The photosynthesis inhibitor metamitron is a highly effective thinner for ‘Golden Delicious’ apple in a warm climate

Abstract

Introduction. Small fruit size is a limiting factor in marketing apple (Malus ×domestica ). Several techniques have been used to improve fruit size, among them blossom and fruit thinning with plant growth regulators such as auxins and cytokinins to reduce crop load. Materials and methods. In the present study, carried out in 3 consecutive years (2012–2014), we evaluated the photosynthesis inhibitor metamitron (MM) as a thinner for ‘Golden Delicious’ apple. Results. MM applied once at 180–225 g ha-1 (150–190 mg L-1 in the form of 0.1%–0.125% Brevis® at 1200 L ha-1 ) at the 6-mm fruit-diameter stage increased dark-adapted chlorophyll fluorescence, measured as a reduction in Fv/Fm values and resulted in significant fruit thinning. Consequently, the average time spent on hand thinning was reduced from 20–30 days ha-1 to 2–5 days ha-1 and there was a considerable and significant shift to larger fruit size. Discussion. The relatively low effective doses of MM compared to those used in Europe and the USA were likely due to the higher night temperatures for 3 weeks postapplication, which increased respiration and caused assimilation deficiencies during that critical period of fruit development. Conclusion. In a warm growing environment the thinner MM is highly effective on apple, since only one application (FB+7) of relatively low dose (190 mg L-1 MM as 0.125% Brevis® ) is enough for optimal fruitlet thinning.