Pollination of Date Palm (Phoenix dactylifera L.) cv. ‘Lulu’
with Pollen Grains-Water Suspension
Mohamed A. Awad
Faculty of Meteorology, Environment and Arid Land Agriculture, King Abdulaziz University, Jeddah, Saudi Arabia.
Email: [email protected]
Abstract. Pollination and thinning are critical processes in the date palm production chain that affect fruit development, quality and yield and regulate tree yearly bearing. Developing a pollination technique that results in an acceptable level of fruit set with a minimum amount of pollen grains and without a further need for thinning is critically required for date palm production, especially under arid conditions.
During 2006 and 2007 seasons, the effect of pollen grains-water suspension application at different concentrations on fruit set, yield and quality were examined on ‘Lulu’ date palm cultivar growing under Al-Ain oasis conditions, UAE. The results showed that pollen grains-water suspension (PGWS) application at 0.5, 1, 1.5 g/l gave fruit set percentage of 75.6, 86.8, and 87.8%, respectively which was lower than control (90.0%),traditionally pollinated). As the mean of both seasons, the PGWS application especially at 0.5 and 1.0g/l significantly decreased both bunch weight (5.52, 6.21, 7.19 and 8.54 kg for 0.5, 1.0, 1.5g/l and control, respectively) and total yield per tree (49.71, 55.92, 64.74 and 76.89 Kg/tree, for 0.5, 1.0, 1.5g/l and control, respectively) at the Tamer stage. Fruit quality characteristics especially fruit and flesh weight, length and diameter, TSS and dry matter concentration were slightly but significantly increased by the PGWS application treatments during the 2006 season. However, seed weight, acidity and vitamin C concentration were not affected by any of the pollination treatments. More research work is required for such pollination technique to justify the concentration of grains-water suspension and the response of each date palm cultivars to this pollination technique.
Introduction
Pollination and thinning are critical processes in the date palm production chain that affect availability of assimilates, fruit development, quality and yield and regulate tree yearly bearing (Nixon, 1955; Hussein et al., 1993;
and Awad, 2006). Being a dioecious species, commercial date palm production necessitates artificial pollination which ensures enough fertilization and overcomes disadvantages of dichogamy and also reduces the number of required male palms. Artificial pollination could be realized according to a traditional method or by using a mechanized device (Zaid and de Wet, 1999). The most common and primitive pollination technique is to cut the strands of male flowers from a freshly opened male spathe and place 5-10 of these strands, lengthwise and in an inverted position, between the strands of the female inflorescence and tie the pollinated female cluster 5 to 7cm from the outer end with a strip torn from a palm leaflet or a string (Zaid and de Wet, 1999). Fruit set ranges between 85-95% for ‘Lulu’ cultivar, as well as, most other cultivars by such pollination technique. With such a high fruit set percentage, thinning, flower and/ or fruit thinning is a necessary process for improving fruit quality and increasing the marketable yield. Also, this pollination technique requires the availability of large number of male spathes that sometime are not available especially for early flowering season cultivars (Dowson, 1982). The male/female ratio in modern plantations is about 1/50 (2%). Hand or chemical thinning are commonly practiced in different ways and normally at flowering time (flower thinning), or shortly after the completion of fruit setting (fruit thinning) (Awad, 2006; Zaid and de Wet, 1999; Al-Khateeb et al., 1993, and El- Hamady et al., 1993). However, hand or chemical thinning is neither practical nor economic especially under such harsh conditions as in the Gulf region (Awad, 2006). Due to the general concern with respect to the use of chemicals in agricultural production, more save and economic thinners or methods are critically required. In mechanical pollination, dilution of pollen concentration with wheat flour might be a way to regulate fruit set (El-Kassas and Mahmoud, 1986a; and El-Mardi et al., 1995). Intended delay of pollination is another way of regulating fruit set, since the receptivity of pistillate flowers decreases by delaying pollination to one or two weeks from female spathe cracking. However, the results seem inconsistent among seasons and cultivars (El-Kassas and
Mahmoud, 1986b; Moustafa, 1998a; and Shabana et al., 1998).
Moreover, Reuveni (1970) found in the ‘Deglet Nour’ cultivar that the day of optimum receptivity varies among different inflorescences of the same tree. It has been reported that water spray treatment (as a simulation of rainfall) after a specific time following pollination decreased fruit set and induced an acceptable level of thinning, especially in arid regions where rainfall is scarce at the pollination period (Awad, 2006).
Accordingly, the aim of this study was to evaluate a new pollination technique by the application of pollen grains-water suspension at different concentrations on fruit set and fruit quality of ‘Lulu’, a heavy bearing date palm cultivar. Our goal was to develop a pollination technique that should result in an acceptable level of fruit set with a minimum amount of pollen grains and without a further need for thinning especially under such harsh conditions.
Materials and Methods Plant Materials and Experimental Procedure
During the 2006 and 2007 seasons, three uniform ‘Lulu’ date palm trees of 17-years-old, growing in a sandy soil at the experimental orchard of the College of Food and Agriculture, United Arab Emirates University at Al-Ain, United Arab Emirates, were selected. At the middle of the flowering period (March 14th and 19th, 2006 and 2007, respectively), on each tree, four normal bunches that were just cracked on the same day or the day before were randomly selected and labeled. Pollens were collected from the spathes of the male cultivar ‘Sekkaa’, air dried at room temperature and pollen grains were extracted from strands, then suspended in water at the rate of 0.5, 1.0 and 1.5 g/l directly before the pollination process. Each of the four bunches on each tree were treated with one of the following treatments: traditional pollination (control) by placing 7 strands of male flowers, lengthwise and in an inverted position, between the strands of the female inflorescence and tie the pollinated female cluster 5 cm from the outer end with a strip torn from a palm leaflet, pollination with pollen grains-water suspension (PGWS) spray at 0.5, 1.0 or 1.5 g/l. The PGWS was applied to the bunches with a plastic hand sprayer (Matabi Style 1,5 Sprayer-1L, Goizper, Spain) by targeting the cone nozzle close to the bunch. The pressure during delivery of suspension was about 30-40 PSI. Each bunch received about 100ml of
suspension. The same male cultivar was used during both seasons of the study. All the other developed bunches on each tree were subsequently pollinated in a similar way as for the control bunches. Shortly after pollination, the number of bunches was adjusted to 9 bunches per tree.
Fruit Set Percentage
In both 2006 and 2007 seasons, at the middle of the kimri stage (about 7-8 weeks from pollination), 20 strands per bunch were randomly selected and labeled. The number of attached fruitlets and the number of dropped fruitlets were counted and the percentage of fruit set was calculated.
Bunch Weight and Total Yield per Tree
In both 2006 and 2007 seasons, the total bunch weight at the Tamer stage (about 22 weeks from pollination), was recorded for each bunch.
The total yield (Kg/ tree) was calculated by multiplying the mean bunch weight by the total bunches per tree.
Fruit Quality Measurements
Only in the 2006 season, at the Tamer stage, 20 fruits per tree (from all bunches) were collected and immediately transported to the Horticulture Laboratory at the College of Food and Agriculture for quality measurements. Fruit, flesh and seed weight (g), diameter and length (cm), and dry matter percentage were recorded. Total soluble solids (TSS) were measured in fruit juice with a hand refractometer.
Titratable acidity was determined in juice by titrating with 0.1N sodium hydroxide in the presence of phenolphthalein as indicator (Ranganna, 1979), and the results were expressed as a percentage of malic acid.
Ascorbic acid (vitamin C) was measured, according to Ranganna (1979), by the oxidation of ascorbic acid with 2,6-dichlorophenol endophenol dye. The results were expressed as mg/ 100ml juice.
Statistical Analysis of Data
All data were statistically analyzed, as a completely randomized design with three replicates per treatment, by analysis of variance (ANOVA) using the statistical package MSTATC Program (Michigan State University, East Lansing, MI). Comparisons between means were made by F-test and the least significant differences (LSD) at P = 0.05.
Results
Because of similarity between the results of the two seasons (no significant interactions between seasons), data were presented as the means of both seasons for fruit set, bunch weight and yield per tree. Data of Table 1 and Fig. 1 show that, pollen grains-water suspension (PGWS) application at 0.5, 1, and 1.5 g/l gave fruit set percentages of 75.6, 86.8, and 87.8%, respectively which were lower than control (90.0%) (traditionally pollinated). In this respect, the lowest fruit set percentage obtained with the PGWS application at 0.5g/l was significantly lower than all other pollination treatments. There were no significant differences among the other PGWS applications and control.
At the Tamer stage, the PGWS application especially at 0.5 and 1.0g/l significantly decreased both bunch weight (5.52, 6.21, 7.19 and 8.54 kg for 0.5, 1.0, 1.5g/l and control, respectively) and total yield per tree (49.71, 55.92, 64.74 and 76.89 Kg/tree, for 0.5, 1.0, 1.5g/l and control, respectively).
In this respect, there were significant differences among all the applied pollination treatments (Table 1 and Fig. 1).
Table 1. Fruit set percentage, bunch weight and total yield per tree at the Tamer stage of
‘Lulu’ date palm as affected by pollen grains-water suspension.
Treatments Fruit set
(%)
Bunch weight (Kg)
Yield/tree (Kg/tree)
Control (traditional pollination) 91.0 8.54 76.9
Pollen grains-water suspension at:
0.5 g/l 76.6 5.52 49.7
1.0 g/l 86.8 6.21 55.9
1.5 g/l 87.8 7.19 64.7
F-test ** *** ***
LSD 0.05 4.63 0.69 6.27
Data are the mean of 2006 and 2007 seasons. ** and ***, significant at levels P = 0.01 and 0.001, respectively.
Data of Table 2 show that during the 2006 season, fruit and flesh weight, length, diameter, and dry matter and TSS concentration were significantly higher at the PGWS application of 0.5 and 1.0 g/l compared to the other treatments. However, seed weight, acidity and vitamin C concentration were not affected by any of the pollination treatments (Table 2).
Fig. 1. Effect of pollen grains-suspension application on fruit set of ‘Lulu’ date palm cultivar. From left to right, the concentration of pollen grains-suspension was 0.5, 1.0, 1.5 g/l and control.
Table 2. Fruit quality of ‘Lulu’ date palm at the Tamer stage as affected by pollen grains- water suspension.
Treatments Fruit quality
Fruit weight
(g)
Flesh weight
(g)
Seed weight
(g)
Fruit length
(cm)
Fruit diameter
(cm)
Dry matter
(%) TSS
(%)
Acidity (%)
Vit. C (mg/
100ml juice) Control (traditional pollination)
6.22 5.34 0.88 2.23 1.72 75.6 81.4 4.86 2.32 Pollens grain-water suspension at:
0.5g/ l 7.10 6.20 0.90 2.48 1.96 83.2 88.3 4.95 2.44 1.0g/ l 6.91 6.11 0.80 2.45 1.89 80.7 82.8 4.91 2.36 1.5g/ l 6.27 5.45 0.82 2.33 1.78 71.6 80.4 4.85 2.34 F-test *** *** NS * * * ** NS NS LSD0.05 0.15 0.10 - 0.16 0.18 1.88 1.39 - - Data are for the 2006 season. NS, not significant; *, ** and ***, significant at level P = 0.05, 0.01 and 0.001, respectively; (-), not calculated.
Discussion
Pollination and thinning are critical processes in the date palm production chain that affect fruit development, quality and yield and regulate tree yearly bearing. Developing a pollination technique that results in an acceptable level of fruit set with a minimum amount of
pollen grains and without a further need for thinning is critically required for date palm production, especially under arid conditions (Awad, 2006).
The results of this study showed that pollen grains-water suspension (PGWS) application at 0.5g/l significantly decreased fruit set percentage, in contrast to 1.0 and 1.5g/l which gave fruit set percentage close to control (traditionally pollinated) (Table 1 and Fig. 1). Generally, ‘Lulu’ is classified as a heavy bearing date palm cultivar and fruit set ranges between 85-95% by traditional pollination technique (Awad, 2006). With such a high fruit set percentage, thinning (flower and/ or fruit thinning) is a necessary process for protecting bunches breaks, improving fruit quality and increasing the marketable yield. Thus, application of PGWS especially at 0.5g/l resulted in an acceptable level of fruit set (75.6%) with a minimum amount of pollen grains and without a further need for thinning (Table 1 and Fig. 1). The effectiveness of the PGWS application was also consistent for the two years. Moreover, in this technique, the pollen grains of excellent male might be stored and used for pollination of the early flowering season cultivars (Dowson, 1982). The total bunch weight and yield per tree were significantly decreased by the PGWS application (Table 1 and Fig. 1). It is known that yield is a result of combined factors such as fruit number, size, weight and other related variables (Al-Khateeb et al., 2001). The parameters of this factor are often proportionally inversely correlated with each other. One of the main objectives of bunch thinning is to obtain more uniform bunch sizes and increase fruit size and weight depending on the level of fruit set. In this experiment, the obtained levels of thinning, especially with the PGWS application at 0.5 and 1.0 g/l, had a significant positive effect on most fruit quality characteristics at the Tamer stage (Table 2). This might be due to more availability of assimilates by lower percentage of fruit set (Nixon, 1955; Hussein et al., 1993; and Awad, 2006). In confirmation to our results, it has been reported that fruit thinning increased fruit size, weight and sugar level but decreased total yield per tree of Deglet Noor (Nixon, 1955), ‘Seewy’ (Moustafa, 1998b), ‘Samany’ (Hussein et al., 1993) and ‘Khalas’ (Al-Khateeb et al., 1993) date palm cultivars. In the present study, seed weight, acidity and vitamin C concentration were not affected by any of the pollination treatments (Table 2). It was also reported that, for some date cultivars under certain climatic conditions, thinning had no pronounced effect on fruit quality (Al-Bekr and Al- Azzaoui, 1965, Awad, 2006). Generally, cultivars which produce
shorter and more round fruit such as ‘Lulu’ and ‘Barhee’ showed no or little response to thinning, in contrast to those that produce a more elongate fruit type (Marashi and Mousavi, 2006). In conclusion, this study investigated the possibility to pollinate date palm flowers with pollen grains-water suspension to control fruit set in heavy bearing cultivars such as ‘Lulu’ to save further thinning cost and pollens of excellent males, preventing bunches breaks, improving fruit quality and produce consistent results among seasons. However, more research work is required to justify the concentration of grains-water suspension and the response of each date palm cultivars to this pollination technique.
Acknowledgements
The author would like to thank Mrs. Abou-messallam Azab and Rasheed Hamed at the Arid Land Agriculture Department, College of Food and Agriculture, United Arab Emirates University, for their indispensable technical support.
References
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