Evaluating the Pollen Tube Growth Characteristics of Different Crabapple Cultivars

Reducing crop load improves apple (Malus xdomestica Borkh.) size and color, and increases return bloom therefore minimizing biennial bearing. Chemically thinning apple trees during bloom can be an efficient way to reduce crop load. Most bloom thinning chemicals are caustic, causing damage to floral organs, thus preventing fertilization. However, the precise timing of bloom thinning applications has been somewhat subjective. Understanding apple pollination and fertilization, specifically pollen tube growth rates, can help improve application timing, greatly improving chemical thinning efficacy and reliability. Crabapples are often used as pollinizers in commercial orchards. The crabapple ‘Snowdrift’ has been used to develop a Pollen Tube Growth Model for estimating application time of bloom thinning chemicals. However, other crabapple genotypes may have different pollen tube growth rates. In 2014, emasculated apple blossoms located on ‘Fuji', ‘Golden Delicious', and ‘Pink Lady’ dwarfed trees grown in root bags were hand pollinated with pollen from one of five crabapple cultivars, (Evereste, Indian Summer, Snowdrift, Selkirk, and Thunderchild) and placed into growth chambers for 24 hours at temperatures of 12 °C, 18 °C, 24 °C, or 30 °C. Pollen tube growth was ceased after 24 hours by placing the blossoms in a 5% sodium sulfite preservative solution. The styles from pollinated blossoms were stained with a water-soluble fluorescent solution containing 0.01% Aniline Blue dye in 0.067M K₂HPO₄ for pollen tube visualization using a Nikon Eclipse Ci microscope (Tokyo, Japan) and a Nikon Intensilight C-HGFI (Tokyo, Japan). Data collection included pollen germination on the stigmatic surface, number of pollen tubes penetrating through the base of the stigma, length of the longest pollen tube, style length, and the number of pollen tubes that grew to the base of the style. The General Linear Model Procedure (SAS Inst., Cary, NC) was used to perform an analysis of variance on the collected data. Preliminary results from 2014 suggest pollen from ‘Selkirk’ and ‘Thunderchild’ grew significantly further down the styles of all three maternal cultivars followed by ‘Indian Summer', ‘Evereste', and ‘Snowdrift’.  On average ‘Selkirk’ and ‘Thunderchild’ pollen tubes grew 57.2% and 53.6% of the total style length, respectively. This project is being replicated in 2015. Significant differences between the pollen tube growth rates of pollen genotypes will help improve chemical thinning application timing and therefore improve crop load management in apple orchards.