Evaluation of the Reproductive Process as the Cause for Low Fruit Set in Two Japanese Plum Cultivars

The reproductive process has been evaluated to ascertain the causes of low fruit set in two Japanese plum cultivars, 'Sweet August' and 'Rubirosa'. In order to evaluate pollen germination, pollen-pistil incompatibility and ovule viability, different experiments were performed in orchard conditions. Pollen viability was determined by assessing the percentage of in vitro pollen germination. The self-(in) compatibility of both cultivars and their cross-compatibility with 'Champion' were examined by self-and cross-pollination experiments followed by observation of pollen tube growth under the microscope. Likewise, the S-RNase alleles of each cultivar were determined by PCR amplification of the S-RNase gene. Finally, ovule development was examined under the microscope. While 'Sweet August' behaved as self- incompatible, 'Rubirosa' appears to be self- compatible. The observation of pollen germination and pollen tube growth in the different crosses analysed showed that fruit set was not limited by the absence of compatible pollen. However, a high percentage of degenerated ovules were observed in flowers from both cultivars. This premature ovule degeneration is likely to be the cause of low fruit set in both cultivars.