IMPROVEMENT OF TROPICAL HORTICULTURAL CROPS AND POTENTIAL OF BIOTECHNOLOGY APPLICATIONS

1. Citrus

The term ‘citrus’ corresponds to three genera: Citrus, Fortunella and Poncirus, all members of the Rutaceae family. The great majority of citrus are diploid. Only a few natural polyploids are known such as the Tahiti lime.

Citrus originate from South-East Asia where they have been domesticated several thousand years ago.
The Mediterranean basin constitutes a second important centre of diversification for certain species (sweet orange, mandarin and lemon). The pomelo is the only species that does not come from Asia as it first appeared in the Carribean region (CIRAD, 1995).

In the 12^th century, citrus fruits were introduced into the northern part of the Mediterranean basin (lime, sour orange, lemon). Sophisticated horticultural practices such as grafting had been mastered at that time.
In the 15^th century, sweet orange and mandarin were brought back from Asia by Portugese navigators.
From the end of the 15^th century, citrus fruits were introduced into the Carribean region and the American continent.
In the 17^th and 18^th centuries, ‘orangeries’ were fashionable in the great European courts; cultivation in pots was widespread and citrus were thus used for decoration and perfumes (CIRAD, 1995).

At the end of the 19^th century and the beginning of the 20^th century, the first industrial orchards were planted in Italy, Spain, North Africa and America.
In the Mediterranean basin, sweet orange was obtained by grafting onto the runners of citron. In the 1920s, the Phytophthora epidemic led to the adoption of rootstock cultivation, generally on sour orange.
During the 1930s, tristeza viral disease caused great damage to the sweet-sour orange associations.
The sour orange was therefore replaced by other rootstocks, such as Poncirus trifoliata and its hybrids with Citrus (CIRAD, 1995).

The 1950s saw the development of modern, efficient citrus fruit growing with the setting up of the first cleansing programmes for plant material. As many citrus diseases are transmitted by grafting, the first cleansing technique used was the selection of nucella plants, as citrus diseases could not be transmitted by seed. Since 1970, this technique has been replaced by thermotherapy and shoot-tip (meristem) micrografting (CIRAD, 1995).

Clonal selection of natural mutants, the usual method for improving citrus tends to increase the variability of such traits as earliness, fruit calibre and yield. This leads to strong diversification, especially for sweet orange, clementin and lemon.
A few colour mutants (natural or induced) have also been created: pink or red pomelos came quickly onto the market in the 1970s.

Sexual hybridization has been practised since the end of the 19^th century. It turns out to be profitable for easy-peeler citrus such as mandarins, which are consumed fresh.
Interspecific hybrids between pomelo and mandarin have been created (tangelo Nova, Orlando, Minneola). A few hybrids among mandarin are also interesting.