A comprehensive study of the effect of meteorological conditions on fruit abscission and metamitron thinning efficacy in Malus domestica Borkh

Doutoramento em Engenharia Agronómica / Instituto Superior de Agronomia. Universidade de Lisboa

Metamitron is an apple (Malus domestica Borkh.) fruit thinner that acts by reducing the photosynthetic capacity of trees. Thinning is a common practice among apple growers however, the effect of meteorological conditions, per si, and after metamitron spraying, is far from elucidated. In order to investigate the triggered physiological and biochemical processes, trials with shading nets (SN) during 5 days and high nighttime temperature (HNT) during 5 nights, along with high humidity (HH) for 3 h prior spraying were set in three locations, within 3 years. Lower irradiance caused a momentaneous photosynthetic decrease that led to reduced levels of leaf sucrose and sorbitol and consequent increases in fruit abscission rates. HNT pathway is a slight increment in the respiratory mechanisms, but mainly in the whole tree metabolic activity, finally resulting in a shortage in sucrose and sorbitol levels, reducing fruit growth and promoting higher abscission rates. Metamitron acts through a photosynthetic inhibition, including reductions in RuBisCO total activity, translating in less CH production. In addition, it stimulates vegetative growth, consuming CH and slowing down fruit growth rate, and finally enhancing of abscission. The combined treatments resulted in the strongest thinning efficacy, through a suppression of the CH production caused by metamitron in combination with low radiation and or through enhanced expenditure cause by HNT, frequently over-thinning. The effect of HH may enhance fruit drop under humid climates and in younger trees, more susceptible to variations in CH content. This work shows meteorological conditions, namely cloudy days, periods of HNT and increased humidity levels, may affect metamitron thinning efficacy and must be considered in order to decide which rate to apply to achieve an optimum crop load

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