Developing Self-Fertile Almonds
June 1, 2015 More than 1.5 million honeybee colonies are necessary to pollinate California’s almond crop. But, University of California-Davis, through its almond genetic improvement program, is working to develop genetic options that could require fewer pollinators while also improving productivity and quality. That includes work toward developing self-fruitful almonds.
UC-Davis’s Tom Gradziel says that to be fully self-fruitful, an almond needs to be both self-compatible (its own pollen must be capable of fertilizing its own flowers) and self-pollinating (the structure of the flower must be such that self-pollination occurs at a high frequency over the range of different climates and weather conditions encountered in the wide range of California production regions, as well as over the 20-plus year orchard life expectancy).
“Self-compatibility is a relatively simple genetic trait controlled mainly by a major gene,” Gradziel says. “Consequently, it can be incorporated by simply transferring this major gene conferring self-compatibility. However, the gene for self-compatibility is not naturally present in almond, which is naturally self-incompatible to enforce outcrossing and to reduce the risk of genetic inbreeding… The gene for self-compatibility thus has to be transferred from wild almond or related species.”
Gradziel says European breeders have used the wild almond species Prunus webbii as the source of their self-compatible gene, while Zaiger Genetics (developers of Independence), has used peach Prunus persica.
“In our program at UCD, we have identified and incorporated genes for self-compatibility from multiple sources including peach Prunus webbii, Prunus mira, Prunus argentea, European cultivars as well as induced and natural mutations,” Gradziel says. “In addition, we have done research to identify the mechanisms involved in each distinct source, including development of genetic markers for their reliable identification and transfer.”
Gradziel and the team at UC-Davis are currently developing advanced breeding lines combining multiple sources for self-compatibly to ensure a more consistent expression of the trait over the wide range of growing environments in California.
But self-compatibility is just one piece of the puzzle. Self-pollination is the other piece, and, according to Gradziel, it’s a very complex trait controlled by many genes and influenced by a number of environmental and developmental variables.
“To be consistently self-pollinating, the flower structure must be such that the stigma (the floral structure that receives the pollen and directs it toward fertilization) must be adjacent to the floral anthers (structures that produce the pollen) at anthesis or pollen shedding,” he says. “However, any casual examination of almond flowers at bloom will show that, depending on the particular environment, the style and stigma may remain adjacent to the anthers or may grow out well beyond the anther position. How these differences affect the quality and consistency of self-pollination without insect pollinators remains to be seen.”
Trialing New Selections
Gradziel says there are more than a dozen advanced self-compatible almond selections now in regional testing. The trials are located in all major growing regions and are replicated to allow precise statistical testing of yield differences. An evaluation period of eight to 10 years is desired before any recommendation for release to growers, he adds.
“While tedious and time-consuming, we feel a thorough evaluation of candidate varieties is needed to identify any significant deficiencies before large-scale grower plantings,” Gradziel says. “Most varieties that have been released to growers without such thorough testing have eventually failed commercially.” Common reasons for commercial failure include lack of consistent yields, defects, susceptibility to pests and diseases and bud failure, to name a few.
“In addition, we have also developed test agreements with interested growers to allow them to evaluate these advanced breeding selections on a smaller scale under their own orchard management conditions,” he says.
The ultimate goal, though, is to develop a range of different self-compatible almond varieties that also offer improved nut quality, disease and pest resistance and water use efficiency.
“For example, a self-compatible almond cultivar blooming just before Nonpareil could maximize successful cross-pollination of the commercially critical early Nonpareil bloom, yet, since it is self-compatible, would not require an additional variety to consistently cover its early boom,” Gradziel notes. “Because we have good genetic markers as well as good testing protocols for self-compatibility, we can readily maximize this trait by combining the appropriate genetic sources.”
However, the genetic control and field expression of self-pollination are more complex and remain largely unknown, Gradziel adds. “Thus we are working to optimize aspects of flower structure that would maximize pollen-stigma contact under normal flower development conditions as well as under breezy and windy conditions and/or reduced pollinator densities,” he says. (One beehive per acre or every 2 acres, etc., for example).
Gradziel says recommendations for single variety plantings and/or reduction or elimination of honeybees would only be made after extensive testing with growers.
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