International Symposium on Asian Pears@(2001.8.25-29) 

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A New Approach to Controlling the Flowering Date for Japanese Pear Using Reflective Film Technique

H. Honjo*, Y. Kobayashi, M. Watanabe and R. Fukui

Faculty of Agriculture, Utsunomiya University, Utsunomiya 321-8505, Japan

     For many fruit growers, high dependence on outdoor cultivation produced new variable factors in their management. This recent situation increased the importance of protected cultivation, which can be controlled from the onset of flowering to the time of harvest. Although a large number of studies have been made, it is not definite that we can perfectly control their growth and development because of the lack of sufficient information on dormancy of deciduous fruit tree crops.

     The aim of this study is to improve the method of controlling the flowering date of Japanese pear without using any chemicals, heating equipment or extensive labor.

Materials and Methods

     The experiment was conducted at the test field of Utsunomiya University using potted Japanese pear trees (cvs. Kosui & Hosui) grown in outdoors. Exp.1: In early November 1998, 40 trees each were used for 4 treatments. Non-woven fabric made of white porous reflective polyethylene film (Du Pont, Tyvek) was used as a covering material. Ten trees each were assigned for 4 treatments; 1) covered above the top of the trees with reflective film, 2) mulched under the tree canopy, 3) covered above and under the tree canopy, and 4) Control (no covering or mulching applied). Covering treatments were continued until early March (Fig.1), and then all trees were kept free in the open field. Exp.2: In early November 1999, 40 trees each were assigned for 3 treatments; 1) covered above the top of the trees, 2) mulched under the tree, and 3) Control. A half of the 1) and 2) trees were switched to 3) at the end of December, and the rest of the 3) trees were converted into 1) or 2) in early February. Air temperatures were recorded with thermometers at intervals of 30 minutes. Bud temperatures were measured with 0.1mmcopper-constantan thermocouples. Buds were observed every 2 days until April.

Results and Discussion

     Tyvek film was characterized with a high albedo (reflectivity of short- and long-wave radiations). It was reported in the preceding report (Honjo et al., 1999) that the film covered above the top of the trees (named floating cover, Fig.2), inhibited temperatures to rise at daytime and kept canopy temperature warmer than the open field at night. The diurnal range of temperature under the floating cover became smaller than that in the open field. Thus, it was expected that the prolonged period of low temperatures was effective in breaking the endodormancy in terms of its chilling requirement. But on the contrary, the diurnal range for the film mulching under the tree canopy was expanded more than that in the open field, as a consequence of the rise in daytime temperature due to the high reflectance of solar radiation.

     In Exp.1, eHosuif and eKosuif started flowering first in treatment 3), and thereafter in the order of 1), 2) and 4). Flowering in treatment 3) began 4 days earlier as compared with control. In Exp.2, eKosuif started flowering first in 1) to 3) switch treatment and the 3) to 2) switch, followed by 2) to 3) switch, 3), 2), 1), and 3) to 1) switch. A similar result was obtained for eHosuif. Difference between the first and last flowering treatments was about 10 days.

Covering over the trees before endodormancy break forced flowering, but covering during ecodormancy retarded flowering. By contrast, mulching under the trees during ecodormancy forced sprouting and flowering. In conclusion, we had developed a new labor-saving method of controlling (forcing and/or retarding) the flowering of Japanese pear.    

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Fig.1. Japanese pear trees at the test field (1998).           Fig.2. Covering over the trees (1998).