Background and Aims: Ripening of table grapes is routinely followed by measurement of soluble solids, acidity, colour and firmness. Non-destructive means to measure ripening can be of great value to determine optimal harvest time and to compare the effect of treatments on ripening.
Methods and Results: The portable fluorescence detector (Multiplex III, Force A, France) generates 12 signals which are processed to ratios which compensate for the structural complexity of the cluster. Sampling was carried out in a Thompson Seedless vineyard in four blocks and over 9 weeks. The simple fluorescence ratio (SFR_R) (correlated to chlorophyll level) decreased exponentially with an R2 value of 0.97, while flourscence excitation ratio (FER_RG) (correlated to anthocyanin level) displayed an increasing linear trend with an R2 of 0.98. The flavonoids (FLAV) ratio which was shown to correlate to the level of flavonoids increased during the first 5 weeks of veraison and then reached a steady level. In an experiment on Crimson Seedless grapes, application of abscisic acid (ABA) resulted in a dose response using the anthocyanin (ANTH) ratio which is the log expression of FER_RG. In a further experiment on Crimson Seedless, the clusters were separated into three colour groups after veraison; ABA and Ethrel were applied, leaving untreated clusters in each colour group as control. The ANTH ratio was able to quantify a significant increase in colour for each group with respect to its control.
Conclusions: The results suggest that portable fluorescence detectors may become important tools to study ripening of table grapes.
Significance of the Study: This is the first report of the use of fluorescence to follow ripening of table grapes.