Metamitron and Shade Effects on Leaf Physiology and Thinning Efficacy of Malus × domestica Borkh

Nídia Rosa,Paula Scotti-Campos,Glória Àvila,Luísa Cristina Carvalho,Cristina Moniz Oliveira,Joaquim Carbó,Miguel Pedro Mourato,Anabela Bernardes Da Silva,José Cochicho Ramalho,Isabel Pereira Pais,Wim Verjans,Luís Asín,Joan Bonany,Luísa Louro Martins

doi:10.3390/agronomy10121924

Abstract

Thinning strategies, namely shade or photosynthetic inhibitors, rely on the reduction of carbon supply to the fruit below the demand, causing fruit abscission. In order to clarify the subject, seven field trials were carried out in Lleida, Girona, and Sint-Truiden (2017 + 2018), using orchards of ‘Golden’ and ‘Gala’ apple trees. At the stage of 9–14-mm fruit diameter, four treatments were implemented: (A) CTR-control, trees under natural environmental conditions; (B) SN-shaded trees, trees above which shading nets reducing 50% of irradiance were installed 24 h after metamitron application date—without application of metamitron—and removed after five days; (C) MET-trees sprayed with 247.5 ppm of metamitron; (D) MET + SN-trees submitted to the combined exposure to metamitron application and shading nets. Low radiation significantly increased metamitron absorption (36–53% in the three locations in 2018) and reduced its degradation. Net photosynthesis and stomatal conductance were strongly reduced in all treatments, with minimum values 2 days after spraying (DAS) and incomplete recovery 10 DAS in MET + SN. All treatments resulted in leaf sucrose and sorbitol decreases, leading to a negative carbon balance. SN and MET + SN promoted the highest thinning efficacy, increasing fruit weight and size, with MET + SN causing over-thinning in some trials. Leaf antioxidant enzymes showed moderate changes in activity increases under MET or MET + SN, accompanied by a rise of glutathione content and a reduction in ascorbate, however without lipid peroxidation. This work shows that environmental conditions, such as cloudy days, must be carefully considered upon metamitron application, since the low irradiance enhances metamitron efficacy and may cause over-thinning.

Highlights

Annual apple (Malus domestica Borkh.) production has increased steadily, becoming the third most produced fruit in the world in 2018, with 86 million metric tons [1]
Our results show that metamitron, shade, and their combination increasingly reduced the number of fruits
Shading net and/or metamitron application significantly limited C-assimilation, with after effects observed until 10 days after spraying (DAS)

Summary

Introduction

Annual apple (Malus domestica Borkh.) production has increased steadily, becoming the third most produced fruit in the world in 2018, with 86 million metric tons [1]. There are several widely used chemical thinning agents, including metamitron This triazinone herbicide is a systemic xylem-translocated compound, which inhibits photosystem (PS) II by blocking the electron transfer between the primary and secondary quinones, leading to the closure of the reaction centres and disrupting thylakoid electron transport [4,5], which, will greatly reduce photosynthetic carbon fixation [4,6]. The photochemical reactions provide reducing power, nicotinamide adenine dinucleotide phosphate (NADPH), and chemical energy molecules, adenosine triphosphate (ATP), both of which are essential for sugar production resulting from the carboxylation capability of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and Calvin-Benson cycle functioning

Methods

Results

Discussion

Conclusion