Apple Pollen Tube Growth and Spur Leaf Injury in Response to Thermal Shock Temperature and Duration

Blossom thinning can confer significant benefits to apple growers, including increased fruit size and annual bearing. Early thinning reduces competition among fruit, which increases cell division and fruit size potential. Blossom thinners are not used in apple crop load management programs in the mid-Atlantic region. Disincentives for use include a lack of registered products, the potential for leaf and fruit injury, and inconsistent results. In 2014, we evaluated the use of short duration forced heated air treatments (thermal shock) as a blossom thinning strategy. In the field setting, 80 uniform ‘York’ spurs were selected when king blossoms were at late balloon stage. On each spur, the side blossoms were removed and each king blossom was emasculated. Spurs were isolated from pollinators with insect netting. On the following day, blossoms were hand pollinated with ‘Rome’ pollen.  Using a variable temperature heat gun, thermal shock treatments were applied to solitary blossoms 24 hours after pollination. The effects of output temperature (five levels) and treatment duration (four levels) were evaluated in a completely randomized design with a factorial treatment structure. A data logging thermometer recorded output temperature of the heat gun, and the distance between the heat gun aperture and blossom or leaf was held constant (2 cm). Blossoms were harvested 96 h after pollination, placed in a labeled vial containing 5% sodium sulfite, and stored at 4 °C until microscopic analysis. Pollen tube growth was observed via fluorescence microscopy, and damage to spur leaves was quantified using a visual rating scale. Short duration treatments (0.5 and 1.0 s) did not reduce pollen tube length or the number of pollen tubes to reach the base of the style. At 2- and 4-s durations, temperatures greater than 56 °C arrested pollen tube growth and prevented pollen tubes from reaching the base of the style.  Visible leaf injury exceeded 10% with the following treatment combinations: 2 s duration at >70 °C and 4 s duration at >56 °C. Pollen tube growth was reduced or arrested at temperature and duration combinations that caused minimal visible injury to spur leaves. Outcomes of this trial suggest that thermal shock may have potential as an apple blossom thinning strategy.