Controlled Pollination Methods for Creating Corymbia Hybrids

Abstract

Inter-specific Corymbia hybrids are of increasing interest to plantation forestry, yet there is little knowledge of the most suitable controlled pollination methods for this genus. Inter-specific crosses were made between C. torelliana [CT(maternal parent)] and C. citriodora subsp. variegata (CCV), C. henryi (CH) and C. citriodora subsp. citriodora (CCC) using conventional pollination, one-stop pollination (OSP) and artificially-induced protogyny on yellow buds (AIP Y) pollination methods. Additional treatments included AIP on green buds (AIP G) and the use of exclusion bags for the OSP and AIP methods. Inter-specific hybrids (CT x CCV, CT x CH and CT x CCC) were successfully created using all three pollination methods. The AIP Y treatment provided the highest seed yields and achieved time savings of >41% over the conventional and OSP methods, resulting in up to five-fold increases in operator productivity. However, the AIP Y treatment also had the highest C. torelliana contamination levels (9.3-13.2%). The use of exclusion bags with the AIP method had minimal effect on contamination rates, indicating a high proportion of selfpollen contamination. Contamination rates varied between maternal parents, suggesting variation in selfcompatibility for C. torelliana individuals. AIP using semi-ripe green buds was not effective at reducing selfing and had low operator productivity. The AIP method is suitable for use in a large-scale hybrid breeding program for C. torelliana. When self-pollination effects are managed, it could greatly reduce the costs associated with the production of seed of elite family crosses for commercial forestry deployment.

Comparison of three pollination methods for Eucalyptus argophloia, a small-flowered eucalypt

“…The controlled pollination methods (AIP unpollinated, AIP and three-stop pollination) retained 12 % or less of their capsules, whereas open pollination retained ∼40 %, similar to 6 % capsule retention in E. grandis , 5.6-15.5 % for Corymbia hybrids (Dickinson et al 2010) and 5-19 % for some E. grandis hybrids (Horsley et al 2010), but lower than 21 % capsule retention for E. grandis hybrids (Assis et al 2005), 64 % for Eucalyptus urophylla , 71 % for E. grandis (Horsley et al 2010) and 73 % for E. globulus (Suitor et al 2008) using the three-stop method.…”

“…Horsley et al (2010) suggested that damage to the buds of E. grandis, E. smithii (0.7×0.7 cm) and E. macarthurii during emasculation due to small size of the buds was the cause of reduced seed set. Dickinson et al (2010) also found lower capsule retention following AIP treatments in Corymbia hybrids which have large buds compared with E. argophloia and suggested that lower capsule retention may be due to increased physical damage resulting from the pollination method. The number of seeds per retained capsule from the threestop treatment was similar to open pollination, indicating that this procedure was capable of producing seed if capsules were retained.…”

“…Seeds in the AIP unpollinated treatment suggest that the polyester pollination bags were not effective at excluding small pollinators, and self or foreign pollen was introduced. Horsley et al (2010) and Dickinson et al (2010) both report that unpollinated treatments produced seeds confirming a pollen source.…”

“…The three-stop pollination method requires flowers to be emasculated and bagged to prevent unwanted self-or cross-pollen from accessing the stigma. This system is expensive and labour-intensive with risks of increased mortality due to damage to buds during emasculation (Dickinson et al 2010;Van Wyk 1977). Most eucalypts are protandrous, and the stigma is not receptive for up to 10 days after anthesis (Harbard et al 1999;Randall et al 2012).…”

“…This method also reduces the number of operator visits and therefore is less expensive than the three-stop method (Williams et al 1999). The tip of the style is also removed in the AIP method and it has similar advantages to one-stop pollination (Dickinson et al 2010;Tibbits 1989); however, there may be an increased risk of self-pollination because the flowers are not emasculated (Assis et al 2005;Randall et al 2012). This method allows pollination before stigma receptivity and increases the quantity of seed produced in Eucalyptus globulus, E. grandis, E. smithii and E. macarthurii (Horsley et al 2010;Patterson et al 2004;Williams et al 1999).…”