2017 ASHS Annual Conference

Apple production in Washington State occurs under semi-arid climate characterized by high temperatures and solar radiation. Heat stress events occur regularly during the growing season. Historical weather data showed that on average, 33% of the days during the growing season exceed ambient air temperature of 30 °C. Apple growers in Washington State are increasingly turning towards photoselective protective netting to reduce the occurrence of sunburn in apple fruit. Our goal was to understand the ecophysiological response of apple under blue photoselective protective netting during a heat stress event. An experiment was conducted on a day that had been predicted to be over 30 °C. The experiment was conducted on potted ‘Honeycrisp’ apple trees during a heat stress event on June 6, 2016 at the WSU Tree Fruit Research and Extension Center in Wenatchee, Washington State, USA (Latitude: 47°26’29.1” N; Longitude 120°20’90.1”W). Two treatments were evaluated, an uncovered control and blue photoselective protective netting. Plant water status, chlorophyll fluoresce, leaf gas exchange and leaf spectral reflectance measurements were done at two-hour intervals from 08:00 to 16:00 HR. Results showed no differences in ambient temperature, soil temperature and above canopy relative humidity between the treatments, whilst photosynthetic photon flux density was reduced by 22% under blue photoselective netting. Midday stem water potential and leaf water potential at 08:00 and 12:00 HR were significantly higher under photoselective netting compared to the uncovered control. Maximum photochemical efficiency of PSII (F_v/F_m) was significantly higher at all measurement times under blue photoselective protective netting compared to control. F_v/F_m went below 0.79, which is considered the threshold for stress at 10:00 HR in the control and at 12:00 HR under blue photoselective protective netting. Quantum photosynthetic yield of PSII (Y(II)) was significantly higher under blue photoselective protective netting at 08:00, 12:00 and 16:00 HR compared to the control. Results from leaf spectrometry showed that photochemical reflectance index (PRI) which is a light use efficiency index was better under the blue photoselective compared to the control. From the results, apple trees under blue photoselective protective netting performed much better during a heat stress event compared to an uncovered control. Photoselective protective netting is a potential tool for apple growers to reduce tree stress under harsh conditions.