Acclimation of apple peel to light and temperature and the effect thereof on red colour development and tolerance to sunburn

Sunburn and poor red colour are the two biggest abiotic fruit quality defects in South African apples. Apples exposed to different microclimates respond differentially to solar radiation and temperature because of biochemical and physiological adjustments (“acclimation”) in the peel. This influences susceptibility to sunburn and poor colour. The overall objective of the study was to assess the influence of rootstock on the innate peel sensitivity to photothermal damage, sunburn and ability to develop red colour, and the role of acclimation to microclimatic conditions in the colour response.

1. Trials on peel photothermal damage (change in F_v/F_m) and visible sunburn damage (2015/16, Objectives 1, 2): These used ‘Rosy Glow’ and ‘Golden Delicious’ fruit from more dwarfing to more vigorous rootstocks, exposed to stress for 1-5 hours and monitored over a five-day recovery period.
2. Laboratory trials on the influence of rootstock and temperature on peel red colour (hue angle) development (2016/17, Objective 3): ‘Rosy Glow’ and ‘Fuji’ fruit on various rootstocks were cold-treated followed by treatment (under lights) of peel discs to a range of six temperatures.
3. Trials to assess the influence of reduced peel temperatures (in situ) in ‘Cripps Pink’ during the mid to late season (Jan-April, Jan-Feb, Feb-March, March-April) on short-term and final red colour development (2016/17, Objective 4).

1. The duration of exposure, recovery period, and canopy light exposure influenced damage to peel photosystems and visible damage. Apples exposed for one hour generally recovered whereas apples exposed for two hours and longer did not recover fully. Shaded inside canopy fruit on all rootstocks responded differently to sun-exposed fruit. The rootstock did not influence photosystem damage and recovery and only minor rootstock effects were found for visible damage.
2. Rootstocks differed in their red colouring ability, with RN29 showing the largest change, M793 significantly less, and G007 and ‘Rosy Glow’ (own roots) the lowest colouring ability. Rootstocks did not differ in the response of colour development over different temperatures. A broad optimum temperature range for red colour development was found (peak at 23°C) for all rootstocks.
3. In situ fruit cooling stimulated red colour development during and sometimes after the cooling period. No final colour differences were found between treatments in 2016. In 2017, fruit cooled between mid-January and mid-March had better final colour (hue angle, % red cover and anthocyanin concentration) than those not cooled.

Objectives 1 and 2: Rootstock did not affect the innate apple peel sensitivity to photothermal stress, with only minor rootstock effects on sunburn symptoms. However, sun/shade canopy position and stress duration influenced photodamage irrespective of rootstock.

Objective 3: Rootstock influenced the innate ability of fruit to develop red colour but did not influence the temperature response of colour development.

Objective 4: Mid- to late-season peel colour responses to lowered temperature did not decrease the red colour development at harvest but increased final red colour in 2017.

Rootstock choices are not constrained by microclimate effects on sunburn and poor red colour, and red colour development following cool weather or evaporative cooling does not jeopardise final fruit colouring.