R.A. Parra-Quezada, T.L. Robinson, J.L. Osborne, A.N. Lakso, L.B. Parra-Bujanda
A growth chamber (GC) study of the interaction of temperature and chemical thinners on fruit set and growth was conducted in 2004 using 4-year-old potted ‘Empire’/M.9 apple trees. Trees which had been grown outside were sprayed with one of two chemical thinning treatments when fruits were 12.5 mm in diameter or left unsprayed. Thinning treatments were Benzyladenine (BA)+Carbaryl or Naphtaleneacetic acid (NAA)+ Carbaryl. Immediately after spraying, trees were either placed in growth chambers with one of four temperature treatments or kept outside with ambient outside temperatures for the next 5 days. Growth chamber day/night temperatures regimes were 15˚/10˚C, 22˚/17˚C and 29˚/24˚C. The average outside day/night temperature was 20˚/9˚C. After 5 days in the GC, the trees were moved back to the field for the remainder of the season. Both spray treated and unthinned control trees were affected by temperature and lower light levels in the GC. Daily fruit diameter increase dropped sharply when trees were moved into the GC regardless of temperature. Net Pn of unsprayed trees in the GC was around 10 µmol CO2•m-2•s-1 compared to 2 for the chemically thinned trees. Six days after thinner application and one day after the trees were moved outside of the GC, fruits from the highest temperature regime (29˚/24˚C) stopped growth and almost all fruits on this treatment abscised on the 13th day after chemical application and 8 days after removal from the GC. BA+Carbaryl and NAA+Carbaryl spray treatments reduced fruit set by 44 and 59% respectively compared to the unthinned control trees. Fruit size and return bloom were also affected by temperature. The effect of the temperature regimes was modeled and the results suggest that the cool temperature regime of 15˚/10˚C or the outside temperatures of 20˚/9˚C showed no carbon deficits for fruit development; while the high temperature regime of 29˚/24˚C resulted in a significant carbon deficit for fruit development, resulting in almost all of the fruits abscising. This was primarily due to higher demand from competing organs.
Parra-Quezada, R.A., Robinson, T.L., Osborne, J.L., Lakso, A.N. and Parra-Bujanda, L.B. (2006). TEMPERATURE AND CHEMICAL THINNER AFFECT FRUIT SET, AND GROWTH OF ´EMPIRE´ APPLE. Acta Hortic. 727, 327-336
DOI: 10.17660/ActaHortic.2006.727.39
Malus × domestica, Benzyladenine, Maxcel, Naphtaleneacetic Acid, Fruitone, Carbaryl, Sevin, cropload, yield, fruit size, growth chambers, return bloom

Acta Horticulturae