Effect of fruit thinning on fruit yield and quality of cactus pear Opuntia ficus-indica (L.) Mill. in a semi-arid area

The aim of this work was to study the effect of fruit thinning on fruit yield and quality of cactus pear. To meet this objective, trials on fruit thinning were conducted on an adult plantation of cactus pear in a semi-arid region in southern Morocco. The fruit thinning consisted of reducing the fruit load of overcharged cladodes by reducing the number of fruits to 6 or 12 per cladode. It was practiced in early May on three types of one-year old cladodes (small cladodes, the medium ones and large cladodes), by removing some floral buds or young fruits at the flowering stage. The control cladodes had an average number of 18 fruits per cladode. Obtained results showed that the reduction in the load of cladodes improved fruit size and quality. In thinned plants to 6 fruits per cladode and for the three types of cladodes, average fruit weight was 160.44 g and average fruit length and diameter was 8.64 and 6.38 cm respectively. While in not thinned plants and for the three types of cladodes, average fruit weight was only 74.53 g and average fruit length and diameter was 6.40 and 4.46 cm respectively. Fruit thinning increased the content of sugar in the fruits and decreased the number of seeds in the fruits and the fruit peel thickness, but did not affect the juice content in the fruits and the pH and titratable acidity of the juice. Fruit thinning also improved the economic income of the yield and the gain in the yield of thinned plants to 6 fruits per cladode was 33912 DH per hectare compared to not thinned plants.


Introduction
Opuntias are typical species that are perfectly suited to the development of arid and semi-arid regions thanks to their adaptation to drought and their use in human and animal nutrition. Their culture is not very demanding in water and investments and the income they can generate is important. Cactus pear is considered one of the pillars of the local economy of the arid and semi-arid regions. In the southern Mediterranean countries, as in Morocco, the most plantations of cactus pear are traditional and don't benefit from cultivation practices. The yield is low and the fruits are often of small size and low quality (not very juicy and tasty). However, cultivation techniques which can improve fruit quality, mainly fruit thinning by reducing the fruit load of cladodes, are the practices which are decisive in the marketing of the crop yield. What will improve the market value of the crop and the economic income of the farmers and rural populations of the arid and semi-arid regions. The aim of this work was to study the effect of fruit thinning on the improvement of fruit yield and quality of cactus pear and to evaluate the economic income of this thinning operation for the farmers.
Several authors reported that the improvement of fruit yield and quality of cactus pear requires appropriate technical management of the orchard, especially the fruit load of cladodes and parameters related to the fruit, such as fruit size, the edible fresh matter content and the organoleptic components of the fruit [1,2,3,4,5]. A fruit size of 120 g weight is a positive attribute for the marketing of cactus pear fruits. The seed content in the fruit and the fruit peel thickness also have an impact on the marketing of fruits [6,7]. Cactus pear fruits can be of good quality when they have criteria which are requested by the producer and the consumer, such as fruit size, taste (flavor and sugar content or °Brix ˃ 15) and edible fresh matter [8]. The selling price of the fruits on the local market could be interesting by improving fruit quality using cultivation practices, mainly fruit thinning [5,9]. Several authors also reported that fruit size of cactus pear depends on the cultivar, water availability, plant mineral nutrition and fruit load of the cladodes [10,11,12,13,14].
Most of emitted flowers by cactus pear are transformed into fruits, and in a year of high production, the cladodes are too heavy and if this load is not reduced by thinning, the fruits are of small size and the cladodes can be damaged [15]. In cactus pear, the emission of floral buds is often done on one year old cladodes, on the upper half part of the cladodes. Terminal and peripheral cladodes on the plant are the most fertile because they are well exposed to the sun [11,16,17,18,19]. In a year of high production, a cladode can produce 25 to 40 fruits [20], what leads to the reduction in fruit growth and low fruit quality, and a late and irregular fruit ripening [11,17]. And in order to produce homogeneous good fruit size, it is necessary to reduce the fruit load of cladodes [16]. However, a severe thinning of 4 fruits per cladode can greatly reduce the yield up to 58% and can even lead to a second flowering or reflowering [14]. Fruit thinning can be practiced manually using gloves that protect against spines and glochids and the appropriate period for thinning is located between two weeks before flowering until three weeks after flowering or two weeks after fruit set. Early bud thinning is difficult to achieve and late thinning does not improve fruit size [11,21]. Blanco-Macías et al. [22] reported that determining the optimal number of fruits to be removed per cladode depends on the surface of the cladode and its fruit load. On an adult plantation of 335 plants per hectare, a thinning of 6 fruits per cladode yielded 20 tons per hectare and yielded fruits have an average size of 100-120 g [23]. Several studies have shown that leaving a load of 6, 9 or 15 fruits per cladode, the fruit and pulp fresh weight increases as the number of removed fruits per cladode increases [11,24].

Material and methods
Trials were carried out on an adult plantation of cactus pear Opuntia ficus-indica (L.) Mill. cv 'Aissa' 19 years old at the experimental station of the Hassan II Institute of Agronomy and Veterinary Medicine in Agadir area, latitude 30 ° 22 'North, longitude 9 ° 39 West and altitude 32 m. Plants have an average length of 2 m and average width of 1.6 m. The planting density is 3 m between rows and 2 m between plants in the rows, i.e. 1666 plants/ha. The soil of the parcel is a moderate alkaline soil with a pH of around 8.4 and electrical conductivity of 0.16 mmhos/cm. It has a sandy-silty texture and an apparent density of 1.4 g/cm 3 . It is composed of 4.5% coarse sand, 30.3% fine sand, 28.3% coarse silt, 21.3% fine silt and 15.6% clay. The site of trials is characterized by a semi-arid climate with hot and dry summer and mild and relatively wet winter. The annual rainfall is 207 mm. The mean annual temperature is 22°C, the annual maximum is 31°C and the minimum is 12.5°C. This research work consisted in studying the effect of fruit thinning on fruit yield and quality of an adult plantation under the natural conditions of the environment of the site of trials. Two types of fruit thinning were used: light thinning with 12 fruits per cladode and severe thinning with 6 fruits per cladode. The not thinned plants (control) had an average load of 18 fruits per cladode. Fruit thinning was practiced on three types of one year old cladodes: small cladodes with an area of 509-735 cm 2 , medium cladodes with an area of 736-920 cm 2 and large cladodes with an area of more than 920 cm 2 . The area of the cladode is determined according to Martin et al. [25]: S = (w /2) x (L/2) x π  S: cladode area  W: cladode width  L: cladode length  π = 3.14 The experimental design adopted was a randomized block design with 4 blocks. An experimental unit consisted of two plants and studied factors focused on:  The fruit thinning at the stage of 82% open flowers on one year old cladodes. Fruit thinning was carried out on May 01, 2019. Fruit thinning treatments used were: T: not thinned plants (the control); T1: thinned plants to 6 fruits per cladode; and T2: thinned plants to 12 fruits per cladode.  Type of thinned cladodes: three types of thinned cladodes according to their surface were used in the study: C1: small cladodes with an area of 509-735 cm 2 , C2: medium cladodes with an area of 736-920 cm 2 and C3: large cladodes with an area of more than 920 cm 2 .
For each type of cladodes, four cladodes were randomly chosen from the four orientations of the plant (north, south, east, and west). Studied parameters focused on:  The fruit quality: fruit size (fruit weight and dimensions: fruit length and diameter) and the organoleptic compounds in the fruits (juice and sugar content and titratable acidity and the pH of juice). Fruit quality parameters were carried out on a sample of 24 mature fruits per type of cladodes and per experimental unit. Fruit length and diameter and fruit peel thickness were measured with a caliper. Fruit and pulp weight was measured using an electronic balance with an accuracy of 0.01 g. The content of juice in the fruits was determined according to the formula: And the content of sugars in the fruits or degree Brix was measured with a refractometer. The pH of juice was determined with a pH meter and the titratable acidity was carried out by the titration of the juice using NaOH 0,1 N and phenolphthalein as indicator of color change. It is calculated according to IFU [26]: QAC: the titratable acidity expressed in g of citric acid per liter of juice VNaOH: the volume of NaOH used in the titration (in ml) For each studied organoleptic parameter, measures were repeated 3 times and the mean value of the three measures was taken into account.
 Fruit yield: it was measured on the two plants of each experimental unit and fruit yield per hectare was calculated on the basis of the density of plantation and average yield per plant.  Fruit maturation: The monitoring of fruit maturation began after fruit set on a sample of 4 cladodes per plant of each experimental unit and for each type of cladodes C1, C2 and C3 of thinned and not thinned plants, and observations were made with a rate of one week to ten days. The fruit ripening period is the period which extends from the date of 5% mature fruits until the date of 100% ripening fruit and the date of full ripening fruit corresponds to 50% of mature fruits [27]. Fruits reached the stage of maturity when the color of the fruit peel changes from green to yellowish-green. The number of ripe fruits is determined by counting mature fruits in relation to the total number of the fruits. The % of mature fruits is determined according to Oelofse et al. [28]: % fruits at ripening stage = number of fruits at ripening stage / total number of fruits on the plant x 100  The seed content in the fruit: on the samples of fruits used in the study of the fruit quality, 10 g of pulp per fruit were used for the separation of the seeds from the pulp and their subsequent counting.  The fruit peel thickness: it is measured on the fruit peel using a caliper after peeling the mature fruits.

Effect of fruit thinning and the type of cladodes on fruit size
Obtained results showed that for fruit size (fruit weight and fruit length and diameter) there was a significant difference (p ≤ 0.001) between fruit thinning treatments, the type of cladodes and the interaction of the two factors. The thinning treatment T1 (thinned plants to 6 fruits per cladode) gave the highest fruit weight and fruit length and diameter for the three types of cladodes (small cladodes, the mediums and large cladodes) with an average fruit lenght and diameter of 8.64 and 6.33 cm respectively and average fruit and pulp weight of 160.44 and 99.86 g respectively. While in not thinned plants, average fruit lenght and diameter for the three types of cladodes was 6,40 and 4,66 cm respectively and average fruit and pulp weight was only 74,53 and 48,42 g respectively. The T1/C3 combination of thinned plants to 6 fruits per cladode and large cladodes yielded fruits with the largest size (fruit and pulp weight of 180.50 and 112.92 g respectively and fruit length and diameter of 9.25 and 6.71 cm respectively) ( Table 2). This has shown that the large severe thinned cladodes have yielded fruits with the largest size due to the low fruit load of the cladodes and large photosynthetic surface of the C3 cladodes.

Effect of fruit thinning and the type of cladodes on the seed content in the fruits and fruit peel thickness
Fruit thinning also had a significant effect (p ≤ 0.001) on the seed content in the fruits. The average number of seeds in the fruits of not thinned plants was 53 per 10 g pulp, while average number of seeds in the fruits of thinned plants was 42 and 36 per 10 g pulp respectively for thinned cladodes to 12 fruits and thinned ones to 6 fruits, a decrease of 14.27% compared to thinned plants to 12 fruits per cladode and 32.13% compared to thinned plants to 6 fruits per cladode. However, the type of cladodes and the interaction of the two factors have no significant effect (p > 0.05) on the seed content in the fruits (Table 4). Fruit thinning and the type of cladodes, and the interaction of the two factors have a significant effect (p ≤ 0.001) on the fruit peel thickness. The mean fruit thickness of not thinned plants was the highest with 4.99 mm for the three types of cladodes, followed by the fruit peel thickness of thinned plants to 12 fruits per cladode with 4.61 mm for the three types of cladodes and the peel fruit thickness of thinned plants to 6 fruits per cladode was the lowest with 4.13 mm for the three types of cladodes. The best result was obtained with the C1/T3 combination with a mean fruit peel thickness of 3.89 mm (Table 4). Several authors also reported that fruit thinning in cactus pear reduces the content of seeds in the fruits and the fruit peel thickness (     The selling price of cactus pear fruits on the market depends essentially on the yield and fruit quality and the economic income of the yield for the farmer depends on the quality of the fruits and the selling price of the fruits on the local market. The purpose of studying the economic income of the thinning operation is to assess its impact on the gain to be made compared to not thinned plants. The production yield per hectare was determined on the basis of the mean fruit yield per plant of each thinning treatment and the planting density used in the parcel of trials (1666 plants per hectare) ( Table 1). The selling price of the production is determined on the basis of the production yield of each thinning treatment and the mean selling price of one kilogram of each fruit size during the fruit harvest season in Morocco (early July-mid August). The mean selling price of the fruits in the 2019 year was 5.71 DH/kg for large size fruits (fruit weight ˃ 120 g) (fruits of the plants T1) and 3.75 DH/kg for small and medium size fruits (fruit weight < 120 g) (fruits of the plants T and T2). The selling price of 1 hectare fruit yield of thinned and not thinned plants for the 2019 year is presented in Figure 2. The highest selling price was obtained with the plants T1, with a mean value of 281242 DH/ha. It exceeded the selling price of the fruit production of the plants T2 with 15570 DH/ha and the selling price of the fruit yield of not thinned plants with 33912 DH/ha. This has shown that the thinning operation increases the economic income of the production yield by increasing the gain of production compared to not thinned plants, although the fruit production of thinned plants was lower than that of not thinned plants, but the selling prices of the production of thinned plants were higher than the selling prices of the production of not thinned plants thanks to good size of the fruits of thinned plants.

Effect of fruit thinning and the type of cladodes on the fruit maturation
Our results are similar to those of several authors who reported that the fruits of thinned plants, which are of good size and good quality (good sugar content, low seed content and thin fruit peel), improve the marketing of the fruit production and the market value of the fruits (Makoboki et al., 2009; FIA, 2010).

Conclusion
Fruit thinning has improved the quality of the fruits, mainly fruit size and the sugar content in the fruits. What improves the selling price of the fruit production on the market and the rentability of farmers and rural populations in the arid and semi-arid regions, despite the negative effect of the thinning operation on fruit yield. Fruit thinning also made it possible to obtain an earlier and more regular ripening of the fruits in time, mainly on large cladodes, and to avoid the alternation of the production by reducing the number of fruits on loaded cladodes. The best results in terms of fruit size and quality were obtained in thinned plants to 6 fruits per cladode. And this has increased the gain of the fruit production of thinned plants by increasing its economic income compared to the production of not thinned plants thanks to attractive selling prices of the good size fruits of thinned plants on the market. The fruit thinning operation in cactus pear is a cultural practice which is easy to achieve and economically not expensive because it can be practiced by the farmers and their family members without calling to specialized workforce. It can be therefore considered as a cultural operation which is within the reach of farmers and rural populations of the arid and semi-arid regions and which can improve their income by the benefit they gain in practicing this operation.