Almost all commercial fruit trees are grafted.

They are in fact made up of two welded plants:

- rootstock for the root system

- the graft for the trunk and canopy

You can see the grafting point on each tree: a stitch down the trunk, still visible even on mature trees.

This ancient technique combines :

- the rootstock's qualities of vigour and resistance

- productivity and flavour of grafted fruit

Citrus trees in general lend themselves well to grafting, but the wrong combination can weaken the resulting tree.

Example:
A Yuzu (up to -16°C) grafted on a Volkameriana (about -5°C) will lose much of its natural resistance to cold.

Extensive commercial cultivation of citrus fruit has spread across all continents over the last two centuries.

In the 19th century, the first rootstocks were mainly bitter orange trees capable of transmitting their increased vigor to the graft without altering the taste of the fruit. Another example is Volkameriana, a natural hybrid (Lemon x Mandarin) probably of Italian origin.

Scientific research and systematic testing began in the USA at the end of the 19th century, organized by the US Department of Agriculture (USDA). USDA botanists, Walter T. Swingle in particular, selected new species and varieties from wild citrus. Among them is Poncirus Trifoliata, native to northern China, the only deciduous citrus whose fruit is inedible. Hybrids were created as early as 1897 and tested for decades, leading to new, ever more productive rootstocks such as Citrumelo Swingle (Poncirus x Pomelo) (1974) and numerous Citrange (Poncirus x Sweet Orange) including Carrizo (1938) and C35 (1987). All of them have become widespread in mass-production regions: Spain and Italy, the United States and Australia.

But researchers never stop, and cross-breeding has continued, with citrus fruits being particularly well-suited.

Since large-scale commercial cultivation has long been, and still is today, limited to areas with a mild climate (in Europe, the Mediterranean rim), the selection of new rootstocks has focused on tolerance to limestone, salt, flooding and the most common diseases and parasites.

In 2003, the University of Valencia obtained Forner-Alcaide 5 (Poncirus x Mandarin), resistant not only to the Tristeza virus, but also to Phytophthora, a root fungus, and to the nematode Tylenchulus semipenetrans, a rootworm. The FA5 also proved to have good tolerance of calcareous soils, moderately salty soils and repeated flooding, as well as exceptional resistance to cold. Thanks to its versatility, it has the potential to become the rootstock of choice north of the Pyrenees and the Alps.

Poncirus trifoliata, the genetic parent of hardy rootstocks, is one of the most cold-hardy species in the Citrus genus. Up to -18°C, even higher for some cultivars. He also passed on this genetic trait, to a greater or lesser extent, to his hybrids:

-14°C for Citrumelo 4475 (Citrus paradisi x Poncirus trifoliata)

-14°C for Citrange C35 (Citrus sinensis Ruby x Poncirus trifoliata)

-15°C to -18°C for Carrizo (Citrus sinensis Washington x Poncirus trifoliata)

-16°C for FornerAlcaide 5 (Citrus Reshni Cleopatra x Poncirus trifoliata)

Citrus trees grafted on these species benefit from a cold-resistant root system.
The graft used must also be resistant to obtain a hardy tree.