Fruit and Leaf Characteristics of ‘Granny Smith' Apple Trees as Affected by Early Shading

Shade nets can be an effective option to protect fruit trees from hail and sunburn, which have increased as a consequence of climate change. The effects of early shading on fruit and leaf characteristics of Malus ×domestica Borkh cv. ‘Granny Smith’ on MM.111 rootstock were evaluated in the High Valley region, Río Negro, Argentina. The trees were planted in 1988, trained to palmette leader, spaced 4.0 ´ 3.0 m and grown in sandy loam, at the orchard of the Comahue National University (lat. 38º56’ S, long. 67º59’ W). Row orientation was north-south. The experimental site was located in an arid region, with average annual rainfall of 250 mm. Relative humidity, sunshine duration and temperature were continuously monitored. Treatments were: a) application of shade, by covering individual branches with an 80 % black shade net, from 35 to 56 days after full bloom (DAFB), and b) control. Full bloom occurred on 2 October 2009. Fruit diameter (FD) was measured at two-weekly intervals. Additionally, fruit weight (FW), leaf area (LA), specific leaf weight (SLW), and chlorophyll content (CC) were determined at 56 DAFB. LA and CC were measured with a Cid – 202 leaf area meter and a Minolta SPAD – 502 portable apparatus, respectively. Means were compared using Student’s t-test to determine significant differences (P ≤ 0.05). Regression models were estimated using the Statistical Analysis System software. Lineal models best fit fruit growth and substantial differences were found between treatments: a) FD = 0.8093 DAFB – 4.6037, R² = 0.81 (n = 264), and b) FD = 0.6528 DAFB + 0.1672, R² = 0.77 (n = 254). Light reduction decreased FD by 7.57 % and FW by 21.2 %, as compared to the control. However, under the same conditions, LA, SLW and CC were not affected: 32.1 vs. 34.4 cm², 5.11 vs. 6.19 mg/cm², and 39.2 vs. 39.0 SPAD values for treatments a) and b), respectively. Further studies are needed to determine how shade conditions affect vegetative and reproductive growth of ‘Granny Smith’ apple trees.