Soil reflectance and colouring of grape. Vine red light excitation could be decisive for grape berry quality

Various conditions of vine « solarisation » with sun light reflecting clothes partially colored in blue, green or red, were performed in field conditions during 1998 and 1999 years.The experiments were conducted both on table grapes (Muscat de Hambourg , Italia and Danuta) and on wine grapes varieties (Carignane, Mourvedre, Shiraz, Muscat a petits grains). The « solarisation » clothes were set up at the cluster closure stage and removed at the harvest time. Effect of various experimental conditions on changes in situ berry colour was examined. The chromatic parameters using the Lab and LCH conventional systems were weekly measured on representative berry samples, and levels of significance of the obtained differences were determined by a variance analysis. Differences in the colouring dynamics and in the values of the chromatic parameters at harvest were statistically significant, in particular in function of the importance of the reflecting area set up per vinestock and also in function of the color of the reflecting cloth used. The results were compared to those obtained from various other analyses : reflectance spectra and composition of berries at harvest, tastings of the table grapes. As a main result, the red colour with a maximum of reflectance at 680 nm appeared as the more effective in improving quality of the berries. These results demonstrate the importance of the reflectance properties of the neighbours of the vine, in particular those of the soil of the parcel, on the composition and the quality of the grape. Moreover, they underline that these properties should be taken in account to explain some misunderstood terroir effects. Finally, they strongly suggest that the excitation of vine photoreceptor systems, specially with the clear red light, could be decisive in the signaling and regulatory mechanisms leading to an optimal ripeness of the grape.