Abstract EN:

At the beginning of spring growth, grapevine (Vitis vinifera L. ) mobilise both carbohydrates and nitrogen reserves from the perennial parts before assimilation takes place. The transition between these two nutritional modes, (heterotroph and autotroph respectively), stands as a crucial point, when fast growth of canes and ovule meiosis begins. In turns, a competition in the allocation of assimilates used either for cane growth or flower formation is occurring which may lead to flower senescence and "coulure". Given that the growth of the canes remains prior to inflorescence development until the phenomena of the "nouaison", any delay in the process of assimilation or any lack as far as the reserves are concerned end up with a lack of nutrition, detrimental to inflorescence formation. In that particular case, a strong remobilisation of carbohydrates and nitrogenous reserves takes place in the flowers, and is characterised by a weak plastidic GS (GS2) activity, a strong GDH and proteolytic activity, concomitant to an accumulation of ammonium. Although the cepage sensitive to "coulure" is all the more sensitive as it is more strongly dependent of its reserves and than it has a deficit in glutamate synthase activity with regard to the cepage non sensitive to coulure. In experiments where sources/sink relationships were modified by shading of the leaves or root removal, we found that reserves bears a determining function in flowers production, while primary assimilation strongly determines the rate of nouaison. In particular, following removal, of the roots, a lack of nitrogen within the whole plant was observed. Nitrogen plays a significant role in the "coulure" of the fruiting cuttings. Its excess leads to a total "coulure" and a necrosis of the whole inflorescence. Carbohydrates are also likely to play a role, as well as the enzyme glutamate synthase, which seems inhibited by high nitrogen concentrations. Nevertheless, no strict correlation between the nitrogenous compounds and "nouaison" rates could be established. Using immunocytochemical experiments we showed that cytosolic glutamine synthetase and mitochondrial glutamate dehydrogenase are mostly present in the phloem companion cells suggesting that they both play a major role during the transport of nitrogenous compounds to the flowers. This finding strengthens the current hypothesis that C and N molecules translocation is one of the limiting factors that controls flower formation.