Responses of green kiwifruit grown in low-chill area to hydrogen cyanamide application

Kiwifruit presently has become an economic fruit in many countries due to advanced technology of cultural practices. However, such temperate fruit is often affected by irregular environment when grown outside its native area. In Thailand, kiwifruit is grown under low-chill climate, and thus hydrogen cyanamide (HC) is widely applied to compensate for cold winter and release dormancy. Nonetheless, complete bud breaking was found to be variable. This study aimed to find the response of two performances; low and high chill requirement to HC application on bud break and carbohydrate level during annual regrowth period. Two commercial green kiwifruits (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson 'Hayward' and 'Bruno') in a uniform plot located in a highland area (1400 m a.s.l.) with good adaptation to the environment of Thailand were chosen for this study. Dormant buds on 1-year-old fruiting canes at the phenological stage of bud swelling were painted once with 2.5% HC (Dormex®). Bud break rate and number of flower bud at each bud position were recorded and compared with the control (non-HC) canes. Bark at each bud position was collected for soluble sugar analysis at 2-week intervals. Sucrose, fructose and glucose levels were measured by HPLC. The results showed that the first bud position from the apical portion of the cane was able to burst at a similar rate in both HC and non-HC treatments. Bud break rate dramatically decreased with each position toward the base of the cane. For non-HC canes, no bud burst was observed in the 6^th to 8^th positions. HC application significantly increased the bud breaking rate at the 2^nd to 8^th bud position. The high-chill cultivar was more responsive to HC than lowchill one. HC-treated canes showed low sucrose level in the bark. Specifically, sucrose loss followed by the rise of glucose and fructose levels happened earlier in the lowchill cultivar than in the high-chill one. In addition, both glucose and fructose in the bark increased at the 3^rd to 7^th bud positions after HC treatment, resulting in bud development increase when compared with the non-HC canes.