray pollination followed by the (PP) and (PT) method at ‘Tamar’ stage. The results obtained by the principal component analysis of pollination techniques considering the pomological attributes are presented in Fig. 3. This figure shows that 63.45% of the total variation was explained by the first three principal components (PCA 1 = 29.55%, PCA 2 = 23.26% and PCA3 = 10.65%) which indicates a strong pattern of differentiation between techniques. The PCA 1 was highly correlated with ‘Khalal’ weight (+0.83), length (+0.73), width (+0.78), diameter (+0.72), volume (+0.87), ‘Tamer’ weight (+0.80), diameter (+0.66) and volume (+0.60). The PCA 2 was correlated with ‘Khalal’ width (–0.53), seed weight (–0.70), ‘Tamer’ length (+0.49), width (+0.45), seed length (+0.72), seed width (+0.54), seed weight (+0.65) and bunch yield (+0.76). The PCA 3 was correlated with ‘Khalal’ seed length (+0.77). According to the PCA plan 1–2, Fruits obtained by PS pollination technic were closely associated which indicate the homogeneity of the measured fruits. The obtained fruits by PS technique were characterized by a high ‘Khalal’ weight, length, width, diameter, volume, ‘Tamer’ weight, diameter and volume. However, the other pollination technics gave lower values of these ‘Khalal’ and ‘Tamer’ fruit parameters. The fruits obtained by PT and PP pollination technics were not tightly grouped but they were scattered along the PCA 2 and 3. This indicated the heterogeneity of the fruits produced after the latter pollination technique.

Table 3.

Effects of different pollination methods on physical characteristics of the date palm fruit ‘Deglet Nour’ at different development stage as ‘Kmiri’, ‘Khlal’ and ‘Tamar’. PP: dusting Pollination (Pollen / talcum), PS: Spray Pollination (Pollen / water), PT: Traditional (manual) pollination

Weight (g) Length (mm) Width (mm) Diameter (mm) Volume (mL) Thickness (mm)
‘Kmiri’
PP 9.19 b 40.46 a 18.58 a 33.4 a 25.22 ab 5.34 b
PT 9.47 b 40.36 a 20.05 a 35 a 21.77 b 5.95 a
PS 11.69 a 40.59 a 18.82 a 28.4 b 28.34 a 6.23 a
‘Khalal’
PP 10.59 b 36.59 b 17.22 c 30.4 c 21.14 c 5.30 b
PT 11.28 b 40.96 ab 20.25 b 33.1 b 33.98 a 6.28 a
PS 13.53 a 44.51 a 21.61 a 35.3 a 27.51 b 5.34 b
‘Tamar’
PP 8.16 b 37.09 a 14.89 a 3.18 a 17.32 a 4.13 a
PT 7.73 b 37.93 a 15.46 a 2.79 b 14.10 b 3.53 b
PS 10.95 a 35.95 a 15.01 a 2.61 b 15.06 b 4.22 a
Stages ***ns**********
Methods **nsns****ns
Stage × Methods **ns******ns

Means within each column followed by the same letter are no tsignificantly different at level p < 0.05.

NS: not significant,*Significant at p < 0.05, **Significant at p < 0.01,***Significant at p < 0.001.

https://static.apub.kr/journalsite/sites/kshs/2022-040-02/N0130400201/images/HST_40_02_01_F3.jpg
Fig. 3.

Representation of pollination technics on the plane 1–2 and 1–3 of principal component analysis.PT (Blue), PP (Green) PS (Golden Yellow). The PCA 1 was highly correlated with ‘Khalal’ weight (+0.83), length (+0.73), width (+0.78), diameter (+0.72), volume (+0.87), ‘Tamer’ weight (+0.80), diameter (+0.66) and volume (+0.60). The PCA 2 was correlated with ‘Khalal’ width (–0.53), seed weight (–0.70), ‘Tamer’ length (+0.49), width(+0.45), seed length (+0.72), seed width (+0.54), seed weight (+0.65) and bunch yield (+0.76). The PCA 3 was correlated with ‘Khalal’ seed length (+0.77). The PCA results showed that (PS) group was separated along PCA1. The obtained fruit by (PS) technique were characterized by a high ‘Khalal’ weight, length, width, diameter, volume, ‘Tamer’ weight, diameter and volume.

Effect of Pollination Methods on Date Palm Seeds

The effect of pollination methods on physical parameters of the dates seeds for each growth stage of fruit development are displayed in the Table 4 and in the Suppl. Fig. 2. No significant variation in seed weight was noted between the different pollination techniques throughout fruit ripening stages. Seed length was not significantly affected by pollination techniques for the development of fruit ripening stages ‘Kmiri’ and ‘Khalal’. The same observation was noted with no significant effects (p < 0.05) of pollination techniques at ‘Tamar’ stage growth, the longest seed length was obtained under suspension technique (22.76 mm) followed by the dusting (21.59 mm) and traditional technique (20.30 mm) at ‘Tamar’ stage, (Table 4 and Suppl. Fig. 2.4). Regarding the date seed width, no significant effect of pollination techniques was recorded on for the three stages. However, maximum width were noted at ‘Kmiri’ fruit stage under traditional technique (7.77 mm) followed by the suspension (7.65 mm) and dusting method (7.24 mm).

Table 4.

Effects of different pollination methods on the physical characteristics of ‘Deglet Nour’ seeds at different fruit growth development stage. PP: dusting Pollination (Pollen / talcum), PS: Spray Pollination (Pollen / water), PT: Traditional (manual) pollination

Techniques Seed weight (g) Seed length (mm) Seed width (mm)
‘Kmiri’
PP 1,46 a 27,93 a 7,24 a
PT 1,44 a 29,39 a 7,77 a
PS 1,70 a 28,83 a 7,65 a
‘Khalal’
PP 1,29 a 26,95 a 7,02 a
PT 1,38 a 26,15 a 7,11 a
PS 1,26 a 26,75 a 6,09 b
‘Tamar’
PP 0,79 a 21,59 ab 5,49 a
PT 0,60 a 20,30 b 4,96 b
PS 0,79 a 22,76 a 5,71 a
Stages ********
Methods nsnsns
Stage × Methods nsnsns

Means within each column followed by the same letter are not significantly different at level p < 0.05.

NS: not significant, *Significant at p < 0.05, **Significant at p < 0.01,***Significant at p < 0.001.

Effect of Pollination Techniques on Dates Chemical Characteristics

The total soluble solids (TSS) reflect the richness of the dates in carbohydrate content. The analysis of variance demonstrated the absence of significant effects of pollination on TSS (Table 5).The results showed that the pollination technique affects significantly (p < 0.01) acidity only at the end of the maturation stages as ‘Tamar’ growth stage. The highest acidity was obtained with spraying suspension pollination technique (0.20%) followed consecutively by handmade traditional (0.18%) and dusting pollination methods (0.17%) (Table 5).The dates moisture was affected significantly (p < 0.05) by pollination technique only at ‘Khalal’ fruit ripening stage (Table 5). Moreover, the highest fruit humidity (62.34%) was obtained under pollination method followed by dusting (61.34%) and traditional technique (61.11%) (Table 5).The influence of different pollination technique on the water activity was significant (p < 0.05) only for ‘Khalal’ fruit ripening stage with a maximum under both spraying suspension and traditional pollination technique (0.93) (Table 5).The obtained results showed that fruit pH underwent significant effects (p < 0.001) according to pollination techniques during all three stages of fruit growth.

Table 5.

Effects of different pollination methods on the chemical characteristics of ‘Deglet Nour’ dates at different development stage. PP: dusting Pollination (Pollen / talcum), PS: Spray Pollination (Pollen / water), PT: Traditional (manual) pollination

Treatments TSS (°Brix %) Acidity (%) moisture (%) Water activity pH
‘Kmiri’
PP 9,17 a 0,090 a 65,19 a 0,92 b 5,37 b
PT 7,73 a 0,098 a 65,01 ab 0,93 ab 5,52 a
PS 6,40 a 0,098 a 63,97 b 0,94 a 5,46 a
‘Khala’
PP 16,88 a 0,0.09 a 61,34 b 0,91 a 5,62 b
PT 20,82 a 0,10 a 61,11 b 0,93 a 5,96 a
PS 28,98 a 0,11 a 62,34 a 0,93 a 5,69 b
‘Tamar’
PP 19,33 a 0,17 b 20,88 a 0,64 b 5,67 a
PT 20,36 a 0,18 b 22,77 a 0,70 a 5,65 a
PS 18,20 a 0,20 a 22,22 a 0,65 ab 5,63 a
Stages ***************
Methods nsnsns*****
Stage × Methods ns**nsns***

Means within each column followed by the same letter are not significantly different at level p < 0.05.

NS: not significant, *Significant at p < 0.05, **Significant at p < 0.01, ***Significant at p < 0.001.

Discussions

The current investigation was the first study to unveil the effect of the pollination technique on the ‘Deglet Nour’ productivity. The obtained results confirmed that the pollination method affected the percentage of fruit set, fruit retention and yield. Indeed, the pollination dusting method (PP) was proven to have the best result compared to traditional pollination (PT) and spray pollination (PS). These results could be attributed to the non-homogenized mixture of water/pollen (Sedra, 2003). Besides, traditional pollination (PT) induces heterogeneous distribution of pollen grains to female flowers (Sedra, 2003). Therefore, dusting pollination method (PP) is among pollen grain homogeneity, ensuring a largest contact surface between female flower and pollen grain (Kadri et al., 2019). Our results are in agreement with those of Mostaan et al. (2010) for the date palm cultivar ‘Barhee’, for the cultivar ‘Hillawi’ (Ullah et al., 2018) and even for the variety ‘Deglet Nour’ (Nourani et al., 2017; Kadri et al., 2019).

Nevertheless, Shaaban et al. (2019) reported that the highest fruit setting rate in the variety ‘Saidy’ was obtained with manual pollination (86.2%) followed by dusting method (72.11%) then by water pollen mixture spraying (62.76%). Thus, the pollination of the variety ‘Khalas’ was more efficient with suspension technique (85.7%) followed by the dusting technique (82.07%) (Munir et al., 2020). It is trusty to note that the retention rate is considered as a good yield indicator (Sedra, 2003). According to El-Mardi et al. (2002), while manual pollination is among 21% drop in date palm fruits after the end of the ‘Kmiri’ fruit growth stage, dusting pollination induces 2% increase in fruit set. The manual pollination of ‘Medjol’ variety was among the highest fruit retention rate (885 fruit / bunch) compared to the suspension technique using 1g of pollen per liter (853 fruit / bunch) (Taleb and Mohamed, 2019). Similar results were observed for the variety ‘Khalas’ with maximum retention rate under water mixture spraying technique (85%) followed by the dusting pollination (83 %) (Munir et al., 2020). Moreover, our results are aligned with those of Munir et al. (2020) on the same date palm variety ‘Khalas’ where dusting method showed maximum yield per bunch (6.14 kg) followed by the suspension method (6.14 kg) then traditional technique (4 kg). The same results were obtained for the variety ‘Hillawi’ in the research work of (Ullah et al., 2018).

The results of the collected dates at the end of ‘Tamar’ fruit growth stage indicated the highest yield for dusting pollination method (5.5 kg per bunch) as a direct consequence of high fruit set and retention rates. The same results were observed for the bunch weight in the ‘Medjol’ and ‘Barhee’ varieties and the values were 22.3 kg and 48.1 kg, respectively, obtained after traditional pollination method (Taleb and Mohamed, 2019). Similar results were also obtained for ‘Lulu’ date palm cultivar, where traditional pollination technique led to 8.54 kg dates per branch, while the suspension method with the doses of 0.5, 1 and 1.5 g / l induced respective yields of 5.52, 6.21 and 7.19 kg dates per branch (Awad, 2011). The observed variability of yield across pollination technique emphasizes the importance of varietal component on date palm yield despite the applied pollination technique. Our study confirmed that the rate of parthenocarpic fruits depended on pollination technique. These results do not agree with those of Munir et al. (2020) who showed that the lowest level of parthenocarpic fruits was obtained with the suspension technique by spraying water, followed by the dusting and traditional pollination technique. The presence of parthenocarpic fruits is a real threat affecting the yield and market value of the date palm fruits. Thus, the spray pollination technique is economically the best pollination for farmers (Kadri et al., 2019). The development of the parthenocarpic in date palm depends on many environmental factors (Pandolfini et al., 2018) such as low temperatures (8–20°C) during pollination (Cohen et al., 2016) as well as male or female incompatibility (Zaid and Dewet, 2002) and hormonal deregulation (Jacobsen and Olszewski, 1993; Smith and Koltunow, 1999). In addition, the (PS) technique caused an increase in the average weight followed by powdering method. These results are closely related to fruit set rate. In fact, a negative correlation exists between the fruit set rate and the fruit weight. Similar results were obtained with the sprinkle pollination method for the ‘Khalas’ variety (Munir et al., 2020) and for the ‘Hillawi’ variety (Ullah et al., 2018). Other results have been recorded in previous studies. In fact, dusting pollination technique used for the ‘Hillawi’ variety, showed significant effects on the length and width of the fruit compared to manual pollination. Dusting technique offered the highest length (4.18 cm) and width (3.68 cm) values compared to other techniques (Ullah et al., 2018). Besides, Refay and Dengay (2017) working on ‘Hayany’ cultivar found that manual and dusting techniques have improved the physical characteristics of fruit. However, Awad and Al-Qurashi (2012) found no significant difference in fruit length, weight and diameter between the traditional and the dusting techniques.

The volume of dates at ‘Khalal’ and ‘Tamar’ fruit growth stage were significantly affected by the pollination techniques. The same results were observed for both ‘Medjol’ and ‘Barhee’ varieties (Abu-Zahra and Shatnawi, 2019). However, in previous studies, the maximum fruit volume of 10 mL and fresh fruit weight of 9.15 g were obtained under dusting pollination technique (El-Mardi et al., 2002). The same result was observed for the date palm ‘Khalas’ (Munir et al., 2020) and ‘Saidy’ varieties (Shaaban et al., 2019).The results of our study showed that there was no significant variation in the weight, length, and width of the seeds between the different pollination techniques throughout the stages of fruit ripening. These results are in accordance with those of Awad (2010) and Al-Qurashi (2011). However, other studies have shown contrasting results. In fact, Ullah et al. (2018) showed that the watering technique led to a higher seed weight (1.67 g) compared to the traditional technique (1.3 g). While Munir et al. (2020) found that the traditional method is among the highest seed weight (1.15 cm) and seed length (20.4 mm), followed by the traditional and the sprinkler pollination technique.

During our study, no significant effect of the different pollination methods was recorded on TSS, acidity and fruit humidity. These results agree with those of Awad and Al-Qurashi (2012) on the varieties ‘Sbbakah’ and ‘Nabut Ali’. However, other studies have shown that pollination techniques influence TSS like Ullah et al. (2018) who investigated the ‘Hillawi’ variety and found that the highest rate of TSS was obtained by the sprinkle pollination method (38.43%) compared to the traditional technique (35.77%). Likewise, Munir et al. (2020) reported that the highest TSS was recorded on the ‘Khalas’ variety with the sprinkle technique (71.62%) followed by the suspension (69.60%) and traditional pollination method (67.51%).

Regarding fruit acidity, previous studies such as Awad (2010) related to the ‘Kenazy’ variety and Awad and Al-Qurashi (2012) pertaining to the ‘Nabut Ali’ and ‘Sebbaka’ varieties, have shown that pollination techniques were not involved in fruit acidity. In fact, it is important to note that the acidity of date palm fruits can be affected by other factors such as variety, growing conditions, maturity, season, geographic origin, fertilization and soil type (Youssef et al., 1992; Ahmed et al., 1995; Al-Farsi et al., 2005). High acidity is often associated with poor quality. Booij et al. (1992) have proven that fruit acidity is negatively correlated with maturity. On the other hand, Munir et al. (2020) revealed that traditional pollination was among the maximum water contents of the ‘Khalas’ variety (18.04%), followed by dusting (15.71%) and the suspension spray pollination method (15.64%). According to Hussein and Hussein (1983), the moisture content of ripened dates depends on the frequency and volume of irrigation at the ‘Bser’ fruit ripening stage. During fruit ripening process, the water content decreases. Our results showed that sprinkle pollination (PP) improved water activity, especially for the ripening stage of ‘Kmiri’ fruits. These data agree with those of Besbes et al. (2009) who found that the water activity of dates is between 0 and 1. Furthermore, the fruit of the ‘Deglet Nour’ date is considered as a semi-soft cultivar, with a fruit moisture content ranging from 20 to 30% (Munier, 1973; Ibrahim and Khalif, 1998).

The essential aim of this study was to assess the effect of the different pollination techniques on the agronomic, pomological and chemical characteristics of the fruits obtained; these results show that the technique of mechanized pollination by dusting gave the most satisfactory. In fact this technique in addition to its advantages on the yield it also presents benefits on the economic and social plan, this technique is of significant economic profitability for the farmers especially with the increasing shortage of the labor force and its high cost. Kadri et al. (2019) have shown that mechanized pollination offers farmers a higher yield of dates than those obtained by the traditional method, in addition it also offers human security risks (absence of palm fall accidents) and also financially due to the high cost of labor and low yield of dates caused by traditional pollination.

Conclusion

Optimizing pollination techniques is one of the key factors in improving the production and quality of date palm fruits. The results clearly showed that the dusting technique is the best pollination method for the ‘Deglet Nour’ variety in comparison with the traditional and spray pollination methods. Together with its profitability, the pollen sprinkling method is more promising due the limited availability of skilled labour and to the low cost compared to manual pollination technique. However, other studies can be carried out to explore these different pollination techniques on other varieties grown in different Tunisian oasis systems.

Supplementary Material

Supplementary materials are available at Horticultural Science and Technology website (https://www.hst-j.org).

  • 1.1. Diameter at ‘Khalal’ stage. 1.2. Diameter at ‘Tamar’ stage. 1.3. Thickness at ‘Tamar’ stage. 1.4. Width fruit at ‘Tamer’ stage. 1.5. Length fruit at ‘Tamar’. 1.6. Weight fruit at ‘Khalal’ stage. 1.7. Volume fruit at ‘Khalal’ stage. 1.8.Volume fruit at ‘Tamar’ stage.

    HORT_20220012_Figure_1s.pdf
  • 2.1. Width Seed at ‘Khalal’ stage. 2.2. Width Seed at ‘Tamer’ stag. 2.3. Length Seed at ‘Khalal’ stage. 2.4. Length Seed at ‘Tamar’ stage. 2.5. Weight Seed at ‘Khalal’. 2.6. Weight Seed at ‘Tamar’ stage.

    HORT_20220012_Figure_2s.pdf

Conflict of Interests

The authors declare no conflict of interest.

Acknowledgements

Kadri Karim and Aounallah Karim have designated the experimental protocol, Abdelhafidh Manar conducted the trial, Hamza Hammadi performed statistical analyses. Kadri Karim investigated literature and wrote the original draft. Chahine Karmous and Taoufik Bettaieb revised the draft. Aounallah Karim responsible for research planning, administration and funding acquisition. All authors read and approved the submitted version.

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