N content of phloem and xylem exudates during the transition to flowering in Sinapis alba and Arabidopsis thaliana

The involvement of nitrogenous substances in the transition to flowering wa s investigated in Sinapis alba and Arabidopsis thaliana (Columbia). Both sp ecies grown in short days (SD) are induced to flower by one long day (LD). In S. alba, the phloem sap (leaf and apical exudates) and the xylem sap (ro ot exudate) were analysed in LD versus SD. In A. thaliana, only the leaf ex udate could be analysed but an alternative system for inducing flowering wi thout day-length extension was used: the displaced SD (DSD). Significant re sults are: (i) in both species, the leaf exudate was enriched in Gln during the inductive LD, at a time compatible with export of the floral stimulus; (ii) in S. alba, the root export of amino acids decreased in LD, whereas t he nitrate remained unchanged - thus the extra-Gln found in the leaf exudat e should originate from the leaves; (iii) extra-Gln was also found very ear ly in the apical exudate of S. alba in LD, together with more Glu; (iv) in A. thaliana induced by one DSD, the leaf export of Asn increased sharply, i nstead of Gln in LD. This agrees with Asn prevalence in C-limited plants. T he putative role of amino acids in the transition to flowering is discussed .