Effect of timing of root pruning and root IBA application on leaf mineral concentration and canopy light interception of 'Cox's Orange Pippin' apple trees

The effects of timing of root pruning (11,P) and application of IBA to roots of 15-year-old 'Cox's Orange Pippin'/M.9 were investigated. Trees were maintained in a weed free strip and received overhead irrigation. Treatments applied were: I Control; 2 RP on 9 May (RP1); 3 RIP on 29 June (RP2); 4 RP1 + 5000 ppm IBA, and 5 RP2 + 5000 ppm IBA. Root pruning was carried out by trenching and cutting (35 cm deep), 70 cm from the tree; trenches were refilled within 6 h. IBA was sprayed immediately after RP with a handheld sprayer. The experiment was a randomized complete block design with 7 replicates. RP1 (9 May) significantly reduced leaf N, K, and Cu concentrations measured on 4 June with respect to control treatment, while IBA had no significant effect. On 3 August, the main effect of both RIP treatments was to reduce significantly leaf N, P and K concentrations. Trees that received RP2 treatment had significantly lower leaf Mg and Ca concentrations than those that received RP1. The mean area of mid-season growth leaves was reduced significantly by RP. For this variable, a significant interaction occurred between IBA application and RIP timing; thus, IBA application decreased mean leaf area at RP1, but it had the opposite effect when applied at RP2. In both vertical and horizontal shoots, RP1 had the greatest reduction in growth either with or without IBA. The effect of treatments on canopy development were indirectly established by measuring non-intercepted radiation through two visible radiation sensors placed on 29 June under each tree at 30 cm above ground level and 30 cm from tree trunk. Non-intercepted radiation was not affected by RP treatments when measured before RP2. Afterwards, both R-P treatments, as well as IBA application reduced canopy development. When radiation interception was measured again, before harvest, only RP timing significantly affected this variable.