The Rg-1 encoded regeneration capacity of tomato is not related to an altered cytokinin homeostasis.

• Cytokinin (CK) metabolism was analyzed in tomato (Lycopersicon esculentum) Rg-1 hybrids during in vitro shoot organogenesis from root explants. • Data were obtained by combining physicochemical analysis with quantification and in situ detection methods. • Although exogenous zeatin is added in all classical regeneration protocols, we show here that regenerating (Rg+ ) tomato explants did not require an exogenous CK source for regeneration. Irrespective of the presence or absence of exogenous zeatin, the endogenous CK levels were not affected by Rg-1 in the initial explants or in the early callus phase. In a later stage, and related to the presence of numerous shoots, the Rg+ explants showed much lower endogenous CK concentrations than the nonregenerating (rg- ) explants. Cells of rg- explants were not able to differentiate, despite their high endogenous CK content, and did not respond to exogenously applied CKs. • We show that the insensitivity of rg- explants to a hormonal signal, normally initiating regeneration, is not related to an altered endogenous CK metabolism. We therefore postulate that Rg-1 action involves a regeneration-specific CK receptor or a regeneration-specific CK signal transduction pathway.

Selection for aluminium tolerance in embryogenic lines of Citrus sinensis L. and C. aurantium L.

The Citrus cultivars 'Amargo' and 'Pineapple' produce embryogenic cultures on two types of sugars, galactose and lactose. Two percent lactose is optimal for both cultivars; embryogenic callus is produced 8-10 weeks after inoculation. Whereas 2% galactose is optimal for 'Amargo', it is 3% for 'Pineapple'. Three types of embryos are recognized: whitish, greenish and heart shaped. In 8 to 10 week old cultures we do also observe a black spotted type of embryos. A high percentage of plant conversion for both cultivars is observed on a medium containing 30 mg/l of adenine and 2 mg/l of GA3 for 'Amargo'; for 'Pineapple' it is necessary to add BA. We tested two techniques for selection of Al-tolerant lines: one on stagnant liquid medium, and another on shaken liquid medium. The second technique gives a more serious selection pressure; there are almost no surviving calli. In 'Amargo' more calli survived than in 'Pineapple'. Selected calli had to be subcultured nine times to determine their stability. Four callus types of the two cultivars grew normally on selective media containing 0, 400, 1000 or 2000 mumol of aluminium.

Propagation of 'Nam Roi' pummelo (Citrus grandis (L.) Osbeck.) by cuttings.

'Nam Roi' pummelo is a famous fruit tree in the Mekong Delta of Viet Nam. Therefore, in recent years, growers extended the cultured area because it guarantees a high income. As a consequence there is a high demand for planting material, and we studied the possibilities of vegetative propagation by cuttings during the period from March 1999 to February 2000. The results of our study showed that a rice husk substrate was better than sand, and that naphthalene acetic acid (NAA) 1000 ppm stimulated rooting (84% rooted). The type of cutting was also a factor affecting the success rate; we studied three types of cuttings.