Document Type : Research Article
Authors
1 Department of Horticulture, Faculty of Agriculture, University of Zanjan
2 Zanjan University
Abstract
Introduction Apple (Malus damestica L.) is one of the most popular temperate fruits in the world as well as Iran. One of the challenges in managing apple orchards is excessive fruit formation or excessive fruit drop especially in the pre-harvest stage, which affects the performance and marketable product. Therefore, the control fruit set is required to help regular fruit production. Currently, the application of synthetic auxins as plant growth regulators are successfully practiced and mainly used in various countries, including all aspects of modern apple production to control and manipulate vegetative growth and regulation of flowering, reduce immature fruit drop, fruit maturity, firmness, and manage apple harvest. Among the auxin-type growth regulators, NAA is a synthetic auxin analogue that may down-regulate abscission-related genes and reduce the sensitivity of the abscission zone to ethylene. It has long been used to reduce or totally prevent pre-harvest fruit drops, to preserve fruit flesh firmness, and to prevent starch degradation in apples. Therefore, this experiment was conducted to evaluate the effects of NAA foliar application at different days after full bloom on controlling fruit drop and improving physical and biochemical attributes of ‘Red Delicious’ apple fruits. Findings described here may help to understand the effects this PGR on apple quality and also help on the development of strategies to reduce post-harvest losses of apple fruits.
Materials and Methods In order to study the effect of NAA on controlling fruit drop, fruit yield and quality, a factorial experiment based on randomized complete blocks design with three replications was conducted under Zanjan climatic conditions during 2023. Ten year-old ‘Red Delicious’ Standard apple trees grafted on M.9 rootstock were used as plant material. Treatments consisted of different concentration of NAA (15, 30, 50 and 75 mg L-1) sprayed at different days after full bloom (10, 25 and 40 DAFB). The date of full bloom was 04/22/2023. The fruit growth pattern was based on fruit weight and diameter during DAFB. According to fruit growth pattern, fruit were harvested at physiological maturity stage. Fruit number per replicate, fruit weight, diameter and shape index, fruit firmness, total soluble solids content and taste index were measured. Also, fruit drop percentage, fruit yield as well as fruit efficiency was estimated.
Results and discussion Advanced knowledge of apple fruit development from fruitlet to maturity is crucial for optimal prediction of year-to-year yields and fruit quality. Apple fruit growth has been defined as sigmoidal increase in fruit diameter or fresh weight. Studying two fruit growth patterns based on fruit weight and diameter data showed that fruit weight is a more appropriate indicator for drawing fruit growth patterns, and according to that, the fruits were harvested at the physiological maturity stage. Foliar spray of NAA during different DAFB showed different significant effects on fruit drop and yield efficiency. The highest rate of fruit drop (95.7 % and 85.9 %) was observed with foliar application of NAA at 10 and 25 DAFB, respectively, but its application at 40 DAFB reduced fruit drop by 46.9% compared to the control, and correspondingly the fruit yield increased by 22% compared to the control treatment. The lowest number and weight of fruits per unit shoot cross sectional area was observed in trees treated with different concentrations of NAA at 10 DAFB. The maximum fruit length (72.3 mm), diameter (75.8 mm), weight (180 g) and fruit volume (240 cm3) were obtained with application of 75 mg L-1 NAA at 10 DAFB. The foliar spray of NAA at 10 DAFB significantly increased the fruit density, flash firmness and TSS. But their use in 25 and 40 DAFB had no significant effect on fruit density and flash firmness, and significantly decreased fruit TSS by 23.9% compared to the control trees. Unlike the TSS, the value of fruit titratable acidity showed a significant increase in all three times of naphthalene acetic acid treatment compared to the control, and as a result, the fruit taste index decreased.
Conclusions The present study concluded that the positive effects of NAA in controlling fruit drop and improving the yield and fruit quality will be different depending on the time of application and the weather conditions of the region under study. According to the results, the application of NAA at 40 DAFB reduced fruit drop and increased fruit yield efficiency.
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