2018 ASHS Annual Conference

Apple production in Washington State (WA) occurs under semi-arid climate characterized by high temperatures and solar radiation. As a result, apple growers in WA are increasingly turning towards photoselective protective netting (PN) to reduce the occurrence of sunburn in apple fruit and reduce tree stress. Previous studies on the ecophysiological response of apple tree under netting have just looked at response under PN on short time scales, but not the entire growing season. Our goal was to understand the ecophysiological response of apple tree under PN over the growing season. The experiment was carried out in a fourth leaf ‘Honeycrisp’ apple commercial orchard on B9 rootstock at Quincy, WA. Measurements were done at 32, 66, 100 and 132 days after full bloom (DAFB). Four treatments were evaluated; an uncovered control, 22% blue, 22% red and 19% pearl PN. Leaf gas exchange measurements were done between 08:00 and 11:00 Pacific Standard Time (PST) before midday depression, whilst leaf spectral reflectance, chlorophyll fluorescence and plant water status were measured between 12:00 and 14:00 PST. Data was analyzed using a two-way analysis of variance with PN treatments and time (DAFB) as factors. There was a signification interaction between PN treatments and time for maximum photochemical efficiency of PSII (F_v/F_m). There were no significant differences between treatments in F_v/F_m at 32, 66 and 100 DAFB. Later in the season at 132 DAFB, F_v/F_m was significantly lower for uncovered trees (0.68) compared to PN treatments and had fallen below the threshold of a healthy leaf (0.79). Quantum photosynthetic yield of PSII had significant effects for PN treatments, it was higher under PN treatments compared to the uncovered control. PN treatment also affected leaf gas exchange. Net carbon assimilation was greater under 22% blue and 19% pearl PN compared to the control. The 22% red PN was not significantly different from the control. Stomatal conductance and leaf transpiration followed the same trend as net carbon assimilation. Midday stem water potentials showed a significant effect for time, it was more negative at 66 and 100 DAFB compared to 32 and 132 DAFB. The reduction in solar radiation stress under protective netting increased light-use efficiency over the growing season. This resulted in improved leaf gas exchange under 22% blue and 19% pearl PN. PN is as a potential tool to alleviate tree stress in apple under high light conditions in WA.