Effects of Metamitron under Different Relative Humidity Conditions on the Fruit Abscission of Malus domestica Borkh. Cultivars

Abstract

Metamitron is an apple (Malus domestica Borkh.) fruit thinner that acts by reducing the photosynthetic capacity of trees. Relative humidity may influence thinning efficacy; however, the broadness of this effect is not yet fully understood. Trials were set in Sint-Truiden (Belgium) in 2018 and Lleida and Girona (Spain) in 2019, using 4-year-old cvs. Braeburn and Elstar trees in Belgium, and 16-year-old cv. Golden Reinders trees in Spain. Four treatments were implemented at the stage of 12–14 mm fruit diameter: (a) CTR—control, trees under natural environmental conditions; (b) HH—high humidity, trees submitted to artificially increased air relative humidity applied for 3 h prior to the beginning of the experiment; (c) MET—trees sprayed with 247.5 mg/L metamitron; (d) MET + HH—trees submitted to the combination of increased humidity (HH) and metamitron (MET) application. In Belgium, metamitron absorption by leaves was greater than in Spain. This might be related to morphological characteristics of the leaves which developed under greater natural relative humidity levels in Belgium than in Spain. Compared to MET alone, ‘Braeburn’ and ‘Elstar’ demonstrated significantly greater metamitron absorption, 59% and 84%, respectively, under MET + HH, accompanied by declines in leaf net photosynthesis (10% and 32%, respectively) and sucrose (31% and 26%, respectively). At harvest, MET + HH treatment reduced yield by 24% and 32% in ‘Braeburn’ and ‘Elstar’, respectively, when compared with MET alone. A large reduction (considered over thinning) in the yield of ‘Elstar’ occurred. In contrast, metamitron absorption by ‘Golden Reinders’ using MET alone was similar to MET + HH; however, there was a slight foliar sugar reduction in the latter treatment. In addition, both treatments enhanced shoot growth and increased fruit abscission with similar improvements in fruit weight and size. In this study, high relative humidity enhanced fruit thinning efficacy under certain circumstances, such as age or genetic predisposal, which left the tree more susceptible to a negative carbohydrate balance. For instance, ‘Braeburn’ and ‘Elstar’ were easier to thin when compared to ‘Golden Reinders’. In addition, this study raises a question that requires further research regarding the impact of HH before and after spraying as well as its effect in combination with higher temperatures.

Response of Malus x domestica Borkh to metamitron and high night temperature: Effects on physiology and fruit abscission

“…In Sint-Truiden, MET+HNT showed a significantly lower P n at 2 DAS and incomplete recovery at 10 DAS, explained by the increase in metamitron absorption verified under these conditions. Metamitron application also reduced g s which is in line with a study developed by Rosa et al (2020) in "Golden" and "Gala" trees, after the application of 247.5 ppm of metamitron.…”

“…The strong P n reduction caused by metamitron would likely limit photoassimilates production, which is in concordance with the lower sugar content observed in MET treatment in both years. A study developed by Rosa et al (2020) in three apple cultivars shows significant decreases in sucrose, sorbitol and total sugars between 20-50% five days after the application of 247.5 ppm of metamitron. In mandarin, Stander et al (2018) reported a 12% shortage in total sugars one day after spraying 300 ppm of metamitron, which persisted until 7 DAS.…”

“…Metamitron is a triazinone herbicide that inhibits photosystem (PS) II and disrupts thylakoid electron transport by blocking the electron transfer between the primary and secondary quinones of PSII (Abbaspoor et al, 2006;Guidi and Degl"Innocenti, 2011). It reduces net photosynthesis and induces a soluble sugar shortage due to a limited carbon fixation (Stander et al, 2018;Rosa et al, 2020), causing an enhancement in fruit abscission (Basak, 2011;Brunner, 2014;Gabardo et al, 2017). The electron transport impairment caused by metamitron, leads to an excess of excited energy that cannot be consumed via CO 2 assimilation (Foyer and Noctor, 2000).…”

Metamitron Thinning Efficacy of Apple Fruitlets Is Affected by Different Rates, Timings and Weather Factors in New York State

“…In New York State, it is common to have cloudy days during the spring period. These cloudy days cause shortages of carbohydrates required to support fruit growth [37]. Our results suggest that cloudy days (total daily solar radiation less than 16 MJ m −2 day −1 ) after metamitron application increased fruit drop.…”