Abstract
The chilling requirement and optimum time for hydrogen cyanamide (HC) application were determined for Superior Seedless table grape grown in southern Tunisia, an arid mild winter region. The reliability of five models to predict chilling accumulation for this cultivar was also investigated. In mid-November, current season shoots were excised and subjected to artificial chilling at 7 degrees C for different lengths of time. Each time, half the shoots were treated with a 2% (v/v) aqueous solution of HC, the others were sprayed with distilled water. Thereafter, these shoots were forced to budburst. Rest intensity gradually declined due to chilling accumulation. We estimated that the cultivar needed approx. 440 hours (h) of chilling, or chilling requirement (CR), to overcome endodormancy. During two dormant seasons, estimation of chilling accumulation showed that the Positive Chill Unit model was the most suitable to predict rest completion for Superior Seedless grown under our climatic conditions. Using this model, we found that the variety's CR was not always met by mid-February. In both laboratory and field trials, HC was most effective in enhancing and advancing budbreak if applied when approx. 2/3 of the cultivar's CR were met. Moreover, by this application berry weight and diameter were increased and fruit maturity was advanced. Our study indicated that HC (2%) was effective in advancing budbreak and fruit maturity of Superior Seedless table grape although its effectiveness depended on application date.