Sensitivity of apple (Malus domestica Borkh.) fruit peel to high irradiance and temperature as influenced by a range of rootstocks in South Africa

Exposure of developing apple fruit to high irradiation can result in high sunburn incidence in warm production areas such as South Africa. The influence of the rootstock on this response is poorly studied. We determined the photothermal sensitivity (as indicated by chlorophyll fluorescence, F_v/F_m) of peel photosystems of 'Rosy Glow' apples on a range of nine current and new rootstocks varying from dwarfing to vigorous, during 2014-2015 and 2015-2016. This was complemented by in-field measurements on an extended set of 16 rootstocks of leaf and fruit surface temperature and red colour (hue angle) one to three months prior to harvest, and fruit colour, sunburn incidence and maturity at harvest. Fruit from either sun-exposed or shaded canopy positions were subjected to five durations (1-5 h) of exposure to ambient high irradiance and high temperature. Measurements of F_v/F_m were conducted immediately after each exposure duration and repeated over the course of the following four days in the laboratory. Damage to peel photosystems, as indicated by reduced F_v/F_m values, occurred after all exposure durations. Duration of exposure, the recovery period, and previous shading or exposure in the canopy were the dominant influences on photothermal stress response. Apples exposed to photothermal stress for one hour showed a general recovery over the five-day period, whereas apples exposed for two hours and longer did not recover fully. The rootstock did not influence in-field measurements of colour or surface temperature. However, differences between rootstocks were found for final colour, sunburn incidence and maturity. Results indicate that apple peel photosystem sensitivity to high irradiance and temperature is not influenced by the rootstock.