Effects of temperature and light level on efficiency of chemical thinner on 'Empire' apple trees

Two studies of the interaction of temperature and sunlight on chemical thinning efficacy were conducted in 2006 and 2007 using six, or seven-year old potted ‘Empire’ apple trees on M.9 rootstock. Trees were sprayed with 7.5 mg∙L-1 Napthaleneacetic acid (NAA, Fruitone N) plus 600 mg∙L-1 Carbaryl (Sevin XLR) (2006 and 2007), or 100 mg∙L-1 Benzyladenine (BA, Maxel) plus 600 mg∙L-1 Carbaryl (2007) when king fruits were 10-12 mm in diameter. Following the application of chemical treatments, the trees were placed in one of three glasshouses with different temperature regimes for 4 days (2006) or 5 days (2007). The day/night temperature regimes were: 29/21°C (HT), 26/18°C (IT) and 24/13°C (LT). Half of the trees in each room were covered with 70% shade net and the other half received full sunlight. After the 4 or 5 day treatment period in the glasshouse, the trees were moved back to the field. In 2007, net photosynthesis (Pn) was significantly reduced under 70% shade (6.1-9.8 μmol∙m-1∙s-1) compared to the full sunlight treatment (9.0-11.9 μmol∙m-1∙s-1) at each of the temperature regimes regardless of chemical treatments. Pn was dramatically reduced by the HT treatment. No significant differences in Pn were observed among chemical treatments. The daily increase in fruit diameter was essentially stopped by the LT and in the HT treatments after 2 days in the glass house, while fruits continued to grow in IT treatment. Fruit growth under 70% shade decreased to almost zero after 1 day in the glasshouse; while fruits of fully exposed trees continued to grow. There was no difference in fruit growth between NAA+cabaryl and BA+carbaryl. Fruit set was negatively related to day and night temperature. Regardless of temperature regime, fruit set on trees under 70% shade was significantly lower (0.2-6.2%) compared to trees in full sunlight (8.6-46.4%). Fruit set of trees in full sunlight was much higher in the LT treatment (33.2-46.4.%) than in the IT treatment (8.6-27%) or in the HT treatment (15.8-22.7%). Fruit set was not different among chemical thinner treatments.