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

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.

Effect of Time of Girdling on Leaf Photosynthetic Performance and Kiwifruit Quality Characteristics at Harvest and Post-Storage

“…Both in September as well as at harvest, the leaves in shoots sprouting from girdled canes presented lower photosynthetic rates without any significant changes in intercellular CO 2 or stomatal conductance. Similar results concerning the reduction of photosynthetic rates have been reported by many other researchers, both in kiwifruit [36] as well as in other fruit species [24,[37][38][39][40][41][42], while non-significant impacts have also been observed [31,43]. The reduction of the photosynthetic capacity of the leaf has been attributed to a feedback inhibition mechanism (or feedback sink regulation) [41,42] due to the accumulation of excessive amounts of carbohydrates in the leaves above the girdle zone [24,42,44].…”

“…This increased accumulation of carbohydrates in the growing fruit leads to the advancement of maturation [12,18,31,32,43,59]. This was not clear in the present experiment, as TSS, chlorophylls, TA, as well as sugar concentration of the pulp and fruit firmness did not exhibit significant differences among treatments.…”

“…Higher antioxidant capacity of fruits produced after girdling has been reported in mandarin [4] and plum [16]. Non-significant differences after girdling treatment have been reported, too [43,64], as well as reduced antioxidant capacity [64], revealing the complexity of the effects of this horticultural technique.…”

Assessing the Effect of Plant Biostimulants and Nutrient-Rich Foliar Sprays on Walnut Nucleolar Activity and Protein Content (Juglans regia L.)

“…Juglans regia L., commonly known as walnut, is one of the most cultivated nutproducing species in the world. Walnuts are healthy due to their high nutritional value [1][2][3][4]. The interest in cultivating walnut trees for fruit production in Portugal has risen due to recent market tendencies [5].…”

Foliar Fertilizer Application Alters the Effect of Girdling on the Nutrient Contents and Yield of Camellia oleifera

“…In recent years, scholars have conducted extensive research on the fertilization of C. oleifera forests [ 5 ]; however, fertilization in forestland was found to be difficult and expensive. Tree girdling, as a means of nutrient content regulation, has been widely used in citrus [ 6 ], grape [ 7 ], apple [ 8 ], kiwi [ 9 ], and other fruit trees [ 10 ]. The objective of girdling is to sever the phloem and prevent the flow of carbohydrates to the underground plant parts, thus promoting reproductive organ growth, flowering, and fruit development and quality [ 11 , 12 ].…”