A Comparison between Semi- and Fully Compatible Apple Pollinators Grown under Suboptimal Pollination Conditions

set Abstract. Apple (Malus domestica) has a gametophytic self-incompatibility (GSI) system. Consequently, fertilization is achieved by cross-pollination with a compatible pollinator. Compatibility is governed by a multiallelic S locus. Cultivars are fully compatible when both of their S-loci differ and are semi compatible when one locus is identical and the other differs. In a previous study we found that the fruit set and yield of the apple cultivar 'Topred' was reduced when it was pollinated by a semi compatible cultivar. To examine if this occurrence is a general feature in apples grown under suboptimal conditions, three additional cultivars, 'Golden Delicious', 'Granny Smith' and 'Royal Gala', were studied as pollen recipients of semi and fully compatible pollinators. Based on PCR analysis of the S-RNase allele, it was determined that the pollination rate of the semi compatible was signifi cantly lower than that of the fully compatible pollinator in all cases. This was refl ected by the lower fruit set and seed set of 'Golden Delicious' and 'Royal Gala', but not of 'Granny Smith'. In hand pollination experiments, where pollen was in excess, no difference was found between the semi and fully compatible pollinators in all three cases. These results indicate that the low yield, conferred by semi compatible pollinators, is due to insuffi cient cross-pollination (and not to cultivar characteristics). Thus, low yields due to semi compatibility may be avoided by appropriate honeybee management that will increase pollination. Still, under suboptimal conditions, for growth and pollination, full compatibility is preferable. Apples (Malus domestica) are self-incom- patible, and therefore cross-pollination with a compatible cultivar is essential to achieve fruit set and yield (Dennis, 1979, 1986). In the gametophytic self-incompatibility (GSI) system, found in Solanaceae, Scrophularia- ceae and Rosaceae, pollen growth inhibition is controlled by the S locus, which contains, among other genes, a multiallelic RNase gene (S-RNase) (Anderson et al., 1986). The S-RNases play a crucial role in conferring the incompatibility: they enter the pollen tubes during their growth in the style (Luu et al., 2000), resulting in pollen growth arrest. Recent fi ndings suggest that the non-self S-RNases are degraded by the ubiquitin/26S proteasome pathway, while the self S-RNase remains active (Qiao et al., 2004). Depending on their S loci, pairs of apple cultivars can be either fully compatible, when they differ in both S loci, semi compatible, when they carry one different and one identi- cal S locus or incompatible, when both loci are identical. Use of the S-RNase alleles as markers provides an effi cient tool for deter- mining the level of compatibility. In addition, these markers also serve to identify the pollen source of a given fertilization event, and thus enable the measurement of the pollen fl ow in the orchard. Goldway et al. (1999) have found that the major reason for the low yields of 'Topred' apple (a sport of 'Red Delicious') in Israel was due to semi-compatibility with one of its pollinators ('Jonathan'). In the present study we questioned whether this observation was a common feature in apples cultivated in re- gions with suboptimal condition for growth and pollination. Materials and Methods Orchard design. All experiments were conducted in 10-ha commercial apple orchards in the north of Israel. Trees from all cultivars ('Golden Delicious', 'Red Delicious', 'Granny Smith', 'Royal Gala', and 'Jonathan') were of the same age (10 to 13 years), grafted on the rootstock Hashabi 13-4 and planted at a spacing of 2 × 4 m (1250 trees/ha). Row direction was north to south. The experimental plots consisted of a block of two rows of the fertilized cultivar and two adjacent rows of pollinator, semi or fully compatible, on each side.