Girdling Effects on Fruit Maturity, Kernel Quality, and Nutritional Value of Walnuts (Juglans regia L.) alongside the Effects on Leaf Physiological Characteristics

Miltiadis V Christopoulos,Mina Kafkaletou,Athanasia D Karantzi,Eleni Tsantili

doi:10.3390/agronomy11020200

Miltiadis V Christopoulos, Mina Kafkaletou + Show 2 more

Open Access

https://doi.org/10.3390/agronomy11020200

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Abstract

Girdling, based on the accumulation of photosynthetic products above the girdling zone, is a common technique applied to tree species to increase the yield and fruit quality. The aim of this study was to investigate the girdling effects on photosynthesis and soluble sugars (Sols) of leaves in fruiting shoots and, for the first time, on quality variables of ‘Franquette’ walnuts harvested at two harvests, early and late, 43 and 84 d after treatment, respectively. Girdling was conducted on a part of four-year old branches at the onset of kernel growth. At late harvest, coinciding with commercial harvest, girdling advanced the mature fruit percentage, kernel weight, and oil content, whereas decreased kernel moisture and slightly affected the color brightness, although kernels remained with an extra light color. Advanced maturation increased linoleic acid and polyunsaturated fatty acid (PUFA) fraction in oil, whereas it decreased palmitic acid and the saturated fatty acid (SFA) fraction in oil, kernel total antioxidants, and Sols (sucrose, glucose, fructose) in kernels and leaves and leaf photosynthesis (Pn), whereas girdling had no effect on these variables. Oil rancidity remained stable throughout the experiment. Development of callus was observed on tree wounds 84 d after treatment. Conclusively, girdling enhanced the fruit maturation, resulting in higher uniformity of maturity and increased oil yield with no quality deterioration or apparent tree damage.

Highlights

English or Persian walnut (Juglans regia L.) trees are cultivated worldwide, withChina being the leading producer with 1,586,367 tons in 2018, corresponding to 43.3%of the global production [1]
Walnuts contain a high percentage (w/w) of oil, which is enriched in polyunsaturated fatty acid (PUFA) [6,7], phytosterol, squalene, and tocopherol [8], and they are a good source of melatonin, serotonin, and magnesium [7]
All early harvested walnuts (EHW) had an intact pericarp and belonged to Class 1 in contrast to LHW when the percentage of Class 1 was less than 20% (Figure 1), a criterion recommended for commercial harvest [12]

Summary

Introduction

English or Persian walnut (Juglans regia L.) trees are cultivated worldwide, withChina being the leading producer with 1,586,367 tons in 2018, corresponding to 43.3%of the global production [1]. There has been an increasing production of walnuts, reflecting on the increasing consumption due to consumer awareness of the beneficial effects on health and prevention of many diseases, such as diabetes, hypercholesterolemia, and cardiovascular disease [2]. Walnuts contain a high percentage (w/w) of oil, which is enriched in PUFA [6,7], phytosterol, squalene, and tocopherol [8], and they are a good source of melatonin, serotonin, and magnesium [7]. They are dense foods consumed as whole kernels (raw or roasted) or as walnut oil [9]. A high quality of walnuts has become challenging [10]

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