Desperately Seeking Seedless

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Citrus Nursery Source: Desperately Seeking Seedless

In spite of citrus greening and canker being the focus of most current citrus research in Florida, work to develop resistance to these two diseases has been possible only for a few years due to regulatory issues. On the other hand, efforts to produce seedless scion varieties have been ongoing for decades, and products of those efforts have just become available. Over the years, a number of important citrus scion varieties including Minneola (1930), Sunburst (1979), and Fallglo (1987) have been developed and released from the USDA citrus breeding program in Florida. These varieties have had significant impact on fresh fruit trade in Florida as well as other citrus producing regions. However, a common fault shared by each of these important varieties is that they are seedy; whereas in the current market place, seedless citrus fruit is highly desired and can command higher prices than does seedy fruit.

Two new seedless citrus scion varieties, US Seedless Pineapple and US Early Pride, generated by our program were officially released in 2009. Irradiation of seeds was used to induce the seedless character in Pineapple sweet orange (an important midseason processing orange) resulting in US Seedless Pineapple.

Irradiation of budwood was used to induce the seedless character in Fallglo (an important early season mandarin in Florida) resulting in US Early Pride. Results of extensive testing indicate Seedless Pineapple and Early Pride are essentially identical to their seedy progenitors except for the absence of seeds. Seedlessness is a desirable trait in citrus fruit and having seedless variants of otherwise high-quality, established cultivars is of value to the citrus industry.

Seedless Pineapple budwood is available to the public from the Florida Department of Agriculture and Consumer Services Division of Plant Industry Budwood program Chiefland. Early Pride is being considered for patent protection based on requests from the Florida, California, Arizona, and Texas citrus industries. US Early Pride trees are available through an agreement between the USDA-ARS and the New Varieties Development & Management Corp.

It’s In The Genes

The utility of producing seedless variants of standard citrus scion varieties, as demonstrated with Seedless Pineapple and Early Pride, has caused us to devote considerable effort to irradiation of standard seedy scion varieties as well as seedy, but otherwise promising new hybrids in our breeding program. We currently have about 2,000 grafted trees produced from irradiated budwood of several selections in the field in an effort to identify seedless variants. Fruit produced from the irradiated material is evaluated for the presence of seeds. When low-seeded or seedless variants are identified, they are propagated for further evaluation, a process that requires several years.

Handle On Hybrids

Another approach used in our effort to generate new, valuable citrus varieties is to produce hybrids using standard but seedy cultivars as the female (fruit producing) parent in combination with Seedless Kishu, a citrus variety that is naturally seedless, as the male (pollen) parent. Our hypothesis is by using Seedless Kishu as the pollen parent, we should be able to generate at least some hybrids that produce seedless fruit. We are eagerly awaiting production of fruit by these hybrids so we may determine if any of them are seedless. As with all citrus hybrids, these seedling trees must overcome juvenility before they begin to flower and produce fruit, a process that can take numerous years. In an effort to reduce the length of the juvenile phase and thereby shorten the time to fruit production, we have been growing our seedless Kishu hybrids in the greenhouse and pruning them to a single stem, a technique that has been reported to reduce the length of the juvenile phase.

Continued Development

Regardless of techniques used, development of new citrus varieties is an inherently lengthy process, typically requiring decades from creation of new germplasm, thorough evaluation, and eventual release. Objectives that seem important today may be insignificant in five, 10, or 50 years. The need to develop citrus varieties with resistance to citrus canker and citrus greening were not apparent until just recently, and in fact, efforts to produce new varieties resistant to these diseases were not previously possible. Developing varieties with resistance to canker and greening by utilizing the most advanced technologies available has become the focus of our most recent efforts and already progress has been made. It is virtually certain sustainable solutions to these two diseases will become available. When they do, there will still be the need for seedless, high-quality scion varieties to ensure the U.S. citrus industry maintains a competitive advantage. The USDA citrus improvement program is well positioned to continue to contribute such varieties.

Greg McCollum is a plant physiologist at the USDA-ARS laboratory in Ft. Pierce, FL.

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