Assessing the Pistil's Role in Sweet Cherry Cultivars Exhibiting Variable Fruit Set

Several sweet cherry (Prunus avium L.) cultivars have compelling quality attributes but are beset with poor productivity (e.g., ‘Benton’, ‘Tieton’, and ‘Regina’). Our current research is investigating the role of temperature on stigma receptivity and ovule viability on fertilization in sweet cherry. We studied four cultivars, ‘Sweetheart’ (self-fertile, high productivity), ‘Benton’ (self-fertile, low productivity), ‘Rainier’ (self-sterile, high productivity), and ‘Tieton’ (self-sterile, low productivity) using controlled temperature chambers. The development of stigma surface, pollen hydration level in vivo, pollen germination in vivo and tube growth in vivo were all observed and counted for the evaluation of stigma receptivity. The degeneration of both the primary and secondary ovules of individual flowers was assessed using fluorescence microscopy. We found that the structure of stigmatic papillae begin to break down by the second day after opening and continued until six days post anthesis at which point there were rarely integral papillae.  The greatest extent of pollen hydration and germination occurred on the stigmas at 2-3 days post anthesis depending on cultivar and temperature. Warm temperatures improved pollen hydration and germination and hastened ovule senescence. After 7 days of flower opening under warm temperatures the proportion of fully viable ovules was as high as 80% in ‘Rainier’ whereas only ca. 25 – 35% were viable in other cultivars. In addition, stigma receptivity of ‘Sweetheart’ was better than other cultivars under low temperature conditions.  Combined, the results suggest that low production of some sweet cherry cultivars in the Pacific Northwest is due to rapid ovule senescence.