Citrus is unusual in that it produces nucellar seeds. An ordinary seed is produced by the fusion of a pollen cell with an ovule, and this leads to genetic recombination. A nucellar seed is produced asexually, from maternal tissue only. Nucellar seeds are valuable because they do not differ genetically, either among themselves, or from their maternal parent. This means, in effect, that a citrus clone can be produced with nucellar seedlings, but without all the diseases, particularly the virus diseases, that are transmitted by grafts and cuttings, but which are not seed-transmitted (see 11.3.5).

Nucellar seeds can cause confusion because they can give an entirely false indication that a citrus cultivar is homozygous and breeding true to type. They can also be a nuisance in a breeding progeny, because they have to be detected (they are morphologically identical) and removed. In some citrus species, such as oranges, grapefruit, and mandarins, nucellar seedlings often dominate the breeding progeny almost entirely.

Otherwise, citrus breeding is rather like grape breeding (see 11.19.6). There are usually plenty of fertile, non-nucellar seeds, but the variation among them is enormous, and it is difficult to find a new seedling that equals an existing cultivar, let alone surpasses it. Improvements in quality are thus likely to be difficult. However, like grapes, citrus has been plagued by new encounter parasites. A breeding program might be justified on the grounds of attempting to accumulate horizontal resistance in order to reduce or eliminate the use of crop protection chemicals. But such a program will be difficult, and it is a task for specialists. A rootstock resistance breeding program could be very useful, and would be within the capacity of a plant breeding club made up of experienced members, such as citrus farmers who do their own grafting.

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