Abstract: In this study artificial pollination has been studied in pistachio orchards. Six shoots that different pollinizers were applied in each tree; the experiment consisted of six treatments; natural pollination as control (covered with plastic mesh), honeybee plus natural pollination (kept uncovered) and four mixtures of pollen (2, 4, 6 and 8%) with soft wheat flour have been prepared for application. Results obtained showed that; artificial pollination of pistachio has both positive and some times negative effects on number of fruits per cluster. Two percent pollen mixture could be used as an effective treatment in pistachio orchards pollination. Artificial pollination not only increased number of fruits per cluster and total yield per cluster but also improved nut size and kernel dry weight in pistachio trees.
INTRODUCTION
Pistachio Pistacia vera L. is a deciduous wind-pollinated woody tree species member of the Anacardiaceae family. It is cultivated mainly in the Mediterranean regions of Europe, the Middle East and California (Vaknin et al., 2001; Vaknin et al., 2002). Its origin is still uncertain, but most experts agree that it probably originated in Central Asia (Herrera, 1997). Pistachio tree is dioecious; male and female flowers are born on different trees, for this reason, cross pollination is necessary (Acar et al., 2001; Ak et al., 1996; Iisfendiyaroglugluglu et al., 2001). Pistachio flowers has no petals to attract insect pollinators, therefore, pollination and fertilization occurs by the transport of pollens from male to female trees by wind (Ak et al., 1996). It is necessary to have enough male trees to insure adequate pollination and to get maximum nut production. Male and female trees are usually interplanted in the orchard with one male to eight or eleven females depending on the orchard (Herrera, 1997; Acar et al., 2001; Ak et al., 1996).
Female flowers do not have petals that could attract bees for insect pollination and pollen transfer depends entirely on the wind (Herrera, 1997). Artificial pollination appears temporary solution to the problem in orchards that under insufficient pollination conditions (Acar et al., 2001). It is worth mentioning that the artificial pollination involves three major steps: (I) pollen collection; (ii) pollen storage; (iii) pollen deposition on receptive stigmas (Vaknin et al., 2001, 2002).
Most of male trees bloom earlier than female pistachio trees and generally spread their pollen while the stigmas are not receptive in female trees (Ak et al., 1996; Iisfendiyaroglugluglu et al., 2001), therefore, artificial pollination gives good result and it should be used as a temporary solution when natural pollination is insufficient (Tasias and Valls, 1990).
Kuru (1995) used different blending materials (corn starch, rice flour, soft and hard wheat flour) in artificial pollination of pistachio and found that the successful results were obtained from the mixtures with 1% pollen concentrations.
Pollination is one of the most important factors in pistachio culture, it is mainly associated to the time difference between flowering of male and female trees, besides pollination may not be satisfactory since male trees are not planted in suitable ratio and in appropriate direction in orchards, this situation results reduction of the production and great proportion of empty fruits (Iisfendiyaroglugluglu et al., 2001). Pollen germination on the stigma was negatively correlated with yield (Vaknin et al., 2002) and the large increases in pollen deposition on the stigma might, in some cases, decrease female reproductive success by causing interference between pollen grains at high densities (Young and Young, 1992).
This study has been conducted in pistachio orchards where the differences in maturity periods of the male and female flowers exist. Therefore, it was aimed to contribute artificial pollination with a practical way on yield of pistachio, since this method of application is suitable for improving pollination in wide pistachio areas on open pollinated trees as recommended by Acar et al. (2001).
MATERIALS AND METHODS
This study was carried out on Ashori cultivar at pistachio orchards in Daba area; about 75 km to the south of Amman. Six pistachio trees (Cultivar Ashori), 12 years old, were selected randomly in the field at the 12th April and considered as blocks. In each tree six shoots were randomly selected, that different pollinizers can be applied with two inflorescence clusters represent each treatment, whereas the other clusters were removed from the selected shoots.
To prevent open pollination, female trees were isolated by using paper bags before blooming (Iisfendiyaroglugluglu et al., 2001). At the beginning of blooming male flowers were collected and the inflorescences were cut and spread over white paper at constant room temperature (25°C). Pollen shed overnight was cleaned and collected by passing it through a 100 μm mesh sieve. The pollen was then sealed in plastic bags and stored in 4°C in the refrigerator until the female blooming started.
Pollen germinability was tested in vitro which were performed at 25°C according to protocol described by Polito and Luza (1998) and Iisfendiyaroglugluglu et al. (2001) on a medium containing Agar (1%), Sucrose (15%) and Boric Acid (0.01%). Pollen grains with germinability more than 50% were used for the experiment.
The experiment consisted of six treatments; natural pollination as control (covered with plastic mesh), honeybee plus natural pollination (kept uncovered) and four mixtures of pollen (2, 4, 6 and 8%) with soft wheat flour have been prepared for application and covered with paper bags, according to procedures outlined by Kuru (1995) and Acar et al. (2001).
Treatments application time was done when the female flowers opened and stigma become receptive during the morning and at the afternoon hours of 15th, 16th and 17th April/2006. The applications of the pollen grains were done by using a drawing brush; four brushes were used (one/treatment). The pollen treated clusters were covered with paper bags until 26th April. Control treatment (wind pollination) was covered with plastic mesh to prevent honeybees from visiting it. On the other hand, the wind and honeybee treated clusters were left uncovered and sprayed with light sugar solution (1:1) as honeybee attractant.
Harvesting was done at 20th September/2006 when the hulls separates easily from the shell as recommended by Herrera (1997). Each two clusters (replicate) was collected in a paper bags for analysis.
Measured Parameters
The parameters were taken into consideration to evaluate the efficiency
of pistachio pollination were:
Number of Fruits per Cluster
The number of produced fruits per two clusters was counted and then the
average readings were considered per one cluster.
Total Production per Cluster
The total freshly harvested fruits per two clusters were weighed (g) using
a digital scale balance and then the average readings were considered per one
cluster.
Nut Size
Ten harvested nuts per replicate were used to determine the nut size using
water displacement method. The volume of displaced water was divided over the
number of the used nuts (Kramer and Twigg, 1973), the average volume (mL) readings
were considered per one nut.
Nut Fresh Weight
Ten harvested nuts per replicate were weighed and the average weight (g)
of one nut was calculated.
Nut Dry Weight
The nut dry weight was determined by weighing ten freshly harvested nuts
per replicate using a digital scale balance, then placed in an oven at 60°C
for 48 h (Leskinen et al., 2002), then average readings were considered
per one nut.
Kernel Fresh Weight
Ten harvested kernels per replicate were weighed after removing their shells
and hulls and the average weight of one kernel was calculated.
Kernel Dry Weight
Kernels were freshly weighed using a digital scale balance and then placed
in an oven at 60°C for 48 h (Leskinen et al., 2002), then average
readings were considered per one kernel.
Experimental Design and Statistical Analysis
A Randomized Completely Block Design (RCBD), with six treatments per six
replicates (trees) were used. All data obtained were statistically analyzed
by variance, according to the procedure outlined by Steel and Torrie (1980).
The differences between means of the different treatments were compared by the
Least Significant Difference (LSD) test using SAS and differences with probability
value at p = 0.05 were considered significant.
RESULTS AND DISCUSSION
Number of Fruits per Cluster
The treatments of artificial pollination (2, 4 and 6%) resulted in significantly
higher number of fruits per cluster (Table 1); the highest
number (69.5) was obtained by 4% pollen mixture treatment. Pollen mixture (8%)
was resulted in a lower number of fruits per cluster in compare to 2 and 6%
pollen mixture treatments, which may be due to the large increases in pollen
deposition on the stigma that decreases female reproductive success by causing
inferences between pollen grains on the stigma at high densities and this is
similar to the previous suggestions by Young and Young (1992). The same results
were found by Vaknin et al. (2002) who concluded that pollen germination
on the stigma was negatively correlated with yield. Control treatment resulted
in the lowest number of fruits per cluster (43.67), using of honeybees in addition
to the natural pollination increased the number of fruits per cluster, but it
still lower than that obtained by artificial pollination mixture treatments.
Total Yield per Cluster
Total yield per cluster was improved by artificial pollination mixture treatments,
since all treatments of artificial pollination resulted in significantly higher
total yield per cluster, in compare to the control or natural pollination plus
honeybee treatments (Table 1).
| Table 1: | Effects of artificial pollination on fruit number per cluster, total production and nut size of pistachio trees* |
| *Values are the mean of six replicates; **Means within each column having different letter(s) are significantly different according to LSD at 5% level | |
| Table 2: | Effects of artificial pollination on nut and kernel fresh and dry weight of pistachio trees* |
| *Values are the mean of six replicates; **Means within each column having different letter(s) are significantly different according to LSD at 5% level | |
The highest yield (123.27 g) was obtained by the 4% pollen mixture treatment, whereas the lowest one (63.18 g) was obtained by the control treatment. Furthermore the use of honeybees in addition to natural pollination increased the total yield per cluster in compare to control treatment, but this increase is still lower than that obtained by artificial pollination treatments (Table 1).
Nut Size
The biggest nut size (2.55 mL) was obtained by the 4 or 6% pollen mixture
treatments, which were not significantly different to 8% pollen mixture treatment
(Table 1). On the other hand the smallest nut size (2.102
mL) was obtained by the control treatment, which was not statistically different
with the natural plus honeybee, 2 and 8% pollen mixture treatments.
Nut and Kernel Weights
No statistical differences were observed between all pollinated treatments
of pistachio trees in respect to nut fresh and dry weight and to kernel fresh
weight (Table 2), even though the highest nut fresh weight
(2.027 g), nut dry weight (0.985 g) and kernel fresh weight (0.665 g) was obtained
by the 6, 4 and 6% pollen mixture treatments, respectively, while the lowest
nut fresh weight (1.653 g) was obtained by the control treatment.
On the other hand, the use of honeybees and or artificial pollination treatments improved kernel dry weight in compare to the control treatment which produced the lowest kernel dry weight (0.373 g), while the highest kernel dry weight (0.462 g) was obtained by the 4% pollen mixture treatment which does not show any significant differences with all other treatments, except the control treatment.
CONCLUSIONS
Artificial pollination is an effective method in pistachio trees, which can be used, to replace natural pollination and ensure high yields with high nut quality. Artificial pollination of pistachio has both positive and some times negative effects on number of fruits per cluster and the 2% pollen mixture could be used as an effective treatment in pistachio orchards pollination.
Artificial pollination; improved number of fruits per cluster, total yield per cluster, nut size and kernel dry weight in pistachio trees, on the other hand, fruit set and yield can be increased with regular soil fertilization, irrigation, pruning, etc.
ACKNOWLEDGMENTS
Special thanks to Mr. Hussain Kamel Anan, for hosting of this work and Mr. Sufian Al-Hanbaly for his valuable help.