Impact of Some Fertilization Treatments on Crop and It's Atrebuites on "Fuerte" Avocado Trees

This study was carried out throughout two successive seasons 2015 and 2016 at Horticulture Research Station at El-Kanater El-Khayria, Qalyubeia Governorate on 20-year-old avocado trees (Persea americana Mill.) “Fuerte” cultivar grafted on Dayouk rootstock and irrigated with through farrow (surface) irrigation system. In this sequence (N1) as the control or untreated trees and other trees were treated with four treatments of different addition times of nitrogen soil fertilization (N2, N3, N4 and N5) all only once and once with boron and zinc as foliar spraying in concentrations (1, 2 and 3 g/L) beside combination between them. Nitrogen fertilization rated 1200g /tree in 3 times as (NH4No3) 33. 5%. Boron was used as sulaphate boron (17, 5%) and zinc was used as sulphate zinc (34%) each treatment was sprayed independently or in combination three times during (October, January, April). Pollen germination, fruit set as well as yield, fruit weight, flesh weight, oil content percentage and vitamin C were determined to assess the effect of the treatments. The obtained results showed that nitrogen soil application time and boron and zinc foliar spraying were significantly affected on improving all the tested parameters compared with control trees. The study also showed that, nitrogen soil application time N2 with boron and zinc combination at 1g/L/tree was more effective than the other treatments and gave significantly the highest values in comparison of other testes treatments in both seasons of study.


Introduction
The avocado Persea Americana, Mill belongs to the family Lauraceae. It has developed into three horticultural races (West Indian, Guatemalan and Mexican [1], which are adaptable to a wide range of soil and climatic conditions. Avocado which has been referred to as the most nutritious of all fruits [2], has gained worldwide recognition and significant volume in international trade. Although relatively new in international commerce, this unique fruit has been appreciated and utilized for at least 9000 years in and near its center of origin in Meso-America [3]. Avocado is a relatively new crop in areas of the world outside its native range in the American tropics. In 2013, world production of avocados was 4.7 million tons, with Mexico alone accounting for 32% (1.47 stage of ripening [6]. In Egypt, "Fuerte" is harvested all year round but its' main season is from October to December. Main problems facing avocado plantations are slow to reach production, low yields in cooler climates with a marked tendency for erratic cropping and sensitivity to low temperatures during flowering and fruit set [7]. Nitrogen seems to be the most important element in avocado nutrition. Deficiencies of nitrogen in avocado result in small, pale leaves, early leaf drop, and smaller and fewer fruits [8]. In addition, nitrogen deficient trees were found to be more susceptible to frost damage [9]. Boron is essential for pollen germination, for successful growth of the pollen tube through the stigma, style and ovary to the ovule [10]. On worldwide basis zinc (Zn) is a very critical microelement because the avocado is very susceptible to their deficiency. Symptoms of Zn deficiency are observed in acid soils from which it is easily leached at a low pH and in calcareous soils in which it is fixed in unavailable forms. Early deficiency symptoms are mottled, narrow, disproportionately small leaves at the terminals, usually light green or chlorotic in color. Leaf margins are necrotic, and internodes are shortened in advanced cases [11].
Numerous fertilization regimes were proposed by several scientists to overcome cropping problems [12] studied the effect of nitrogen fertilizer application times and rates on "Hass" avocado to increase total yield without reducing fruit size and found that application time proved to be an important determinant of total yield lower annual N would reduce fertilizer expense and protect the environment. Boron sprays applied either during fall or spring on trees not deficient in boron (based on leaf analyses) have been effective in increasing fruit set in a number of deciduous tree fruit, nut crops and in avocado [13]. [14] on avocado trees proved that B and Zn were significantly improved pollen germination; fruit set number as well as yield per tree and increased fruit weight, length and breadth of fruits. They showed that the combination of B+Zn had positive synergistic effect and gave the highest values in the tested parameters. According to [15] Zn level at (0.5 %) improved fruit set wheras levels (0.25, 0.5 %) were more effective on fruit drop number and enhanced production of piryform fruits with more elongation. The scope of the present study was to illustrate the impact of nitrogen fertilization regimes with or without foliar sprays of both zinc and boron on the performance of Fuerte avocado trees.

Materials and Methods
This investigation was carried out through the two successive El-Khayreia, Qalubia Governorate, Egypt. Trees were planted at 7x7 meters (86 trees/ feddan (. One hundred and fifty Feurte cultivar trees grafted on Dayouk rootstock were chosen for this study. The chosen trees for the investigation were uniform in their vigor, size, shape and disease free, grown on loamy clay soil and irrigated with a farrow (surface) irrigation system. Trees were subjected to normal cultural practices recommended by the Ministry of Agriculture except for the treatments of this investigation. Experimental design followed the complete randomized block design. The following regimes were conducted each on three separate trees (each acting as a replicate).

Considered fertilization regimes
Nitrogen fertilization regimes: All trees used in this investigation were fertilized by broadcast with 1200 gm N as the recommendation of ministry of Agriculture (the fertilizer ammonium sulfate 20% N was used). Five regimes were considered based on percentage and time of application. The considered regimes were: N 1 : Control as farm's regime. Fertilizer was split into 3 doses i.e.

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The following parameters were assessed to evaluate the comparative effects of the conducted treatments.

a) Pollen grains germination percentage
Five inflorescences were chosen randomly on each of the considered trees to assess comparative effects of conducted treatments on this parameter and the fruiting parameters. Pollen germination (%), Pollen grains were collected during anthesis stage. Flower in the male stage of the reproductive cycle were collected in paper bags then transferred to the laboratory. After anther dehiscence when pollen shed they were collected and incubated in Petri dishes on a medium containing 15% sucrose and 0.8% agar according to [16]. Pollen germination was recorded after 6 hours as the percentage of germinated pollen in a total of 500 grains from different areas of plat. Each pollen sample was replicated three times. Pollen was considered to have germinated if pollen tube length was at least twice as long as the diameters of grain, samples were observed by Optical microscope.

b) Yielding Parameters
In both seasons, fruit set was determined by marking five flowering branch ends around the circumference of each treated trees two weeks after full bloom and fruit set percentage was calculated. On the last week of August just at harvest time the number of fruit/ branches was counted to estimate the final fruit set (number of fruits per branch/number of initial flowers *100).
At harvest, fruits of each tree were picked, counted and weighed with a digital balance in Kgs. The yield (Kg) was determined as total number of fruit / tree *Average fruit weight (gm)/1000).

c) Fruit quality Parameters
Mature Fuerte fruits were harvested at the 3 rd week of September maturity according to [17]. Samples of five representing fruits from each considered tree are harvested, cleaned packed in carton boxes in one layer and transferred to laboratory then both of physical and chemical parameters were assessed.

i. Physical Parameters
The following parameters were determined: fruit weight (g) and flesh weight (g) by using a digital balance.

ii. Chemical Parameters
Free fatty acids were determined by comparison of retention time of the gas chromatographic peaks with these of commercial free fatty acid methyl ester standards, then automatically computed as a percentage by the data processor (Chrom card) from the ratio of individual peak area to the total peaks area of fatty acids. Vitamin C as mg ascorbic acid/100 gm fruit weight was determined and estimated/ 100 ml fruit juice, according to [18].

d) Statistical design and data analysis
Experimental design followed the complete randomized block design. The obtained data was subjected to factorial analysis according to [19]. Attained means were compared by using New LSD method at 5%.

Results and Discussion
Fruit set parameters respectively. The obtained results are in line with the finding of [20] who proved that effect of combination of these nutrients positively affected pollen germination. [21] reported that boron plays an important role in pollen germination and pollen tube growth. Table 2 showed that the on the average the applied nitrogen regimes in the both seasons were more effective  Furthermore, interaction between nitrogen soil application regimes and boron and zinc foliar spraying application during both seasons was significant. Data showed that the combined (N 2 +B 1 +Zn 1 ) induced the highest fruit set percentage amounting to (57.2 & 60.37 %) in both seasons respectively. These findings are in agreement with [22] who found that increase in fruit set due to boron might be attributed to its role in maintaining high pollen viability and germination. also, it seems that the improvement in fruit set percentage could be explained as a result of increase pollen tube elongation due to boron treatments [23]. [24] on date palm found that (N, P, K and Zn) spray application can improve fruit set, yield and fruit size without thinning. In addition, zinc is involved in protein synthesis, influence on electron transfer reaction including those in the Kreb's cycle and subsequently on energy production in the plant and also directly involved in the synthesis of indole acetic acid [11].   Fruit weight (g): Table 4  with [25] and [26,27]. Table 5 showed that flesh weight was significantly affected by applied nitrogen regimes on the average.  treatments with insignificant differences between them. While the lowest content was attributed to N 1 & no spray treatment in both seasons. These results are in no agreement with those of [15] who illustrated that there was no significant different were observed in fat percentage, however this result in the line with agree with [28] and [29].  [30] reported that B and Zn sprays enhanced ascorbic acid content in guava.

In Conclusion the Present Study Clearly
Illustrate that nitrogen fertilization regimes clearly affect the cropping and its' attributes in avocados. also, for foliar application of boron and zinc in combination, it showed clear enhancements in terms of increasing pollen grains germination percentage leading to increasing the crop. Also, their application showed enhancements in crop physical and chemical characteristics [31][32][33].

As a Recommendation
It is preferable to fertilize avocado trees cv. Feurte with nitrogen at 240g/tree during (January), 600g/tree during (May) and 360 g/ tree during (August) combined with 3 foliar application of boron and zinc at 1g/L at for three times, the first at the beginning of flower bud induction in (October), the second spray at bud burst during (January) and the last and third one was at anthesis in (April).