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Question:
Can citrus be grown from seed?

Answer:
Cloning and vegetative reproduction eliminate the variable risks involved with the sexual reproduction of using seed. This is why seeds of good-tasting apples, peaches, pears, or pecans should never be planted in hopes of growing a tree which will produce the same fruit. ONLY the seed of citrus will produce an identical type as the original fruit since citrus seed produce somatic (from a cell) seedlings which do not result from cross-pollination. However, to fruit a seedling citrus may require 5-7 years of thorny juvenile growth.

Nucellar embryony (apomixis) is a form of seed reproduction that occurs in certain plant species, including many citrus varieties. During the development of seeds from plants that possess this genetic trait, the nucellar tissue which surrounds the megagametophyte can produce additional embryos (polyembryony) which are genetically identical to the parent plant. These nucellar seedlings are clones of the parent. By contrast, zygotic seedlings are sexually produced and inherit genetic material from both parents. Zygotic and nucellar embryos can occur in the same seed, and a zygotic embryo can divide to produce multiple embryos.

Nucellar embryony is important to the citrus industry, as it allows for the production of uniform rootstock which yields consistent results in fruit production. However, this trait can interfere with progress in cross-breeding; most commercial scion varieties produce mainly nucellar seedlings which do not inherit any of the traits of the "father" plant.

In botany, apomixis was defined by Hans Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttings or leaves, has never been considered to be apomixis, but replacement of the seed by a plantlet or replacement of the flower by bulbils are types of apomixis. Apomictically produced offspring are genetically identical to the parent plant.

In flowering plants, the term "apomixis" is commonly used in a restricted sense to mean agamospermy, i.e. clonal reproduction through seeds. Apogamy is a related term that has had various meanings over time. In plants with independent gametophytes (notably ferns), the term is still used interchangeably with "apomixis", and both refer to the formation of sporophytes by parthenogenesis of gametophyte cells.

Because apomictic plants are genetically identical from one generation to the next, each lineage has some of the characters of a true species, maintaining distinctions from other apomictic lineages within the same genus, while having much smaller differences than is normal between species of most genera.

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This is why the breeding program of Dr. Moy was so impressive, i.e., he had to physically extract and grow the true embryonic cross (hybrid) to produce ‘Orange Frost’, ‘Arctic Frost’ and ‘Lemon Frost’.

Trifoliate orange is the most cold hardy citrus though its fruit are for the most part non-edible ( Nagle, 1997). Numerous crossed have been made with trifoliate orange to capitalize on this cold hardiness, however for the most part the fruit quality remains poor (Nagle, 1997). ‘Changsha’ tangerine which originated in the foothills of China also has exceptional cold tolerance, but has fair fruit quality having numerous seeds per locule (Sauls, 1981) ‘Changsha’ appears to a hybrid which includes Ichang Papeda (Nagle, 1997). Ichang Paapeda is probably the cold hardiest of the evergreen species of citrus and it is native to the warm temperate inland areas of China (Nagle, 1997). Crosses between ‘Changsha’ and Satsuma could increase cold tolerance and maintain fruit quality. Such a cross resulted from a controlled breeding program in San Antonio, TX, U.S.A. The overall purpose of the breeding program was the creation of Satsuma and tangerine hybrids that exhibit the cold tolerance of the tangerine, but maintain the Satsuma fruit quality.

After he retired from the San Antonio Botanical Garden in 1999, Mr. Moy returned as a part-time employee in 2004 to evaluate his new hardy Satsuma-Changsha seedling crosses (called Changsats). Mr. Moy had genetically crossed the ‘Changsha’ tangerine with a Mandarin Orange (Satsuma) to produce a high quality fruit which is mostly seedless and more cold hardy than Satsuma. This was no small feat and has never been accomplished by any other plant breeder in the world. His hardy Satsuma-Changsha seedling crosses will provide delicious fruit to people who have never been able to reliably grow citrus in their area. The first release named ‘Orange Frost’ (Mr. Moy named it ‘Flourishing’) and his high-yielding, precocious variety he named ‘Bumper’ will be available all over Texas in 2013 and are already being sold in San Antonio in 2011. These selections will be designated Texas SuperStars by Texas A&M.





Two nucellar citrus seedlings surrounding
the true embryonic seedling

White albino true embryonic seedling
surrounded by two green nucellar seedling
 


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