Christian Magné

Flower abscission and inflorescence carbohydrates in sensitive and non-sensitive cultivars of grapevine

The Gewurztraminer (GW) and the Pinot noir (PN) cultivars of grapevine differ in their sensitivity to environmental factors that can cause flower abscission, cv. GW being the most sensitive. In order to further define the mechanisms leading to abscission, and owing to the importance of sugars in the achievement of sexual organ ontogenesis, we attempted to correlate the chronology of flower ontogenesis with the variations of carbohydrates in the inflorescence. In the vineyard, under optimal climatic conditions, fruit set of cv. GW and cv. PN was 82% and 65%, respectively. The sugar distribution was different in their inflorescences during the entire duration of flower development. Between stages 15 and 17, flowers of GW and PN reached the crucial meiosis stage. At that time, the inflorescences of cv. GW exhibited higher concentrations of starch and sucrose, whereas those of PN presented higher levels of glucose and fructose. Despite higher starch concentrations in GW inflorescences, starch reserves were present in the ovules and anthers of PN but not in those of GW. These results suggest that the higher content of reserve and transport carbohydrates in the inflorescences of GW favour flower development and fruit set under optimal environmental conditions. Furthermore, since meiosis represents a key step of female development, the different sugar concentrations in the inflorescences of the two cultivars at stages 15 and 17 could be related to the sensitivity to flower abscission under climatic stress. In particular, the presence of starch granules in PN ovules and anthers might explain the higher resistance of this cultivar to flower abscission.

Mobilisation and distribution of starch and total N in two grapevine cultivars differing in their susceptibility to shedding

As a part of a project aimed at elucidating the causal relationship between reserve mobilisation and the extent of shedding in Vitis vinifera L., we compared storage and fate of carbon (C) and nitrogen (N) reserves in two varieties differing in their susceptibility to fruitlet abscission. Merlot (susceptible) and Pinot Noir (P. Noir, not susceptible) vines were grown in trenches under semi-controlled conditions over a 3-y period after planting. Mobilisation of stored C and N, distribution of reserve materials within the vines and 15N uptake were followed particularly during the spring growth flush and floral development in the third year. At dormancy, starch levels in the perennial tissues (roots, trunk, canes) were higher in Merlot than in P. Noir. During the spring growth flush, starch level decreased markedly in the roots of both cultivars until early bloom. At that time, starch started to accumulate in P. Noir but not in Merlot. Similar variations were found with total N. Accordingly, 15N analysis showed that translocation of storage N to the annual tissues was nearly achieved at early bloom in P. Noir while it continued until pea berry size in Merlot. In parallel, N uptake increased during the spring growth flush, and it was higher in P. Noir than in Merlot. These results indicate that transition between heterotrophic (root) and autotrophic (leaf) mode of nutrient allocation towards the developing inflorescences occurs earlier in P. Noir. Possible consequences are discussed in relation to the susceptibility of each cultivar to shedding.

Stress reactions in Vitis vinifera L. following soil application of the herbicide flumioxazin.

In order to evaluate the stress effects of flumioxazin (fmx) on grapevine, a non-target plant (Vitis vinifera L.), physiological parameters such as carbohydrate content, water status or nitrogenous metabolites were investigated on fruiting cuttings and plants grown in vineyard. In the leaves of cuttings, the soil-applied herbicide induced stress manifestations including a decrease of the dry weight percentage and the soluble carbohydrate content during the first week after treatment. Thereafter, a decrease of the osmotic potential was observed, as well as a decrease of total protein content and a parallel accumulation of free amino acids, including proline. Altogether, these results suggest that soil-applied fmx induced a stress in grapevines, leading to leaf proteolysis. However, this stress was partially recovered 3 weeks after herbicide application, suggesting that the cuttings were capable to adapt to the fmx exposure. In the vineyard, the flumioxazin effects were still significant 5 months after the treatment, particularly in the CH cv. They included a decrease of the leaf dry weight percentage and soluble carbohydrate content, as well as an increase of the osmotic potential. The decrease of leaf soluble carbohydrates may have dramatic consequences for the berry growth and the reserve constitution. Moreover, treated plants were characterized by a decrease of the free amino acid content and an accumulation of ammonium, while the protein level did not significantly increase, suggesting a degradation of amino acids. The alteration of carbon and nitrogen status after herbicide treatment may affect the grapevine vigour in a long term.